While chemotactic drug targeting holds great promise for drug delivery, there are key advantages and limitations that must be considered. One main advantage is that these systems can precisely target specific cells, tissues, or organs within the body while minimizing their disruptive effects on healthy tissue. Moreover, by delivering the drug directly to the desired target, researchers can effectively reduce the required drug dosage needed. However, some limitations to chemotactic drug targeting include issues with biocompatibility, drug-carrying capacity, and the life span of specific carriers. Another major challenge with this approach is motility, when either the chemical stimuli diminish, or the attached enzymes become oversaturated. This can limit the effectiveness of the drug delivery system and may require additional modifications to improve its performance. Thus, although these approaches have shown great promise, more research is still needed to fully understand chemotaxis mechanisms and optimize this property for targeted drug delivery strategies.


In  pharmacokinetics , a  compartment  is a defined volume of  body fluids , typically of the  human body , but also those of other animals with multiple  organ systems . The meaning in this area of study is different from the concept of  anatomic compartments , which are bounded by  fasciae , the sheath of  fibrous tissue  that enclose  mammalian  organs. Instead, the concept focuses on broad types of fluidic systems. This analysis is used in attempts to mathematically describe distribution of small molecules throughout organisms with multiple compartments. Various  multi-compartment models  can be used in the areas of pharmacokinetics and  pharmacology , in the support of  efforts in  drug discovery , and in  environmental science .


In humans and related organisms, there are five major body compartments: the  blood plasma ,  interstitial fluids ,  fat tissues ,  intracellular fluids , and  transcellular fluids , the latter of which includes fluids in the  pleural  ( peritoneal ) cavity. [ 1 ] [ verification needed ]  The relative percents of body mass of these are included in the following table.


Context-sensitive half-life  or  context sensitive half-time  is defined as the time taken for  blood plasma  concentration of a drug to decline by one half after an infusion designed to maintain a  steady state  (i.e. a constant plasma concentration) has been stopped.  The "context" is the duration of infusion.


When a drug which has a multicompartmental  pharmacokinetic  model is given by  intravenous   infusion  it initially will distribute to the central compartment and then move out of this compartment into one or two peripheral compartments.  Once this infusion is discontinued, drug continues to move into the peripheral compartments until an  equilibrium  is reached.  At this  time, the only way drug may leave plasma is by  metabolism  or  excretion .  As the plasma concentration falls, the  concentration gradient  of drug reverses and drug moves from peripheral compartments back into plasma, maintaining the plasma concentration of the drug, often prolonging the pharmacological effect.  If an infusion has reached steady state then the context-sensitive half-life is equal to the  terminal plasma half-life  of the drug.  Otherwise  it will be shorter than the terminal elimination half-life.


Remifentanil  is relatively context insensitive whilst  fentanyl  and  thiopentone  are examples of drugs which have significant context-sensitive changes in their half-life. [ 1 ]


The Context-Sensitive Half-Time describes the time required for the plasma drug concentration to decline by 50% after terminating an infusion of a particular duration. [ 2 ]


It is the time required for drug plasma concentration to decrease by 50% after stopping administration. The drug is administered continuously.


Remifentanil is relatively context insensitive. Fentanyl and thiopental are examples of drugs which have significant context-sensitive changes in their half-life.


The  Cooperstown cocktail [ 1 ]  refers to a panel of four drug probes used in human  pharmacokinetic  studies to determine the activity of  drug metabolising  enzymes. The terminology 'cocktail' refers to the fact that the drug probes are given together.


The Cooperstown cocktail consists of four drugs that are considered specific substrates for four  cytochrome P450  (CYP) isoforms. One of the drugs ( caffeine ) provides, through its  metabolites , substrates for two additional enzymes.


The drugs and the enzymes they probe are as follows - caffeine (probes  CYP1A2 ,  N-acetyltransferase 2 ,  xanthine oxidase ),  midazolam  (probes  CYP3A ),  omeprazole  (probes  CYP2C19 ) and  dextromethorphan  (probes  CYP2D6 ). After giving the cocktail, the concentrations of the drugs and their metabolites in plasma (for midazolam and omeprazole) and urine (for caffeine and dextromethorphan) are determined at various times. By analysing these concentrations, it is possible to determine the activity (i.e. the  phenotype ) of the relevant enzyme. Caffeine can be used as a probe for three different enzymes by measuring several of its urinary metabolites and comparing their relative concentrations.


The 'Cooperstown 5 + 1 cocktail', [ 2 ]  in addition to the four drug probes mentioned above, incorporates  warfarin  as well. Warfarin (actually the S-warfarin  enantiomer ) is a specific probe for CYP2C9. The '+ 1' refers to the vitamin K that is given together with the warfarin to prevent any anticoagulant effect.


The Cooperstown cocktail and the Cooperstown 5 + 1 cocktail are powerful tools for investigating the activity of important drug metabolising enzymes. They are used in human  drug interaction  studies in which the ability of a study drug to inhibit or induce cytochrome p450 enzymes is studied.


Cytochromes P450  ( P450s  or  CYPs ) are a  superfamily  of  enzymes  containing  heme  as a  cofactor  that mostly, but not exclusively, function as  monooxygenases . [ 1 ]  However, they are not omnipresent; for example, they have not been found in  Escherichia coli . [ 2 ]  In mammals, these enzymes oxidize  steroids ,  fatty acids ,   xenobiotics , and participate in many biosyntheses. [ 1 ]  By hydroxylation, CYP450 enzymes  convert xenobiotics into hydrophilic derivatives, which are more readily excreted.


P450s are, in general, the terminal oxidase enzymes in  electron transfer  chains, broadly categorized as  P450-containing systems . The term "P450" is derived from the  spectrophotometric  peak at the  wavelength  of the  absorption maximum  of the enzyme (450  nm ) when it is in the  reduced  state and complexed with  carbon monoxide . Most P450s require a protein partner to deliver one or more  electrons  to reduce the  iron  (and eventually molecular  oxygen ).


Genes  encoding P450 enzymes, and the enzymes themselves, are designated with the  root symbol   CYP  for the  superfamily , followed by a number indicating the  gene family , a capital letter indicating the subfamily, and another numeral for the individual gene. The convention is to  italicize  the name when referring to the gene. For example,  CYP2E1  is the gene that encodes the enzyme  CYP2E1 —one of the enzymes involved in  paracetamol  (acetaminophen) metabolism. The  CYP  nomenclature is the official naming convention, although occasionally  CYP450  or  CYP 450  is used synonymously. These names should never be used as according to the nomenclature convention (as they denote a P450 in family number 450). However, some gene or enzyme names for P450s are also referred to by historical names (e.g. P450 BM3  for CYP102A1) or functional names, denoting the catalytic activity and the name of the compound used as substrate. Examples include  CYP5A1 ,  thromboxane  A 2  synthase, abbreviated to  TBXAS1  ( T hrom B o X ane  A 2   S ynthase  1 ), and  CYP51A1 , lanosterol 14-α-demethylase, sometimes unofficially abbreviated to LDM according to its substrate ( L anosterol) and activity ( D e M ethylation). [ 3 ]


The current nomenclature guidelines suggest that members of new CYP families share at least 40%  amino-acid  identity, while members of subfamilies must share at least 55% amino-acid identity. Nomenclature committees assign and track both base gene names ( Cytochrome P450 Homepage   Archived  2010-06-27 at the  Wayback Machine ) and  allele  names ( CYP Allele Nomenclature Committee ). [ 4 ] [ 5 ]


Based on the nature of the electron transfer proteins, P450s can be classified into several groups: [ 6 ]


The most common reaction catalyzed by cytochromes P450 is a monooxygenase reaction, e.g., insertion of one atom of oxygen into the aliphatic position of an organic substrate (RH), while the other oxygen atom is  reduced  to water:


Many  hydroxylation  reactions (insertion of  hydroxyl  groups) use CYP enzymes, but many other hydroxylases exist.   Alpha-ketoglutarate-dependent hydroxylases  also rely on an Fe=O intermediate but lack hemes.  Methane monooxygenase, which converts methane to methanol, are non-heme iron-and iron-copper-based enzymes. [ 7 ]


The active site of cytochrome P450 contains a heme-iron center. The iron is tethered to the protein via a  cysteine   thiolate   ligand . This cysteine and several flanking residues are highly conserved in known P450s, and have the formal  PROSITE  signature consensus pattern [FW] - [SGNH] - x - [GD] - {F} - [RKHPT] - {P} -  C  - [LIVMFAP] - [GAD]. [ 8 ]   In general, the P450 catalytic cycle proceeds as follows:


Mechanistic details, including the  oxygen rebound mechanism , have been investigated with synthetic analogues, consisting of iron oxo heme complexes. [ 14 ]


Binding of substrate is reflected in the spectral properties of the enzyme, with an increase in absorbance at 390 nm and a decrease at 420 nm. This can be measured by difference spectroscopies and is referred to as the "type I" difference spectrum (see inset graph in figure).  Some substrates cause an opposite change in spectral properties, a "reverse type I" spectrum, by processes that are as yet unclear.  Inhibitors and certain substrates that bind directly to the heme iron give rise to the type II difference spectrum, with a maximum at 430 nm and a minimum at 390 nm (see inset graph in figure).  If no reducing equivalents are available, this complex may remain stable, allowing the degree of binding to be determined from absorbance measurements  in vitro [ 13 ]
C: If carbon monoxide (CO) binds to reduced P450, the catalytic cycle is interrupted. This reaction yields the classic CO difference spectrum with a maximum at 450 nm. However, the interruptive and inhibitory effects of CO varies upon different CYPs such that the CYP3A family is relatively less affected. [ 15 ] [ 16 ]


In  biochemistry ,  cytochrome P450 enzymes  have been identified in all  kingdoms  of life:  animals ,  plants ,  fungi ,  protists ,  bacteria , and  archaea , as well as in  viruses . [ 1 ]  As of 2018 [update] , more than 300,000 distinct CYP proteins are known. [ 2 ] [ 3 ]


Human P450s are primarily membrane-associated proteins [ 4 ]  located either in the inner membrane of  mitochondria  or in the  endoplasmic reticulum  of cells. P450s metabolize thousands of  endogenous  and  exogenous  chemicals. Some P450s metabolize only one (or a very few) substrates, such as  CYP19  ( aromatase ), while others may metabolize multiple  substrates . Both of these characteristics account for  medicinal interest . Cytochrome P450 enzymes play roles in  hormone  synthesis and breakdown (including  estrogen  and  testosterone  synthesis and metabolism),  cholesterol  synthesis, and  vitamin D  metabolism. Cytochrome P450 enzymes also function to metabolize potentially toxic compounds, including  drugs  and products of endogenous metabolism such as  bilirubin , principally in the  liver .


The  Human Genome Project  has identified 57 human genes coding for the various cytochrome P450 enzymes. [ 5 ]


P450s are the major enzymes involved in  drug metabolism , accounting for about 75% of the total metabolism. [ 7 ]  Most drugs undergo deactivation by P450s, either directly or by facilitated  excretion  from the body. However, many substances are  bioactivated  by P450s to form their active compounds like the  antiplatelet drug   clopidogrel  and the opiate  codeine .


The CYP450 enzyme superfamily comprises 57 active subsets, with seven playing roles in the metabolism of most pharmaceuticals. [ 8 ]  The fluctuation in the amount of CYP450 enzymes (CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP3A4, and CYP3A5) in phase 1 (detoxification) can have varying effects on individuals, as genetic expression varies from person to person. This variation is due to the enzyme's genetic polymorphism, which leads to variability in its function and expression. To optimize drug metabolism in individuals, genetic testing should be conducted to determine functional foods and specific phytonutrients that cater to the individual's CYP450 polymorphism. Understanding these genetic variations can help personalize drug therapies for improved effectiveness and reduced adverse reactions. [ 9 ]


Many drugs may increase or decrease the activity of various P450  isozymes  either by inducing the biosynthesis of an isozyme ( enzyme induction ) or by directly inhibiting the activity of the P450 ( enzyme inhibition ). A classical example includes  anti-epileptic drugs , such as  phenytoin , which induces  CYP1A2 ,  CYP2C9 ,  CYP2C19 , and  CYP3A4 .


Effects on P450 isozyme activity are a major source of adverse  drug interactions , since changes in P450 enzyme activity may affect the  metabolism  and  clearance  of various drugs. For example, if one drug inhibits the P450-mediated metabolism of another drug, the second drug may accumulate within the body to toxic levels. Hence, these drug interactions may necessitate dosage adjustments or choosing drugs that do not interact with the P450 system.


Many substrates for CYP3A4 are drugs with a narrow  therapeutic index , such as  amiodarone [ 10 ]  or  carbamazepine . [ 11 ]  Because these drugs are metabolized by CYP3A4, the  mean plasma levels  of these drugs may increase because of enzyme inhibition or decrease because of enzyme induction.


Naturally occurring compounds may also induce or inhibit P450 activity. For example,  bioactive  compounds found in  grapefruit juice  and some other fruit juices, including  bergamottin ,  dihydroxybergamottin , and  paradicin-A , have been found to inhibit CYP3A4-mediated metabolism of  certain medications , leading to increased  bioavailability  and, thus, the strong possibility of  overdosing . [ 12 ]  Because of this risk, avoiding grapefruit juice and fresh grapefruits entirely while on drugs is usually advised. [ 13 ]


Other examples:


A subset of cytochrome P450 enzymes play roles in the synthesis of  steroid hormones  ( steroidogenesis ) by the  adrenals ,  gonads , and peripheral tissue:


Certain cytochrome P450 enzymes are critical in metabolizing  polyunsaturated fatty acids  (PUFAs) to biologically active, intercellular  cell signaling  molecules ( eicosanoids ) and/or metabolize biologically active metabolites of the PUFA to less active or inactive products.  These CYPs possess  cytochrome P450 omega hydroxylase  and/or  epoxygenase  enzyme activity.


Humans have 57 genes and more than 59  pseudogenes  divided among 18 families of cytochrome P450 genes and 43 subfamilies. [ 23 ]  This is a summary of the genes and of the proteins they encode. See the homepage of the cytochrome P450 Nomenclature Committee for detailed information. [ 5 ]


Other animals often have more P450 genes than humans do. Reported numbers range from 35 genes in the sponge  Amphimedon queenslandica  to 235 genes in the cephalochordate  Branchiostoma floridae . [ 24 ]   Mice  have genes for 101 P450s, and  sea urchins  have even more (perhaps as many as 120 genes). [ 25 ]
Most CYP enzymes are presumed to have monooxygenase activity, as is the case for most mammalian CYPs that have been investigated (except for, e.g.,  CYP19  and  CYP5 ).  Gene  and  genome sequencing  is far outpacing  biochemical  characterization of enzymatic function, though many genes with close  homology  to CYPs with known function have been found, giving clues to their functionality.


The classes of P450s most often investigated in non-human animals are those either involved in  development  (e.g.,  retinoic acid  or  hormone  metabolism) or involved in the metabolism of toxic compounds (such as  heterocyclic amines  or  polyaromatic hydrocarbons ). Often there are differences in  gene regulation  or  enzyme function  of P450s in related animals that explain observed differences in susceptibility to toxic compounds (ex. canines' inability to metabolize xanthines such as caffeine). Some drugs undergo metabolism in both species via different enzymes, resulting in different metabolites, while other drugs are metabolized in one species but excreted unchanged in another species. For this reason, one species's reaction to a substance is not a reliable indication of the substance's effects in humans. A species of Sonoran Desert Drosophila that uses an upregulated expression of the  CYP28A1  gene for detoxification of cacti rot is  Drosophila mettleri . Flies of this species have adapted an upregulation of this gene due to exposure of high levels of alkaloids in host plants.


P450s have been extensively examined in  mice ,  rats ,  dogs ,  zebrafish , and turkeys. [ 26 ]   CYP1A5  and  CYP3A37  in turkeys were found to be very similar to the human  CYP1A2  and  CYP3A4  respectively, in terms of their kinetic properties as well as in the metabolism of aflatoxin B1. [ 27 ]


CYPs have also been extensively studied in  insects , often to understand  pesticide resistance .  For example,  CYP6G1  is linked to insecticide resistance in  DDT -resistant  Drosophila melanogaster [ 28 ]  and  CYP6M2  in the mosquito  malaria  vector  Anopheles gambiae  is capable of directly metabolizing  pyrethroids . [ 29 ]  Other cytochromes, such as those in  Anopheles gambiae , are under preliminary research for their potential role in  pesticide resistance ,  infectious diseases , and  malaria . [ 30 ]


Microbial cytochromes P450 are often soluble enzymes and are involved in diverse metabolic processes. In bacteria the distribution of P450s is very variable with many bacteria having no identified P450s (e.g.  E.coli ). Some bacteria, predominantly actinomycetes, have numerous P450s (e.g., [ 31 ] [ 32 ] ). Those so far identified are generally involved in either biotransformation of xenobiotic compounds (e.g.  CYP105A1  from  Streptomyces griseolus  metabolizes  sulfonylurea herbicides  to less toxic derivatives, [ 33 ] ) or are part of specialised metabolite biosynthetic pathways (e.g.  CYP170B1  catalyses production of the  sesquiterpenoid  albaflavenone in  Streptomyces albus [ 34 ] ). Although no P450 has yet been shown to be essential in a microbe, the  CYP105 family  is highly conserved with a representative in every  streptomycete  genome sequenced so far. [ 35 ]  Due to the solubility of bacterial P450 enzymes, they are generally regarded as easier to work with than the predominantly membrane bound eukaryotic P450s. This, combined with the remarkable chemistry they catalyse, has led to many studies using the  heterologously expressed proteins  in vitro. Few studies have investigated what P450s do in vivo, what the natural substrate(s) are and how P450s contribute to survival of the bacteria in the natural environment.Three examples that have contributed significantly to structural and mechanistic studies are listed here, but many different families exist.


The commonly used  azole  class of antifungal drugs works by inhibition of the fungal  cytochrome P450 14α-demethylase . [ 40 ] [ better source needed ]


Cytochromes P450 are involved in a variety of processes of plant growth, development, and defense. It is estimated that P450 genes make up approximately 1% of the plant genome. [ 41 ] [ 42 ]  These enzymes lead to various  fatty acid  conjugates,  plant hormones ,  secondary metabolites ,  lignins , and a variety of defensive compounds. [ 43 ]


Cytochromes P450 play roles in plant defense– involvement in phytoalexin biosynthesis, hormone metabolism, and biosynthesis of diverse secondary metabolites. [ 44 ]  The expression of cytochrome p450 genes is regulated in response to environmental stresses indicative of a critical role in plant defense mechanisms. [ 45 ]


The biosynthesis of phytoalexins, antimicrobial compounds produced by some plants, involves the P450 enzymes CYP79B2, CYP79B3, CYP71A12, CYP71A13, and CYP71B15. The first step of camalexin biosynthesis produces indole-3-acetaldoxime (IAOx) from tryptophan and is catalyzed by either CYP79B2 or CYP79B3. IAOx is then immediately converted to indole-3-acetonitrile (IAN) and is controlled by either CYP71A13 or its homolog CYP71A12. The last two steps of the biosynthesis pathway of camalexin are catalyzed by CYP71B15. In these steps, indole-3-carboxylic acid (DHCA) is formed from cysteine-indole-3-acetonitrile (Cys(IAN)) followed by the biosynthesis of camalexin. There are some intermediate steps within the pathway that remain unclear, but it is well understood that cytochrome P450 is pivotal in camalexin biosynthesis and that this phytoalexin plays a major role in plant defense mechanisms. [ 46 ]


Cytochromes P450 are largely responsible for the synthesis of the jasmonic acid (JA), a common hormonal defenses against abiotic and biotic stresses for plant cells. For example, a P450, CYP74A is involved in the dehydration reaction to produce an insatiable allene oxide from hydroperoxide. [ 47 ]  JA chemical reactions are critical in the presence of biotic stresses that can be caused by plant wounding, specifically shown in the plant, Arabidopsis. As a prohormone, jasmonic acid must be converted to the JA-isoleucine (JA-Ile) conjugate by JAR1 catalysation in order to be considered activated. Then, JA-Ile synthesis leads to the assembly of the co-receptor complex compo`sed of COI1 and several JAZ proteins. Under low JA-Ile conditions, the JAZ protein components act as transcriptional repressors to suppress downstream JA genes. However, under adequate JA-Ile conditions, the JAZ proteins are ubiquitinated and undergo degradation through the 26S proteasome, resulting in functional downstream effects. Furthermore, several CYP94s (CYP94C1 and CYP94B3) are related to JA-Ile turnover and show that JA-Ile oxidation status impacts plant signaling in a catabolic manner. [ 41 ]  Cytochrome P450 hormonal regulation in response to extracellular and intracellular stresses is critical for proper plant defense response. This has been proven through thorough analysis of various CYP P450s in jasmonic acid and phytoalexin pathways.


Cytochrome P450 aromatic O-demethylase , which is made of two distinct promiscuous parts: a cytochrome P450 protein (GcoA) and three domain reductase, is significant for its ability to convert Lignin, the aromatic biopolymer common in plant cell walls, into renewable carbon chains in a catabolic set of reactions. In short, it is a facilitator of a critical step in Lignin conversion.


InterPro  subfamilies:


Clozapine, imipramine, paracetamol, phenacetin Heterocyclic aryl amines
Inducible and CYP1A2 5-10% deficient
oxidize uroporphyrinogen to uroporphyrin (CYP1A2) in heme metabolism, but they may have additional undiscovered endogenous substrates.
are inducible by some polycyclic hydrocarbons, some of which are found in cigarette smoke and charred food.


These enzymes are of interest, because in assays, they can activate compounds to carcinogens.
High levels of CYP1A2 have been linked to an increased risk of colon cancer. Since the 1A2 enzyme can be induced by cigarette smoking, this links smoking with colon cancer. [ 48 ]


In the pharmaceutical industry, drug  dissolution testing  is routinely used to provide critical  in vitro  drug release information for both  quality control  purposes, i.e., to assess batch-to-batch consistency of solid oral dosage forms such as tablets, and  drug development , i.e., to predict  in vivo  drug release profiles. [ 1 ]  There are three typical situations where dissolution testing plays a vital role: (i) formulation and optimization decisions: during product development, for products where dissolution performance is a critical quality attribute, both the product formulation and the manufacturing process are optimized based on achieving specific dissolution targets. (ii) Equivalence decisions: during generic product development, and also when implementing post-approval process or formulation changes, similarity of in vitro dissolution profiles between the reference product and its generic or modified version are one of the key requirements for regulatory approval decisions. (iii) Product compliance and release decisions: during routine manufacturing, dissolution outcomes are very often one of the criteria used to make product release decisions. [ 2 ] [ 3 ] [ 4 ]


The main objective of developing and evaluating an  IVIVC  is to establish the dissolution test as a surrogate for human studies, as stated by the  Food and Drug Administration  (FDA). [ 5 ]  Analytical data from drug dissolution testing are sufficient in many cases to establish  safety  and  efficacy  of a drug product without  in vivo  tests, following minor formulation and manufacturing changes (Qureshi and Shabnam, 2001). Thus, the dissolution testing which is conducted in dissolution apparatus must be able to provide  accurate  and  reproducible  results.


Several dissolution apparatuses exist.  In  United States Pharmacopeia  (USP) General Chapter <711> Dissolution, there are four dissolution apparatuses standardized and specified. [ 6 ]  They are:


The vessels of the dissolution method are usually either partially immersed in a water bath solution or heated by a jacket. An apparatus is used on solution within the vessels for a predetermined amount of time which depends on the method for the particular drug. The dissolution medium within the vessels are heated to 37 °C with an acceptable difference of ± 0.5 °C  [ 7 ]


The performances of dissolution apparatuses are highly dependent on hydrodynamics due to the nature of dissolution testing.   The designs of the dissolution apparatuses and the ways of operating dissolution apparatuses have huge impacts on the hydrodynamics, thus the performances.  Hydrodynamic studies in dissolution apparatuses were carried out by researchers over the past few years with both experimental methods and numerical modeling such as  Computational Fluid Dynamics  (CFD). The main target was USP Dissolution Apparatus 2. [ 1 ] [ 8 ] [ 9 ] [ 10 ] [ 11 ] [ 12 ] [ 13 ] [ 14 ]  The reason is that many researchers suspect that USP Dissolution Apparatus 2 provides inconsistent and sometimes faulty data. [ 15 ] [ 16 ] [ 17 ] [ 18 ] [ 19 ] [ 20 ] [ 21 ]  The hydrodynamic studies of USP Dissolution Apparatus 2 mentioned above clearly showed that it does have intrinsic hydrodynamic issues which could result in problems. In 2005, Professor Piero Armenante from  New Jersey Institute of Technology  (NJIT) and Professor Fernando Muzzio from  Rutgers University  submitted a technical report to the FDA. [ 22 ]  In this technical report, the intrinsic hydrodynamic issues with USP Dissolution Apparatus 2 based on the research findings of Armenante's group and Muzzio's group were discussed.


More recently, hydrodynamic studies were conducted in USP Dissolution Apparatus 4. [ 23 ] [ 24 ] [ 25 ]


The general procedure for a dissolution involves a liquid known as Dissolution Medium which is placed in the vessels of a dissolution unit. The medium can range from degassed or sonicated deionized water to pH adjusted chemically-prepared solutions and mediums that are prepared with surfactants. [ 26 ]  Degassing the dissolution medium through sonication or other means is important since the presence of dissolved gases may affect results.  The drug is placed within the medium in the vessels after it has reached sufficient temperature and then the dissolution apparatus is operated. Sample solutions collected from dissolution testing are commonly analyzed by  HPLC  or  Ultraviolet–visible spectroscopy . [ 27 ]  There are criteria known as 'release specifications' that samples tested must meet statistically, both as individual values and as average of the whole. [ 28 ] [ 29 ]  One such criteria is the parameter "Q", which is a percentage value denoting the quantity of dissolved active ingredient within the monograph of a sample solution. If the initial sample analysis, known as S1 or stage 1 testing fails to meet the acceptable value for Q, then additional testing known as stage 2 and 3 testing is required. S3 testing is performed only if S2 testing still fails the Q parameter. If there is a deviation from the acceptable Q values at S3, then an OOS (Out of Specification) investigation is generally initiated.


Distribution  in  pharmacology  is a branch of  pharmacokinetics  which describes the reversible transfer of a drug from one location to another within the body.


Once a drug enters into systemic circulation by absorption or direct administration, it must be distributed into interstitial and intracellular fluids. Each organ or tissue can receive different doses of the drug and the drug can remain in the different organs or tissues for a varying amount of time. [ 1 ]  The distribution of a drug between tissues is dependent on  vascular permeability , regional blood flow, cardiac output and  perfusion  rate of the tissue and the ability of the drug to bind tissue and  plasma proteins  and its lipid solubility.  pH partition  plays a major role as well. The drug is easily distributed in highly perfused organs such as the liver, heart and kidney. It is distributed in small quantities through less perfused tissues like muscle, fat and peripheral organs. The drug can be moved from the plasma to the tissue until the equilibrium is established (for unbound drug present in plasma).


The concept of compartmentalization of an organism must be considered when discussing a drug's distribution. This concept is used in pharmacokinetic modelling.


There are many factors that affect a drug's distribution throughout an organism, but Pascuzzo [ 1 ]  considers that the most important ones are the following: an organism's physical volume, the removal rate and the degree to which a drug binds with plasma proteins and / or tissues.


This concept is related to multi-compartmentalization. Any drugs within an organism will act as a  solute  and the organism's tissues will act as  solvents . The differing specificities of different tissues will give rise to different concentrations of the drug within each group. Therefore, the chemical characteristics of a drug will determine its distribution within an organism. For example, a liposoluble drug will tend to accumulate in body fat and water-soluble drugs will tend to accumulate in extracellular fluids.  The  volume of distribution  (V D ) of a drug is a property that quantifies the extent of its distribution. It can be defined as the theoretical volume that a drug would have to occupy (if it were uniformly distributed), to provide the same concentration as it currently is in blood plasma. It can be determined from the following formula:

 V
 d
 =

 A
 b

 C
 p

 {\displaystyle Vd={\frac {Ab}{Cp}}\,}

Where:

 A
 b

 {\displaystyle Ab}

  is total amount of the drug in the body and

 C
 p

 {\displaystyle Cp}

  is the drug's plasma concentration.


As the value for

 A
 b

 {\displaystyle Ab}

  is equivalent to the dose of the drug that has been administered the formula shows us that there is an inversely proportional relationship between

 V
 d

 {\displaystyle Vd}

  and

 C
 p

 {\displaystyle Cp}

 . That is, that the greater

 C
 p

 {\displaystyle Cp}

  is the lower

 V
 d

 {\displaystyle Vd}

  will be and vice versa. It therefore follows that the factors that increase

 C
 p

 {\displaystyle Cp}

  will decrease

 V
 d

 {\displaystyle Vd}

 . This gives an indication of the importance of knowledge relating to the drug's plasma concentration and the factors that modify it.


If this formula is applied to the concepts relating to bioavailability, we can calculate the amount of drug to administer in order to obtain a required concentration of the drug in the organism (' loading dose ):


D
 c
 =

 V
 d
 .
 C
 p

 D
 a
 .
 B

 {\displaystyle Dc={\frac {Vd.Cp}{Da.B}}}


This concept is of clinical interest as it is sometimes necessary to reach a certain concentration of a drug that is known to be optimal in order for it to have the required effects on the organism (as occurs if a patient is to be scanned).


A drug's removal rate will be determined by the proportion of the drug that is removed from circulation by each organ once the drug has been delivered to the organ by the circulating blood supply. [ 1 ]
This new concept builds on earlier ideas and it depends on a number of distinct factors:


Some drugs have the capacity to bind with certain types of proteins that are carried in blood plasma. This is important as only drugs that are present in the plasma in their free form can be transported to the tissues. Drugs that are bound to plasma proteins therefore act as a reservoir of the drug within the organism and this binding reduces the drug's final concentration in the tissues. The binding between a drug and plasma protein is rarely specific and is usually  labile  and reversible. The binding generally involves  ionic bonds ,  hydrogen bonds ,  Van der Waals forces  and, less often,  covalent bonds . This means that the bond between a drug and a protein can be broken and the drug can be replaced by another substance (or another drug) and that, regardless of this, the protein binding is subject to  saturation . An equilibrium also exists between the free drug in the blood plasma and that bound to proteins, meaning that the proportion of the drug bound to plasma proteins will be stable, independent of its total concentration in the plasma.


In vitro studies carried out under optimum conditions have shown that the equilibrium between a drug's plasmatic concentration and its tissue concentration is only significantly altered at binding rates to plasma proteins of greater than 90%. Above these levels the drug is "sequestered", which decreases its presence in tissues by up to 50%. This is important when considering pharmacological interactions: the tissue concentration of a drug with a plasma protein binding rate of less than 90% is not going to significantly increase if that drug is displaced from its union with a protein by another substance.  On the other hand, at binding rates of greater than 95% small changes can cause important modifications in a drug's tissue concentration. This will, in turn, increase the risk of the drug having a toxic effect on tissues.


Perhaps the most important plasma proteins are the  albumins  as they are present in relatively high concentrations and they readily bind to other substances. Other important proteins include the  glycoproteins , the  lipoproteins  and to a lesser degree the  globulins .


It is therefore easy to see that clinical conditions that modify the levels of plasma proteins (for example,  hypoalbuminemias  brought on by renal dysfunction) may affect the effect and toxicity of a drug that has a binding rate with plasma proteins of above 90%.


Highly lipid-soluble drugs given by intravenous or inhalation methods are initially distributed to organs with high blood flow. Later, less vascular but more bulky tissues (such as muscle and fat) take up the drug—plasma concentration falls and the drug is withdrawn from these sites. If the site of action of the drug was in one of the highly perfused organs, redistribution results in termination of the drug action. The greater the lipid solubility of the drug, the faster its redistribution will be. For example, the anaesthetic action of thiopentone is terminated in a few minutes due to redistribution. However, when the same drug is given repeatedly or continuously over long periods, the low-perfusion and high-capacity sites are progressively filled up and the drug becomes longer-acting.


Dosage forms  (also called  unit doses ) are  pharmaceutical drug  products presented in a specific form for use. They contain a mixture of  active ingredients  and inactive components ( excipients ), configured in a particular way (such as a  capsule  shell) and apportioned into a specific  dose . For example, two products may both be  amoxicillin , but one may come in 500 mg capsules, while another may be in 250 mg chewable tablets.


The term  unit dose  can also refer to non-reusable packaging, particularly when each drug product is individually packaged. [ 1 ]  However, the FDA differentiates this by referring to it as  unit-dose "packaging" or "dispensing" . [ 2 ]  Depending on the context,  multi(ple) unit dose  may refer to multiple distinct drug products  packaged  together or a  single  product containing multiple drugs and/or doses.


The term  dosage form  may also sometimes refer only to the  pharmaceutical formulation  of a drug product's constituent substances, without considering its final configuration as a consumable product (e.g., capsule, patch, etc.). Due to the somewhat ambiguous nature and overlap of these terms within the pharmaceutical industry, caution is advisable when discussing them with others who may interpret the terminology differently.


Dosage forms vary depending on the method/route of administration, which can include many types of liquid, solid, and semisolid forms. Common dosage forms include  pills ,  tablets , capsules,  drinks , and  syrups , among others.


A  combination drug  (or fixed-dose combination; FDC) is a product that contains more than one  active ingredient  (e.g., one tablet, one capsule, or one syrup with multiple drugs).


In  naturopathy , dosages can take the form of  decoctions  and  herbal teas , in addition to the more conventional methods mentioned above.


The  route of administration  (ROA) for  drug delivery  depends on the dosage form of the  substance . Different dosage forms may be available for a particular drug, especially if certain conditions restrict the ROA. For example, if a patient is unconscious or experiencing persistent  nausea  and  vomiting , oral administration may not be feasible, necessitating the use of alternative routes, such as  inhalational ,  buccal ,  sublingual ,  nasal ,  suppository , or  parenteral .


A specific dosage form may also be required due to issues such as  chemical stability  or  pharmacokinetic  properties. For instance,  insulin  cannot be given orally because it is extensively  metabolized  in the  gastrointestinal tract  (GIT) before it reaches the  bloodstream , preventing it from reaching therapeutic target destinations. Similarly, the oral and intravenous doses of a drug like  paracetamol  differ for the same reason. [ 3 ]


Talc is an  excipient  often used in pharmaceutical tablets that may end up being crushed to a powder against medical advice or for recreational use. Also,  illicit drugs  that occur as white powder in their pure form are often  cut  with cheap  talc . Natural talc is cheap but contains  asbestos  while asbestos-free talc is more expensive. Inhaled talc that has asbestos is generally accepted as being able to cause lung cancer if it is inhaled. The evidence about asbestos-free talc is less clear, according to the  American Cancer Society . [ 4 ]


These are usually solutions and suspensions.


Eye drops  (normal saline in disposable packages) are distributed to syringe users by  needle exchange programs . [ citation needed ]


The injection of talc from crushed pills has been associated with  pulmonary talcosis  in intravenous drug users. [ 5 ]


The  action of drugs  on the human body (or any other organism's body) is called  pharmacodynamics , and the  body's response to drugs  is called  pharmacokinetics . The drugs that enter an individual tend to stimulate certain  receptors ,  ion channels , act on  enzymes  or  transport proteins . As a result, they cause the human body to react in a specific way.


Based on drug action on receptors, there are 2 different types of drugs:


Once the receptors are activated, they either trigger a particular response directly on the body, or they trigger the release of  hormones  and/or other endogenous drugs in the body to stimulate a particular response.


Drugs interact with receptors by bonding at specific binding sites. Most receptors are made up of proteins, and the drugs can therefore interact with the amino acids to change the conformation of the receptor proteins.


These interactions are very basic, just like that of other chemical bondings:


Mainly occur through attractions between opposite charges; for example, between protonated amino (on  salbutamol ) or quaternary ammonium (e.g.  acetylcholine ), and the dissociated  carboxylic acid  group. Similarly, the dissociated carboxylic acid group on the drug can bind with  amino  groups on the receptor.


This type of bond is very strong, and varies with the inverse of the distance between the atoms so that it can act over large distances.


Cation-π interactions  can also be classified as ionic bonding. This type of interaction occurs when a cation, e.g. acetylcholine, interacts with the negative  π bonds  on an aromatic group of the receptor.


Ion-dipole  and  dipole-dipole  bonds have similar interactions, but are more complicated and are weaker than ionic bonds.


There is a small but significant attraction between  hydrogen atoms  and polar functional groups (e.g. the  hydroxyl  [-OH] group). These so-called hydrogen bonds only act over short distances, and are dependent on the correct alignment between functional groups.


Receptors are located on all cells in the body. The same receptor can be located on different organs, and even on different types of tissues. There are also different subtypes of receptor which elicit different effects in response to the same agonist. For example, there are two types of  histamine receptor :  H1  and  H2 . Activation of the H1 subtype receptor causes contraction of smooth muscle, whereas activation of the H2 receptor stimulates gastric secretion.


It is this phenomenon that gives rise to drug specificity. Of course, drugs do not only act on receptors: they also act on  ion channels ,  enzymes , and cell transporter proteins. These hydrogen bonds are so powerful than the  ionic bonds .


When talking about the  shape of molecules ,  biochemists  are mainly concerned with the three-dimensional conformation of drug molecules. There are many  isomers  of a particular drug, and each one will have its own effects. Differences in isomer affect not only what the drug activates, but also changes the  potency  of each drug.


Potency  is a measure of how much a drug is required in order to produce a particular effect. Therefore, only a small dosage of a high potency drug is required to induce a large response. The other terms used to measure the ability of a drug to trigger a response are:


Drug companies invest significant effort in designing drugs that interact specifically with particular receptors, [ 1 ]  since non-specific drugs can cause more side effects.


An example is the  endogenous  drug acetylcholine (ACh).  ACh is used by the  parasympathetic nervous system  to activate muscarinic receptors and by the neuromuscular system to activate nicotinic receptors.  However, the compounds muscarine and nicotine can each preferentially interact one of the two receptor types, allowing them to activate only one of the two systems where ACh itself would activate both.


The specificity of drugs cannot be talked about without mentioning the  affinity  of the drugs. The affinity is a measure of how tightly a drug binds to the receptor. If the drug does not bind well, then the action of the drug will be shorter and the chance of binding will also be less. This can be measured numerically by using the dissociation constant K D . The value of K D  is the same as the concentration of drug when 50% of receptors are occupied.


The equation can be expressed as K D  =

 [
 d
 r
 u
 g
 ]
 [
 r
 e
 c
 e
 p
 t
 o
 r
 ]

 [
 c
 o
 m
 p
 l
 e
 x
 ]

 {\displaystyle {\frac {[drug][receptor]}{[complex]}}}


But the value of K D  is also affected by the conformation, bonding and size of the drug and the receptor. The higher the K D  the lower the affinity of the drug.


Drug metabolism  is the  metabolic breakdown  of  drugs  by living  organisms , usually through specialized  enzymatic  systems. More generally,  xenobiotic metabolism  (from the Greek  xenos  "stranger"  and biotic "related to living beings") is the set of  metabolic pathways  that modify the chemical structure of  xenobiotics , which are compounds foreign to an organism's normal biochemistry, such as any  drug  or  poison . These pathways are a form of  biotransformation  present in all major groups of organisms and are considered to be of ancient origin. These reactions often act to  detoxify  poisonous compounds (although in some cases the  intermediates  in xenobiotic metabolism can themselves cause toxic effects). The study of drug metabolism is the object of  pharmacokinetics . Metabolism is one of the stages (see  ADME ) of the drug's transit through the body that involves the breakdown of the drug so that it can be excreted by the body.


The metabolism of  pharmaceutical drugs  is an important aspect of  pharmacology  and  medicine . For example, the rate of metabolism determines the duration and intensity of a drug's pharmacologic action. Drug metabolism also affects  multidrug resistance  in  infectious diseases  and in  chemotherapy  for  cancer , and the actions of some drugs as  substrates  or  inhibitors  of enzymes involved in xenobiotic metabolism are a common reason for hazardous  drug interactions . These pathways are also important in  environmental science , with the xenobiotic metabolism of  microorganisms  determining whether a pollutant will be broken down during  bioremediation , or  persist  in the environment. The enzymes of xenobiotic metabolism, particularly the  glutathione S-transferases  are also important in agriculture, since they may produce resistance to  pesticides  and  herbicides .


Drug metabolism is divided into three phases. In phase I, enzymes such as  cytochrome P450 oxidases  introduce reactive or polar groups into xenobiotics. These modified compounds are then conjugated to polar compounds in phase II reactions. These reactions are catalysed by  transferase  enzymes such as glutathione S-transferases. Finally, in phase III, the conjugated xenobiotics may be further processed, before being recognised by  efflux transporters  and pumped out of cells. Drug metabolism often converts  lipophilic  compounds into  hydrophilic  products that are more readily  excreted . [ citation needed ]


The exact compounds an organism is exposed to will be largely unpredictable, and may differ widely over time; these are major characteristics of xenobiotic toxic stress. [ 1 ]  The major challenge faced by xenobiotic detoxification systems is that they must be able to remove the almost-limitless number of xenobiotic compounds from the complex mixture of chemicals involved in normal  metabolism . The solution that has evolved to address this problem is an elegant combination of physical barriers and low-specificity  enzymatic  systems.


All organisms use  cell membranes  as hydrophobic permeability barriers to control access to their internal environment. Polar compounds cannot diffuse across these cell membranes, and the uptake of useful molecules is mediated through  transport proteins  that specifically select substrates from the extracellular mixture. This selective uptake means that most  hydrophilic  molecules cannot enter cells, since they are not recognised by any specific transporters. [ 2 ]  In contrast, the diffusion of  hydrophobic  compounds across these barriers cannot be controlled, and organisms, therefore, cannot exclude  lipid -soluble xenobiotics using membrane barriers.


However, the existence of a permeability barrier means that organisms were able to evolve detoxification systems that exploit the hydrophobicity common to membrane-permeable xenobiotics. These systems therefore solve the specificity problem by possessing such broad substrate specificities that they metabolise almost any non-polar compound. [ 1 ]  Useful metabolites are excluded since they are polar, and in general contain one or more charged groups.


The detoxification of the reactive by-products of normal metabolism cannot be achieved by the systems outlined above, because these species are derived from normal cellular constituents and usually share their polar characteristics. However, since these compounds are few in number, specific enzymes can recognize and remove them. Examples of these specific detoxification systems are the  glyoxalase system , which removes the reactive  aldehyde  methylglyoxal, [ 3 ]  and the various antioxidant systems that eliminate  reactive oxygen species . [ 4 ]


The metabolism of xenobiotics is often divided into three phases: modification, conjugation, and excretion. These reactions act in concert to detoxify xenobiotics and remove them from cells.


In phase I, a variety of enzymes act to introduce reactive and polar groups into their substrates. One of the most common modifications is hydroxylation catalysed by the  cytochrome P-450-dependent mixed-function oxidase system . These enzyme complexes act to incorporate an atom of oxygen into nonactivated hydrocarbons, which can result in either the introduction of hydroxyl groups or N-, O- and S-dealkylation of substrates. [ 5 ]  The reaction mechanism of the P-450 oxidases proceeds through the reduction of cytochrome-bound oxygen and the generation of a highly-reactive oxyferryl species, according to the following scheme: [ 6 ]


Phase I reactions (also termed nonsynthetic reactions) may occur by  oxidation ,  reduction ,  hydrolysis ,  cyclization , decyclization, and addition of oxygen or removal of hydrogen, carried out by mixed function oxidases, often in the liver. These oxidative reactions typically involve a  cytochrome P450  monooxygenase (often abbreviated CYP), NADPH and oxygen. The classes of pharmaceutical drugs that utilize this method for their metabolism include  phenothiazines ,  paracetamol , and steroids. If the metabolites of phase I reactions are sufficiently polar, they may be readily excreted at this point. However, many phase I products are not eliminated rapidly and undergo a subsequent reaction in which an  endogenous   substrate  combines with the newly incorporated functional group to form a highly polar conjugate.


A common Phase I oxidation involves conversion of a C-H bond to a C-OH. This reaction sometimes converts a pharmacologically inactive compound (a  prodrug ) to a pharmacologically active one. By the same token, Phase I can turn a nontoxic molecule into a poisonous one ( toxification ). Simple hydrolysis in the stomach is normally an innocuous reaction, however there are exceptions. For example, phase I metabolism converts  acetonitrile  to HOCH 2 CN, which rapidly dissociates into  formaldehyde  and  hydrogen cyanide . [ 7 ]


Phase I metabolism of drug candidates can be simulated in the laboratory using non-enzyme catalysts. [ 8 ]  This example of a  biomimetic  reaction tends to give products that often contains the Phase I metabolites. As an example, the major metabolite of the pharmaceutical  trimebutine , desmethyltrimebutine (nor-trimebutine), can be efficiently produced by in vitro oxidation of the commercially available drug.  Hydroxylation of an N-methyl group leads to expulsion of a molecule of  formaldehyde , while oxidation of the O-methyl groups takes place to a lesser extent.


Cytochrome P450 reductase, also known as NADPH:ferrihemoprotein oxidoreductase, NADPH:hemoprotein oxidoreductase, NADPH:P450 oxidoreductase, P450 reductase, POR, CPR, CYPOR, is a membrane-bound enzyme required for electron transfer to cytochrome P450 in the microsome of the eukaryotic cell from a FAD- and FMN-containing enzyme NADPH:cytochrome P450 reductase
The general scheme of electron flow in the POR/P450 system is:
NADPH
→
FAD
→
FMN
→
P450
→
O 2


During reduction reactions, a chemical can enter  futile cycling , in which it gains a free-radical electron, then promptly loses it to  oxygen  (to form a  superoxide anion ).


In subsequent phase II reactions, these activated xenobiotic metabolites are  conjugated  with charged species such as  glutathione  (GSH),  sulfate ,  glycine , or  glucuronic acid . Sites on drugs where conjugation reactions occur include  carboxy  (-COOH),  hydroxy  (-OH),  amino  (NH 2 ), and  thiol  (-SH) groups. Products of conjugation reactions have increased  molecular weight and tend to be less active than their substrates, unlike Phase I reactions which often produce  active metabolites . The addition of large anionic groups (such as GSH) detoxifies reactive  electrophiles  and produces more polar metabolites that cannot diffuse across membranes, and may, therefore, be actively transported.


These reactions are catalysed by a large group of broad-specificity transferases, which in combination can metabolise almost any hydrophobic compound that contains nucleophilic or electrophilic groups. [ 1 ]  One of the most important classes of this group is that of the  glutathione S-transferases  (GSTs).


After phase II reactions, the xenobiotic conjugates may be further metabolized. A common example is the processing of glutathione conjugates to  acetylcysteine  (mercapturic acid) conjugates. [ 11 ]  Here, the  γ-glutamate  and  glycine  residues in the glutathione molecule are removed by  gamma-glutamyl transpeptidase  and  dipeptidases . In the final step, the  cysteine  residue in the conjugate is  acetylated .


Conjugates and their metabolites can be excreted from cells in phase III of their metabolism, with the anionic groups acting as affinity tags for a variety of membrane transporters of the  multidrug resistance protein  (MRP) family. [ 12 ]  These proteins are members of the family of  ATP-binding cassette transporters  and can catalyse the ATP-dependent transport of a huge variety of hydrophobic anions, [ 13 ]  and thus act to remove phase II products to the extracellular medium, where they may be further metabolized or excreted. [ 14 ]


The detoxification of endogenous reactive metabolites such as  peroxides  and reactive  aldehydes  often cannot be achieved by the system described above. This is the result of these species' being derived from normal cellular constituents and usually sharing their polar characteristics. However, since these compounds are few in number, it is possible for enzymatic systems to utilize specific molecular recognition to recognize and remove them. The similarity of these molecules to useful metabolites therefore means that different detoxification enzymes are usually required for the metabolism of each group of endogenous toxins. Examples of these specific detoxification systems are the  glyoxalase system , which acts to dispose of the reactive aldehyde  methylglyoxal , and the various  antioxidant  systems that remove  reactive oxygen species .


Quantitatively, the  smooth endoplasmic reticulum  of the  liver  cell is the principal organ of drug metabolism, although every  biological tissue  has some ability to metabolize drugs.
Factors responsible for the liver's contribution to drug metabolism include that it is a large organ, that it is the first organ perfused by chemicals absorbed in the  gut , and that there are very high concentrations of most drug-metabolizing enzyme systems relative to other organs.
If a drug is taken into the GI tract, where it enters hepatic circulation through the  portal vein , it becomes well-metabolized and is said to show the  first pass effect .


Other sites of drug metabolism include  epithelial cells  of the  gastrointestinal tract ,  lungs ,  kidneys , and the  skin .
These sites are usually responsible for localized toxicity reactions.


The duration and intensity of pharmacological action of most lipophilic drugs are determined by the rate they are metabolized to inactive products. The  Cytochrome P450 monooxygenase system  is a crucial pathway in this regard. In general, anything that  increases  the rate of metabolism (e.g.,  enzyme induction ) of a pharmacologically active metabolite will  decrease  the duration and intensity of the drug action. The opposite is also true, as in  enzyme inhibition . However, in cases where an enzyme is responsible for metabolizing a pro-drug into a drug, enzyme induction can accelerate this conversion and increase drug levels, potentially causing toxicity. [ medical citation needed ]


Various  physiological  and  pathological  factors can also affect drug metabolism. Physiological factors that can influence drug metabolism include age, individual variation (e.g.,  pharmacogenetics ),  enterohepatic circulation ,  nutrition ,  sex differences  or  gut microbiota . [ medical citation needed ]  This last factor has significance because gut microorganisms are able to chemically modify the structure of drugs through degradation and biotransformation processes, thus altering the activity and toxicity of drugs. These processes can decrease the efficacy of drugs, as is the case of  digoxin  in the presence of  Eggerthella lenta  in the microbiota. [ 15 ]  Genetic variation ( polymorphism ) accounts for some of the variability in the effect of drugs. [ 15 ]


In general, drugs are metabolized more slowly in  fetal ,  neonatal  and  elderly   humans  and  animals  than in  adults . Inherited genetic variations in drug metabolising enzymes result in their different catalytic activity levels. For example, N-acetyltransferases (involved in  Phase II  reactions), individual variation creates a group of people who acetylate slowly ( slow acetylators ) and those who acetylate quickly ( rapid acetylators ), split roughly 50:50 in the population of Canada. However, variability in  NAT2  alleles distribution across different populations is high and some ethnicities have higher proportion of slow acetylators. [ 16 ]  This variation in metabolising capacity may have dramatic consequences, as the  slow acetylators  are more prone to dose-dependent toxicity.  NAT2  enzyme is a primary metaboliser of antituberculosis ( isoniazid ), some antihypertensive ( hydralazine ), anti-arrythmic drugs ( procainamide ), antidepressants ( phenelzine ) and many more  [ 17 ]  and increased toxicity as well as drug adverse reactions in slow acetylators have been widely reported. Similar phenomenons of altered metabolism due to inherited variations have been described for other drug-metabolising enzymes, like  CYP2D6 ,  CYP3A4 ,  DPYD ,  UGT1A1 .  DPYD  and  UGT1A1  genotyping is now required before administration of the corresponding substrate compounds ( 5-FU  and  capecitabine  for DPYD and  irinotecan  for UGT1A1) to determine the activity of DPYD and UGT1A1 enzyme and reduce the dose of the drug in order to avoid severe adverse reactions. [ 18 ]


Dose, frequency, route of administration, tissue distribution and protein binding of the drug affect its metabolism. [ medical citation needed ]   Pathological factors  can also influence drug metabolism, including  liver ,  kidney , or  heart  diseases. [ medical citation needed ]


In silico  modelling and simulation methods allow drug metabolism to be predicted in virtual patient populations prior to performing clinical studies in human subjects. [ 19 ]  This can be used to identify individuals most at risk from adverse reaction.


Studies on how people transform the substances that they ingest began in the mid-nineteenth century, with chemists discovering that organic chemicals such as  benzaldehyde  could be oxidized and conjugated to amino acids in the human body. [ 20 ]  During the remainder of the nineteenth century, several other basic detoxification reactions were discovered, such as  methylation ,  acetylation , and  sulfonation .


In the early twentieth century, work moved on to the investigation of the enzymes and pathways that were responsible for the production of these metabolites. This field became defined as a separate area of study with the publication by  Richard Williams  of the book  Detoxication mechanisms  in 1947. [ 21 ]  This modern biochemical research resulted in the identification of glutathione  S -transferases in 1961, [ 22 ]  followed by the discovery of cytochrome P450s in 1962, [ 23 ]  and the realization of their central role in xenobiotic metabolism in 1963. [ 24 ] [ 25 ]


This is a table of  drugs  that are  secreted  in the  kidney .


Acid medication are, because of  pH partition , secreted to a higher extent when urine is basic. In the same way, basic medications are secreted to a higher extent when urine is acidic.


In  pharmacology , the  elimination  or  excretion  of a drug is understood to be any one of a number of processes by which a drug is eliminated (that is,  cleared  and  excreted ) from an organism either in an unaltered form (unbound molecules) or modified as a metabolite. The  kidney  is the main excretory organ although others exist such as the  liver , the  skin , the  lungs  or  glandular  structures, such as the  salivary glands  and the  lacrimal glands . These organs or structures use specific routes to expel a drug from the body, these are termed  elimination pathways :


Drugs are excreted from the kidney by  glomerular filtration  and by active  tubular secretion  following the same steps and mechanisms as the products of intermediate metabolism. Therefore, drugs that are filtered by the  glomerulus  are also subject to the process of passive  tubular reabsorption .  Glomerular filtration will only remove those drugs or metabolites that are not bound to proteins present in blood plasma (free fraction) and many other types of drugs (such as the organic acids) are actively secreted.  In the  proximal  and   distal convoluted tubules , non-ionised acids and weak bases are reabsorbed both actively and passively. Weak acids are excreted when the tubular fluid becomes too alkaline and this reduces passive reabsorption. The opposite occurs with weak bases. Poisoning treatments use this effect to increase elimination, by alkalizing the urine causing  forced diuresis  which promotes excretion of a weak acid, rather than it getting reabsorbed. As the acid is  ionised , it cannot pass through the  plasma membrane  back into the blood stream and instead gets excreted with the urine. Acidifying the urine has the same effect for weakly basic drugs.


On other occasions drugs combine with bile juices and enter the intestines. In the intestines the drug will join with the unabsorbed fraction of the administered dose and be eliminated with the faeces or it may undergo a new process of absorption to eventually be eliminated by the kidney.


The other elimination pathways are less important in the elimination of drugs, except in very specific cases, such as the respiratory tract for alcohol or anaesthetic gases. The case of mother's milk is of special importance. The liver and kidneys of newly born  infants  are relatively undeveloped and they are highly sensitive to a drug's  toxic effects . For this reason it is important to know if a drug is likely to be eliminated from a woman's body if she is breast feeding in order to avoid this situation.


Pharmacokinetics studies the manner and speed with which drugs and their metabolites are eliminated by the various excretory organs. This elimination will be proportional to the drug's plasmatic concentrations. In order to model these processes a working definition is required for some of the concepts related to excretion.


The plasma half-life or  half life of elimination  is the time required to eliminate 50% of the absorbed dose of a drug from an organism. Or put another way, the time that it takes for the plasma concentration to fall by half from its maximum levels.


The difference in a drug's concentration in arterial blood (before it has circulated around the body) and venous blood (after it has passed through the body's organs) represents the amount of the drug that the body has eliminated or  cleared . Although clearance may also involve other organs than the kidney, it is almost synonymous with  renal clearance  or  renal plasma clearance .  Clearance is therefore expressed as the plasma volume totally free of the drug per unit of time, and it is measured in units of volume per units of time.
Clearance can be determined on an overall, organism level («systemic clearance») or at an organ level (hepatic clearance, renal clearance etc.). The equation that describes this concept is:


Where:

 C

 L

 o

 {\displaystyle CL_{o}}

  is the organ's clearance rate,

 C

 A

 {\displaystyle C_{A}}

  is the drug's plasma concentration in arterial blood,

 C

 V

 {\displaystyle C_{V}}

  is the drug's plasma concentration in venous blood and

 Q

 {\displaystyle Q}

  an organ's blood flow.


Each organ will have its own specific clearance conditions, which will relate to its mode of action. The «renal clearance» rate will be determined by factors such as the degree of  plasma protein binding  as the drug will only be filtered out if it is in the unbound free form, the degree of  saturation  of the transporters (active secretion depends on transporter proteins that can become saturated) or the number of functioning  nephrons  (hence the importance of factors such as  kidney failure ).


As «hepatic clearance» is an active process it is therefore determined by factors that alter an organism's metabolism such as the number of functioning  hepatocytes , this is the reason that  liver failure  has such clinical importance.


The steady state or  stable concentration  is reached when the drug's supply to the blood plasma is the same as the rate of elimination from the plasma. It is necessary to calculate this concentration in order to decide the period between doses and the amount of drug supplied with each dose in prolonged treatments.


Other parameters of interest include a drug's  bioavailability  and the  apparent volume of distribution .


For elimination via bile please see: Estimation of Biliary Excretion of Foreign Compounds Using Properties of Molecular Structure. 2014. Sharifi M., Ghafourian T. AAPS J. 16(1) 65–78.


An  enteric coating  is a  polymer  barrier applied to oral  medication  that prevents its dissolution or disintegration in the gastric environment. [ 1 ]  This helps by either protecting drugs from the acidity of the stomach, the stomach from the detrimental effects of the drug, or to release the drug after the stomach (usually in the upper tract of the intestine). [ 2 ]  Some drugs are unstable at the  pH  of gastric acid and need to be protected from degradation. Enteric coating is also an effective method to obtain  drug targeting  (such as gastro-resistant drugs). Other drugs such as some  anthelmintics  may need to reach a high concentration in a specific part of the intestine. [ 1 ]  Enteric coating may also be used during studies as a research tool to determine drug absorption. [ 3 ]  Enteric-coated medications pertain to the "delayed action" dosage form category.  Tablets , mini-tablets, pellets and granules (usually filled into  capsule  shells) are the most common enteric-coated  dosage forms . [ 3 ]


Most enteric coatings work by presenting a surface that is stable at the intensely acidic  pH  found in the stomach, but breaks down rapidly at a higher pH (alkaline pH). For example, they will not dissolve in the gastric acids of the  stomach  (pH ~3), but they will in the alkaline (pH 7–9) environment present in the  small intestine . [ 4 ] [ 5 ]  The time required for an enteric-coated dosage form to reach the intestine mostly depends on the presence and type of food in the stomach. It varies from 30 minutes up to 7 hours, with an average time of 6 hours. [ 6 ]  Although some studies indicated that larger sized dosage forms may require additional time for gastric emptying, others suggested that the size, shape, or volume of the tablet possess no significant effects instead. [ 7 ] [ 8 ] [ 9 ]  Enteric coated granules emptying rate is, however, less affected by the presence of food and present the more uniform release and reproducible transit time typical of the multiparticulates  dispersion . [ clarification needed ] [ 3 ] [ 8 ]


By preventing the drug from dissolving into the stomach, enteric coating may protect  gastric mucosa  from the irritating effects of the medication itself. When the drug reaches the neutral or alkaline environment of the intestine, its active ingredients can then dissolve and become available for absorption into the bloodstream. Drugs that have an irritant effect on the stomach, such as  aspirin  or  potassium chloride , can be coated with a substance that will dissolve only in the small intestine. However, it has been shown that enteric coated aspirin may lead to incomplete inhibition of  platelets , [ 10 ]   potentially negating the intended effect for those being treated for vascular disease. Likewise, certain groups of  proton pump inhibitors  ( esomeprazole ,  omeprazole ,  pantoprazole  and all grouped azoles) are acid-activated. For such drugs, enteric coating added to the formulation tends to avoid  activation  in the  mouth  and  esophagus .


Materials used for enteric coatings include  fatty acids ,  waxes ,  shellac ,  plastics , and plant fibers. Conventional materials used are solutions of film resins. However, as the solvents for such solutions are organic solvents, there is a concern about the toxicity potential of the traces of the residual solvents in the tablet coating. [ 11 ]


The first form of gastro-resistant coating was introduced by Unna in 1884 in the form of  keratin -coated pills, although it was later discovered that they were not able to withstand gastric  digestion . [ 12 ]   Salol  was also used by Ceppi as one of the first forms of enteric coating. [ 13 ]  However, the first material that was extensively used as enteric coating agent was  shellac , since its introduction in 1930. Properly treated or hydrolyzed shellac showed different enteric release properties. [ 3 ]


Recently, some companies have begun to apply enteric coatings to  fish oil  ( omega-3 fatty acids ) supplements. The coating prevents the fish oil capsules from being digested in the stomach, which has been known to cause  gastroesophageal reflux .


Sometimes the abbreviation "EC" is added beside the name of the drug to indicate that it has an enteric coating.


Enzyme induction  is a process in which a  molecule  ( e.g.  a drug) induces ( i.e.  initiates or enhances) the  expression  of an  enzyme .


Enzyme inhibition  can refer to


If the molecule induces enzymes that are responsible for its own  metabolism , this is called  auto-induction  (or  auto-inhibition  if there is inhibition). These processes are particular forms of  gene expression regulation .


These terms are of particular interest to  pharmacology , and more specifically to  drug metabolism  and  drug interactions . They also apply to  molecular biology .


In the late 1950s and early 1960s, the French molecular biologists  François Jacob  and  Jacques Monod  became the first to explain enzyme induction, in the context of the  lac operon  of  Escherichia coli . In the absence of lactose, the constitutively expressed  lac repressor  protein binds to the operator region of the DNA and prevents the transcription of the operon genes. When present, lactose binds to the lac repressor, causing it to separate from the DNA and thereby enabling transcription to occur. Monod and Jacob generated this theory following 15 years of work by them and others (including  Joshua Lederberg ), partially as an explanation for Monod's observation of  diauxie . Previously, Monod had hypothesized that enzymes could physically adapt themselves to new substrates; a series of experiments by him, Jacob, and  Arthur Pardee  eventually demonstrated this to be incorrect and led them to the modern theory, for which he and Jacob shared the 1965  Nobel Prize in Physiology or Medicine  (together with  André Lwoff ). [ 1 ]


Index inducer  or just  inducer  predictably induce metabolism via a given  pathway  and are commonly used in prospective clinical  drug-drug interaction  studies. [ 2 ]


Strong, moderate, and weak inducers are drugs that decreases the AUC of  sensitive index substrates  of a given metabolic pathway by ≥80%, ≥50% to <80%, and ≥20% to <50%, respectively. [ 2 ]


A growing body of research has begun to highlight differences in the way racial and ethnic groups respond to psychiatric medication. [ 1 ] [ 2 ]


Understanding the relevance between mental health and cultural associations is key to attempting to understand more about how the brain works for people of different ethnic and cultural groups. Mental health can be attributed to both the brain function but it can also be associated with environmental factors which can have a physiological effect. [ 1 ]


It has been noted that there are "dramatic cross-ethnic and cross-national variations in the dosing practices and side-effect profiles in response to practically all classes of psychotropics." [ 3 ]


It is important to understand epidemiology briefly since it can be connected with ethnopsychopharmacology. Studying how culture impacts the way disease is spread is important to in order to fully understand the racial disparities that impact how Western medication is used and perceived. [ 4 ]


Drug metabolism is controlled by a number of specific enzymes, and the action of these enzymes varies among individuals. For example, most individuals show normal activity of the IID6 isoenzyme that is responsible for the metabolism of many tricyclic antidepressant medications and most antipsychotic drugs. However, studies have found that one-third of Asian Americans and African Americans have a genetic alteration that decreases the metabolic rate of the IID6 isoenzyme, leading to a greater risk of side effects and toxicity. [ 5 ]  The CYP2D6 enzyme, important for the way in which the liver clears many drugs from the body, varies greatly between individuals in ways that can be ethnically specific. [ 2 ]
Though enzyme activity is genetically influenced, it can also be altered by cultural and environmental factors such as diet, the use of other medications, alcohol and disease states.


If two individuals have the same blood level of a medication there may still be differences in the way that the body responds due to  pharmacodynamic  differences; pharmacodynamic responses may also be influenced by racial and cultural factors. [ 6 ]


In addition to biology and environment,  culturally determined  attitudes toward illness may affect how an individual responds to psychiatric medication.


In addition to biology and environment,  culturally determined  attitudes toward illness and its treatment may affect how an individual responds to psychiatric medication. Some cultures see suffering and illness as unavoidable and not amenable to medication, [ 7 ]  while others treat symptoms with polypharmacy, often mixing medications with herbal drugs. [ 8 ]  Cultural differences may have an effect on adherence to medication regimes as well as influence the  placebo effect . [ 9 ]


Further, the way an individual expresses and reacts to the symptoms of psychiatric illness, and the cultural expectations of the physician, may affect the diagnosis a patient receives. For example, bipolar disorder often is misdiagnosed as schizophrenia in people of color.


The differential response of many ethnic minorities to certain psychiatric medications raises important concerns for both research and practice.


Include Ethnic Groups . Most studies of psychiatric medications have white male subjects. [ 6 ]  Because there is often a greater difference  within  racial and ethnic groups than between them, researchers must be certain they choose prototypical representatives of these groups, or use a larger random sample. [ 10 ]


Further, because broad racial and ethnic groups have many different subgroups. For example, in North American research it may not be enough to characterize individuals as Asian, Hispanic, Native American, or African American. [ 10 ]  Even within the same ethnic group, there are no reliable measures to determine important cultural differences. [ 11 ]


" Start Low and Go Slow ." Individuals who receive a higher dose of psychiatric medication than needed may discontinue treatment because of side effects, or they may develop toxic levels that lead to serious complications. A reasonable approach to prescribing medication to any psychiatric patient, regardless of race or culture, is to "start low and go slow". [ 6 ]


Someday there may be a simple blood test to predict how an individual will respond to a specific class of drugs; research in these fields fall in the domain of  pharmacogenomics  and  pharmacometabolomics .


The  first pass effect  (also known as  first-pass metabolism  or  presystemic metabolism ) is a phenomenon of  drug metabolism  at a specific location in the body which leads to a reduction in the  concentration  of the active  drug  before it reaches the site of action or systemic circulation. [ 1 ] [ 2 ]  The effect is most associated with orally administered medications, but some drugs still undergo first-pass metabolism even when delivered via an alternate route (e.g., IV, IM, etc.). [ 3 ]  During this metabolism, drug is lost during the process of absorption which is generally related to the  liver  and gut wall. The liver is the major site of first pass effect; however, it can also occur in the lungs, vasculature or other metabolically active tissues in the body. Notable drugs that experience a significant first pass effect are  buprenorphine ,  chlorpromazine ,  cimetidine ,  diazepam ,  ethanol  (drinking alcohol),  imipramine ,  insulin ,  lidocaine ,  midazolam ,  morphine ,  pethidine ,  propranolol , and  tetrahydrocannabinol  (THC). First-pass metabolism is not to be confused with Phase I metabolism, which is a separate process.


First-pass metabolism may occur in the liver (for propranolol, lidocaine,  clomethiazole , and nitroglycerin) or in the gut (for  benzylpenicillin  and  insulin ). [ 4 ]


After a drug is swallowed, it is absorbed by the  digestive system  and enters the  hepatic portal system . It is carried through the  portal vein  into the  liver  before it reaches the rest of the body. The liver  metabolizes  many drugs, sometimes to such an extent that only a small amount of  active drug  emerges from the liver to the rest of the  circulatory system . This  first pass  through the liver thus may greatly reduce the  bioavailability  of the drug.


An example of a drug where first-pass metabolism is a complication and disadvantage is in the antiviral drug  remdesivir . Remdesivir cannot be administered orally because the entire dose would be trapped in the liver with little achieving systemic circulation or reaching target organs and cells (for example, cells infected with  SARS-CoV-2 ). [ 5 ] [ 6 ]  For this reason, remdesivir is administered by IV infusion, bypassing the portal vein. However, significant hepatic extraction still occurs because of second pass metabolism, whereby a fraction of venous blood travels through the hepatic portal vein and hepatocytes.


The four primary systems that affect the first pass effect of a drug are the  enzymes  of the  gastrointestinal   lumen , gut wall enzymes, bacterial enzymes, and hepatic enzymes.


In  drug design , drug candidates may have good  druglikeness  but fail on first-pass metabolism because it is biochemically selective. [ ambiguous ]


Alternative  routes of administration , such as  insufflation ,  rectal administration , [ 7 ] [ 8 ]   intravenous ,  intramuscular ,  inhalational aerosol ,  transdermal , or  sublingual , avoid or partially avoid the first pass effect because they allow drugs to be absorbed directly into the  systemic circulation . [ 9 ]


Drugs with high first pass effect typically have a considerably higher oral dose than sublingual or  parenteral  dose. There is marked individual variation in the oral dose due to differences in the extent of first-pass metabolism, frequently among several other factors. Oral bioavailability of many vulnerable drugs appears to be increased in patients with compromised liver function. Bioavailability is also increased if another drug competing for first-pass metabolism enzymes is given concurrently (e.g., propranolol and  chlorpromazine ).


The  free fraction  is a parameter in  pharmacokinetics  and  receptor-ligand kinetics .
One speaks of two different free fractions:


The  plasma free fraction  is the fraction of the  ligand  at equilibrium in  blood plasma  that is not bound to  plasma proteins .


Glycopegylation  "is a site-selective  PEGylation  method developed for modifying complex  glycoproteins ". [ 1 ]  It can be useful to improve bioavailability and extend the half-life of various  therapeutic proteins . [ 2 ] [ 1 ] [ 3 ]  Examples of glycopegylated molecules include  pegozafermin [ 4 ]  and  recombinant factor IX . [ 5 ]


Some fruit juices and fruits can  interact  with numerous drugs, in many cases causing  adverse effects . [ 1 ]  The effect is most studied with  grapefruit  and  grapefruit juice , [ 1 ]  but similar effects have been observed with certain other  citrus fruits . [ 1 ] [ 2 ] [ 3 ] [ 4 ]


One whole grapefruit, or a small glass (200 mL, 6.8 US fl oz) of grapefruit juice, can cause drug overdose toxicity. [ 1 ]  Fruit consumed three days before the medicine can still have an effect. [ 5 ]  The relative risks of different types of citrus fruit have not been systematically studied. [ 1 ]  Affected drugs typically have an  auxiliary label  saying "Do not take with grapefruit" on the container, and the interaction is elaborated upon in the package insert. [ 6 ]  People are advised to ask their  physician  or  pharmacist  about drug interactions. [ 6 ]  However, some experts believe that for the majority of patients, complete avoidance of grapefruit is unwarranted. [ 7 ]


Although a  prospective cohort study  of middle-aged women indicated that some  flavonoid -rich foods are associated with a reduction in  all-cause mortality , frequent grapefruit consumption was associated with a small increase in all-cause mortality, possibly because of the clinically significant drug interactions of the non-flavonoid components. [ 8 ]


The effect of grapefruit juice with regard to drug absorption was originally discovered in 1989 by a group led by  pharmacologist David Bailey . Their first published clinical report on grapefruit drug interactions was in 1991. [ 9 ]  The effect was first discovered accidentally in 1989, when a test of drug interactions with alcohol used grapefruit juice to  hide the taste  of the ethanol. [ 9 ] [ 10 ]  A 2005 medical review advised patients to avoid all citrus juices until further research clarifies the risks. [ 11 ]  It was reported in 2008 that similar effects had been observed with apple juice. [ 2 ] [ 12 ] [ 13 ]


Citrus fruits contain various  polyphenols , which may include  furanocoumarins  and  naringin , such as  bergamottin ,  dihydroxybergamottin , and  bergapten . [ 14 ]  Grapefruit,  Seville oranges , [ 15 ]  and  bergamot [ 16 ]  contain  naringin . Furanocoumarins may have a stronger effect than naringin. [ 15 ] [ 17 ]


The effects are caused by  furanocoumarins  (and, to a lesser extent,  flavonoids ) which are compounds produced by many plants including but not limited to grapefruit. [ 18 ]  These chemicals inhibit key  drug metabolizing   enzymes , such as  cytochrome P450 3A4  (CYP3A4). CYP3A4 is a metabolizing enzyme for almost 50% of drugs, and is found in the liver and small intestinal epithelial cells. [ 19 ]  Organic derivatives of furanocoumarin interfere with  liver  and  intestinal  enzyme  CYP3A4  and may be responsible for the effects of grapefruit on the enzyme. [ 20 ]   Cytochrome isoforms  affected by grapefruit components include  CYP1A2 ,  CYP2C9 , and  CYP2D6 , [ 21 ] [ 22 ] [ 23 ] [ 24 ] [ 25 ]  but CYP3A4 is the major CYP enzyme in the intestine. [ 26 ]


Inhibition of enzymes can have two different effects:


Grapefruit or grapefruit juice can reduce the absorption of many drugs by inhibiting transport across cell membranes by the transporters  P-glycoprotein  (a member of the superfamily of  ATP-binding cassette (ABC) transporters ) and members of the  organo anion transporter family . [ 26 ] [ 7 ]  These transporters appear to have a minimal effect on systemic exposure of the drugs they affect, however. [ 7 ]  Many drugs are affected by consumption of citrus juice. When the metabolizing enzyme is inhibited, less of the drug will be metabolized by it in the epithelial cells. [ 19 ]  This interaction is particularly dangerous when the drug in question has a low  therapeutic index , so that a small increase in blood concentration can be the difference between therapeutic effect and toxicity. Citrus juice inhibits the enzyme only within the  intestines  if consumed in small amounts. When larger amounts are consumed they may in addition inhibit the enzyme in the liver. The hepatic enzyme inhibition may cause an additional increase in potency and a prolonged metabolic half-life (prolonged metabolic half-life for all ways of drug administration). [ 28 ]


Grapefruit–drug interactions that affect the pre-systemic metabolism (i.e., the metabolism that occurs before the drug enters the blood) of drugs have a different duration of action than interactions that work by other mechanisms, such as on absorption, discussed below. [ 19 ]


The interaction is greatest when the juice is ingested with the drug or up to 4 hours before the drug. [ 1 ] [ 5 ] [ 29 ]


The location of the inhibition occurs in the lining of the intestines, not within the liver. [ 30 ]  The effects last because grapefruit-mediated inhibition of drug metabolizing enzymes, like  CYP3A4 , is  irreversible ; [ 30 ]  that is, once the grapefruit has "broken" the enzyme, the intestinal cells must produce more of the enzyme to restore their capacity to metabolize drugs that the enzyme is used to metabolize. [ 19 ]  It takes around 24 hours to regain 50% of the cell's baseline enzyme activity and it can take 72 hours for the enzyme activity to completely return to baseline. For this reason, simply separating citrus consumption and medications taken daily does not avoid the drug interaction. [ 5 ]


For medications that interact due to inhibition of  OATP (organic anion-transporting polypeptides) , a relative short period of time is needed to avoid this interaction, and a four-hour interval between grapefruit consumption and the medication should suffice. [ 19 ] [ 31 ]  For drugs recently sold on the market, drugs have information pages (monographs) that provide information on any potential interaction between a medication and grapefruit juice. [ 19 ]  Because there is a growing number of medications that are known to interact with citrus, [ 1 ]  patients should consult a pharmacist or physician before consuming citrus while taking their medications.


Grapefruit is not the only  citrus fruit  that can interact with medications. [ 1 ] [ 2 ] [ 3 ] [ 4 ]  One medical review advised patients to avoid all citrus. [ 11 ]


There are three ways to test if a fruit interacts with drugs:


The first approach involves risk to trial volunteers. The first and second approaches have another problem: the same fruit  cultivar  could be tested twice with different results. Depending on growing and processing conditions, concentrations of the interacting polyphenol compounds can vary dramatically. The third approach is hampered by a paucity of knowledge of the genes in question. [ 32 ]


A descendant of citrus cultivars that cannot produce the problematic polyphenol compounds would presumably also lack the genes to produce them. Many citrus cultivars are hybrids of a small number of ancestral species, which have now been fully genetically sequenced. [ 33 ] [ 34 ]


Many traditional citrus groups, such as true sweet oranges and lemons, seem to be  bud sports , mutant descendants of a single hybrid ancestor. [ 35 ]  In theory, cultivars in a bud sport group would be either all safe or all problematic. Nonetheless, new citrus varieties arriving on the market are increasingly likely to be sexually created hybrids, not asexually created sports.


The ancestry of a hybrid cultivar may not be known. Even if it is known, it is not possible to be certain that a cultivar will not interact with drugs on the basis of taxonomy, as it is not known which ancestors lack the capacity to make the problematic polyphenol compounds. Nonetheless, many of the citrus cultivars known to be problematic seem to be closely related.


Pomelo  (the Asian fruit that was crossed with an orange to produce grapefruit) contains high amounts of  furanocoumarin  derivatives.  Grapefruit relatives  and other varieties of pomelo have variable amounts of furanocoumarin. [ 11 ] [ 2 ] [ 36 ] [ 37 ]


The  Dancy  cultivar has a small amount of pomelo ancestry, [ 34 ]  but is genetically close to a non-hybrid  true mandarin  (unlike most commercial mandarins, which may have much more extensive hybridization). Eight Dancy fruits, all picked at one time from one tree, have been blended and tested for furanocoumarins; none were detectable. [ 36 ]


No  citron  or  papeda  seems to have been tested. [ citation needed ]


Both  sweet oranges  and  bitter oranges  are  mandarin - pomelo  hybrids. [ 34 ] [ 35 ]  Bitter oranges (such as the  Seville oranges  often used in  marmalade ) can interfere with drugs [ 38 ]  including  etoposide , a chemotherapy drug, some  beta blocker  drugs used to treat high blood pressure, and  cyclosporine , taken by transplant patients to prevent rejection of their new organs. [ 12 ]  Evidence on sweet oranges is more mixed. [ 11 ]


Tests on some tangelos (hybrids of mandarins/tangerines and pomelo or grapefruit) have not shown significant amounts of furanocoumarin; these studies were also conducted on eight fruit all picked at one time from one tree. [ 36 ]


Common lemons are the product of orange/citron hybridization, and hence have pomelo ancestry, and although  Key limes  are papeda/citron hybrids, the more commercially prevalent  Persian limes  and similar varieties are crosses of the Key lime with lemons, and hence likewise have pomelo ancestry. [ 33 ] [ 34 ]  These limes can also inhibit drug metabolism. [ 38 ]  Other less-common citrus species also referred to as lemons or limes are genetically distinct from the more common varieties, with different proportions of pomelo ancestry. [ 33 ]


Marketing classifications often do not correspond to taxonomic ones. The "Ambersweet" cultivar is marketed and sold as an orange, but does not descend from the same common ancestor as sweet oranges; it has grapefruit, orange, and mandarin ancestry. Fruits are often sold as mandarin,  tangerine , or  satsuma  (which may be synonyms [ 39 ] ). Fruit sold under these names include many that are, like Sunbursts and  Murcotts , hybrids with grapefruit ancestry. [ 36 ] [ 40 ] [ 41 ]


The discovery that flavonoids are responsible for some interactions make it plausible that other fruit and vegetables are affected. [ 31 ]


Apple juice, especially commercially produced products, interferes with the action of  OATPs . [ 42 ]  This interference can decrease the absorption of a variety of commonly used medications, including  beta blockers  like  atenolol ,  antibiotics  like  ciprofloxacin , and  antihistamines  like  montelukast . [ 42 ] [ 43 ]


Pomegranate juice inhibits the action of the drug metabolizing enzymes  CYP2C9  and  CYP3A4 . [ 44 ]  As of 2014 [update] , the currently available literature does not appear to indicate a clinically relevant impact of pomegranate juice on drugs that are metabolized by CYP2C9 and CYP3A4. [ 44 ]


Researchers have identified over 85 drugs with which grapefruit reacts adversely. [ 45 ] [ 1 ]  According to a review done by the  Canadian Medical Association , [ 1 ]  there is an increase in the number of potential drugs that can interact with grapefruit juice, and of the number of fruit types that can interact with those drugs. From 2008 to 2012, the number of drugs known to potentially interact with grapefruit, with risk of harmful or even dangerous effects ( gastrointestinal bleeding ,  nephrotoxicity ), increased from 17 to 43. [ 1 ]


The interaction between citrus and medication depends on the individual drug, and not the class of the drug. Drugs that interact usually share three common features: they are taken orally, normally only a small amount enters systemic blood circulation, and they are metabolized by CYP3A4. [ 1 ]  The effects on the CYP3A4 in the liver could, in principle, cause interactions with non-oral drugs (e.g.  parenteral ,  inhaled substances ,  transdermal ), [ citation needed ]  and non-CYP3A4-mediated effects also exist. [ 31 ]


Cytochrome isoforms affected by grapefruit components include  CYP3A4 ,  CYP1A2 ,  CYP2C9 , and  CYP2D6 . [ 21 ]  Drugs that are metabolized by these enzymes may have interactions with components of grapefruit.


An easy way to tell if a medication may be affected by grapefruit juice is by researching whether another known CYP3A4 inhibitor drug is already  contraindicated  with the active drug of the medication in question. Examples of such known CYP3A4  inhibitors  include  cisapride  (Propulsid), [ 46 ]   erythromycin ,  itraconazole  (Sporanox),  ketoconazole  (Nizoral), and  mibefradil  (Posicor). [ 47 ]


Drugs that interact with grapefruit compounds at  CYP3A4  include


Drugs that interact with grapefruit compounds at  CYP1A2  include


Drugs that interact with grapefruit compounds at  CYP2D6  include


Research has been done on the interaction between  amphetamines  and CYP2D6 enzyme, and researchers concluded that some parts of substrate molecules contribute to the binding of the enzyme. [ 59 ]


Additional drugs affected by grapefruit juice include, but are not limited to:


clonazepam  (Klonopin)


ketamine


In  toxicology ,  Haber's rule  or  Haber's law  is a  mathematical  statement of the relationship between the  concentration  of a  poisonous   gas  and how long the gas must be breathed to produce death, or other  toxic  effect. The rule was formulated by  German   chemist   Fritz Haber  in the early 1900s.


Haber's rule states that, for a given poisonous gas,

 t
 C
 =
 k

 {\displaystyle tC=k}

 , where

 C

 {\displaystyle C}

  is the concentration of the gas (mass per unit volume),

 t

 {\displaystyle t}

  is the amount of time necessary to breathe the gas to produce a given toxic effect, and

 k

 {\displaystyle k}

  is a constant, depending on both the gas and the effect. Thus, the rule states that doubling the concentration will halve the time, for example.


It makes  equivalent  any two groupings of  dose concentration  and  exposure time  that have equivalent  mathematical products .  For instance, if we assign  dose   concentration  the  symbol  C, and  time  the classic t, then for any two dose  schema , if C 1 t 1 =C 2 t 2 , then under Haber's rule the two dose schema are equivalent.


Haber's rule is an approximation, useful with certain inhaled poisons under certain conditions, and Haber himself acknowledged that it was not always applicable. If a substance is efficiently eliminated in the host, for example, then Haber's Law breaks down in the limit of t approaching the  order  of the  half-life  of the  drug , rewriting the equation as the integral ∫Cdt = constant for arbitrary varying C and elapsed time T.  It is very convenient, however, because its relationship between

 C

 {\displaystyle C}

  and

 t

 {\displaystyle t}

  appears as a straight line in a  log-log plot .


In 1940,  statistician  C. I. Bliss published a study of  toxicity  in  insecticides  in which he proposed more complex  models , for example, expressing the relationship between

 C

 {\displaystyle C}

  and

 t

 {\displaystyle t}

  as two straight line segments in a  log-log plot . [ 1 ]  However, because of its simplicity, Haber's rule continued to be widely used. Recently, some researchers have argued that it is time to move beyond the simple relationship expressed by Haber's rule and to make regular use of more sophisticated models. [ 2 ]


Integrated discrete Multiple Organ Culture (IdMOC)  is an  in vitro , cell culture based experimental model for the study of  intercellular communication .  In conventional  in vitro  systems, each cell type is studied in isolation ignoring critical interactions between organs or cell types. IdMOC technology is based on the concept that multiple organs signal or communicate via the systemic circulation (i.e., blood).


The IdMOC plate consists of multiple inner wells within a large interconnecting chamber. Multiple cell types are first individually seeded in the inner wells and, when required, are flooded with an overlying medium to facilitate well-to-well communication. Test material can be added to the overlying medium and both media and cells can be analyzed individually. Plating of  hepatocytes  with other organ-specific cells allows evaluation of  drug metabolism  and organotoxicity. [ 1 ]


The IdMOC system has numerous applications in  drug development , such as the evaluation of  drug metabolism  and  toxicity . It can simultaneously evaluate the toxic potential of a drug on cells from multiple organs and evaluate drug stability, distribution, metabolite formation, and efficacy. By modeling multiple-organ interactions, IdMOC can examine the  pharmacological  effects of a drug and its metabolites on target and off-target organs as well as evaluate  drug-drug interactions  by measuring  cytochrome P450  (CYP) induction or inhibition in hepatocytes.


IdMOC can also be used for routine and  high throughput screening  of drugs with desirable  ADME  or  ADME-Tox  properties.  In vitro toxicity  screening using hepatocytes in conjunction with other primary cells such as cardiomyocytes ( cardiotoxicity  model), kidney proximal tubule epithelial cells ( nephrotoxicity  model), astrocytes ( neurotoxicity  model), endothelial cells (vascular toxicity model), and airway epithelial cells ( pulmonary toxicity  model) is invaluable to the  drug design  and  discovery  process. [ 2 ]


The IdMOC was patented by Dr.  Albert P. Li  in 2004. [ 3 ]