For synaptic receptors, an agonist is a compound that increases the activation of the receptor by binding directly to it or by increasing the amount of time  neurotransmitters  are in the synaptic cleft. An antagonist is a compound that has the opposite effect of an agonist. It decreases the activation of a synaptic receptor by binding and blocking neurotransmitters from binding or by decreasing the amount of time neurotransmitters are in the synaptic cleft. These actions can be achieved via multiple mechanisms. A common mechanism for agonists is  reuptake inhibition , where the agonist blocks neurotransmitters from reentering the pre-synaptic axon terminal. This gives the neurotransmitter more time in the synaptic cleft to act on the synaptic receptors. Conversely, antagonists often bind directly to receptors in the synaptic cleft, effectively blocking neurotransmitters from binding.


At the  alpha adrenoceptors ,  ( R )-3-nitrobiphenyline  is an α 2C  selective agonist as well as being a weak  antagonist  at the α 2A  and α 2B  subtypes. [ 2 ] [ 3 ]


The best known agonist-antagonists are  opioids . Examples of such opioids are:


Agonist–antagonist opioids usually have a  ceiling effect  – over particular dose they don't increase their potency. [ 7 ]   Hence agonist–antagonist opioids have a lower addiction potential but also lower analgesic efficacy and are more likely to produce  psychotomimetic  effects. [ 8 ]


Agonist–antagonist opioids that block  delta  while activating  mu opioid receptors  produce analgesia without the development of  tolerance . [ 9 ]


Antidepressant treatment tachyphylaxis  ( ADT  tachyphylaxis ), also known as  Prozac poop-out , is a medical condition in which progressive or acute tolerance effects are seen following chronic administration of a drug. [ 1 ]  It occurs more often with  Selective serotonin reuptake inhibitors  (SSRIs), which are the most commonly prescribed antidepressants. [ 2 ] [ 3 ]


Patients affected by ADT tachyphylaxis experience a noticeably sudden progressive decrease in response to SSRIs. The reported rates of this condition vary from 9% to 33% of SSRI users, and the majority of those affected are  responsive to subsequent treatments. [ 4 ]  In most observational studies, these individuals suffer a recurrence or relapse of depression without changing the previously effective dose. [ 5 ]


ADT tachyphylaxis incorporates  drug sensitivity  as a potential causal factor for the decreased response. However,  tolerance  provides a more accurate explanation. While the exact cause of ADT tachyphylaxis in individual cases is unknown, drug tolerance is a more comprehensive model, as it includes mechanisms of  pharmacodynamic tolerance ,  metabolic tolerance , and others. [ 5 ]


ADT tachyphylaxis specifically occurs in patients experiencing depression who are using SSRIs and  Monoamine Oxidase Inhibitors  (MAOIs). Currently, SSRIs are the preferred treatment for depression among clinicians, as MAOIs, despite being very effective, require a few dietary restrictions and some caution when taking with other medications due to the potential for interactions capable of inducing dangerous side effects. [ 6 ]


Following a declination or total extinction in response to a previously therapeutic dose of an antidepressant, the issue is clinically addressed as stemming from tolerance development. Several strategies are available, such as exploring drug options from a different drug class used to treat depression. The patient can also choose to switch to another SSRI (or MAOI, if applicable) while maintaining proportionate dose. If tolerance develops in a drug from the same class, the clinician may recommend a regular cycle consisting of all effective treatments within the SSRI or MAOI classes, in order to minimize transitional side effects while maximizing  therapeutic efficacy . [ 7 ]


Other options include increasing dose of the same medication, or supplementation with another antidepressant. Dual reuptake inhibitors, such as  serotonin–norepinephrine reuptake inhibitors  and some  tricyclic antidepressants , have been preliminarily found to have lower rates of tachyphylaxis. [ 8 ]


Antimicrobial pharmacodynamics  is the relationship between the concentration of an  antibiotic  and its ability to inhibit vital processes of  endo- or ectoparasites  and  microbial  organisms. [ 1 ]  This branch of  pharmacodynamics  relates the concentration of an anti-infective agent to its effect, specifically to its antimicrobial effect. [ 2 ]


The  minimum inhibitory concentration  (MIC) and  minimum bactericidal concentration  are used to measure  in vitro  activity of  antimicrobial  agents. They are good indicators of antimicrobial potency, but don't give any information relating to time-dependent antimicrobial killing (the so-called post antibiotic effect). [ 1 ]


The post-antibiotic effect (PAE) is defined as persistent suppression of  bacterial  growth after a brief exposure (1 or 2 hours) of bacteria to an  antibiotic  even in the absence of  host defense mechanisms . [ 3 ]  Factors that affect the duration of the post-antibiotic effect include duration of antibiotic exposure, bacterial species, culture medium and class of antibiotic. It has been suggested that an alteration of DNA function is possibly responsible for the post-antibiotic effect following the observation that most inhibitors of protein and nucleic acid synthesis ( aminoglycosides ,  fluoroquinolones ,  tetracyclines ,  clindamycin , certain newer  macrolides / ketolides , and  rifampicin  and  rifabutin ) induce long-term PAE against susceptible bacteria. [ 3 ] [ 4 ]  Theoretically, the ability of an antibiotic to induce a PAE is an attractive property since antibiotic concentrations could fall below the MIC for the bacterium yet retain their effectiveness in their ability to suppress the growth. Therefore, an antibiotic with PAE would require less frequent administration and it could improve patient adherence with regard to pharmacotherapy. [ 3 ] [ 5 ]  Proposed mechanisms include (1) slow recovery after reversible nonlethal damage to cell structures; (2) persistence of the drug at a binding site or within the  periplasmic space ; and (3) the need to synthesize new enzymes before growth can resume. Most antimicrobials possess significant in vitro PAEs (≥ 1.5 hours) against susceptible gram-positive  cocci . Antimicrobials with significant PAEs against susceptible gram-negative bacilli are limited to  carbapenems  and agents that inhibit protein or DNA synthesis. [ 6 ]


In  chemistry ,  binding selectivity  is defined with respect to the binding of  ligands  to a  substrate  forming a  complex . Binding selectivity describes how a ligand may bind more preferentially to one  receptor  than another. A selectivity coefficient is the  equilibrium constant  for the reaction of displacement by one ligand of another ligand in a complex with the substrate. Binding selectivity is of major importance in  biochemistry [ 1 ]  and in  chemical separation processes .


The concept of selectivity is used to quantify the extent to which one chemical substance, A, binds each of two other chemical substances, B and C. The simplest case is where the complexes formed have 1:1  stoichiometry . Then, the two interactions may be characterized by  equilibrium constants   K AB  and  K AC . [ note 1 ]

 A
 +
 B

 ↽

 −

 −

 ⇀

 AB

 ;

 K

 A
 B

 =

 [

 AB

 ]

 [
 A
 ]

 [
 B
 ]

 A
 +
 C

 ↽

 −

 −

 ⇀

 AC

 ;

 K

 A
 C

 =

 [
 AC
 ]

 [
 A
 ]

 [
 C
 ]

 {\displaystyle {\begin{aligned}{\ce {A + B <=> AB}};&\quad K_{\rm {AB}}={\frac {[{\ce {AB}}]}{{\ce {[A][B]}}}}\\{\ce {A + C <=> AC}};&\quad K_{\rm {AC}}={\frac {{\ce {[AC]}}}{{\ce {[A][C]}}}}\end{aligned}}}

where [X] represents the  concentration  of substance X (A, B, C, …).


A  selectivity coefficient  is defined as the ratio of the two equilibrium constants.

 K

 B
 ,
 C

 =

 K

 A
 C

 K

 A
 B

 {\displaystyle K_{\rm {B,C}}={\frac {K_{\rm {AC}}}{K_{\rm {AB}}}}}

This selectivity coefficient is in fact the equilibrium constant for the displacement reaction


AB
 +
 C

 ↽

 −

 −

 ⇀

 AC
 +
 B

 ;

 K

 B
 ,
 C

 =

 [
 AC
 ]

 [
 B
 ]

 [
 AB
 ]

 [
 C
 ]

 =

 K

 A
 C

 [
 A
 ]

 [
 B
 ]

 [
 C
 ]

 K

 A
 B

 [
 A
 ]

 [
 B
 ]

 [
 C
 ]

 =

 K

 A
 C

 K

 A
 B

 {\displaystyle {\ce {AB + C <=> AC + B}};\quad K_{\rm {B,C}}={\frac {{\ce {[AC][B]}}}{{\ce {[AB][C]}}}}={\frac {K_{\rm {AC}}{\ce {[A][B][C]}}}{K_{\rm {AB}}{\ce {[A][B][C]}}}}={\frac {K_{\rm {AC}}}{K_{\rm {AB}}}}}


It is easy to show that the same definition applies to complexes of a different stoichiometry, A p B q  and A p C q . The greater the selectivity coefficient, the more the ligand C will displace the ligand B from the complex formed with the substrate A. An alternative interpretation is that the greater the selectivity coefficient, the lower the concentration of C that is needed to displace B from AB. Selectivity coefficients are determined experimentally by measuring the two equilibrium constants,  K AB  and  K AC .


In biochemistry the substrate is known as a receptor. A receptor is a  protein  molecule, embedded in either the  plasma membrane  or the  cytoplasm  of a cell, to which one or more specific kinds of signalling molecules may bind. A  ligand  may be a  peptide  or another small molecule, such as a  neurotransmitter , a  hormone , a pharmaceutical drug, or a toxin. The specificity of a receptor is determined by its spatial geometry and the way it  binds  to the ligand through  non-covalent interactions , such as  hydrogen bonding  or  Van der Waals forces . [ 2 ]


If a receptor can be isolated a synthetic drug can be developed either to stimulate the receptor, an  agonist  or to block it, an  antagonist . The  stomach ulcer  drug  cimetidine  was developed as an  H 2  antagonist  by chemically engineering the molecule for maximum specificity to an isolated tissue containing the receptor. The further use of  quantitative structure-activity relationships  (QSAR) led to the development of other agents such as  ranitidine .


"Selectivity" when referring to a drug is relative. For example, in a higher dose, a specific drug molecule may also bind to other receptors than those said to be "selective".


Chelation therapy is a form of medical treatment in which a  chelating ligand [ note 2 ]  is used to selectively remove a metal from the body. When the metal exists as a divalent ion, such as with  lead , Pb 2+  or  mercury , Hg 2+  selectivity against  calcium , Ca 2+  and  magnesium , Mg 2+ , is essential in order that the treatment does not remove essential metals. [ 3 ]


Selectivity is determined by various factors. In the case of  iron overload , which may occur in individuals with β- thalessemia  who have received  blood transfusions , the target metal ion is in the +3  oxidation state  and so forms stronger complexes than the divalent ions. It also forms stronger complexes with oxygen-donor ligands than with nitrogen-donor ligands.  deferoxamine , a naturally occurring  siderophore  produced by the actinobacter  Streptomyces pilosus  and was used initially as a chelation therapy agent. Synthetic siderophores such as  deferiprone  and  deferasirox  have been developed, using the known structure of deferoxamine as a starting point. [ 4 ] [ 5 ]  Chelation occurs with the two oxygen atoms.


Wilson's disease  is caused by a defect in  copper  metabolism which results in accumulation of copper metal in various organs of the body. The target ion in this case is divalent, Cu 2+ . This ion is classified as borderline in the scheme of Ahrland, Chatt and Davies. [ 6 ]  This means that it forms roughly equally strong complexes with ligands whose donor atoms are N, O or F as with ligands whose donor atoms are P, S or Cl.  Penicillamine , which contains nitrogen and sulphur donor atoms, is used as this type of ligand binds more strongly to copper ions than to calcium and magnesium ions.


Treatment of poisoning by heavy metals such as lead and mercury is more problematical, because the ligands used do not have high specificity relative to calcium. For example,  EDTA  may be administered as a calcium salt to reduce the removal of calcium from bone together with the heavy metal. Factors determining selectivity for lead against zinc, cadmium and calcium have been reviewed, [ 7 ]


In column chromatography a mixture of substances is dissolved in a mobile phase and passed over a stationary phase in a column. A selectivity factor is defined as the ratio of  distribution coefficients , which describe the equilibrium distribution of an  analyte  between the stationary phase and the mobile phase. The selectivity factor is equal to the selectivity coefficient with the added assumption that the  activity  of the stationary phase, the substrate in this case, is equal to 1, the standard assumption for a pure phase. [ 8 ]  The resolution of a chromatographic column,  R S  is related to the selectivity factor by:


where α is selectivity factor,  N  is the number of  theoretical plates   k A  and  k B  are the  retention factors  of the two analytes. Retention factors are proportional to distribution coefficients. In practice substances with a selectivity factor very close to 1 can be separated. This is particularly true in  gas-liquid chromatography  where column lengths up to 60 m are possible, providing a very large number of theoretical plates.


In ion-exchange chromatography the selectivity coefficient is defined in a slightly different way [ 9 ]


Solvent extraction [ 10 ]  is used to extract individual  lanthanoid  elements from the mixtures found in nature in ores such as  monazite . In one process, the metal ions in aqueous solution are made to form complexes with  tributylphosphate  (TBP), which are extracted into an organic solvent such as  kerosene . Complete separation is effected by using a  countercurrent exchange  method. A number of cells are arranged as a  cascade . After equilibration, the aqueous component of each cell is transferred to the previous cell and the organic component is transferred to the next cell, which initially contains only water. In this way the metal ion with the most stable complex passes down the cascade in the organic phase and the metal with the least stable complex passes up the cascade in the aqueous phase. [ 11 ]


If solubility in the organic phase is not an issue, a selectivity coefficient is equal to the ratio of the  stability constants  of the TBP complexes of two metal ions. For lanthanoid elements which are adjacent in the  periodic table  this ratio is not much greater than 1, so many cells are needed in the cascade.


A potentiometric selectivity coefficient defines the ability of an  ion-selective electrode  to
distinguish one particular ion from others. The selectivity coefficient, K B,C  is evaluated by means of the emf response of the ion-selective electrode in mixed solutions of the primary ion, B, and interfering ion, C (fixed interference method) or less desirably, in separate solutions of B and C (separate
solution method). [ 12 ]   For example, a  potassium  ion-selective  membrane electrode  utilizes the naturally occurring macrocyclic  antibiotic   valinomycin . In this case the cavity in the macrocyclic ring is just the right size to encapsulate the potassium ion, but too large to bind the sodium ion, the most likely interference, strongly.


Chemical sensors , [ 13 ] [ 14 ]  are being developed for specific target molecules and ions in which the target (guest) form a complex with a sensor (host). The sensor is designed to be an excellent match in terms of the size and shape of the target in order to provide for the maximum binding selectivity. An indicator is associated with the sensor which undergoes a change when the target forms a complex with the sensor . The indicator change is usually a colour change (gray to yellow in the illustration) seen in  absorbance  or, with greater sensitivity,   luminescence . The indicator may be attached to the sensor via a spacer, in the ISR arrangement, or it may be displaced from the sensor, IDA arrangement.


In  pharmacology , the term  ceiling effect  refers to the property of increasing doses of a given medication to have progressively smaller incremental effect (an example of  diminishing returns ).  Mixed  agonist-antagonist opioids , such as  nalbuphine , serve as a classic example of the ceiling effect; increasing the dose of a narcotic frequently leads to smaller and smaller gains in relief of  pain .  In many cases, the severity of  side effects  from a medication increases as the dose increases, long after its therapeutic ceiling has been reached.


The term is defined as "the phenomenon in which a drug reaches a maximum effect, so that increasing the drug dosage does not increase its effectiveness." [ 1 ]  Sometimes drugs cannot be compared across a wide range of treatment situations because one drug has a ceiling effect. [ citation needed ]


Sometimes the desired effect increases with dose, but  side-effects  worsen or start being dangerous, and  risk to benefit ratio  increases. This is because of occupation of all the receptors in a given specimen.


In  medicine  the term  course  generally takes one of two meanings, both reflecting the sense of " path that something or someone moves along...process or sequence or steps ":


A patient may be said to be at the beginning, the middle or the end, or at a particular stage of the course of a disease or a treatment. A precursor is a sign or event that precedes the course or a particular stage in the course of a disease, for example chills often are precursors to fevers. [ 5 ]


Cross-tolerance  is a phenomenon that occurs when  tolerance  to the effects of a certain drug produces tolerance to another drug. It often happens between two drugs with similar functions or effects—for example, acting on the same  cell receptor  or affecting the transmission of certain  neurotransmitters . Cross-tolerance has been observed with pharmaceutical drugs such as  anti-anxiety agents  and  illicit substances , and sometimes the two of them together. Often, a person who uses one drug can be tolerant to a drug that has a completely different function. [ 1 ]  This phenomenon allows one to become tolerant to a drug that they have never used before. [ 2 ]


Excitation of the GABA receptor produces an influx of negatively charged chloride ions, which hyperpolarizes the neuron and makes it less likely to give rise to an action potential. In addition to  gamma-Aminobutyric acid  (GABA) itself, the GABA A  receptor can also bind barbiturates and benzodiazepines.  Benzodiazepine binding increases the binding of GABA and barbiturates maximize the time the pore is open.  Both of these mechanisms allow for influx of chloride ions.  When these drugs are taken together, especially with  ethanol  (drinking alcohol), there is a disproportionate increase in toxicity because the effects of both occur simultaneously and add up since they act on the same receptor at different sites.  Convergence upon the GABA A  receptor is why tolerance for one drug in the group will most likely cause cross-tolerance for the other drugs in the group. [ 1 ]   However, the barbiturates are also  AMPA receptor  blockers, and in addition interact with the  nAChR  and  voltage-gated calcium channels .  As a result, somebody who is tolerant to benzodiazepines is more sensitive to barbiturates than vice versa.


These drugs block dopamine receptors and some also block serotonin receptors (such as  chlorpromazine , the first antipsychotic used clinically). Having been on one or more antipsychotics for any appreciable amount of time results in dramatically reduced sensitivity to others with similar mechanisms of action. However, an antipsychotic with a substantial disparity in  pharmacology  (e.g.  haloperidol  and  quetiapine ) may retain significant efficacy.


MAO inhibitor drugs block an enzyme system resulting in increased stores of monoamine neurotransmitters. More common antidepressants such as tricyclic antidepressants and SSRIs block reuptake transporters causing increased levels of norepinephrine or serotonin in synapses. [ 1 ]  Mood stabilizers include lithium and many  anticonvulsants , such as  carbamazepine  and  lamotrigine  are also used for mood disorders. This would demonstrate little to zero cross-tolerance with serotonergic or  lithium  treatment.


These drugs mimic three classes of endorphins, such as endomorphins, enkephalins, and dynorphins. All three of these classes each have their own receptor-mu, kappa, and delta. Opioids will bind to the receptor for the endorphin they are most chemically similar to. Tolerance to some effects occurs with regular use, a result of the downregulation of the stimulated opioid receptors. Cross tolerance to analgesia may develop incompletely and less rapidly, allowing rotation between opioid medications be used to compensate somewhat for tolerance. This phenomenon is called  incomplete cross-tolerance . [ 3 ]


Cocaine, amphetamines, methylphenidate and ephedrine block the reuptake of dopamine and  norepinephrine . With increasing doses, amphetamines also cause the direct release of these neurotransmitters.


Serotonergic psychedelics  act through modulation of serotonin receptors. Most of these drugs share a high affinity for the  5-HT 2A  receptor subtype, known to result in their common perceptual and psychological effects.


Sometimes cross-tolerance occurs between two drugs that do not share mechanisms of action or classification. For example, in rats some amphetamine-like stimulants have been shown to exhibit cross-tolerance with  caffeine , though this effect was not observed with amphetamine itself.  It is likely that this mechanism of cross-tolerance involves the  dopamine receptor D 1 . [ 4 ]  Amphetamines also have cross-tolerance with  pseudoephedrine , as pseudoephedrine can block dopamine uptake in the same manner that amphetamines do, but less potently. [ 5 ]


Alcohol is another substance that often cross-tolerates with other drugs. Findings of cross-tolerance with nicotine in animal models suggest that it is also possible in humans, and may explain why the two drugs are often used together. [ 6 ]  Numerous studies have also suggested the possibility of cross-tolerance between alcohol and cannabis. [ 7 ]


Cigarette smoking  produces increased metabolic tolerance to caffeine due to upregulation of the CYP1A enzyme family (see  aryl hydrocarbon receptor ).


In  pharmacology , a  dirty drug  is an informal term for  drugs  that may bind to many different molecular targets or receptors in the body, and so tend to have a wide range of effects and possibly  adverse drug reactions . Today, pharmaceutical companies try to make new drugs as selective as possible to minimise binding to  antitargets  and hence reduce the occurrence of side effects and risk of adverse reactions.


Examples of  compounds  often cited as "dirty drugs" include  tramadol ,  chlorpromazine ,  olanzapine ,  dextromethorphan ,  ibogaine , and  ethanol , all of which bind to multiple receptors or influence multiple receptor systems.  There may be instances of advantages to drugs that exhibit multi-receptor activity such as the anti-addictive drug ibogaine that acts within a broad range of neurohormonal systems where activity is also exhibited by drugs commonly associated with addiction including  opioids ,  nicotine , and  alcohol . [ 1 ] [ 2 ]  Similarly chlorpromazine is primarily used as an  antipsychotic , but its strong serotonin receptor blocking effects make it useful for treating serotonergic crisis such as  serotonin syndrome . Dextromethorphan for its part is widely used as a  cough medication , but its other actions have led to trials for several conditions such as its use as an adjunct to analgesia, and a potential anti-addictive drug, as well as its occasional recreational use as a  dissociative . [ citation needed ]


Kanamycin  is an aminoglycoside  antibiotic  which induces  deafness  through blockage of the  outer hair cells  of the  cochlea ; yet it has many other effects, weakening for instance the  collagen  and  DNA   biosynthesis . It acts by inhibiting the synthesis of  proteins  in susceptible organisms. Kanamycin requires close clinical supervision because of its potential toxicity and adverse side effects to the auditory and vestibular branches of the eighth cranial nerve and to the renal tubules. [ 3 ]


Clozapine  and  latrepirdine  are examples of drugs used in the treatment of CNS disorders that have a superior efficacy precisely  because  of their "multifarious" broadspectrum mode of activity. Likewise, in cancer chemotherapeutics, it has been recognized that drugs active at more than one target have a higher probability of being efficacious. [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ] [ 9 ] [ 10 ] [ 11 ]


The anti-histamine and anti-cholinergic effects of atypical and low potency typical antipsychotics, such as the aforementioned clozapine and chlorpromazine, can also mediate against potentially distressing movement disorders such as extrapyramidal symptoms and akathisia associated with dopamine antagonism. In fact, clozapine may even help treat movement problems associated with Parkinson's disease. [ 12 ]


Examples of "promiscuous" cancer drugs include:  Sutent ,  Sorafenib ,  Zactima , and  AG-013736 . [ citation needed ]


In the field of drugs used to treat depression, the nonselective  MAOIs  and the  TCAs  are sometimes believed to have an efficacy that is superior to the  SSRIs . [ 13 ]  SSRIs are usually nevertheless picked as the first-line choice of agent and not the (less-selective) MAOIs and TCAs for several reasons. Firstly, SSRIs are safer in overdose than TCAs. Secondly, MAOIs can cause serious side effects when mixed with certain foods, including life-threatening  hypertensive crisis . MAOIs and TCAs generally have more side effects than SSRIs. TCAs in particular have  anticholinergic  side effects such as constipation and blurred vision, whereas SSRIs have fewer anticholinergic side effects. [ 14 ]


The  dose–response relationship , or  exposure–response relationship , describes the magnitude of the  response  of an  organism , as a  function  of exposure (or  doses ) to a  stimulus  or  stressor  (usually a  chemical ) after a certain exposure time. [ 1 ]  Dose–response relationships can be described by  dose–response curves . This is explained further in the following sections. A  stimulus response function  or  stimulus response curve  is defined more broadly as the response from any type of stimulus, not limited to chemicals.


Studying dose response, and developing dose–response models, is central to determining "safe", "hazardous" and (where relevant) beneficial levels and dosages for drugs, pollutants, foods, and other substances to which humans or other  organisms  are exposed. These conclusions are often the basis for public policy. The  U.S. Environmental Protection Agency  has developed extensive guidance and reports on dose–response modeling and assessment, as well as software. [ 2 ]  The  U.S. Food and Drug Administration  also has guidance to elucidate dose–response relationships [ 3 ]  during  drug development . Dose response relationships may be used in individuals or in populations. The adage  The dose makes the poison  reflects how a small amount of a toxin has no significant effect, while a large amount may be fatal. This reflects how dose–response relationships can be used in individuals. In populations, dose–response relationships can describe the way groups of people or organisms are affected at different levels of exposure. Dose response relationships modelled by dose response curves are used extensively in pharmacology and drug development. In particular, the shape of a drug's dose–response curve (quantified by EC50, nH and ymax parameters) reflects the biological activity and strength of the drug.


Some example measures for dose–response relationships are shown in the tables below. Each sensory stimulus corresponds with a particular  sensory receptor , for instance the nicotinic acetylcholine receptor for nicotine, or the  mechanoreceptor  for mechanical pressure. However, stimuli (such as temperatures or radiation) may also affect physiological processes beyond sensation (and even give the measurable response of death). Responses can be recorded as continuous data (e.g. force of muscle contraction) or discrete data (e.g. number of deaths).


A  dose–response curve  is a  coordinate graph  relating the magnitude of a dose (stimulus) to the response of a biological system. A number of effects (or  endpoints ) can be studied. The applied dose is generally plotted on the X axis and the response is plotted on the Y axis. In some cases, it is the  logarithm  of the dose that is plotted on the X axis. The curve is typically  sigmoidal , with the steepest portion in the middle. Biologically based models using dose are preferred over the use of log(dose) because the latter can visually imply a  threshold dose  when in fact there is none. [ citation needed ]


Statistical analysis of dose–response curves may be performed by regression methods such as the  probit model  or  logit model , or other methods such as the Spearman–Kärber method. [ 5 ]  Empirical models based on nonlinear regression are usually preferred over the use of some transformation of the data that linearizes the dose-response relationship. [ 6 ]


Typical experimental design for measuring dose-response relationships are  organ bath  preparations,  ligand binding assays ,  functional assays , and  clinical drug trials .


Specific to response to doses of radiation the Health Physics Society (in the United States) has published a  documentary series  on the origins of the linear no-threshold (LNT) model though the society has not adopted a policy on LNT."


Logarithmic dose–response curves are generally  sigmoidal-shape  and monotonic and can be fit to a classical  Hill equation . The Hill equation is a  logistic function  with respect to the logarithm of the dose and is similar to a  logit model . A generalized model for multiphasic cases has also been suggested. [ 7 ]


The  Hill equation  is the following formula, where

 E

 {\displaystyle E}

  is the magnitude of the response,

 [
 A
 ]

 {\displaystyle {\ce {[A]}}}

  is the drug concentration (or equivalently, stimulus intensity) and

 E
 C

 50

 {\displaystyle \mathrm {EC} _{50}}

  is the drug concentration that produces a 50% maximal response and

 n

 {\displaystyle n}

  is the  Hill coefficient .


The parameters of the dose response curve reflect measures of  potency  (such as EC50, IC50, ED50, etc.) and measures of efficacy (such as tissue, cell or population response).


A commonly used dose–response curve is the  EC 50  curve, the half maximal effective concentration, where the EC 50  point is defined as the inflection point of the curve.


Dose response curves are typically fitted to the  Hill equation .


The first point along the graph where a response above zero (or above the control response) is reached is usually referred to as a threshold dose. For most beneficial or recreational drugs, the desired effects are found at doses slightly greater than the threshold dose. At higher doses, undesired  side effects  appear and grow stronger as the dose increases. The more potent a particular substance is, the steeper this curve will be. In quantitative situations, the Y-axis often is designated by percentages, which refer to the percentage of exposed individuals registering a standard response (which may be death, as in  LD 50 ). Such a curve is referred to as a quantal dose–response curve, distinguishing it from a graded dose–response curve, where response is continuous (either measured, or by judgment).


The Hill equation can be used to describe dose–response relationships, for example ion channel-open-probability vs.  ligand  concentration. [ 9 ]


Dose is usually in milligrams,  micrograms , or grams per kilogram of body-weight for oral exposures or milligrams per cubic meter of ambient air for inhalation exposures. Other dose units include moles per body-weight, moles per animal, and for dermal exposure, moles per square centimeter.


The E max  model is a generalization of the Hill equation where an effect can be set for zero dose. Using the same notation as above, we can express the model as: [ 10 ]


Compare with a rearrangement of Hill:


The E max  model is the single most common model for describing dose-response relationship in drug development. [ 10 ]


The shape of dose-response curve typically depends on the topology of the targeted reaction network. While the shape of the curve is often  monotonic , in some cases non-monotonic dose response curves can be seen. [ 11 ]


The concept of linear dose–response relationship, thresholds, and all-or-nothing responses may not apply to non-linear situations. A  threshold model  or  linear no-threshold model  may be more appropriate, depending on the circumstances.
A recent critique of these models as they apply to endocrine disruptors argues for a substantial revision of testing and toxicological models at low doses because of observed non- monotonicity , i.e. U-shaped dose/response curves. [ 12 ]


Dose–response relationships generally depend on the exposure time and exposure route (e.g., inhalation, dietary intake); quantifying the response after a different exposure time or for a different route leads to a different relationship and possibly different conclusions on the effects of the stressor under consideration. This limitation is caused by the complexity of biological systems and the often unknown biological processes operating between the external exposure and the adverse cellular or tissue response. [ citation needed ]


Schild analysis  may also provide insights into the effect of drugs.


Dosing  generally applies to feeding  chemicals  or  medicines  when used in small quantities.


For medicines the term  dose  is generally used. In the case of inanimate objects the word dosing is typical. The term  dose titration , referring to stepwise  adjustment  of doses until a desired level of effect is reached, is common in  medicine .


The word  dosing  is very commonly used by  engineers  in thermal  power stations , in water treatment, in any industry where  steam  is being generated, and in  building services  for heating and cooling water treatment. Dosing procedures are also in vogue in  textile  and similar industries where chemical treatment is involved.


Commercial  swimming pools  also require chemical dosing in order to control  pH  balance,  chlorine  level, and other such water quality criteria. Modern swimming pool plant will have bulk storage of chemicals held in separate  dosing tanks , and will have automated controls and  dosing pumps  to top up the various chemicals as required to control the water quality.


In a power station treatment chemicals are injected or fed to  boiler  and also to feed and make up water under  pressure , but in small dosages or rate of  injection . The feeding at all places is done by means of small capacity dosing  pumps  specially designed for the duty demanded.


In building services the water quality of various pumped fluid systems, including for heating, cooling, and condensate water, will be regularly checked and topped up with chemicals manually as required to suit the required water quality. Most commonly  inhibitors  will be added to protect the pipework and components against corrosion, or a  biocide  will be added to stop the growth of bacteria in lower temperature systems. The required chemicals will be added to the fluid system by use of a  dosing pot ; a multi-valved chamber in which the chemical can be added, and then introduced to the fluid system in a controlled manner.


In food industries, the dosing of ingredients is particularly important in order to ensure the quality of the recipe as well as its food safety where the consumers' health may be directly implicated. Dosing is done in wet processes with dosing pumps but even more importantly in dry processes, just prior to the packaging of the product. Dosing of dry materials is commonly done through Gain-in-Weight dosing or Loss-in-Weight dosing using equipment on load cells. [ 1 ]


See pesticide application


The feeding of chemicals in  agriculture  has also become common due to technology developments. However  agricultural dosing  is done by means of hand held pressure spray pumps.


Sometimes aerial spraying of chemicals by fixed quantities at intervals or dosing is also adopted for agricultural spraying or for atmospheric spraying for eliminating certain types of harmful  insects .


A  drug class  is a group of  medications  and other compounds that share similar  chemical structures , act through the same  mechanism of action  (i.e., binding to the same  biological target ), have similar  modes of action , and/or are used to treat similar diseases. [ 1 ] [ 2 ]  The  FDA  has long worked to classify and license new medications. Its Drug Evaluation and Research Center categorizes these medications based on both their  chemical  and  therapeutic  classes. [ 3 ]


In several major drug classification systems, these four types of classifications are organized into a hierarchy. [ 4 ]  For example,  fibrates  are a chemical class of drugs (amphipathic carboxylic acids) that share the same mechanism of action ( PPAR agonist ), the same mode of action (reducing blood  triglyceride  levels), and are used to prevent and treat the same disease ( atherosclerosis ). However, not all PPAR agonists are fibrates, not all triglyceride-lowering agents are PPAR agonists, and not all drugs used to treat atherosclerosis lower triglycerides.
A drug class is typically defined by a  prototype drug , the most important, and typically the first developed drug within the class, used as a reference for comparison.


This type of categorisation of drugs is from a  chemical  perspective and categorises them by their chemical structure. Examples of drug classes that are based on chemical structures include:


This type of categorisation is from a  pharmacological  perspective and categorises them by their biological target. Drug classes that share a common molecular  mechanism of action  modulate the activity of a specific  biological target . [ 6 ]  The definition of a mechanism of action also includes the type of activity at that biological target. For receptors, these activities include  agonist ,  antagonist ,  inverse agonist , or  modulator .  Enzyme target mechanisms include  activator  or  inhibitor . Ion channel modulators include  opener  or  blocker .  The following are specific examples of drug classes whose definition is based on a specific mechanism of action:


This type of categorisation of drugs is from a  biological  perspective and categorises them by the anatomical or functional change they induce. Drug classes that are defined by common  modes of action  (i.e. the functional or anatomical change they induce) include:


This type of categorisation of drugs is from a  medical  perspective and categorises them by the pathology they are used to treat. Drug classes that are defined by their  therapeutic use  (the pathology they are intended to treat) include:


Some drug classes have been amalgamated from these three principles to meet practical needs. The class of  nonsteroidal anti-inflammatory drugs  (NSAIDs) is one such example. Strictly speaking, and also historically, the wider class of anti-inflammatory drugs also comprises  steroidal  anti-inflammatory drugs . These drugs were in fact the predominant anti-inflammatories during the decade leading up to the introduction of the term "nonsteroidal anti-inflammatory drugs." Because of the disastrous reputation that the corticosteroids had got in the 1950s, the new term, which offered to signal that an anti-inflammatory drug was not a steroid, rapidly gained currency. [ 7 ]  The drug class of "nonsteroidal anti-inflammatory drugs" (NSAIDs) is thus composed by one element ("anti-inflammatory") that designates the mechanism of action, and one element ("nonsteroidal") that separates it from other drugs with that same mechanism of action. Similarly, one might argue that the class of disease-modifying anti-rheumatic drugs (DMARD) is composed by one element ("disease-modifying") that albeit vaguely designates a mechanism of action, and one element ("anti-rheumatic drug") that indicates its therapeutic use.


Other systems of drug classification exist, for example the  Biopharmaceutics Classification System  which determines a drugs' attributes by solubility and intestinal permeability.


Drug intolerance  or  drug sensitivity  refers to an inability to tolerate the  adverse effects  of a medication, generally at therapeutic or subtherapeutic doses. Conversely, a patient is said to be "tolerating" a drug when they can tolerate its adverse effects.  Some instances of drug intolerance are known to result from genetic variations in  drug metabolism .


Drugs in systemic circulation have a certain concentration in the blood, which serves as a surrogate marker for how much drug will be delivered throughout the body (how much drug the rest of the body will "see"). There exists a minimum concentration of drug within the blood that will give rise to the intended therapeutic effect ( minimum effective concentration , MEC), as well as a minimum concentration of drug that will give rise to an unintended  adverse drug event  (minimum toxic concentration, MTC). The difference between these two values is generally referred to as the  therapeutic window . Different drugs have different therapeutic windows, and different people will have different MECs and MTCs for a given drug. If someone has a very low MTC for a drug, they are likely to experience adverse effects at drug concentrations lower than what it would take to produce the same adverse effects in the general populace; thus, the individual will experience significant toxicity at a dose that is otherwise considered "normal" for the average person. This individual will be considered "intolerant" to that drug.


There are a variety of factors that can affect the MTC, which is often the subject of clinical  pharmacokinetics . Variations in MTC can occur at any point in the  ADME  ( absorption ,  distribution ,  metabolism , and  excretion ) process. For example, a patient could possess a genetic defect in a drug metabolizing enzyme in the  cytochrome P450  superfamily. While most individuals will possess the effective metabolizing machinery, a person with a defect will have a difficult time trying to  clear  the drug from their system. Thus, the drug will accumulate within the blood to higher-than-expected concentrations, reaching a MTC at a dose that would otherwise be considered normal for the average person. In other words, in a person that is intolerant to a medication, it is possible for a dose of 10 mg to "feel" like a dose of 100 mg, resulting in an overdose—a "normal" dose can be a "toxic" dose in these individuals, leading to clinically significant effects.


There is also an aspect of drug intolerance that is subjective. Just as different people have different pain tolerances, so too do people have different tolerances for dealing with the adverse effects from their medications. For example, while  opioid-induced constipation  may be tolerable to some individuals, other people may stop taking an  opioid  due to the unpleasantness of the constipation even if it brings them significant pain relief.


Intolerance to  analgesics , particularly  NSAIDs , is relatively common. It is thought that a variation in the metabolism of  arachidonic acid  is responsible for the intolerance.  Symptoms include chronic  rhinosinusitis  with  nasal polyps ,  asthma ,  gastrointestinal ulcers ,  angioedema , and  urticaria . [ 3 ]


Drug tolerance  or  drug insensitivity  is a  pharmacological  concept describing subjects' reduced reaction to a drug following its repeated use. Increasing its dosage may re-amplify the drug's effects; however, this may accelerate tolerance, further reducing the drug's effects. Drug tolerance is indicative of drug use but is not necessarily associated with  drug dependence  or  addiction . [ 4 ]  The process of tolerance development is reversible (e.g., through a  drug holiday [ 5 ] ) and can involve both  physiological  factors and  psychological  factors. [ 6 ]


One may also develop drug tolerance to  side effects , [ 7 ]  in which case tolerance is a desirable characteristic. A medical intervention that has an objective to increase tolerance (e.g.,  allergen immunotherapy , in which one is exposed to larger and larger amounts of  allergen  to decrease one's  allergic reactions ) is called  drug desensitization . [ 8 ]


The opposite concept to drug tolerance is  reverse tolerance , in which case the subject's reaction or effect will increase following its repeated use. The two notions are not incompatible and tolerance may sometimes lead to reverse tolerance. For example, heavy drinkers initially develop tolerance to alcohol (requiring them to drink larger amounts to achieve a similar effect) but excessive drinking can cause  liver damage , which then puts them at risk of intoxication when drinking even very small amounts of alcohol. [ 9 ]


Drug tolerance should not be confused with  drug tolerability , which refers to the degree to which overt  adverse effects  of a drug can be tolerated by a patient.


Tachyphylaxis  is a subcategory of drug tolerance referring to cases of sudden, short-term onset of tolerance following the administration of a drug. [ 10 ]


Pharmacodynamic tolerance begins when the cellular response to a substance is reduced with repeated use. A common cause of pharmacodynamic tolerance is high concentrations of a substance constantly binding with the  receptor , desensitizing it through constant interaction. [ 11 ]  Other possibilities include a reduction in receptor density (usually associated with receptor agonists), other mechanisms leading to changes in action potential firing rate, or alterations in protein transcription among others adaptations. [ 12 ] [ 13 ]  Pharmacodynamic tolerance to a  receptor antagonist  involves the reverse, i.e., increased receptor firing rate, an increase in receptor density, or other mechanisms.


While most occurrences of pharmacodynamic tolerance occur after sustained exposure to a drug, instances of acute or instant tolerance (tachyphylaxis) can occur. [ 14 ]


Pharmacokinetics refers to the absorption, distribution, metabolism, and excretion of drugs (ADME). All psychoactive drugs are first absorbed into the bloodstream, carried in the blood to various parts of the body including the site of action (distribution), broken down in some fashion (metabolism), and ultimately removed from the body (excretion). All of these factors are very important determinants of crucial pharmacological properties of a drug, including its potency, side effects, and duration of action.


Pharmacokinetic tolerance (dispositional tolerance) occurs because of a decreased quantity of the substance reaching the site it affects. This may be caused by an increase in  induction  of the enzymes required for degradation of the drug e.g. CYP450 enzymes. This is most commonly seen with substances such as  ethanol .


This type of tolerance is most evident with oral ingestion, because other routes of drug administration bypass  first-pass metabolism . Enzyme induction is partly responsible for the phenomenon of tolerance, in which repeated use of a drug leads to a reduction of the drug's effect. However, it is only one of several mechanisms leading to tolerance.


Behavioral tolerance occurs with the use of certain psychoactive drugs, where tolerance to a behavioral effect of a drug, such as increased motor activity by methamphetamine, occurs with repeated use. It may occur through drug-independent learning or as a form of pharmacodynamic tolerance in the brain; the former mechanism of behavioral tolerance occurs when one learns how to actively overcome drug-induced impairment through practice. Behavioral tolerance is often context-dependent, meaning tolerance depends on the environment in which the drug is administered, and not on the drug itself. [ 15 ]   Behavioral sensitization  describes the opposite phenomenon.


Half maximal effective concentration  ( EC 50 ) is a measure of the concentration of a drug, antibody or toxicant which induces a  biological response  halfway between the baseline and maximum after a specified exposure time. [ 1 ]  More simply, EC 50  can be defined as the  concentration required to obtain a 50% [...] effect [ 2 ]  and may be also written as [A] 50 . [ 3 ]  It is commonly used as a measure of a drug's  potency , although the use of EC 50  is preferred over that of 'potency', which has been criticised for its vagueness. [ 3 ]  EC 50  is a measure of concentration, expressed in  molar units  (M), where 1 M is equivalent to 1  mol / L .


The EC 50  of a  graded   dose response curve  therefore represents the concentration of a compound where 50% of its maximal effect is observed. [ 4 ]
The EC 50  of a  quantal  dose response curve represents the concentration of a compound where 50% of the population exhibit a response, [ 5 ]  after a specified exposure duration.


For clarification, a graded dose response curve shows the graded effect of the drug (y axis) over the dose of the drug (x axis) in one or an average of subjects. A quantal dose response curve shows the percentage of subjects where a response is noted in an all-or-none manner (y axis) over the dose of the drug (x axis).


For competition binding assays and functional antagonist assays  IC 50  is the most common summary measure of the dose-response curve.  For agonist/stimulator assays the most common summary measure is the EC 50 . [ 6 ]


The EC 50  is also related to  IC 50  which is a measure of a compound's inhibition (50% inhibition).


Biological responses to  ligand  concentrations typically follow a  sigmoidal  function. The  inflection point  at which the increase in response with increasing ligand concentration begins to slow is the EC 50 , which can be mathematically determined by derivation of the best-fit line. While relying on a graph for estimation is more convenient, this typical method yields less accurate and precise results. [ 6 ]


The response or effect, E, is dependent on both the binding of the drug and the drug-bound receptor. The  agonist  that binds to the receptor and initiates the response is usually abbreviated A or D. At low agonist concentrations, [A], the response, E is immeasurably low but at higher [A], E becomes measurable. E increases with [A] until at sufficiently high [A], when E plateaus towards an asymptotic maximum attainable response, E max . The [A] at which E is 50% of E max  is termed the  half maximal effective concentration  and is abbreviated EC 50 , or rarely [A] 50 . The term "potency" refers to the EC 50  value. The lower the EC 50 , the less the concentration of a drug is required to produce 50% of maximum effect and the higher the potency. The EC 10  and EC 90  concentrations to induce 10% and 90% maximal responses are defined similarly.


There is a wide range of EC 50  values of drugs; they are typically anywhere from nM to mM. Hence, it is often more practical to refer to the logarithmically transformed  p EC 50  values instead of EC 50 , where


pEC

 50

 =
 −

 log

 10

 (

 EC

 50

 )

 {\displaystyle {\ce {pEC50}}=-\log _{10}({\ce {EC50}})}

 .


A drug's potency is dependent on the drug's  affinity  and  efficacy .


Affinity  describes how well a drug can bind to a receptor. Faster or stronger binding is represented by a higher affinity, or equivalently a lower dissociation constant. The EC 50  should not be confused with the affinity constant, K d . While the former reflects the drug concentration needed for a level of tissue response, the latter reflects the drug concentration needed for an amount of receptor binding.


Efficacy  is the relationship between receptor occupancy and the ability to initiate a response at the molecular, cellular, tissue or system level.


The EC 50  relates to the  Hill equation , which is a function of the agonist concentration, [A]:


where E is the observed response or effect above baseline, and n, the  Hill coefficient  reflects the slope of the curve. [ 7 ]


The EC 50  represents the  point of inflection  of the Hill equation, beyond which increases of [A] have less impact on E. In  dose response curves , the logarithm of [A] is often taken, turning the Hill equation into a sigmoidal  logistic function . In this case, the EC 50  represents the rising section of the sigmoid curve.


The effects of a stressor or drug generally depend on the exposure time. Therefore, the EC 50  (and similar statistics) will be a function of exposure time. The exact shape of this time function will depend upon the stressor (e.g., the specific toxicant), its mechanism of action, the organism exposed, etc. This time dependency hampers the comparison of  potency  or  toxicity  between compounds and between different organisms.


A drug will not have a single value of EC 50  due to different tissues having different sensitivities to the drug (in part due to tissue specific receptor expression). [ citation needed ]  Furthermore, EC 50  is dependent on many factors including species, tissue and cell type and genetics. [ citation needed ]


Efficacy  is the ability to perform a task to a satisfactory or expected degree. The word comes from the same roots as  effectiveness , and it has often been used synonymously, although in  pharmacology  a  distinction is now often made between efficacy and effectiveness . [ 1 ]


The word  efficacy  is used in  pharmacology  and  medicine  to refer both to the maximum response achievable from a  pharmaceutical drug  in research settings, [ 2 ]  and to the capacity for sufficient therapeutic effect or beneficial change in clinical settings.


In pharmacology, efficacy ( E max ) is the maximum response achievable from an applied or dosed agent, for instance, a small molecule drug. [ 2 ]   Intrinsic activity  is a relative term for a drug's efficacy relative to a drug with the highest observed efficacy. [ 3 ]  It is a purely descriptive term that has little or no mechanistic interpretation.


In order for a drug to have an effect, it needs to bind to its target, and then to affect the function of this target. The target of a drug is commonly referred to as a  receptor , but can in general be any chemically sensitive site on any molecule found in the body. The nature of such binding can be quantified by characterising how tightly these molecules, the drug and its receptor, interact: this is known as the  affinity . Efficacy, on the other hand, is a measure of the action of a drug once binding has occurred. The maximum response,  E max , will be reduced if efficacy is sufficiently low.


The definition of efficacy has been object for discussion. [ 4 ]  The only way in which absolute measures of efficacy have been obtained is by single ion channel analysis of ligand gated ion channels. It is still not possible to do this for G protein-linked receptors.


In the case of the glycine receptor and the nicotinic acetylcholine receptor (muscle type), it has been proposed by Sivilotti  et al . that opening of the ion channel involves two steps after agonist is bound. Firstly a conformation change to a higher affinity (but still shut) form, followed by the conformation change from shut to open. [ 5 ] [ 6 ]  It was found that partial agonism results from deficiency in the first step, and that the opening and shutting steps are essentially the same for both full and partial agonists. This has been confirmed and extended by Sine and colleagues (2009). [ 7 ]  The implication of this work [ 6 ]  is that efficacy has to be defined by at least two equilibrium constants (or, more generally, by four rate constants).


The combined influences of affinity and efficacy determine how effectively a drug will produce a biological effect, a property known as  potency .


In medicine, efficacy is the capacity for beneficial change (or  therapeutic effect ) of a given intervention (for example a drug,  medical device ,  surgical procedure , or a  public health  intervention). [ 8 ]  Establishment of the efficacy of an intervention is often done relative to other available interventions, with which it will be compared. [ 9 ]  Specifically, efficacy refers to "whether a drug demonstrates a health benefit over a placebo or other intervention when tested in an ideal situation, such as a tightly controlled clinical trial." [ 10 ]  These studies focus on a primary parameter to be shown statistically different between placebo and intervention groups. Comparisons of this type are called 'explanatory'  randomized controlled trials , whereas 'pragmatic' trials are used to establish the  effectiveness  of an intervention regarding also non-specific parameters. [ citation needed ]


Effectiveness refers to "how the drug works in a real-world situation", [ 11 ]  and is "often lower than efficacy because of interactions with other medications or health conditions of the patient, sufficient dose or duration of use not prescribed by the physician or followed by the patient, or use for an off-label condition that had not been tested." [ 10 ] [ 12 ]


In agriculture and forestry, efficacy is used to describe whether a  pesticide  is effective in controlling a pest or disease. [ 13 ]


In  Protestant Theology  (esp. in  Lutheran  but also in  Calvinist  doctrine)  efficacy  is an attribute of  Scripture . The efficacy of Scripture means that it is united with the power of the Holy Spirit and with it, not only demands, but also creates the acceptance of its  teaching [ 14 ] [ 15 ] [ 16 ]  and that this teaching produces faith and  obedience . Efficacy further means that Holy Scripture is not a dead letter, but rather, the power of the Holy Spirit is inherent in it [ 17 ] [ 18 ] [ 19 ]  and that Scripture does not compel a mere  intellectual assent  to its doctrine, resting on logical argumentation, but rather it creates the  living agreement of faith . [ 20 ] [ 21 ]  The  Smalcald Articles  affirm, "in those things which concern the  spoken, outward Word , we must firmly hold that God  grants His Spirit  or grace to no one, except through or with the preceding outward Word." [ 22 ]  The  Formula of Concord  teaches that when humans  reject the calling  of the Holy Spirit, it is not a result of the Word being less efficacious. Instead, contempt for the  means of grace  is the result of "the  perverse will  of man, which rejects or perverts the means and instrument of the Holy Ghost, which God offers him through the  call , and resists the Holy Ghost, who wishes to be efficacious, and works through the  Word ..." [ 23 ]


In  pharmacology , an  endogenous agonist  for a particular  receptor  is a compound naturally produced by the body which binds to and  activates  that receptor. For example, the primary endogenous agonist for  serotonin receptors  is  serotonin , and the primary endogenous agonist for  dopamine receptors  is  dopamine . [ 1 ]


In general, receptors for  small molecule   neurotransmitters  such as serotonin will have only one endogenous agonist, but often have many different receptor subtypes (e.g. 13 different  receptors for serotonin ). On the other hand,  neuropeptide  receptors tend to have fewer subtypes, but may have several different endogenous agonists. This allows for a high degree of complexity in the body's signalling system, with different tissues often showing quite distinct responses to a particular  ligand . Some endogenous  antagonists  and  inverse agonists  are also known (e.g.,  kynurenic acid  at the  NMDA receptor ), but these are much less common.


The  eudysmic ratio  (also spelled  eudismic ratio ) represents the difference in  pharmacologic activity  between the two  enantiomers  of a drug. In most cases where a  chiral compound  is biologically active, one enantiomer is more active than the other. The eudysmic ratio is the ratio of activity between the two. A eudysmic ratio  significantly differing  from 1 means that they are statistically different in activity. Eudisimic ratio (ER) reflects the degree of enantioselectivity of the biological systems. [ 1 ] [ 2 ]  For example,  (S)-propranolol (ER = 130) meaning that (S)-propranolol is 130 times more active than its (R)-enantiomer. [ 3 ]


The  eutomer  is the enantiomer having the desired pharmacological activity, [ 4 ]  e.g.,  as an active ingredient in a drug.


The  distomer , on the other hand, is the enantiomer of the eutomer which may have undesired bioactivity or may be bio-inert. [ 5 ]


A  racemic mixture  is an equal mixture of both enantiomers, which may be easier to manufacture than a single enantiomeric form.


It is often the case that only a single one of the enantiomers contains all of the wanted bioactivity, the distomer is often less active, has no desired activity or may even be toxic. [ 6 ]  In some cases, the eudysmic ratio is so high, that it is desired to separate out the two enantiomers instead of leaving it as a racemic product. It is also possible that the distomer is not simply completely inactive but actually  antagonizes  the effects of the eutomer. There are a few examples of chiral drugs where both the enantiomers contribute, in different ways, to the overall desired effect. An interesting situation is that in which the distomer antagonizes a side-effect of the eutomer for the desired action, mutually beneficial action form therapeutic standpoint.  This is convincingly demonstrated by the diuretic  indacrinone . [ 7 ]


The (R)-(+)-isomer, the eutomer, is responsible for the diuretic action and undesired uric acid retention, a side-effect common to many diuretics.  The (S)-(-)-isomer, the distomer, acts as a uricosuric agent and thus antagonizes the side-effect caused by the (R)-isomer.  A superficial examination of these facts might suggest the marketing of this product as a racemate (1:1 mixture of both enantiomers) to be desirable, since both enantiomers are complementing each other, but for optimal action, the ideal eutomer to distomer ratio for indacrinone has been determined to be 9:1. [ 8 ]   [ 9 ] [ 10 ] [ 11 ]  This is a classical case of a non-racemic drug. Alternatively, it is possible that in the body the distomer converts, at least in part, into the eutomer.


One way the eudysmic ratio is computed is by dividing the  EC 50  or the  IC 50  of the eutomer by the same measurement of the distomer. [ 12 ] [ 13 ]   Whether one chooses to use the  EC 50  or  IC 50  depends on the drug in question.


Functional selectivity  (or “agonist trafficking”, “biased agonism”, “biased signaling”, "ligand bias" and “differential engagement”) is the  ligand -dependent selectivity for certain  signal transduction  pathways relative to a reference ligand (often the endogenous hormone or peptide) at the same  receptor . [ 1 ]  Functional selectivity can be present when a receptor has several possible signal transduction pathways. To which degree each pathway is activated thus depends on which ligand binds to the receptor. [ 2 ]  Functional selectivity, or biased signaling, is most extensively characterized at  G protein coupled receptors  (GPCRs). [ 3 ]  A number of biased agonists, such as those at muscarinic M2 receptors tested as analgesics [ 4 ]  or antiproliferative drugs, [ 5 ]  or those at opioid receptors that mediate pain, show potential at various receptor families to increase beneficial properties while reducing side effects. For example, pre-clinical studies with  G protein  biased agonists at the  μ-opioid receptor  show equivalent efficacy for treating pain with reduced risk for addictive potential and  respiratory depression . [ 1 ] [ 6 ]  Studies within the chemokine receptor system also suggest that GPCR biased agonism is physiologically relevant. For example, a beta-arrestin biased agonist of the chemokine receptor  CXCR3  induced greater  chemotaxis  of T cells relative to a G protein biased agonist. [ 7 ]


Functional selectivity has been proposed to broaden conventional definitions of  pharmacology .


Traditional pharmacology   posits  that a  ligand  can be either classified as an  agonist  (full or partial),  antagonist  or more recently an  inverse agonist  through a specific receptor subtype, and that this characteristic will be consistent with all  effector  ( second messenger ) systems coupled to that receptor.  While this dogma has been the backbone of ligand-receptor interactions for decades now, more recent data indicates that this classic definition of ligand-protein associations does not hold true for a number of compounds; such compounds may be termed as  mixed agonist-antagonists .


Functional selectivity  posits that a ligand may inherently produce a mix of the classic characteristics through a single receptor isoform depending on the effector pathway coupled to that receptor. For instance, a ligand can not easily be classified as an agonist or antagonist, because it can be a little of both, depending on its preferred signal transduction pathways. Thus, such ligands must instead be classified on the basis of their individual effects in the cell, instead of being either an agonist or antagonist to a receptor.


These observations were made in a number of different  expression systems , and therefore functional selectivity is not just an  epiphenomenon  of one particular expression system.


One notable example of functional selectivity occurs with the  5-HT 2A  receptor , as well as the  5-HT 2C  receptor .  Serotonin , the main endogenous ligand of  5-HT receptors , is a functionally selective agonist at this receptor, activating  phospholipase C  (which leads to  inositol triphosphate  accumulation), but does not activate  phospholipase A2 , which would result in  arachidonic acid  signaling. However, the other endogenous compound  dimethyltryptamine  activates arachidonic acid signaling at the 5-HT 2A  receptor, as do many exogenous hallucinogens such as  DOB  and  lysergic acid diethylamide  (LSD). Notably, LSD does not activate IP 3  signaling through this receptor to any significant extent. (Conversely, LSD, unlike serotonin, has negligible affinity for the 5-HT 2C-VGV  isoform, is unable to promote calcium release, and is, thus, functionally selective at 5-HT 2C . [ 8 ] ) Oligomers, specifically 5-HT 2A – mGluR2 Tooltip metabotropic glutamate receptor 2   heteromers , mediate this effect. This may explain why some direct 5-HT 2  receptor agonists have  psychedelic  effects, whereas compounds that indirectly increase serotonin signaling at the 5-HT 2  receptors generally do not, for example:  selective serotonin reuptake inhibitors  (SSRIs),  monoamine oxidase inhibitors  (MAOIs), and medications using 5HT 2A  receptor agonists that do not have constitutive activity at the mGluR2  dimer , such as  lisuride . [ 9 ]


Tianeptine , an  atypical antidepressant , is thought to exhibit functional selectivity at the  μ-opioid receptor  to mediate its antidepressant effects. [ 10 ] [ 11 ]


Oliceridine  is a μ-opioid receptor agonist that has been described to be functionally selective towards G protein and away from β-arrestin2 pathways. [ 12 ]  However, recent reports highlight that, rather than functional selectivity or 'G protein bias', this agonist has low intrinsic efficacy. [ 13 ]   In vivo , it has been reported to mediate pain relief without tolerance nor gastrointestinal side effects.


The  delta opioid receptor  agonists  SNC80  and  ARM390  demonstrate functional selectivity that is thought to be due to their differing capacity to cause  receptor internalization . [ 14 ]  While SNC80 causes delta opioid receptors to internalize, ARM390 causes very little receptor internalization. [ 14 ]  Functionally, that means that the effects of SNC80 (e.g.  analgesia ) do not occur when a subsequent dose follows the first, whereas the effects of ARM390 persist. [ 14 ]  However, tolerance to ARM390's analgesia still occurs eventually after multiple doses, though through a mechanism that does not involve receptor internalization. [ 14 ]  Interestingly, the other effects of ARM390 (e.g. decreased anxiety) persist after tolerance to its analgesic effects has occurred. [ 14 ]


An example of functional selectivity to bias metabolism was demonstrated for an electron transfer protein  cytochrome P450 reductase  (POR) with binding of small molecule ligands shown to alter the protein conformation and interaction with various redox partner proteins of POR. [ 15 ]


Half maximal inhibitory concentration  ( IC 50 ) is a measure of the  potency  of a substance in inhibiting a specific biological or biochemical function. IC 50  is a quantitative measure that indicates how much of a particular  inhibitory  substance (e.g. drug) is needed to inhibit,  in vitro , a given biological process or biological component by 50%. [ 1 ]  The biological component could be an  enzyme ,  cell ,  cell receptor  or  microbe . IC 50  values are typically expressed as  molar concentration .


IC 50  is commonly used as a measure of  antagonist drug   potency  in  pharmacological  research. IC 50  is comparable to other measures of potency, such as  EC 50  for  excitatory drugs . EC 50  represents the dose or plasma concentration required for obtaining 50% of a maximum effect  in vivo . [ 1 ]


IC 50  can be determined with functional assays or with competition binding assays.


Sometimes, IC 50  values are converted to the  pIC 50  scale.


Due to the minus sign, higher values of pIC 50  indicate exponentially more potent inhibitors. pIC 50  is usually given in terms of  molar concentration  (mol/L, or M), thus requiring IC 50  in units of M. [ 2 ]


The IC 50  terminology is also used for some behavioral measures in vivo, such as the  two bottle fluid consumption test . When animals decrease consumption from the drug-laced water bottle, the concentration of the drug that results in a 50% decrease in consumption is considered the IC 50  for fluid consumption of that drug. [ 3 ]


The IC 50  of a drug can be determined by constructing a  dose-response curve  and examining the effect of different concentrations of antagonist on reversing agonist activity. IC 50  values can be calculated for a given antagonist by determining the concentration needed to inhibit half of the maximum biological response of the agonist. [ 4 ]  IC 50  values can be used to compare the potency of two antagonists.


IC 50  values are very dependent on conditions under which they are measured. In general,  a higher concentration of inhibitor leads to lowered agonist activity.  IC 50  value increases as agonist concentration increases. Furthermore, depending on the type of inhibition, other factors may influence IC 50  value; for  ATP  dependent enzymes, IC 50  value has an interdependency with concentration of ATP, especially if inhibition is  competitive . [ citation needed ]


In this type of assay, a single concentration of radioligand (usually an agonist) is used in every assay tube. The ligand is used at a low concentration, usually at or below its  K d  value. The level of specific binding of the radioligand is then determined in the presence of a range of concentrations of other competing non-radioactive compounds (usually antagonists), in order to measure the potency with which they compete for the binding of the radioligand. Competition curves may also be computer-fitted to a logistic function as described under direct fit.


In this situation the IC 50  is the concentration of competing ligand which displaces 50% of the specific binding of the radioligand. The IC 50  value is converted to an absolute  inhibition constant  K i  using the  Cheng-Prusoff equation  formulated by   Yung-Chi Cheng  and  William Prusoff  (see K i ). [ 4 ] [ 5 ]


IC 50  is not a direct indicator of  affinity , although the two can be related at least for competitive agonists and antagonists by the Cheng-Prusoff equation. [ 6 ]  For enzymatic reactions, this equation is:


where K i  is the binding affinity of the inhibitor, IC 50  is the functional strength of the inhibitor, [S] is fixed substrate concentration and K m  is the  Michaelis constant  i.e. concentration of substrate at which enzyme activity is at half maximal (but is frequently confused with substrate affinity for the enzyme, which it is not).


Alternatively, for inhibition constants at cellular receptors: [ 7 ]


where [A] is the fixed concentration of agonist and EC 50  is the concentration of agonist that results in half maximal activation of the receptor. Whereas the IC 50  value for a compound may vary between experiments depending on experimental conditions, (e.g. substrate and enzyme concentrations) the K i  is an absolute value. K i  is the inhibition constant for a drug; the concentration of competing ligand in a competition assay which would occupy 50% of the receptors if no ligand were present. [ 5 ]


The Cheng-Prusoff equation produces good estimates at high agonist concentrations, but over- or under-estimates K i  at low agonist concentrations. In these conditions, other analyses have been recommended. [ 7 ]


In  pharmacology , an  indirect agonist  or  indirect-acting agonist  is a substance that enhances the release or action of an  endogenous   neurotransmitter  but has no specific  agonist  activity at the neurotransmitter  receptor  itself. Indirect agonists work through varying mechanisms to achieve their effects, including transporter blockade, induction of transmitter release, and inhibition of transmitter breakdown.


Cocaine  is a  monoamine transporter  blocker and, thus, an indirect agonist of  dopamine receptors . [ 1 ]   Cocaine  binds the  dopamine transporter  (DAT), blocking the protein's ability to uptake  dopamine  from the  synaptic cleft  and also blocking DAT from terminating  dopamine  signaling. Blockage of DAT increases the  extracellular  concentration of  dopamine , therefore increasing the amount of  dopamine  receptor binding and signaling.


Dipyridamole  inhibits reuptake of  adenosine , resulting in greater extracellular concentrations of  adenosine .  Dipyridamole  also inhibits the  enzyme   adenosine deaminase , the  enzyme  that catalyzes the breakdown of  adenosine .


Fenfluramine  is an indirect agonist of  serotonin receptors . [ 2 ]   Fenfluramine  binds to the  serotonin transporter , blocking  serotonin  reuptake. However,  fenfluramine  also acts to induce non- exocytotic   serotonin  release; in a mechanism similar to that of  methamphetamine  in  dopamine  neurons,  fenfluramine  binds to  VMAT2 , disrupting the compartmentalization of  serotonin  into  vesicles  and increasing the concentration of  cytoplasmic   serotonin  available for drug-induced release. [ 3 ]


Intrinsic activity  ( IA ) and  efficacy  ( E max ) refer to the relative ability of a  drug - receptor  complex to produce a maximum functional response. This must be distinguished from the  affinity , which is a measure of the ability of the drug to bind to its molecular target, and the  EC 50 , which is a measure of the  potency  of the drug and which is proportional to both efficacy and affinity. This use of the word "efficacy" was introduced by Stephenson (1956) [ 1 ]  to describe the way in which  agonists  vary in the response they produce, even when they occupy the same number of receptors. High efficacy agonists can produce the maximal response of the receptor system while occupying a relatively low proportion of the receptors in that system. There is a distinction between efficacy and intrinsic activity. [ clarification needed ]


Agonists of lower efficacy are not as efficient at producing a response from the drug-bound receptor, by stabilizing the active form of the drug-bound receptor. Therefore, they may not be able to produce the same maximal response, even when they occupy the entire receptor population, as the efficiency of transformation of the inactive form of the drug-receptor complex to the active drug-receptor complex may not be high enough to evoke a maximal response. Since the observed response may be less than maximal in systems with no spare receptor reserve, some low efficacy agonists are referred to as partial agonists. [ 2 ]  However, it is worth bearing in mind that these terms are relative - even partial agonists may appear as full agonists in a different system/experimental setup, as when the number of receptors increases, there may be enough drug-receptor complexes for a maximum response to be produced, even with individually low efficacy of transducing the response. There are actually relatively few  true  full agonists or silent antagonists; many compounds usually considered to be full agonists (such as  DOI ) are more accurately described as high efficacy partial agonists, as a partial agonist with efficacy over ≈80-90% is indistinguishable from a full agonist in most assays. Similarly many antagonists (such as  naloxone ) are in fact partial agonists or inverse agonists, but with very low efficacy (less than 10%). Compounds considered partial agonists tend to have efficacy in between this range. Another case is represented by silent agonists, [ 3 ]  which are ligands that can place a receptor, typically an ion channel, into a desensitized state with little or no apparent activation of it, forming a complex that can subsequently generate currents when treated with an allosteric modulator. [ 4 ]


Intrinsic activity of a test agonist is defined as:


R. P. Stephenson (1925–2004) was a British pharmacologist. [ 6 ]  Efficacy has historically been treated as a proportionality constant between the binding of the drug and the generation of the biological response. [ 7 ]  Stephenson defined efficacy as:


where

 p

 {\displaystyle p}

  is the proportion of agonist-bound receptors (given by the  Hill equation ) and

 S

 {\displaystyle S}

  is the stimulus to the biological system. [ 8 ]  The response is generated by an unknown function

 f
 (
 S
 )

 {\displaystyle f(S)}

 , which is assumed to be  hyperbolic . [ 8 ]  This model was arguably flawed in that it did not incorporate the equilibrium between the  inactivated  agonist-bound-receptor and the  activated  agonist-bound-receptor that is shown in the del Castillo Katz model.


Robert F. Furchgott  later improved on Stephenson's model with the definition of efficacy, e, as


where

 ε

 {\displaystyle \varepsilon }

  is the  intrinsic efficacy  and

 [

 R

 ]

 T
 o
 t

 {\displaystyle [{\ce {R}}]_{\mathrm {Tot} }}

  is the total concentration of receptors.


Stevenson and Furchgott's models of efficacy have been criticised and many more have been developed. The models of efficacy are shown in Bindslev (2008). [ 9 ] [ 10 ]


In  pharmacology , an  inverse agonist  is a  drug  that binds to the same  receptor  as an  agonist  but induces a pharmacological response opposite to that of the agonist.


A  neutral antagonist  has no activity in the absence of an agonist or inverse agonist but can block the activity of either; [ 1 ]  they are in fact sometimes called  blockers  (examples include  alpha blockers ,  beta blockers , and  calcium channel blockers ). Inverse agonists have opposite actions to those of agonists but the effects of both of these can be blocked by antagonists. [ 2 ]


A prerequisite for an inverse agonist response is that the receptor must have a  constitutive  (also known as  intrinsic  or basal) level of activity in the absence of any  ligand . [ 3 ]  An agonist increases the activity of a receptor above its basal level, whereas an inverse agonist decreases the activity below the basal level.


The  efficacy  of a full agonist is by definition 100%, a neutral antagonist has 0% efficacy, and an inverse agonist has < 0% (i.e., negative) efficacy.


Receptors for which inverse agonists have been identified include the  GABA A ,  melanocortin ,  mu opioid ,  histamine  and  beta adrenergic receptors . Both  endogenous  and  exogenous  inverse agonists have been identified, as have drugs at ligand gated ion channels and at G protein-coupled receptors.


An example of a receptor site that possesses basal activity and for which inverse agonists have been identified is the  GABA A  receptors . Agonists for GABA A  receptors (such as  muscimol ) create a  relaxant  effect, whereas inverse agonists have  agitation  effects (for example,  Ro15-4513 ) or even  convulsive  and  anxiogenic  effects (certain  beta-carbolines ). [ 4 ] [ 5 ]


Two known endogenous inverse agonists are the  Agouti-related peptide  (AgRP) and its associated peptide  Agouti signalling peptide  (ASIP). AgRP and ASIP appear naturally in humans and bind  melanocortin receptors  4 and 1 ( Mc4R  and  Mc1R ), respectively, with nanomolar affinities. [ 6 ]


The  opioid antagonists   naloxone  and  naltrexone  act as  neutral antagonists  of the  mu opioid receptors  under basal conditions, but as inverse agonists when an opioid such as  morphine  is bound to the same channel. 6α-naltrexo,  6β-naltrexol , 6β-naloxol, and 6β-naltrexamine acted  neutral antagonists  regardless of opioid binding and caused significantly reduced withdrawal jumping when compared to  naloxone  and  naltrexone . [ 7 ]


Nearly all antihistamines acting at  H1 receptors  and  H2 receptors  have been shown to be inverse agonists. [ 8 ]


The  beta blockers   carvedilol  and  bucindolol  have been shown to be low level inverse agonists at  beta adrenoceptors . [ 8 ]


Like  agonists , inverse agonists have their own unique ways of inducing pharmacological and physiological responses depending on many factors, such as the type of inverse agonist, the type of  receptor , mutants of receptors, binding affinities and whether the effects are exerted acutely or chronically based on receptor population density. [ 9 ]  Because of this, they exhibit a spectrum of activity below the  Intrinsic activity  level. [ 9 ] [ 10 ]  Changes in constitutive activity of receptors affect response levels from ligands like inverse agonists. [ 11 ]


To illustrate, mechanistic models have been made for how inverse agonists induce their responses on  G protein-coupled receptors  (GPCRs). Many types of Inverse agonists for  GPCRs  have been shown to exhibit the following conventionally accepted mechanism.


Based on the Extended  Ternary complex  model, the mechanism contends that inverse agonists switch the receptor from an active state to an inactive state by undergoing conformational changes. [ 12 ]  Under this model, current thinking is that the  GPCRs  can exist in a continuum of active and inactive states when no ligand is present. [ 12 ]  Inverse agonists stabilize the inactive states, thereby suppressing agonist-independent activity. [ 12 ]  However, the implementation of 'constitutively active mutants' [ 12 ]  of  GPCRs  change their intrinsic activity. [ 9 ] [ 10 ]  Thus, the effect an inverse agonist has on a receptor depends on the basal activity of the receptor, assuming the inverse agonist has the same binding affinity (as shown in the figure 2).


In  pharmacology , the term  mechanism of action  ( MOA ) refers to the specific biochemical  interaction  through which a  drug  substance produces its  pharmacological effect . [ 2 ]   A mechanism of action usually includes mention of the specific molecular targets to which the drug binds, such as an  enzyme  or  receptor . [ 3 ]  Receptor sites have specific affinities for drugs based on the chemical structure of the drug, as well as the specific action that occurs there.


Drugs that do not bind to receptors produce their corresponding therapeutic effect by simply interacting with chemical or physical properties in the body. Common examples of drugs that work in this way are  antacids  and  laxatives . [ 2 ]


In contrast, a  mode of action (MoA)  describes functional or anatomical changes, at the cellular level, resulting from the exposure of a living organism to a substance.


Elucidating the mechanism of action of novel drugs and medications is important for several reasons:


Bioactive compounds induce  phenotypic  changes in target cells, changes that are observable by  microscopy  and that can give insight into the mechanism of action of the compound. [ 13 ]


With  antibacterial agents , the conversion of target cells to  spheroplasts  can be an indication that  peptidoglycan  synthesis is being inhibited, and  filamentation  of target cells can be an indication that  PBP 3,  FtsZ , or  DNA synthesis  is being inhibited. Other antibacterial agent-induced changes include ovoid cell formation, pseudomulticellular forms, localized swelling, bulge formation, blebbing, and peptidoglycan thickening. [ 4 ]   In the case of  anticancer agents ,  bleb  formation can be an indication that the compound is disrupting the  plasma membrane . [ 14 ]


A current limitation of this approach is the time required to manually generate and interpret data, but advances in automated microscopy and  image analysis  software may help resolve this. [ 4 ] [ 13 ]


Direct biochemical methods include methods in which a protein or a small molecule, such as a drug candidate, is labeled and is traced throughout the body. [ 15 ]  This proves to be the most direct approach to find target protein that will bind to small targets of interest, such as a basic representation of a drug outline, in order to identify the  pharmacophore  of the drug. Due to the physical interactions between the labeled molecule and a protein, biochemical methods can be used to determine the toxicity, efficacy, and mechanism of action of the drug. [ citation needed ]


Typically, computation inference methods are primarily used to predict protein targets for small molecule drugs based on computer based pattern recognition. [ 15 ]  However, this method could also be used for finding new targets for existing or newly developed drugs. By identifying the  pharmacophore  of the drug molecule, the profiling method of pattern recognition can be carried out where a new target is identified. [ 15 ]  This provides an insight at a possible mechanism of action since it is known what certain functional components of the drug are responsible for when interacting with a certain area on a protein, thus leading to a therapeutic effect. [ citation needed ]


Omics based methods use omics technologies, such as  chemoproteomics ,  reverse genetics  and  genomics ,  transcriptomics , and  proteomics , to identify the potential targets of the compound of interest. [ 16 ]  Reverse genetics and genomics approaches, for instance, uses genetic perturbation (e.g.  CRISPR - Cas9  or  siRNA ) in combination with the compound to identify genes whose knockdown or  knockout  abolishes the pharmacological effect of the compound. On the other hand, transcriptomics and proteomics profiles of the compound can be used to compare with profiles of compounds with known targets. Thanks to computation inference, it is then possible to make hypotheses about the mechanism of action of the compound, which can subsequently be tested. [ 16 ]


There are many drugs in which the mechanism of action is known. One example is aspirin. [ citation needed ]


The  mechanism of action of aspirin  involves irreversible inhibition of the enzyme  cyclooxygenase ; [ 17 ]  therefore suppressing the production of  prostaglandins  and  thromboxanes , thus reducing pain and inflammation. This mechanism of action is specific to aspirin and is not constant for all  nonsteroidal anti-inflammatory drugs  (NSAIDs). Rather, aspirin is the only NSAID that irreversibly inhibits COX-1. [ 18 ]


Some drug mechanisms of action are still unknown.  However, even though the mechanism of action of a certain drug is unknown, the drug still functions; it is just unknown or unclear how the drug interacts with receptors and produces its therapeutic effect.


In some literature articles, the terms "mechanism of action" and " mode of action " are used interchangeably, typically referring to the way in which the drug interacts and produces a medical effect. However, in actuality, a mode of action describes functional or anatomical changes, at the cellular level, resulting from the exposure of a living organism to a substance. [ 19 ]  This differs from a mechanism of action since it is a more specific term that focuses on the interaction between the drug itself and an  enzyme  or  receptor  and its particular form of interaction, whether through  inhibition ,  activation ,  agonism , or  antagonism . Furthermore, the term "mechanism of action" is the main term that is primarily used in pharmacology, whereas "mode of action" will more often appear in the field of microbiology or certain aspects of biology. [ citation needed ]


In statistics, in particular in the  design of experiments , a  multi-valued treatment  is a treatment that can take on more than two values. It is related to the  dose-response model  in the medical literature.


Generally speaking, treatment levels may be finite or infinite as well as ordinal or cardinal, which leads to a large collection of possible treatment effects to be studied in applications. [ 1 ]  One example is the effect of different levels of program participation (e.g. full-time and part-time) in a job training program. [ 2 ]


Assume there exists a finite collection of multi-valued treatment status

 T
 =
 {
 0
 ,
 1
 ,
 2
 ,
 …
 ,
 J
 ,
 }

 {\displaystyle T=\{0,1,2,\ldots ,J,\}}

  with J some fixed integer. As in the potential outcomes framework, denote

 Y
 (
 j
 )
 ⊂
 R

 {\displaystyle Y(j)\subset R}

  the collection of potential outcomes under the treatment  J , and

 Y
 =

 ∑

 j
 =
 0

 J

 D

 j

 Y
 (
 j
 )

 {\displaystyle Y=\textstyle \sum _{j=0}^{J}\displaystyle D_{j}Y(j)}

  denotes the observed outcome and

 D

 j

 {\displaystyle D_{j}}

  is an indicator that equals  1  when the treatment equals  j  and  0  when it does not equal  j , leading to a fundamental problem of  causal inference . [ 3 ]   A general framework that analyzes ordered choice models in terms of marginal treatment effects and average treatment effects has been extensively discussed by Heckman and Vytlacil. [ 4 ]


Recent work in the econometrics and statistics literature has focused on estimation and inference for multivalued treatments and ignorability conditions for identifying the treatment effects. In the context of program evaluation, the propensity score has been generalized to allow for multi-valued treatments, [ 5 ]  while other work has also focused on the role of the conditional mean independence assumption. [ 6 ]   Other recent work has focused more on the large sample properties of an estimator of the marginal mean treatment effect conditional on a treatment level in the context of a difference-in-differences model, [ 7 ]  and on the efficient estimation of multi-valued treatment effects in a semiparametric framework. [ 8 ]


Non-competitive inhibition  is a type of  enzyme inhibition  where the inhibitor reduces the activity of the enzyme and binds equally well to the enzyme whether or not it has already bound the substrate. [ 1 ]  This is unlike  competitive inhibition , where binding affinity for the substrate in the enzyme is decreased in the presence of an inhibitor.


The inhibitor may bind to the enzyme whether or not the substrate has already been bound, but if it has a higher affinity for binding the enzyme in one state or the other, it is called a  mixed inhibitor . [ 1 ]


During his years working as a physician  Leonor Michaelis   and a friend Peter Rona built a compact lab, in the hospital, and over the course of five years – Michaelis successfully became published over 100 times. During his research in the hospital, he was the first to view the different types of inhibition; specifically using fructose and glucose as inhibitors of  maltase  activity. Maltase breaks maltose into two units of  glucose . Findings from that experiment allowed for the divergence of non-competitive and  competitive inhibition . Non-competitive inhibition affects the  k cat  value (but not the K m ) on any given graph; this inhibitor binds to a site that has specificity for the certain molecule. Michaelis determined that when the inhibitor is bound, the enzyme would become inactivated. [ 2 ]


Like many other scientists of their time, Leonor Michaelis and  Maud Menten  worked on a reaction that was used to change the composition of  sucrose  and make it  lyse  into two products – fructose and glucose. [ 2 ]  The enzyme involved in this reaction is called  invertase , and it is the enzyme the kinetics of which have been supported by Michaelis and Menten to be revolutionary for the kinetics of other enzymes. While expressing the rate of the reaction studied, they derived an equation that described the rate in a way which suggested that it is mostly dependent on the enzyme concentration, as well as on presence of the substrate, but only to a certain extent. [ 2 ] [ 3 ]


Adrian John Brown  and  Victor Henri  laid the groundwork for the discoveries in enzyme kinetics that Michaelis and Menten are known for. [ 4 ]  Brown theoretically envisioned the mechanism now accepted for enzyme kinetics, but did not have the quantitative data to make a claim. [ 4 ]  Victor Henri made significant contributions to enzyme kinetics during his doctoral thesis, however he lacked noting the importance of hydrogen ion concentration and mutarotation of glucose. The goal of Henri's thesis was to compare his knowledge of enzyme-catalysed reactions to the recognized laws of physical chemistry. [ 2 ]  Henri is credited with being the first to write the equation that is now known as the Michaelis-Menten equation. Using glucose and fructose in the catalytic reactions controlled by maltase and invertase, Leonor Michaelis was the first scientist to distinguish the different types of inhibition by using the pH scale which did not exist in Henri's time. [ 2 ]


Particularly during their work on describing the rate of this reaction they also tested and extrapolated on the idea of another scientist,  Victor Henri , that enzyme they were using had some affinity for both products of this reaction – fructose and glucose. [ 2 ] [ 3 ]  Using Henri's methods, Michaelis and Menten nearly perfected this concept of initial-rate method for steady-state experiments. They were studying inhibition when they found that non-competitive (mixed) inhibition is characterized by its effect on  k cat  (catalyst rate) while competitive is characterized by its effect on velocity (V). [ 2 ]  In the Michaelis and Menten experiments they heavily focused on pH effects of invertase using hydrogen ions. [ 2 ]  Invertase is an enzyme found in extracellular yeast and catalyzed reactions by hydrolysis or inverting a sucrose (mixture of sucrose and fructose) to “ invert sugar .” The main reason for using invertase was that it could be easily assayed and experiments could be done in quicker manner.  Sucrose rotates in  polarimeter  as  dextroratatory-D  whereas invert sugar is  levorotatory-L . This made tracking the inversion of sugar relatively simple. They also found that  α-D-glucose  is released in reactions catalyzed by invertase which is very unstable and spontaneously changes to  β-D-glucose . [ 4 ]  Although, these are both in the dextrorotatory form, this is where they noted that glucose can change spontaneously, also known as mutarotation. Failing to take this into consideration was one of the main reasons Henri's experiments fell short. Using invertase to catalyze sucrose inversion, they could see how fast the enzyme was reacting by polarimetry; therefore, non-competitive inhibition was found to occur in the reaction where sucrose was inverted with invertase. [ 2 ]


It is important to note that while all non-competitive inhibitors bind the enzyme at allosteric sites (i.e. locations other than its  active site )—not all inhibitors that bind at allosteric sites are non-competitive inhibitors. [ 1 ]   In fact, allosteric inhibitors may act as  competitive , non-competitive, or  uncompetitive  inhibitors. [ 1 ]


Many sources continue to conflate these two terms, [ 5 ]  or state the definition of allosteric inhibition as the definition for non-competitive inhibition.


Non-competitive inhibition models a system where the inhibitor and the substrate may both be bound to the enzyme at any given time. When both the substrate and the inhibitor are bound, the enzyme-substrate-inhibitor complex cannot form product and can only be converted back to the enzyme-substrate complex or the enzyme-inhibitor complex. Non-competitive inhibition is distinguished from general mixed inhibition in that the inhibitor has an equal affinity for the enzyme and the enzyme-substrate complex.


For example, in the enzyme-catalyzed reactions of  glycolysis , accumulation phosphoenol is catalyzed by  pyruvate kinase  into  pyruvate .  Alanine  is an amino acid which is synthesized from pyruvate also inhibits the enzyme pyruvate kinase during glycolysis. Alanine is a non-competitive inhibitor, therefore it binds away from the active site to the substrate in order for it to still be the final product. [ 6 ]


Another example of non-competitive inhibition is given by  glucose-6-phosphate  inhibiting hexokinase in the brain. Carbons 2 and 4 on glucose-6-phosphate contain hydroxyl groups that attach along with the phosphate at carbon 6 to the enzyme-inhibitor complex. The substrate and enzyme are different in their group combinations that an inhibitor attaches to. The ability of glucose-6-phosphate to bind at different places at the same time makes it a non-competitive inhibitor. [ 7 ]


The most common mechanism of non-competitive inhibition involves reversible binding of the inhibitor to an  allosteric site , but it is possible for the inhibitor to operate via other means including direct binding to the active site. It differs from competitive inhibition in that the binding of the inhibitor does not prevent binding of substrate, and vice versa, but simply prevents product formation for a limited time.


This type of inhibition reduces the  maximum rate  of a  chemical reaction  without changing the apparent binding  affinity  of the  catalyst  for the  substrate  (K m app  – see  Michaelis-Menten kinetics ). When a non-competitive inhibitor is added the Vmax is changed, while the Km remains unchanged. According to the  Lineweaver-Burk plot  the Vmax is reduced during the addition of a non-competitive inhibitor, which is shown in the plot by a change in both the slope and y-intercept when a non-competitive inhibitor is added. [ 8 ]