Drug metabolism
Encyclopedia
Drug metabolism is the biochemical modification of pharmaceutical substances by living organism
s, usually through specialized enzymatic
systems. This is a form of xenobiotic metabolism
. Drug metabolism often converts lipophilic chemical compound
s into more readily excreted
polar products. Its rate is an important determinant of the duration and intensity of the pharmacological action of drugs.
Drug
metabolism can result in toxication
or detoxication
- the activation or deactivation of the chemical.
While both occur, the major metabolites of most drugs are detoxication products.
Drugs are almost all xenobiotic
s. Other commonly used organic chemicals are also xenobiotics, and are metabolized by the same enzyme
s as drugs. This provides the opportunity for drug-drug and drug-chemical interactions
or reactions.
, hydrolysis
, cyclization, and decyclization addition of oxygen or removal of hydrogen, carried out by mixed function oxidases, often in the liver. These oxidative reactions typically involve a cytochrome P450
monooxygenase (often abbreviated CYP), NADPH and oxygen. The classes of pharmaceutical drugs that utilize this method for their metabolism include phenothiazine
s, paracetamol, and steroids. If the metabolites of phase I reactions are sufficiently polar, they may be readily excreted at this point. However, many phase I products are not eliminated rapidly and undergo a subsequent reaction in which an endogenous substrate combines with the newly incorporated functional group to form a highly polar conjugate.
A common Phase I oxidation involves conversion of a C-H bond to a C-OH. This reaction sometimes converts a pharmacologically inactive compound (a prodrug
) to a pharmacologically active one. By the same token, Phase I can turn a nontoxic molecule into a poisonous one (toxification
). Simple hydrolysis in the stomach transforms , which are comparatively innocuous. But Phase I metabolism converts acetonitrile
to HOCH2CN, which rapidly dissociates into formaldehyde
and hydrogen cyanide, both of which are toxic.
Phase I metabolism of drug candidates can be simulated in the laboratory using non-enzyme catalysts. This example of a biomimetic reaction tends to give products that often contains the Phase I metabolites. As an example, the major metabolite of the pharmaceutical trimebutine
, desmethyltrimebutine (nor-trimebutine), can be efficiently produced by in vitro oxidation of the commercially available drug. Hydroxylation of an N-methyl group leads to expulsion of a molecule of formaldehyde
, while oxidation of the O-methyl groups takes place to a lesser extent.
Cytochrome P450 reductase also known as NADPH:ferrihemoprotein oxidoreductase, NADPH:hemoprotein oxidoreductase, NADPH:P450 oxidoreductase, P450 reductase, POR, CPR, CYPOR, is a membrane-bound enzyme required for electron transfer to cytochrome P450 in the microsome of the eukaryotic cell from a FAD- and FMN-containing enzyme NADPH:cytochrome P450 reductase
The general scheme of electron flow in the POR/P450 system is:
NADPH
→
FAD
→
FMN
→
P450
→
O2
During reduction reactions, a chemical can enter futile cycling, in which it gains a free-radical electron, then promptly loses it to oxygen
(to form a superoxide anion).
glucuronic acid
, sulfonates (commonly known as sulfation), glutathione
or amino acids) — are usually detoxicating
in nature, and involve the interactions of the polar functional groups of phase I metabolites. Sites on drugs where conjugation reactions occur include carboxyl (-COOH), hydroxyl
(-OH), amino (NH2), and sulfhydryl (-SH) groups. Products of conjugation reactions have increased molecular weight and are usually inactive unlike Phase I reactions which often produce active metabolites.
cell is the principal organ of drug metabolism, although every biological tissue
has some ability to metabolize drugs.
Factors responsible for the liver's contribution to drug metabolism include that it is a large organ, that it is the first organ perfused by chemicals absorbed in the gut
, and that there are very high concentrations of most drug-metabolizing enzyme systems relative to other organs.
If a drug is taken into the GI tract, where it enters hepatic circulation through the portal vein, it becomes well-metabolized and is said to show the first pass effect
.
Other sites of drug metabolism include epithelial cells of the gastrointestinal tract
, lung
s, kidney
s, and the skin
.
These sites are usually responsible for localized toxicity reactions.
The Cytochrome P450 monooxygenase system is the most important pathway in this regard.
In general, anything that increases the rate of metabolism (e.g., enzyme induction
) of a pharmacologically active metabolite will decrease the duration and intensity of the drug action.
The opposite is also true (e.g., enzyme inhibition
). However, in cases where an enzyme is responsible for metabolizing a pro-drug into a drug, enzyme induction can speed up this conversion and increase drug levels, potentially causing toxicity.
Various physiological and pathological factors can also affect drug metabolism.
Physiological factors that can influence drug metabolism include age, individual variation (e.g., pharmacogenetics
), enterohepatic circulation
, nutrition
, intestinal flora, or sex differences.
In general, drugs are metabolized more slowly in fetal, neonatal and elderly human
s and animal
s than in adult
s.
Genetic variation (polymorphism) accounts for some of the variability in the effect of drugs.
With N-acetyltransferases (involved in Phase II reactions), individual variation creates a group of people who acetylate slowly (slow acetylators) and those who acetylate quickly, split roughly 50:50 in the population of Canada
.
This variation may have dramatic consequences, as the slow acetylators are more prone to dose-dependent toxicity.
Cytochrome P450 monooxygenase system enzymes can also vary across individuals, with deficiencies occurring in 1 - 30% of people, depending on their ethnic background.
Pathological factors can also influence drug metabolism, including liver
, kidney
, or heart
diseases.
In silico modelling and simulation methods allow drug metabolism to be predicted in virtual patient populations prior to performing clinical studies in human subjects. This can be used to identify individuals most at risk from adverse reaction.
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s, usually through specialized enzymatic
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
systems. This is a form of xenobiotic metabolism
Xenobiotic metabolism
Xenobiotic metabolism is the set of metabolic pathways that modify the chemical structure of xenobiotics, which are compounds foreign to an organism's normal biochemistry, such as drugs and poisons...
. Drug metabolism often converts lipophilic chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s into more readily excreted
Excretion
Excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism. This is primarily carried out by the lungs, kidneys and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell...
polar products. Its rate is an important determinant of the duration and intensity of the pharmacological action of drugs.
Drug
Medication
A pharmaceutical drug, also referred to as medicine, medication or medicament, can be loosely defined as any chemical substance intended for use in the medical diagnosis, cure, treatment, or prevention of disease.- Classification :...
metabolism can result in toxication
Toxication
Toxication is the process of metabolism in which the metabolite of a compound is more toxic than the parent drug or chemical. A parent drug or chemical that was previously non-toxic may be called a protoxin.Toxication may involve:...
or detoxication
Detoxication
Detoxication products are the major metabolites formed from most drug metabolism.There are two common patterns observed:#A drug with potent pharmacological activity is converted to a major metabolite with markedly reduced or no pharmacological activity #A drug is converted to a metabolite with...
- the activation or deactivation of the chemical.
While both occur, the major metabolites of most drugs are detoxication products.
Drugs are almost all xenobiotic
Xenobiotic
A xenobiotic is a chemical which is found in an organism but which is not normally produced or expected to be present in it. It can also cover substances which are present in much higher concentrations than are usual...
s. Other commonly used organic chemicals are also xenobiotics, and are metabolized by the same enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s as drugs. This provides the opportunity for drug-drug and drug-chemical interactions
Drug interaction
A drug interaction is a situation in which a substance affects the activity of a drug, i.e. the effects are increased or decreased, or they produce a new effect that neither produces on its own. Typically, interaction between drugs come to mind...
or reactions.
Phase I
Phase I reactions (also termed nonsynthetic reactions) may occur by oxidation, reductionRedox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
, hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
, cyclization, and decyclization addition of oxygen or removal of hydrogen, carried out by mixed function oxidases, often in the liver. These oxidative reactions typically involve a cytochrome P450
Cytochrome P450 oxidase
The cytochrome P450 superfamily is a large and diverse group of enzymes. The function of most CYP enzymes is to catalyze the oxidation of organic substances. The substrates of CYP enzymes include metabolic intermediates such as lipids and steroidal hormones, as well as xenobiotic substances...
monooxygenase (often abbreviated CYP), NADPH and oxygen. The classes of pharmaceutical drugs that utilize this method for their metabolism include phenothiazine
Phenothiazine
Phenothiazine is an organic compound that occurs in various antipsychotic and antihistaminic drugs. It has the formula S2NH. This yellow tricyclic compound is soluble in acetic acid, benzene, and ether. The compound is related to the thiazine-class of heterocyclic compounds...
s, paracetamol, and steroids. If the metabolites of phase I reactions are sufficiently polar, they may be readily excreted at this point. However, many phase I products are not eliminated rapidly and undergo a subsequent reaction in which an endogenous substrate combines with the newly incorporated functional group to form a highly polar conjugate.
A common Phase I oxidation involves conversion of a C-H bond to a C-OH. This reaction sometimes converts a pharmacologically inactive compound (a prodrug
Prodrug
A prodrug is a pharmacological substance administered in an inactive form. Once administered, the prodrug is metabolised in vivo into an active metabolite, a process termed bioactivation. The rationale behind the use of a prodrug is generally for absorption, distribution, metabolism, and...
) to a pharmacologically active one. By the same token, Phase I can turn a nontoxic molecule into a poisonous one (toxification
Toxification
Toxification is the conversion of chemical compounds into more toxic forms. This conversion can take place in the body as well as in substrates such as soil or water...
). Simple hydrolysis in the stomach transforms , which are comparatively innocuous. But Phase I metabolism converts acetonitrile
Acetonitrile
Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...
to HOCH2CN, which rapidly dissociates into formaldehyde
Formaldehyde
Formaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...
and hydrogen cyanide, both of which are toxic.
Phase I metabolism of drug candidates can be simulated in the laboratory using non-enzyme catalysts. This example of a biomimetic reaction tends to give products that often contains the Phase I metabolites. As an example, the major metabolite of the pharmaceutical trimebutine
Trimebutine
Trimebutine is a drug with antimuscarinic and weak mu opioid agonist effects. The maleic acid salt of trimebutine is marketed under the trademark of Debridat, Recutin, Polybutin, or Modulon for treatment of irritable bowel syndrome and other gastrointestinal disorders...
, desmethyltrimebutine (nor-trimebutine), can be efficiently produced by in vitro oxidation of the commercially available drug. Hydroxylation of an N-methyl group leads to expulsion of a molecule of formaldehyde
Formaldehyde
Formaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...
, while oxidation of the O-methyl groups takes place to a lesser extent.
Oxidation
- Cytochrome P450 monooxygenase systemCytochrome P450 oxidaseThe cytochrome P450 superfamily is a large and diverse group of enzymes. The function of most CYP enzymes is to catalyze the oxidation of organic substances. The substrates of CYP enzymes include metabolic intermediates such as lipids and steroidal hormones, as well as xenobiotic substances...
- Flavin-containing monooxygenase system
- Alcohol dehydrogenaseAlcohol dehydrogenaseAlcohol dehydrogenases are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide...
and aldehyde dehydrogenaseAldehyde dehydrogenaseAldehyde dehydrogenases are a group of enzymes that catalyse the oxidation of aldehydes.- Function :Aldehyde dehydrogenase is a polymorphic enzyme responsible for the oxidation of aldehydes to carboxylic acids, which leave the liver and are metabolized by the body’s muscle and heart... - Monoamine oxidaseMonoamine oxidaseL-Monoamine oxidases are a family of enzymes that catalyze the oxidation of monoamines. They are found bound to the outer membrane of mitochondria in most cell types in the body. The enzyme was originally discovered by Mary Bernheim in the liver and was named tyramine oxidase...
- Co-oxidation by peroxidasePeroxidasePeroxidases are a large family of enzymes that typically catalyze a reaction of the form:For many of these enzymes the optimal substrate is hydrogen peroxide, but others are more active with organic hydroperoxides such as lipid peroxides...
s
Reduction
- NADPH-cytochrome P450 reductase
Cytochrome P450 reductase also known as NADPH:ferrihemoprotein oxidoreductase, NADPH:hemoprotein oxidoreductase, NADPH:P450 oxidoreductase, P450 reductase, POR, CPR, CYPOR, is a membrane-bound enzyme required for electron transfer to cytochrome P450 in the microsome of the eukaryotic cell from a FAD- and FMN-containing enzyme NADPH:cytochrome P450 reductase
The general scheme of electron flow in the POR/P450 system is:
NADPH
→
FAD
→
FMN
→
P450
→
O2
- Reduced (ferrous) cytochrome P450
During reduction reactions, a chemical can enter futile cycling, in which it gains a free-radical electron, then promptly loses it to oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(to form a superoxide anion).
Phase II
Phase II reactions — usually known as conjugation reactions (e.g., withglucuronic acid
Glucuronic acid
Glucuronic acid is a carboxylic acid. Its structure is similar to that of glucose. However, glucuronic acid's sixth carbon is oxidized to a carboxylic acid...
, sulfonates (commonly known as sulfation), glutathione
Glutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...
or amino acids) — are usually detoxicating
Detoxication
Detoxication products are the major metabolites formed from most drug metabolism.There are two common patterns observed:#A drug with potent pharmacological activity is converted to a major metabolite with markedly reduced or no pharmacological activity #A drug is converted to a metabolite with...
in nature, and involve the interactions of the polar functional groups of phase I metabolites. Sites on drugs where conjugation reactions occur include carboxyl (-COOH), hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
(-OH), amino (NH2), and sulfhydryl (-SH) groups. Products of conjugation reactions have increased molecular weight and are usually inactive unlike Phase I reactions which often produce active metabolites.
| Mechanism | Involved enzyme | Co-factor | Location |
|---|---|---|---|
| Methylation Methylation In the chemical sciences, methylation denotes the addition of a methyl group to a substrate or the substitution of an atom or group by a methyl group. Methylation is a form of alkylation with, to be specific, a methyl group, rather than a larger carbon chain, replacing a hydrogen atom... |
Methyltransferase Methyltransferase A methyltransferase is a type of transferase enzyme that transfers a methyl group from a donor to an acceptor.Methylation often occurs on nucleic bases in DNA or amino acids in protein structures... |
S-adenosyl-L-methionine | Liver, kidney, lung, CNS |
| Sulphation | Sulfotransferase Sulfotransferase Sulfotransferases are transferase enzymes that catalyze the transfer of a sulfate group from a donor molecule to an acceptor alcohol or amine. The most common sulfate donor is 3'-phosphoadenosine-5'-phosphosulfate... s |
3'-phosphoadenosine-5'-phosphosulfate 3'-Phosphoadenosine-5'-phosphosulfate 3'-Phosphoadenosine-5'-phosphosulfate is a derivative of adenosine monophosphate that is phosphorylated at the 3' position and has a sulfate group attached to the 5' phosphate. This anion, abbreviated PAPS, serves as a coenzyme in sulfotransferase reactions... |
Liver, kidney, intestine |
| Acetylation Acetylation Acetylation describes a reaction that introduces an acetyl functional group into a chemical compound... |
|
Acetyl coenzyme A | Liver, lung, spleen, gastric mucosa, RBCs Red blood cell Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system... , lymphocytes |
| Glucuronidation Glucuronidation Glucuronidation is the addition of glucuronic acid to a substrate. Glucuronidation is often involved in xenobiotic metabolism of substances such as drugs, pollutants, bilirubin, androgens, estrogens, mineralocorticoids, glucocorticoids, fatty acid derivatives, retinoids, and bile acids... |
UDP-glucuronosyltransferases | UDP-glucuronic acid | Liver, kidney, intestine, lung, skin, prostate, brain |
| Glutathione conjugation | Glutathione S-transferase Glutathione S-transferase Enzymes of the glutathione S-transferase family are composed of many cytosolic, mitochondrial, and microsomal proteins. GSTs are present in eukaryotes and in prokaryotes, where they catalyze a variety of reactions and accept endogenous and xenobiotic substrates.GSTs can constitute up to 10% of... s |
Glutathione Glutathione Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain... |
Liver, kidney |
Sites
Quantitatively, the smooth endoplasmic reticulum of the liverLiver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
cell is the principal organ of drug metabolism, although every biological tissue
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
has some ability to metabolize drugs.
Factors responsible for the liver's contribution to drug metabolism include that it is a large organ, that it is the first organ perfused by chemicals absorbed in the gut
Gut (zoology)
In zoology, the gut, also known as the alimentary canal or alimentary tract, is a tube by which bilaterian animals transfer food to the digestion organs. In large bilaterians the gut generally also has an exit, the anus, by which the animal disposes of solid wastes...
, and that there are very high concentrations of most drug-metabolizing enzyme systems relative to other organs.
If a drug is taken into the GI tract, where it enters hepatic circulation through the portal vein, it becomes well-metabolized and is said to show the first pass effect
First pass effect
The first-pass effect is a phenomenon of drug metabolism whereby the concentration of a drug is greatly reduced before it reaches the systemic circulation. It is the fraction of lost drug during the process of absorption which is generally related to the liver and gut wall...
.
Other sites of drug metabolism include epithelial cells of the gastrointestinal tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
, lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
s, kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
s, and the skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
.
These sites are usually responsible for localized toxicity reactions.
Factors that affect drug metabolism
The duration and intensity of pharmacological action of most lipophilic drugs are determined by the rate they are metabolized to inactive products.The Cytochrome P450 monooxygenase system is the most important pathway in this regard.
In general, anything that increases the rate of metabolism (e.g., enzyme induction
Enzyme induction and inhibition
Enzyme induction is a process in which a molecule induces the expression of an enzyme.Enzyme inhibition can refer to* the inhibition of the expression of the enzyme by another molecule...
) of a pharmacologically active metabolite will decrease the duration and intensity of the drug action.
The opposite is also true (e.g., enzyme inhibition
Enzyme induction and inhibition
Enzyme induction is a process in which a molecule induces the expression of an enzyme.Enzyme inhibition can refer to* the inhibition of the expression of the enzyme by another molecule...
). However, in cases where an enzyme is responsible for metabolizing a pro-drug into a drug, enzyme induction can speed up this conversion and increase drug levels, potentially causing toxicity.
Various physiological and pathological factors can also affect drug metabolism.
Physiological factors that can influence drug metabolism include age, individual variation (e.g., pharmacogenetics
Pharmacogenetics
The terms pharmacogenomics and pharmacogenetics tend to be used interchangeably, and a precise, consensus definition of either remains elusive...
), enterohepatic circulation
Enterohepatic circulation
Enterohepatic circulation refers to the circulation of biliary acids from the liver, where they are produced and secreted in the bile, to the small intestine, where it aids in digestion of fats and other substances, back to the liver....
, nutrition
Nutrition
Nutrition is the provision, to cells and organisms, of the materials necessary to support life. Many common health problems can be prevented or alleviated with a healthy diet....
, intestinal flora, or sex differences.
In general, drugs are metabolized more slowly in fetal, neonatal and elderly human
Human
Humans are the only living species in the Homo genus...
s and animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s than in adult
Adult
An adult is a human being or living organism that is of relatively mature age, typically associated with sexual maturity and the attainment of reproductive age....
s.
Genetic variation (polymorphism) accounts for some of the variability in the effect of drugs.
With N-acetyltransferases (involved in Phase II reactions), individual variation creates a group of people who acetylate slowly (slow acetylators) and those who acetylate quickly, split roughly 50:50 in the population of Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
.
This variation may have dramatic consequences, as the slow acetylators are more prone to dose-dependent toxicity.
Cytochrome P450 monooxygenase system enzymes can also vary across individuals, with deficiencies occurring in 1 - 30% of people, depending on their ethnic background.
Pathological factors can also influence drug metabolism, including liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
, kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
, or heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
diseases.
In silico modelling and simulation methods allow drug metabolism to be predicted in virtual patient populations prior to performing clinical studies in human subjects. This can be used to identify individuals most at risk from adverse reaction.