Drug metabolism
From Wikipedia, the free encyclopedia
Drug metabolism is the metabolism of drugs, their biochemical modification or degradation, usually through
specialized enzymatic systems. This is a form of xenobiotic metabolism. Drug metabolism often
converts lipophilic chemical compounds into more readily excreted polarproducts. 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 xenobiotics. Other commonly used organic chemicals are also xenobiotics, and are metabolized
by the same enzymes as drugs. This provides the opportunity for drug-drug and drug-chemical interactions or
reactions.
Contents
[hide]

1 Phase I vs. Phase II

2 Sites

3 Major enzymes and pathways
o
3.1 Phase I

3.1.1 Oxidation
o

3.1.2 Reduction

3.1.3 Hydrolysis
3.2 Phase II

3.2.1 Methylation

3.2.2 Sulphation

3.2.3 Acetylation

3.2.4 Conjugation

4 Factors that affect Drug Metabolism

5 See also

6 References

7 External links
[edit]Phase
I vs. Phase II
Phase I and Phase II reactions are biotransformations of chemicals that occur during drug metabolism.
Phase I reactions usually precedes Phase II, though not necessarily. During these reactions, polar bodies are either
introduced or unmasked, which results in (more) polar metabolites of the original chemicals. In the case of
pharmaceutical drugs, Phase I reactions can lead either to activation or inactivation of the drug.
Phase I reactions (also termed nonsynthetic reactions) may occur by oxidation, reduction, hydrolysis, cyclization,
and decyclization reactions. Oxidation involves the enzymatic 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 haemoprotein, NADPH and oxygen. The classes of pharmaceutical drugs that utilize this method for their
metabolism include phenothiazines, paracetamol, and steroids. If the metabolites of phase I reactions are sufficiently
polar, they may be readily excreted at this point. However, many phase I products are not eliminated rapidly and
undergo a subsequent reaction in which an endogenous substrate combines with the newly incorporated functional
group to form a highly polar conjugate.
Phase II reactions — usually known as conjugation reactions (e.g., with glucuronic acid, sulfonates (commonly
known as sulfation) , glutathioneor amino acids) — are usually detoxication 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.
[edit]Sites
Quantitatively, the smooth endoplasmic reticulum of the liver cell is the principal organ of drug metabolism,
although every biological tissue has some ability to metabolize drugs. Factors responsible for the liver's contribution
to drug metabolism include that it is a large organ, that it is the first organ perfused by chemicals absorbed in
the gut, and that there are very high concentrations of most drug-metabolizing enzyme systems relative to other
organs. If a drug is taken into the GI tract, where it enters hepatic circulation through the portal vein, it becomes
well-metabolized and is said to show the first pass effect.
Other sites of drug metabolism include epithelial cells of the gastrointestinal tract, lungs, kidneys, and the skin.
These sites are usually responsible for localized toxicity reactions.
[edit]Major
enzymes and pathways
Several major enzymes and pathways are involved in drug metabolism, and can be divided into Phase I and Phase II
reactions:
[edit]Phase
I
[edit]Oxidation

Cytochrome P450 monooxygenase system

Flavin-containing monooxygenase system

Alcohol dehydrogenase and aldehyde dehydrogenase

Monoamine oxidase

Co-oxidation by peroxidases
[edit]Reduction

NADPH-cytochrome P450 reductase

Reduced (ferrous) cytochrome P450
It should be noted that 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).
[edit]Hydrolysis

Esterases and amidases

Epoxide hydrolase
[edit]Phase
II
[edit]Methylation

Methyl Transferase
[edit]Sulphation

Glutathione S-transferases

Sulfotransferases
[edit]Acetylation

N-acetyltransferases

Amino acid N-acyl transferases
[edit]Conjugation

UDP-glucuronosyltransferases

Mercapturic acid biosynthesis
[edit]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) of a
pharmacologically active metabolite will decrease the duration and intensity of the drug action. The opposite is also
true (e.g., enzyme inhibition).
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 humans and animals than in adults.
Genetic variation (polymorphism) accounts for some of the variability in the effect of drugs. With Nacetyltransferases (involved in Phase IIreactions), 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.[1] This can be used to identify individuals most at risk from
adverse reaction.
[edit]See

also
Active metabolite

Simcyp Simulator

Simulations Plus

Drug reaction testing

SPORCalc, an example process for exploring drug metabolism databases [2]
[edit]References
1. ^ Amin Rostami-Hodjegan; Geoffrey Tucker (2007). "Simulation and prediction of in vivo drug metabolism in human
populations from in vitro data". Nature Reviews Drug Discovery 6 (2): 140–8. doi:10.1038/nrd2173. PMID 17268485.
2. ^ James Smith; Viktor Stein (2009). "SPORCalc: A development of a database analysis that provides putative
metabolic enzyme reactions for ligand-based drug design". Computational Biology and Chemistry 33 (2): 149159. doi:10.1016/j.compbiolchem.2008.11.002. PMID 19157988.

Basic and Clinical Pharmacology (9th Edition; Katzung): 1.4. Drug Biotransformation
[edit]External
links

University of Minnesota Biocatalysis/Biodegradation Database

Drug Metabolism Database
Physiologic pharmacokinetic