Download Factors that affect Drug Metabolism

II. Where does metabolism occur in the body?
The liver is the primary site for metabolism. Liver contains the necessary
enzymes for metabolism of drugs and other xenobiotics. These enzymes
induce two metabolism pathways: Phase I (functionalization reactions) and
Phase II (biosynthetic reactions) metabolism. Some typical examples of Phase
I metabolism include oxidation and hydrolysis. The enzymes involved in Phase
I reactions are primarily located in the endoplasmic reticulum of the liver cell,
they are called microsomal enzymes. Phase II metabolism involves the
introduction of a hydrophilic endogenous species, such as glucuronic acid or
sulfate, to the drug molecule. Enzymes involved in phase II reactions are
mainly located in the cytosol, except glucuronidation enzyme, which is also a
microsomal enzyme.
Drugs are usually lipophilic substances (Oil-like) so they can pass plasma
membranes and reach the site of action. Drug metabolism is basically a process
that introduces hydrophilic functionailities onto the drug molecule to facilitate
excretion. When the drug molecule is oxidized, hydrolyzed, or covalently
attached to a hydrophilic species, the whole molecule becomes more
hydrophilic, and is excreted more easily. Drugs often undergo both Phase I and
II reactions before excretion. The Phase I reaction introduces a functional
group such as a hydroxyl group onto the molecule, or exposes a preexisting
functional group, and Phase II reaction connects this functional group to the
endogenous species such as a glucuronic acid. The modified drug molecule
may then be hydrophilic enough to be excreted.
Although liver is the primary site for metabolism, virtually all tissue cells have
some metabolic activities. Other organs having significant metabolic activities
include the gastrointestinal tract, kidneys, and lungs. When a drug is
administrated orally, it undergoes metabolism in the GI track and the liver
before reaching systemic circulation. This process is called first-pass
metabolism. First-pass metabolism limits the oral bioavailability of drugs,
sometimes significantly.
Why are drugs metabolized? Small molecule drugs are xenobiotics, foreign
molecules, that the human body attempts to deal with through a number of
responses. Some drugs are excreted from the human body intact. Most drugs,
however, need to be modified structurally to facilitate excretion. These
modification processes are called drug metabolism. Drug metabolism is a
detoxification function the human body possesses to defend itself from environment
hostility. When a person is sick, however, the body needs some kind of medication
to fight the disease. Ideally, a drug should reach the site of action intact, cure the
disease, and leave the body after it completes its mission. However, drug
developers often face the dilemma that a potential drug is either
metabolized/excreted from the body too fast, that the drug can not reach its
therapeutic effect, or too slow, that it stays in the body for a long time, causing side
effects. (Remember the drug is a xenobiotic that the normal human body doesn't
need.) The study of drug metabolism, therefore, serves primarily two purposes: to
elucidate the function and fate of the drug, and to manipulate the metabolic process
of a potential drug.
III. What happens to drugs once they are metabolized?
Drugs are ultimately excreted from the body through various routes. The
kidney is the major organ for drug excretion. It excretes hydrophilic drug and
drug metabolites through glomerular filtration. Macromolecules such as
proteins are retained. Lipophilic drug molecules are not directly excreted from
the kidney. Only after they are metabolized into more hydrophilic molecules,
can they be excreted through the kidneys into the urine. Drugs and their
metabolites are also excreted into bile. This is usually mediated by protein
transporters. Drugs and their metabolites in bile are eventually released into
the intestinal tract. The drugs may be reabsorbed into the body from the
intestine. Drug metabolites such as glucuronide conjugates, may be converted
back to the parent drug in the intestine through glucuronidase enzyme, and
then reabsorbed into systemic circulation. This drug recycling process is
called enterohepatic recycling. This process, if extensive, may prolong the
half-life of the drug. The bile drugs and drug metabolites, if not reabsorbed by
intestine, are excreted from the body through feces. Also, a variety of orally
administrated drugs are excreted through feces because they are not absorbed
through the intestine. Oral bioavailability constitutes a major challenge for
drug developers. Other routes of excretion, such as sweat, tears, and saliva,
are quantitatively less important. Excretion through breast milk is not
important to the mother, but may be of key importance to the baby, because
the drug may be toxic to the baby. Pulmonary excretion is important for
anesthetic gases and vapor drugs.
IV. Are there common motifs (consensus sites) on molecules where
metabolism occurs?
As is pointed out, small molecule drugs are usually lipophilic substances that
can penetrate cell membranes to reach the site of action, and drug metabolism
is a process of introducing hydrophilic functional groups onto the drug
molecule. The most common phase I reactions are oxidative processes that
involve cytochrome P450 enzymes. These enzymes are a super family of
proteins found in all living organisms. These enzymes catalyze the following
reactions: aromatic hydroxylation; aliphatic hydroxylation; N-, O-, and Sdealkylation; N-hydroxylation; N-oxidation; sulfoxidation; deamination; and
dehalogenation.
These enzymes are also involved in a number of reductive reactions, generally
under oxygen-deficiency condition. Hydrolysis is also observed for a wide
variety of drugs. The enzymes involved in hydrolysis are esterases, amidases,
and proteases. These reactions generate hydroxyl or amine groups, which are
suitable for phase II conjugation.
Phase II conjugation introduces hydrophilic functionalities such as glucuronic
acid, sulfate, glycine, or acetyl group onto the drug or drug metabolite
molecules. These reactions are catalyzed by a group of enzymes called
transferases. Most trasferases are located in cytosol, except the one facilitates
glucuronidation, which is a microsomal enzyme. This enzyme, called uridine
diphosphate glucuronosyltransferase (UGTs), catalyzes the most important
phase II reaction: glucuronidation. Glucuronic acid contains a number of
hydroxyl groups and one carboxylic acid functionality. This molecule is
extremely hydrophilic, and improves the hydrophilicity of a drug molecule
when they are covalently bound.
The following is a partial list of common metabolism motifs :
1.
2.
3.
4.
5.
6.
Aliphatic/Aromatic carbons: hydroxylation.
Methoxyl/methylamine group: demethylation.
Amine: N-oxidation, or deamination.
Sulfur: S-oxidation.
Phenol/alcohol: glucuronidation/sulphation.
Esters/amides: hydrolysis.
V. The top changes that occur and their mass shifts
Below is a brief list of mass shifts caused by metabolism of common
functional groups.
1.
2.
3.
4.
5.
Glucuronidation: plus 176 u.
Sulfation: plus 80 u.
Oxidation (N-, S-): plus 16 u.
Hydroxylation (aliphatic, aromatic): plus 16 u (or 32, if two sites).
Dealkylation: minus the alkyl group: minus 14 u for a methyl group, and
28 u for an ethyl group.
6. Hydrolysis: minus R-1 for ester hydrolysis into the acid.
VI. Conclusion
In reality, drug metabolism is an extremely complicated process, and the
picture can be very messy. Often, a drug is metabolized into many products,
some major, others minor. A complete picture of the metabolism of a drug is,
in many cases, not possible, and not usually necessary.
Abstract
Drug-metabolizing enzymes are called mixed-function oxidase or monooxygenase and
containing many enzymes including cytochrome P450, cytochrome b5, and NADPHcytochrome P450 reductase and other components. The hepatic cytochrome P450s
(Cyp) are a multigene family of enzymes that play a critical role in the metabolism of
many drugs and xenobiotics with each cytochrome isozyme responding differently to
exogenous chemicals in terms of its induction and inhibition. For example, Cyp 1A1 is
particularly active towards polycyclic aromatic hydrocarbons (PAHs), activating them
into reactive intermediates those covalently bind to DNA, a key event in the initiation of
carcinogenesis. Likewise, Cyp 1A2 activates a variety of bladder carcinogens, such as
aromatic amines and amides. Also, some forms of cytochrome P450 isozymes such as
Cyp 3A and 2E1 activate the naturally occurring carcinogens (e.g. aflatoxin B1) and Nnitrosamines respectively into highly mutagenic and carcinogenic agents. The
carcinogenic potency of PAHs, and other carcinogens and the extent of binding of their
ultimate metabolites to DNA and proteins are correlated with the induction of
cytochrome P450 isozymes. Phase II drug-metabolizing enzymes such as glutathione
S-transferase, aryl sulfatase and UDP-glucuronyl transferase inactivate chemical
carcinogens into less toxic or inactive metabolites. Many drugs change the rate of
activation or detoxification of carcinogens by changing the activities of phases I and II
drug-metabolizing enzymes. The balance of detoxification and activation reactions
depends on the chemical structure of the agents, and is subjected to many variables
that are a function of this structure, or genetic background, sex, endocrine status, age,
diet, and the presence of other chemicals. It is important to realize that the enzymes
involved in carcinogen metabolism are also involved in the metabolism of a variety of
substrates, and thus the introduction of specific xenobiotics may change the operating
level and the existence of other chemicals. The mechanisms of modification of drugmetabolizing enzyme activities and their role in the activation and detoxification of
xenobiotics and carcinogens have been discussed in the text.
Drug metabolism
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 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 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.
Phase I vs. Phase II
Phase I reactions usually precede 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 monooxygenase (often abbreviated CYP), NADPH and oxygen.
The classes of pharmaceutical drugs that utilize this method for their metabolism
include phenothiazines, paracetamol, and steroids. If the metabolites of phase I
reactions are sufficiently polar, they may be readily excreted at this point.
However, many phase I products are not eliminated rapidly and undergo a
subsequent reaction in which an endogenous substrate combines with the newly
incorporated functional group to form a highly polar conjugate.
A common Phase I oxidation involves conversion of a C-H bond to a C-OH. This
reaction sometimes converts a pharmacologically inactive compound (a prodrug)
to a pharmacologically active one. By the same token, Phase I can turn a nontoxic
molecule into a poisonous one (toxification). A famous example is acetonitrile,
CH3CN. Simple hydrolysis in the stomach transforms acetonitrile into acetate and
ammonia, 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.[1] This example of a biomimetic reaction tends to give a
mixture of products that often contains the Phase I metabolites.
Phase II reactions — usually known as conjugation reactions (e.g., with
glucuronic acid, sulfonates (commonly known as sulfation) , glutathione or 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.
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.
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:
Phase I
Oxidation





Cytochrome P450 monooxygenase system
Flavin-containing monooxygenase system
Alcohol dehydrogenase and aldehyde dehydrogenase
Monoamine oxidase
Co-oxidation by peroxidases
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).
Hydrolysis


Esterases and amidases
Epoxide hydrolase
Phase II
Methylation

methyltransferase
Sulphation


Glutathione S-transferases
Sulfotransferases
Acetylation


N-acetyltransferases
Amino acid N-acyl transferases
Glucuronidation

UDP-glucuronosyltransferases
o Mercapturic acid biosynthesis
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 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.[2] This can be used to identify individuals most at risk from adverse
reaction.