Chapter 13. Drug Metabolism Introduction: the process of drugs in the body includes absorption, distribution, metabolism and elimination. Drug metabolism is also named “drug biotransformation”

Important Terms • Biotransformation: Processes of drugs or toxins in the body, which may change the physical, chemical or biological properties of the drugs or toxins. • Bioavailability: F, the fraction of the dose that reaches the systemic circulation. F=1 for IV administration. • Distribution: Movement of drug from the central compartment (tissues) to peripheral compartments (tissues) where the drug is present.

• Elimination: The processes that encompass the effective "removal" of drug from "the body" through excretion or metabolism. • Half-Life: the length of time necessary to eliminate 50% of the remaining amount of drug present in the body.

Routes of Administration • Oral • Injection: Intravenous, Subcutaneous, Intramuscular, Intraperitoneal • Transdermal (patch) • Mucous membranes of mouth or nose (includes nasal sprays) • Inhalation • Rectal or vaginal

1. Biotransformation and the enzymes The major site for drug biotransformation is the liver. The extrahepatic sites include: the lung, kidney, intestine, brain, skin, etc. The major organelles for drug biotransformation is microsome, and others include cytosol and mitochondria. The major enzymes for drug biotransformation are microsomal enzymes.

Drug Metabolism Extrahepatic microsomal enzymes (oxidation, conjugation) Hepatic microsomal enzymes (oxidation, conjugation) Hepatic non-microsomal enzymes (acetylation, sulfation,GSH, alcohol/aldehyde dehydrogenase, hydrolysis, ox/red)

Reactions in biotransformation Include Phase 1 & Phase 2 Reactions. Phase 1: involves metabolic oxygenation, reduction, or hydrolysis; result in changes in biological activity (increased or decreased) Phase 2: conjugation—bound by polar molecules or modified by functional groups, in almost all cases results in detoxication.

1) The first phase reactions Metabolic oxygenation Microsomal enzymes catalyze hydroxylation, dealkylation, deamination, S-oxidation, N-oxidation and hydroxylation, dehalogenation, etc.

a) Hydroxylation Hydroxylations include aliphatic and aromatic hydroxylation

b) Dealkylation Dealkylations include N-, O- and S-dealkylation. R-X-CH2-R’ [R-X-CH(OH)-R’] R-XH + O=CH-R’ [O] X = O, N, S

N-dealkylation Dealkylation of secondary or tertiary amines will produce primary amines and aldehydes.

O-dealkylation Dealkylation of ethers or esters will produce phenols and aldehydes. Codeine Morphine

S-dealkylation S-dealkylation usually produces sulfhydryl group and aldehyde. R-S-CH3 [R-S-CH2OH] R-SH + HCHO [O] 6-methylthiopurine 6-thiopurine

c) Deamination Deamination may produce ketone and ammonia. For example, deamination of amphetamine:

d) S-oxidation For example, S-oxidation of chlorpromazine:

e) N-oxidation For example, N-oxidation of chlorpheniramine

B. Microsomal oxidases and their action mechanisms The enzymes that catalyze the above oxygenation of drugs are called “mixed- function oxidase” or “monooxygenase”. In the reactions, one oxygen is reduced into water and the other is integrated into the substrate molecule. RH + O2 + NADPH + H+ ROH + NADP+ + H2O

Mixed-function oxidase contains cytochrome P450 (CYP) and NADPH as electron carrier and hydrogen provider. The CYP family: Human CYPs – have several types and subtypes, named CYP1, 2, 3…; CYP1a, 1b, and so on. They are important in drug metabolism.

Human Liver CYPs S. Rendic & F.J. DiCarlo, Drug Metab Rev 29:413-80, 1997

C. Other oxidases Monoamine oxidase These enzymes exist in mitochondria. They catalyze oxidation of amines into aldehyde and ammonia. For example, degradation of 5-hydroxytryptamine. RCH2-NH2 RCH=NH RCHO + NH3 [O] H2O

Alcohol and aldehyde oxidases R-CHOH R-CHO R-COOH Alcohol dehydrogenase Aldehyde dehydrogenase

D. Reductions Aldehyde and ketone reductases: these enzymes catalyze reduction of ketones or aldehydes to alcohols. For example: CCl3CHO CCl3CH2OH The coenzyme may be NADH or NADPH. 2H Trichloroacetaldehyde Trichloroethanol

Reductases for Azo or nitro compounds These reductases mainly exist in hepatic mitochondria with NADH or NADPH as coenzyme. Azo Aniline Nitrobenzene

E. Hydrolysis Esters and amides may be hydrolyzed to produce acids and alcohol or amine. Para-aminobenzoic acid Ester(Procain) Amide(Procainamide)

2) The second phase reactions The second phase reactions of drugs are also named “Conjugation Reactions” . These reactions include glucuronidation, sulfation, acetylation, methylation and amino acid binding.

Glucuronidation

Sulfation PAPS is the phosphate donor. (PAPS, 3’-phosphoadenosine- 5’-phosphosulfate)

Acetylation Acetylation may reduce the water solubility of the compounds.

Methylation Methylation of phenols, amines and biologically active molecules may change their activity or toxicity. Generally, methylation reduces the hydrophilicity of the compound. S-adenosylmethionine (SAM) is the donor of methyl group. Methylation includes N- or O-methylation.

Methylation RH R-CH3 SAM

2. Factors that affect drug metabolism Inducers Inducers are those that promote drug metabolism in the body. Most inducers are lipophilic compounds and have no specificity in actions. Examples: barbital, ether, amidopyrine, miltown (meprobamate), glucocorticoids, vit. C, etc. Repeated administration of these drugs may result in drug-resistance.

The mechanism by which inducers enhance drug metabolism in the body is believed to be the induction of the enzymes involved in the drug metabolism. For example, phenobarbital stimulates proliferation of SER and increases production of some enzymes in the metabolisn of drugs, such as liver CYPs and UDP-glucuronate transferase, both of which enhance metabolism of many drugs in the liver (oxygenation and conjugation).

Inhibitors Inhibitors are those that inhibit drug metabolism in the body. Include competitive and non-competitive inhibitors. a) A drug inhibits the metabolism of other drugs: such as chloramphenicol and isoniazid. They inhibit hepatic microsomal enzymes. Combined administration of these drugs and others such as barbitals may increase the toxicity of the latter.

b) Non-drug compounds inhibit the metabolism of drugs: such as pyrogallol (没食子酚). This compound inhibits o-methylation of epinephrine and thus enhances the activity of the hormone in body (it competes with epinephrine for methyltransferase).

Other factors Species difference. Sex, age, nutrition conditions have effects on drug metabolism. Hepatic functions.

3. Significance of drug biotransformation Effective removal of drug from the body through excretion or metabolism. For example, sulfation and glucuronidation increase secretion of the drug in urine. Change of the biological activity or toxicity of drugs in the body. For example, trichloroacetaldehyde is first reduced into trichloroethanol and then conjugated by glucuronate to become a non-toxic compound.

Inactivation of bioactive molecules in the body Inactivation of bioactive molecules in the body. For example, some hormones are inactivated through biotransformation in the liver (epinephrine, steroid hormones). Exploration of new drugs. Based on the mechanisms of biotransformation, it is possible to design new drugs with longer half-lives and fewer side-effects. Explanation for the carcinogenic property of some drugs. For example, after biotransformation some “non-toxic” drugs may become toxic or carcinogenic.

N-acetylation may form nitrenium ion which is a potent carcinogenic agent

The mechanisms of biotransformation may be used to improve the efficacy of drugs. For example, those that are mainly metabolized in the liver may have less efficacy through oral administration than IV route.