5. Drug Metabolism

Drug elimination Drug elimination is the irreversible loss of drug from the body. It occurs by two processes: metabolism and excretion. Metabolism involves enzymic conversion of one chemical entity to another within the body, whereas excretion consists of elimination from the body of chemically unchanged drug or its metabolites.

Introduction Renal excretion terminates the actions of relatively few drugs, such as:  Small, hydrophilic drugs  Those remain fully or nearly fully ionized at physiological pH  Those are not protein bound However, most pharmacologically active molecules tend to be:  Lipophilic  Remain unionized or only partially ionized at physiological pH  Often bound strongly to plasma proteins

These are not good candidates for renal elimination. Metabolism is the more important mechanism of termination for most drugs. Lipophilic, unionized or bound drugs would remain in the body for prolonged periods if their actions were not terminated by some alternate process. For example- Pentobarbital Highly lipophilic anesthetic agent would exert its pharmacological effect for 100 years if it weren’t for drug metabolism.

Drug metabolism Drug metabolism refers to the biochemical changes that drugs undergo in the body, leading to the formation of different metabolites with different effects. Rarely is only one metabolite produced from a single drug. In general, most drugs are metabolized into molecules that are more water soluble. Usually, drug metabolites are less active pharmacologically than the parent compound.

Possible types of metabolites a. Inactive metabolites: Atropine → Tropic acid + Tropine 6-mercaptopurine → 6-mercapturic acid (Active drug)

(Inactive)

b. Metabolites that retain similar activity: →

Propranolol (Active)

Codeine (Active)

Hydroxypropranolol (Active)

→

Morphine (More active)

c. Metabolites with altered activity: Retinoic acid (vitamin A) → Isoretinoic acid (Anti-acne agent)

Iproniazid

→

(Anti-acne agent)

Isoniazid (Antitubercular agent)

d. Bioactivated metabolite: Enalapril

→

(Prodrug)

L-Dopa (Prodrug)

Enalaprilat (Potent antihypertensive)

→

Dopamine (Antiparkinson drug)

Organ sites of drug metabolism The liver is the major site for drug metabolism, but specific drugs may undergo biotransformation in other tissues: GI mucosa (tyramine, salbutamol) Kidneys Skin Lung (various prostanoids) Nasal mucosa Plasma (suxamethonium)

Cellular sites of drug metabolism » Cytosol or cytoplasm or intracellular fluid » Mitochondria » Lysosomes

Kinetics of metabolism First order kinetics: Rate of drug metabolism is directly proportional to the concentration of free drug and first order kinetics are observed. This means that a constant fraction of remaining drug is metabolized per unit time.

At low doses drug metabolism is first order, that is proportional to drug dose. 80 70

Velocity

(ng/g tissue/min)

60

zero order metabolism

50 40 30 20 10 0 0

first order metabolism 5 10 15 20 25 30 35 40 45 50 55 60

[Drug] mM

Zero order kinetics: Rate of drug metabolism is not dependent on the concentration of free drug and rate of metabolism remain constant over time. The enzyme is saturated by a high freedrug concentration, and the rate of metabolism remains constant over time. This is called zero order kinetics. A constant amount of drug is metabolized per unit time. At high doses drug (aspirin) metabolism is zero order, that is constant and independent of drug dose.

Reactions of drug metabolism The kidney cannot efficiently eliminate lipophilic drugs that readily cross cell membranes and are reabsorbed in the distal tubules of kidney. Therefore, lipid-soluble agents must be metabolized for excretion. Drug metabolism reactions are divided into two main categories: Phase I Phase II

Fig: Nephron

Phase I reaction Phase I reactions function to convert lipophilic molecules into more polar molecules by introducing or unmasking a polar functional group, such as -OH, -NH2 etc. Phase I reactions are catabolic. Phase I metabolism may increase, decrease, or leave unaltered the drug's pharmacologic activity. Common metabolism hydrolysis.

reactions that occur during are oxidation, reduction,

Phase I reactions utilizing the P450 system Cytochrome P450 system Group of 12 closely related enzyme families in the liver. CYP1, CYP2 and CYP3 metabolize drugs. The Phase I reactions most frequently involved in drug metabolism are catalyzed by the cytochrome P450 system (also called microsomal mixed function oxidase). The P450 system is important for the metabolism of many endogenous compounds (steroids, lipids, etc.), and for the detoxication of exogenous substances.

Cytochrome P-450, designated as CYP, is composed of many families of heme-containing isozymes that are located in most cells, but mainly those in the liver and intestinal tract. Substrate specificity is very low and have powerful oxidizing property. Oxidation by Cytochrome P450 system

e-

The cycle involves four steps: Oxidized (Fe3+) cytochrome P450 combines with a drug substrate to form a binary complex. NADPH donates an electron to the cytochrome P450 reductase, which in turn reduces the oxidized cytochrome P450-drug complex.

A second electron is introduced from NADPH via the same cytochrome P450 reductase, which serves to reduce molecular oxygen and form an "activated oxygen"-cytochrome P450substrate complex. This complex in turn transfers "activated" oxygen to the drug substrate to form the oxidized product. The potent oxidizing properties of this activated oxygen permit oxidation of a large number of substrates.

Drug-drug interactions: Some drugs can induce the synthesis of many different CYP isozymes, thereby accelerating the clearance of substrates of those metabolic pathways and decreasing their levels. Common CYP inducers rifampin, carbamazepine Rifampin

→

are:

phenobarbital,

↓ HIV protease inhibitors in plasma

(Antitubercular drug)(saquinavir, ritonavir, indinavir, nelfinavir etc.) used to treat or prevent infection by viruses

Some drugs can inhibit the synthesis of many different CYP isozymes, clearance of substances can be lowered & their levels elevated by inhibition of their metabolism. Common CYP inhibitors ketoconazole, omeprazole

are:

erythromycin,

Omeprazole →

↑ warfarin concentration in plasma

(Antiulcerant)

(an anticoagulant)

Phase I reactions not involving the P-450 system: Amine oxidation: Oxidation of catecholamines or histamine Alcohol dehydrogenation: Ethanol oxidation Hydrolysis: Hydrolysis of Procain

Phase II Phase II consists of conjugation reactions. If the metabolite from Phase I metabolism is sufficiently polar, it can be excreted by the kidneys. However, many metabolites are too lipophilic to be retained in the kidney tubules. A subsequent conjugation reaction with an endogenous substrate, such as glucuronic acid, sulfuric acid, acetic acid or an amino acid results in polar, usually more water-soluble compounds that are most often therapeutically inactive. Glucuronidation is the most common and the most important conjugation reaction.

Neonates are deficient in this conjugating system making them particularly vulnerable to drugs such as chloramphenicol. Drugs already possessing an -OH, -HN2, or -COOH group may enter Phase II directly, and become conjugated without prior Phase I metabolism. The highly polar drug conjugates may then be excreted by the kidney.

Phases of drug metabolism

Example of the whole system Benzene– very lipid soluble compound First hydroxylated by phase I reactions to phenol Hydroxylation doubles (2X) the elimination of the compound Phenol is then conjugated with sulfate and glucuronic acid (phase II) Resulting compound has an excretion rate which is 10 to 20 fold greater than benzene

Endogenous substances for conjugation Glucorinic acid Sulfate

Glycin Acetate

The biotransformation of drugs Directly

Drug

Phase I

Oxidation, reduction and/or hydrolysis

Following Phase I, the drug may be activated, unchanged, inactivated.

Some drugs directly Enter Phase II metabolism

Phase II

Conjugation products

Conjugated drug is usually inactive.

Summary of drug metabolism: Phase I reactions often introduce a reactive group, such as hydroxyl, into the molecule. This process is known as 'functionalisation'. This group then serves as the point of attack for the conjugating system to attach a substituent such as glucuronide. Both phases decrease lipid solubility, thus increasing renal elimination.