LR

19 Comprehensive Notes on Drug Metabolism

Drug Metabolism

Learning Outcomes

  • Explain drug metabolism and its location.
  • Define Phase I and Phase II metabolism.
  • Name and describe Phase I reactions for lidocaine, codeine, ethanol, chloramphenicol, procaine, and aspirin.
  • Recall main Phase II conjugation pathways.
  • Recall aspirin's structure, its metabolism, and general structures of its metabolites (not full conjugates).
  • Describe paracetamol metabolism, its toxicity at high doses, and N-acetylcysteine as an antidote.
  • Define prodrug.
  • Explain the use of prodrugs and understand what happens to cefamandole nafate and sulfasalazine.

Introduction to Drug Metabolism

  • Drug metabolism is part of pharmacokinetics.
  • Pharmacokinetics: movement of drugs into, around, and out of the body, involving:
    • Absorption
    • Distribution
    • Metabolism
    • Excretion
    • Elimination

What is Drug Metabolism?

  • Metabolism: process where drugs undergo transformations, catalyzed by enzymes.
  • Products of these transformations are called metabolites.
  • Generally, lipophilic drugs are converted into hydrophilic metabolites, which are excretable.
  • Water solubility increases renal excretion and decreases tubular re-absorption.

Consequences of Drug Metabolism

  1. Drug converted to excretable form.
  2. Drug action terminated.
  3. Drug converted to metabolite with pharmacological activity.
  4. Inactive pro-drug converted to active metabolite.

Location of Drug Metabolism

  • Liver is the principal organ.
  • Liver cells contain efficient enzymes for metabolizing foreign materials.
  • Other active tissues: gastrointestinal tract, lungs, skin, and kidneys.

Drugs and the Liver

  • Liver cells contain efficient enzymes for metabolism of foreign materials.
  • Reactions in the liver produce more water-soluble materials for easier excretion.
  • Drugs should be metabolized and eliminated, but rapid metabolism can reduce benefit.
  • First-pass effect: orally administered drugs rendered inactive upon first pass through the liver.

Metabolic Processes: Overview

  • Phase 1: Drug with functional group introduced
  • Phase 2: Conjugation with polar groups (water soluble)
  • Hydrophilic metabolites are then excretable.

Phase I and Phase II Metabolism

  • Phase I: Transformation of drug into a more polar metabolite by introducing a functional group (e.g., oxidation, reduction, or hydrolysis).
    • Oxidation reactions often catalyzed by cytochrome P450 family (mostly in the endoplasmic reticulum of hepatocytes).
  • Phase II: Combination of glucuronic acid, sulfate, acetic acid, or amino acid with a functional group (may or may not result from Phase I) to form a polar conjugate that can be readily excreted.
    • Enzymes catalyzing these reactions are mostly in the cytosol of the hepatocytes.

Phase I Chemical Reactions

  • Many oxidation reactions are catalyzed by the mixed function oxidase (MFO) system (Cytochrome P–450, NADPH, molecular O_2).
  • Phase I reactions include oxidation, dealkylation, reduction, and hydrolysis.
  • Functional groups such as -OH, -NH_2, -COOH are introduced into the drug molecule.
  • Main function of phase I metabolism is to prepare drugs for phase II metabolism.
  • NADPH: nicotinamide adenine dinucleotide phosphate.

Phase I Oxidation Reactions – Oxidation / Hydroxylation

  • Hydroxylation involves addition of an -OH group and is a type of oxidation.
  • P-450, NADPH, O_2
  • Lidocaine metabolism involves hydroxylation as a first step.

Metabolism of Ethanol

  • Most ethanol is metabolized by the liver.
  • Stage 1: Oxidation of ethanol to ethanal (acetaldehyde) by alcohol dehydrogenase (ADH).
    • Dehydrogenation implies the removal of two hydrogens from the molecule.
    • NADH: (nicotinamide adenine dinucleotide).
  • Stage 2: Oxidation of acetaldehyde to acetate occurs enzymatically by aldehyde dehydrogenase (ALDH) in the mitochondria.
  • Excess ethanal causes hangover.

Disulphiram—Treatment of Alcohol Abuse

  • Disulphiram (Antabuse) helps maintain abstinence in alcoholism management.
  • It is highly lipid soluble (accumulates in adipose tissue) and has 80% bioavailability after an oral dose.
  • Alcohol is mainly metabolized in the liver to acetaldehyde by alcohol dehydrogenase (ADH), then oxidized to acetate by aldehyde dehydrogenase (ALDH).
  • Disulphiram irreversibly inhibits the oxidation of acetaldehyde by competing with the cofactor nicotinamide adenine dinucleotide (NAD) for binding sites on ALDH.
  • It causes a 5- to 10-fold increase in acetaldehyde concentration, producing unpleasant side effects.

Phase I Reactions – Oxidation / Dealkylation

  • 10% of codeine is converted to morphine.
  • The analgesic effect of codeine is due to its conversion to morphine by oxidative demethylation.
  • This reaction is an oxidative dealkylation because the alkyl group of codeine is oxidized to an aldehyde.
  • Codeine (Pro-drug) → Morphine (Active) + Methanal.
  • Dealkylation implies the removal of an alkyl group (usually methyl).

Phase I Reactions - Reduction

  • Chloramphenicol is a broad-spectrum antibiotic (more commonly used in developing countries).
  • Reduction implies the addition of two hydrogens to the molecule.
  • Nitro is the chemistry name given to the –NO_2 functional group.
  • Inhibits protein synthesis by preventing peptide bond formation.

Phase I Reactions - Hydrolysis

  • Procaine (local anesthetic).
  • Hydrolysis implies the addition of water (H_2O) to the molecule, resulting in the breaking of the molecule into two parts.
  • Common for drugs with ester functional groups: Ester + H_2O → Acid + Alcohol.
  • Aspirin at pH ~ 7 → salicylate + acetate.

Phase I: Summary

  • Most drugs can undergo modifications by drug-metabolizing enzyme systems.
  • Drugs can be subject to several Phase I pathways.
  • These reactions create functional groups for Phase II conjugative mechanisms.
  • The main function of phase I reactions is to prepare chemicals for phase II metabolism and subsequent excretion.
  • Phase II is the true “detoxification” step in the metabolism process.

Phase II Metabolism

  • The main conjugation reactions are:
    • Glucuronic acid conjugation (on -OH, -COOH, -NH_2, -SH groups)
    • Glycine conjugation (on -COOH groups)
    • Sulfate conjugation (on -NH_2, -OH groups)
    • Glutathione conjugation (organic halides or electrophilic compounds)
  • These conjugation reactions are catalyzed by transferase enzymes, and various coenzymes are also needed.
  • In phase II metabolism, a hydrophilic group is conjugated with a group already in the molecule (may or may not result from phase I metabolism) giving a water-soluble product which is excreted in bile or urine.

Phase II: Conjugation with Glucuronic Acid

  • Glucuronic acid is transferred to the drug from uridine diphosphate glucuronic acid (UDP-GA) in the presence of glucuronyl transferase.
  • Glucuronic acid, C6H{10}O_7, abbreviated as RO-GA
  • Common for drugs with -OH, -COOH, -NH_2 groups
  • Conjugation with glucuronic acid (glucuronidation) is the most common form of conjugation and gives a glucuronide (quantitatively most important Phase II pathway).

Phase II: Conjugation with Sulfate (Sulfation)

  • Sulfation is common for phenols (aromatic alcohol).
  • Sulfate is transferred to the drug from the reactive intermediate 3-phosphoadenosine-5’-phosphosulfate, (PAPS), in the presence of sulfotransferase (Enzyme).
  • Glucuronide formation and sulfation often compete for the same substrate, e.g., paracetamol.

Conjugation with Glycine

  • Glycine (amino acid).

Metabolism of Aspirin

  1. At low doses of aspirin, this is the main metabolic pathway.
  2. At higher doses, 1 becomes saturated, and glucuronide conjugation occurs.
  3. At top doses of aspirin, direct urinary excretion occurs after saturation of conjugation pathways.
    • Hydrolysis (Phase I) → Salicylate + acetate
    • Glycine Conjugation (Phase II)
    • Glucuronic acid (GA) conjugation (Phase II)

Paracetamol and its Metabolism

  • Paracetamol has analgesic and antipyretic effects but weak anti-inflammatory activity.
  • Several possible mechanisms of action.
  • Paracetamol (Acetaminophen) already contains a phenol (aromatic alcohol) functional group, so Phase II conjugations can occur directly.

Paracetamol – Two Main Phase II Metabolic Pathways

  • Both of these are Phase II reactions.
  • 45-50% UDP-glucuronyl transferase (UDP-GA) → Excretion
  • 45-50% Sulfotransferase (PAPS) → Excretion

Paracetamol – Another Minor (4-5%) Phase I Metabolic Pathway

  • N-Acetyl-p-benzoquinone imine, NAPQI
  • NAPQI is toxic to the liver, but at low doses (1-2 g), this is not a problem as it conjugates with glutathione (GSH) and is excreted.
  • Phase I: P450 system (oxidation)
  • Phase II: Glutathione transferase
  • Glutathione (GSH): The tri-peptide glutathione is found in virtually all mammalian tissues. It contains a reactive thiol group (SH), and one of its functions is to react with harmful electrophiles that are produced by metabolism.

Paracetamol – Fatal at High Doses

  • At high doses of paracetamol (10-15g), the glutathione pathway becomes saturated.
  • NAPQI amounts increase and attack the liver, which can be fatal.
  • Low dose < 2 g: GSH conjugates and is excreted.
  • High dose 10 - 15 g: Accumulates in the liver, and liver reserve is depleted.

Paracetamol Toxicity Antidote: N-acetylcysteine

  • N-acetylcysteine, CH3CONHCH(CH2SH)COOH is an antidote to paracetamol poisoning.
  • It acts by stimulating the production of Glutathione (GSH).
  • Note: Cys is a constituent of glutathione (γ-Glu-Cys-Gly).

Pro-drugs

  • A pro-drug is a pharmacologically inert precursor to an active drug.
  • Prodrug → Active drug (Metabolic enzymatic conversion)
  • IUPAC definition: A prodrug is any compound that undergoes biotransformation before exhibiting its pharmacological effects.
  • Prodrugs can thus be viewed as drugs containing specialized protective groups used in a transient manner to alter or to eliminate undesirable properties in the parent molecule.

An Ester Prodrug – Cefamandole Nafate

  • Prodrug is called Cefamandole Nafate (Kefadol)
  • The ester formed between cefamandole and methanoic acid (HCOOH) is highly pure, and the ester is cleaved rapidly in the blood by esterase enzymes to give the active drug.

Pro-drug for Site-Specific Drug Delivery

  • Sulfasalazine for Ulcerative Colitis (in colon, azoreductases).
  • Sulfasalazine → 5-Aminosalicylic acid (5-ASA) + Sulfapyridine
  • 5-Aminosalicylic acid (5-ASA) is the active therapeutic moiety.
  • Sulfapyridine is the carrier for 5-ASA but is responsible for the side effects of sulfasalazine.
  • Sulfasalazine was for 50 yrs. the drug of choice for ulcerative colitis (still used but now there are others).
  • The active therapeutic moiety is 5-aminosalicylic acid (5-ASA), but this, if administered directly, cannot reach the colon due to absorption at prior sites.
  • However, sulfasalazine, which can reach the colon, contains 5-ASA linked to sulfapyridine by an azo linkage, which is broken down by azoreductases in the colon.

Appendix 1(a) – a Note on Esters

  • Some of the drugs mentioned in this lecture are esters.
  • The general formula of an ester is RC(=O)OR’ where R = -H, -CH3, -CH2CH3, -C6H5 etc. R’ = -CH3, -CH2CH3, -C6H5 etc.
  • Aspirin conforms to the above formula with R and R’ shown below and is therefore an ester.
  • Esters are formed from the acid R(C=O)OH and the alcohol R’OH.

Appendix 1(b) – a note on Ester Hydrolysis

  • Esters undergo hydrolysis to give acids and alcohols.
  • Aspirin undergoes hydrolysis to give ethanoic acid and salicylic acid (the alcohol in this case).

Sample MCQs

  • Alcohol abusers are prescribed disulphiram for the management of the addiction. This works by inhibiting which enzyme?
    • B. Aldehyde dehydrogenase.
  • A 43-year-old male takes low dose Nu-Seals® aspirin (75 mg daily) for secondary prophylaxis following a myocardial infarction. Aspirin undergoes phase I metabolism by:
    • B Hydrolysis.
  • At normal doses of paracetamol, the metabolite NAPQI is removed by which of the following metabolic reactions?
    • D. Conjugation with glutathione.
  • The most common phase II reaction in the metabolism of xenobiotics is conjugation with:
    • A Glucuronic acid.