Drug Metabolism: Phase 1 and Phase 2 Reactions GPAT Notes

Introduction to Drug Metabolism in the Human Body

Drug metabolism, also known as biotransformation, is the biochemical modification of pharmaceutical substances by living organisms. The human body treats most therapeutic drugs as xenobiotics. Because many active drug molecules are lipophilic to cross lipid rich cell membranes, they cannot be filtered and excreted efficiently by the kidneys. The renal tubules would simply reabsorb these nonpolar compounds back into systemic circulation.

To solve this challenge, specialized enzymatic machinery primarily located in the liver converts lipophilic molecules into polar, water soluble compounds. Understanding the precise pathways of drug biotransformation is indispensable for B.Pharm exams, NIPER entrance tests, and targeted GPAT preparation. This comprehensive guide breaks down Phase 1 and Phase 2 metabolic pathways, enzyme kinetics, and clinical drug interactions.

Primary Sites of Drug Metabolism

While the liver serves as the central clearance organ, biotransformation occurs across several tissue beds:

• Hepatic tissue: The smooth endoplasmic reticulum of hepatocytes holds the highest density of metabolic enzymes, notably the Cytochrome P450 superfamily.
• Gastrointestinal tract: Epithelial enterocytes express high levels of CYP3A4 and carboxylesterases, providing significant pre systemic clearance.
• Lungs: Pulmonary endothelial cells clear circulating amines and volatile anesthetics.
• Kidneys: Proximal tubular cells possess both oxidative and conjugative enzymes that process filtered drugs.
• Blood plasma: Soluble esterases and pseudocholinesterases metabolize drugs such as suxamethonium, procaine, and remifentanil in seconds.

Phase 1 Metabolism: Functionalization Reactions

Phase 1 reactions convert the parent drug into a more polar metabolite by introducing or unmasking a functional group such as hydroxyl, amino, carboxyl, or thiol groups. These metabolites may be pharmacologically active, inactive, or occasionally more toxic than the parent molecule.

Oxidation Reactions

Oxidation is the most prevalent Phase 1 pathway, largely catalyzed by mixed function oxidases on the smooth endoplasmic reticulum.

• Cytochrome P450 Monooxygenase System: This electron transport chain requires molecular oxygen, NADPH, and NADPH Cytochrome P450 reductase. The iron atom in the heme ring alternates between ferric and ferrous states to transfer one oxygen atom to the substrate while reducing the other to water.
• Flavin containing Monooxygenases (FMO): Located in the endoplasmic reticulum, FMO3 metabolizes nitrogen, sulfur, and phosphorus compounds without relying on CYP machinery.
• Alcohol and Aldehyde Dehydrogenases: Cytoplasmic enzymes that oxidize ethanol to acetaldehyde and subsequently to acetic acid.
• Monoamine Oxidase (MAO): Mitochondrial outer membrane enzymes that degrade endogenous catecholamines and exogenous tyramine.

Major Cytochrome P450 Isoenzymes

For your Pharmacology cheat sheet, remember the relative contribution and typical substrates of the major human CYP isoforms:

• CYP3A4 and CYP3A5: Responsible for metabolizing roughly fifty percent of all marketed drugs, including atorvastatin, cyclosporine, clarithromycin, and nifedipine.
• CYP2D6: Metabolizes approximately twenty percent of drugs, including codeine, metoprolol, haloperidol, and tricyclic antidepressants. This enzyme shows significant genetic polymorphism.
• CYP2C9: Processes narrow therapeutic index agents including warfarin, phenytoin, and tolbutamide.
• CYP2C19: Responsible for activating the prodrug clopidogrel and metabolizing omeprazole.
• CYP1A2: Metabolizes caffeine, theophylline, and paracetamol.
• CYP2E1: Induced by chronic ethanol consumption and converts paracetamol to its toxic intermediate, NAPQI.

Reduction and Hydrolysis

• Reduction: Carried out by nitro and azo reductases under low oxygen tension, such as the reduction of chloramphenicol and prontosil.
• Hydrolysis: Cleaves ester, amide, and epoxide bonds. Plasma pseudocholinesterase hydrolyzes succinylcholine, while tissue amidases hydrolyze lidocaine.

Phase 2 Metabolism: Conjugation Reactions

Phase 2 reactions involve the covalent attachment of an endogenous hydrophilic substrate to the functional group created during Phase 1. These reactions produce large, highly polar, biologically inactive molecules that are readily cleared in bile or urine.

Glucuronidation

• Enzyme: UDP glucuronosyltransferase (UGT), located within the luminal surface of the endoplasmic reticulum.
• Activated Donor: Uridine diphosphate glucuronic acid (UDPGA).
• Substrates: Morphine, bilirubin, paracetamol, and chloramphenicol.
• Clinical High Yield: Deficiencies in neonates cause Grey Baby Syndrome due to chloramphenicol accumulation, as newborn livers lack mature UGT enzymes.

Sulfation

• Enzyme: Sulfotransferases (SULT), located in the cytoplasm.
• Activated Donor: 3 prime phosphoadenosine 5 prime phosphosulfate (PAPS).
• Substrates: Paracetamol, minoxidil, steroids, and methyldopa.
• Kinetics: Sulfation has high affinity but low capacity, becoming saturated quickly at high substrate concentrations.

Glutathione Conjugation

• Enzyme: Glutathione S transferase (GST).
• Donor: Reduced glutathione (GSH), a tripeptide composed of glutamate, cysteine, and glycine.
• Substrates: Reactive electrophilic intermediates such as NAPQI generated from paracetamol overdose. Conjugates are excreted as mercapturic acid derivatives.

Acetylation

• Enzyme: N acetyltransferase (NAT1 and NAT2).
• Donor: Acetyl Coenzyme A.
• Substrates: Isoniazid, hydralazine, procainamide, and dapsone.
• Genetics: Slow acetylators have defective NAT2 enzymes, predisposing them to isoniazid induced peripheral neuropathy and drug induced lupus erythematosus.

Methylation

• Enzymes: Thiopurine S methyltransferase (TPMT) and Catechol O methyltransferase (COMT).
• Donor: S adenosylmethionine (SAMe).
• Substrates: 6 mercaptopurine, dopamine, and adrenaline.

CYP Enzyme Modulators: High Yield Exam Concepts

Competitive pharmacy tests frequently probe enzyme induction and inhibition.

Potent Enzyme Inducers

Enzyme inducers accelerate protein synthesis or reduce enzyme degradation, accelerating the clearance of co administered medications and leading to therapeutic failure:

• Rifampicin
• Phenytoin
• Carbamazepine
• Phenobarbital
• St Johns Wort
• Chronic ethanol ingestion
• Tobacco smoke (via CYP1A2)

Potent Enzyme Inhibitors

Enzyme inhibitors compete directly for binding sites or form stable inactive complexes, rapidly increasing plasma concentrations of co administered drugs to toxic levels:

• Ketoconazole and itraconazole
• Clarithromycin and erythromycin
• Cimetidine
• Ritonavir
• Ciprofloxacin
• Grapefruit juice (furanocoumarins targeting intestinal CYP3A4)
• Sodium valproate

Prodrugs and First Pass Metabolism

Not all parent drugs enter the body in their active form. Prodrugs require in vivo biotransformation to exert pharmacological activity:

• Levodopa: Converted to dopamine by DOPA decarboxylase in the central nervous system.
• Enalapril: Hydrolyzed by hepatic esterases into active enalaprilat.
• Codeine: Demethylated via CYP2D6 into active morphine.
• Clopidogrel: Oxidized through CYP2C19 to form its active thiol metabolite.

First pass metabolism denotes the rapid degradation of orally administered drugs during their initial transit through intestinal walls and the hepatic portal system before reaching systemic circulation. Drugs with high first pass clearance like lidocaine, propranolol, and nitroglycerin require non oral administration or substantially higher oral doses.

Summary

Drug biotransformation preserves physiological balance by turning nonpolar active molecules into polar inert products for renal and biliary elimination. Phase 1 functionalization introduces polar anchors via oxidation, reduction, and hydrolysis, predominantly driven by the hepatic Cytochrome P450 system. Phase 2 synthetic reactions conjugate these intermediates with endogenous donors to ensure rapid excretion. Reviewing these enzymatic pathways and specific modulators provides a rock solid foundation for your ongoing Pharmacy study notes.