Comprehensive Pharmacology Notes on Enzyme

This presentation provides a comprehensive overview of enzyme induction, enzyme inhibition, and kinetics of drug elimination as described in KD Tripathi’s Essentials of Medical Pharmacology. Key concepts of drug metabolism, enzyme modulation, pharmacokinetic principles, and clinical implications are explained in a clear, point-wise manner for better understanding. Suitable for M.Pharm, B.Pharm, MBBS, Nursing, and other life science students preparing for exams, seminars, or competitive tests. Includes detailed notes, examples of enzyme inducers and inhibitors, and differences between first-order and zero-order elimination kinetics.

1. Pharmacokinetics RAJKISHOR SAHU M PHARM (PHARMACOLOGY) CHITKARA UNIVERSITY, PUNJAB

2. Introduction Pharmacokinetics is the quantitative study of the time course of drug movement within the biological system , encompassing the sequential processes of absorption, distribution, metabolism, and excretion (ADME) . While pharmacodynamics focuses on the biochemical and physiological effects of a drug and its mechanism of action, pharmacokinetics explains how the body handles the drug . Together, they provide the complete picture of drug action. The ultimate objective of pharmacokinetics is to establish the relationship between dose, plasma concentration, and therapeutic response , often referred to as the dose–concentration–effect correlation . This understanding is crucial for designing optimal dosage regimens, predicting onset and duration of action, and preventing toxicity. Pharmacokinetics is governed by several fundamental principles: Membrane Transport: Drugs must cross biological membranes by passive diffusion, carrier-mediated transport, or vesicular mechanisms to reach systemic circulation and target sites. Plasma Protein Binding: Only the free (unbound) fraction of drug is pharmacologically active, able to diffuse into tissues, undergo metabolism, and be excreted. Compartmental Distribution: The body is conceptualized as compartments (central and peripheral) through which drugs distribute based on solubility, tissue affinity, and perfusion. Biotransformation (Metabolism): Drugs are chemically modified mainly in the liver to inactive or more polar metabolites; occasionally, active or toxic metabolites may be formed.

3. Membrane Transport of Drugs Most drugs are weak electrolytes and must cross lipid membranes to reach systemic circulation or intracellular sites. Drug transport is broadly classified as: (a) Passive Diffusion Most common process. Driven by concentration gradient (Fick’s law). Lipid-soluble, non-ionized drugs diffuse readily. Rate depends on lipid solubility, surface area, and membrane thickness. Example: absorption of ethanol, barbiturates. (b) Facilitated Diffusion Carrier-mediated, selective, saturable. Does not require energy; occurs down concentration gradient. Example: glucose transport via GLUT. Few drugs (like levodopa, gabapentin) use facilitated diffusion. (c) Active Transport Carrier-mediated, saturable, requires energy (ATP). Occurs against concentration gradient. Important for absorption of nutrients and some drugs. Examples: uptake of methyldopa, 5-fluorouracil, penicillin (via organic anion transporters). (d) Endocytosis and Exocytosis For macromolecules and particulate drugs. Pinocytosis (uptake of fluid) and phagocytosis (solid). Example: vitamin B12 absorption (via intrinsic factor).

4. Absorption of Drugs Definition Absorption is the process by which a drug moves from its site of administration into systemic circulation. Factors Affecting Absorption Physicochemical Properties of Drug Lipid solubility, molecular size, ionization ( pKa , pH of medium). Unionized, lipid-soluble drugs are absorbed faster. Formulation Factors Particle size, disintegration/dissolution rate, excipients, sustained release designs. Physiological Factors Gastric emptying, intestinal motility, food, pH, first-pass metabolism. Routes of Drug Absorption Oral (enteral): Most common; influenced by gastric acid, enzymes, intestinal transit. Parenteral (IV, IM, SC): Avoids first-pass effect; bioavailability is high. Inhalation, Sublingual, Rectal, Transdermal: Each has advantages in bypassing metabolism or providing rapid absorption. Bioavailability Fraction of unchanged drug reaching systemic circulation after administration. Oral drugs may have reduced bioavailability due to incomplete absorption or first-pass metabolism

5. Distribution of Drugs Definition Distribution is the reversible transfer of a drug between systemic circulation and body tissues/fluids. Determinants of Distribution Physicochemical Properties – lipid solubility, ionization, molecular size. Plasma Protein Binding Drugs may bind to albumin (acidic drugs), α1- acid glycoprotein (basic drugs), or lipoproteins. Bound drug is inactive, cannot cross membranes, not excreted. Only free fraction is pharmacologically active. Tissue Binding – e.g., tetracyclines to bone, chloroquine to melanin-containing tissues. Perfusion Rate – well-perfused organs (liver, kidney, brain) receive drugs rapidly. Volume of Distribution ( Vd ) Theoretical volume in which the drug appears to be distribution Vd = {Dose}/{Plasma concentration} High Vd → extensive tissue binding (e.g., digoxin). Low Vd → confined to plasma (e.g., warfarin). Special Barriers Blood–Brain Barrier (BBB): restricts polar and ionized drugs; lipophilic drugs cross easily. Placental Barrier: relatively permeable, allows most lipid-soluble drugs to enter fetus .

6. Metabolism of Drugs (Biotransformation) Metabolism (biotransformation) is the chemical modification of drugs by enzymatic systems to facilitate excretion. Objectives of Metabolism Convert lipophilic drugs into hydrophilic metabolites for excretion. Usually results in inactivation , but some drugs are activated (prodrugs). Can produce toxic metabolites (e.g., paracetamol → NAPQI). Sites of Metabolism Liver is primary organ (smooth endoplasmic reticulum, microsomal enzymes). Other sites: intestine, kidney, lung, skin, plasma. Phases of Metabolism Phase I (Functionalization Reactions): Introduce or expose functional groups (–OH, –NH2, –COOH). Mainly by cytochrome P450 (CYP450) oxidases. Reactions: oxidation, reduction, hydrolysis. Examples: hydroxylation of phenytoin, oxidation of barbiturates. Phase II (Conjugation Reactions): Coupling of drug or Phase I metabolite with endogenous substrate. Reactions: glucuronidation, sulfation, acetylation, methylation, glutathione conjugation. Usually produces inactive, water-soluble metabolites. Example: morphine → morphine-6-glucuronide.

7. Factors Affecting Metabolism Genetic polymorphism: slow vs fast acetylators. Age: neonates have immature enzymes; elderly have reduced metabolism. Enzyme induction: rifampin , carbamazepine increase metabolism → reduced drug action. Enzyme inhibition: cimetidine, ketoconazole inhibit metabolism → drug accumulation. Disease states: liver disease, heart failure reduce metabolism . Excretion of Drugs Excretion is the removal of drug and its metabolites from the body. Routes of Excretion Renal Excretion (Primary route) Involves: Glomerular filtration (free drug only). Tubular secretion (active transport of weak acids/bases). Tubular reabsorption (lipid-soluble drugs reabsorbed passively). Example: penicillins excreted unchanged. Biliary Excretion Drugs excreted into bile may undergo enterohepatic circulation (e.g., oral contraceptives, morphine). Other Routes Lungs: volatile anesthetics , alcohol. Sweat, saliva: minor routes, sometimes used for drug testing. Breast milk: weak bases accumulate (e.g., codeine, chloramphenicol).

8. Enzyme Induction Definition Enzyme induction refers to the increase in the synthesis or activity of drug-metabolizing enzymes in response to repeated exposure to certain drugs, xenobiotics, or environmental chemicals. This enhances the metabolism of both the inducer and other co-administered drugs. Mechanism of Enzyme Induction Genomic Regulation Many inducers act by binding to nuclear receptors that regulate gene transcription. Examples: CAR (Constitutive Androstane Receptor): Activated by phenobarbital, rifampicin. PXR ( Pregnane X Receptor): Activated by rifampicin, St. John’s Wort. AhR (Aryl Hydrocarbon Receptor): Activated by polycyclic hydrocarbons, dioxins. Increased mRNA transcription and translation → elevated enzyme levels in smooth endoplasmic reticulum. Non-genomic mechanisms (less common): stabilization of enzymes, altered degradation. Characteristics Slow onset (2–7 days) due to requirement for new protein synthesis. Sustained effect; may persist for 1–3 weeks after withdrawal. Usually selective for specific CYP isoforms. Examples of Enzyme Inducers CYP3A4: rifampicin, carbamazepine, phenytoin, St. John’s Wort. CYP1A2: smoking, omeprazole, chargrilled meat. CYP2E1: chronic alcohol consumption. UGTs ( glucuronyl transferases): phenobarbital.

9. Consequences of Enzyme Induction Reduced Therapeutic Efficacy Faster metabolism → lower plasma levels → loss of effect. Example: rifampicin reduces oral contraceptive efficacy → unintended pregnancy. Shortened Duration of Action E.g., barbiturates accelerate metabolism of warfarin, leading to subtherapeutic INR. Increased Production of Toxic Metabolites Example: paracetamol metabolism enhanced → increased NAPQI → hepatotoxicity. Autoinduction Inducer enhances its own metabolism, requiring dose escalation. Example: carbamazepine. Tolerance Development Example: chronic barbiturate use → tolerance due to enhanced metabolism. Clinical Relevance Critical in polypharmacy, especially in tuberculosis, epilepsy, HIV, and cancer chemotherapy. Requires dose adjustment or alternative drug selection

10. Enzyme Inhibition Definition Enzyme inhibition is the decrease in the activity of drug-metabolizing enzymes by another drug or chemical, leading to slower metabolism , prolonged drug action, and risk of toxicity. Mechanism of Enzyme Inhibition Competitive Inhibition Two substrates compete for the same active site of CYP450 enzyme. Example: ketoconazole inhibits metabolism of cyclosporine. Non-competitive/Allosteric Inhibition Inhibitor binds to a different site, altering enzyme conformation. Example: non-competitive inhibition of CYP enzymes by macrolides. Mechanism-Based (Suicide) Inhibition Irreversible inhibition via enzyme destruction after metabolism of the inhibitor. Example: erythromycin, chloramphenicol, ritonavir. Cofactor Inhibition Some drugs interfere with enzymes by reducing availability of cofactors (e.g., NADPH). Characteristics Rapid onset (within hours). Reversible or irreversible depending on the inhibitor. Effects cease once the inhibitor is cleared or enzyme levels are restored.

11. Examples of Enzyme Inhibitors CYP3A4 inhibitors: ketoconazole, itraconazole, erythromycin, clarithromycin, ritonavir, grapefruit juice. CYP2C9 inhibitors: fluconazole, sulfonamides . CYP2D6 inhibitors: quinidine, fluoxetine, paroxetine. CYP1A2 inhibitors: ciprofloxacin, cimetidine. Consequences of Enzyme Inhibition Prolonged Drug Action and Toxicity Example: erythromycin inhibits metabolism of theophylline → seizures. Ketoconazole + cyclosporine → nephrotoxicity. Drug Interactions of Clinical Concern Ritonavir is intentionally used as a "booster" for protease inhibitors (HIV therapy). Grapefruit juice enhances statin toxicity. Narrow Therapeutic Index Drugs at High Risk Warfarin, digoxin, phenytoin, theophylline, cyclosporine. Clinical Relevance Requires monitoring, dose reduction, or drug substitution. Useful strategy: using inhibition intentionally (e.g., ritonavir boosting).

12. Kinetics of Drug Elimination Drug elimination refers to irreversible removal of drug from the body , by metabolism and/or excretion. The kinetics of elimination describes how plasma concentration decreases over time. First-Order Kinetics Definition: The rate of drug elimination is proportional to plasma concentration . Characteristics: Constant fraction of drug is eliminated per unit time. Half-life (t½) is constant, independent of dose. Most drugs at therapeutic concentrations follow first-order kinetics. Equation:dc /dt= -kc Where k is the first-order rate constant. Examples: penicillin, paracetamol, digoxin (within therapeutic range). Clinical implication: Predictable kinetics → easier dose adjustment. Zero-Order Kinetics Definition: The rate of elimination is constant regardless of plasma concentration (enzyme-saturated kinetics). Characteristics: Constant amount of drug is eliminated per unit time. Half-life is not constant; increases with dose. Small dose increase → disproportionate rise in plasma concentration.

13. Mixed-Order (Michaelis–Menten) Kinetics Some drugs exhibit first-order kinetics at low concentration and shift to zero-order once metabolic pathways saturate. Examples: phenytoin, theophylline. Clinical implication: Non-linear dose–concentration relationship → careful therapeutic drug monitoring required . Clinical Importance of Elimination Kinetics Dosing interval selection: Based on half-life and clearance. Steady-state concentration: Achieved after ~4–5 half-lives. Drug accumulation: Directly related to elimination kinetics. Adjustment in renal/hepatic disease: Clearance is reduced → dose modification required. Drug interactions: Induction or inhibition alters elimination → risk of failure or toxicity.