Unit 2: Drug Elimination & Bioavailability/Bioequivalence

Semester 6

BP604T

Drug Elimination & Bioavailability/Bioequivalence

This unit covers two major areas: (1) Drug Elimination — metabolism (Phase I and Phase II reactions, CYP450 system) and excretion (renal clearance, glomerular filtration, tubular secretion/reabsorption, non-renal routes). (2) Bioavailability and Bioequivalence — definitions, absolute and relative bioavailability calculations, methods of measurement, in-vitro dissolution models, In-Vitro In-Vivo Correlations (IVIVC), bioequivalence study design, and strategies to enhance dissolution and bioavailability of poorly soluble drugs.

Syllabus & Topics

1Drug Metabolism – Overview: Metabolism (biotransformation): chemical modification of drugs in the body → more polar → more easily excreted. Primary site: Liver (hepatocytes — smooth endoplasmic reticulum). Other sites: GI mucosa (intestinal CYP3A4), kidneys, lungs, plasma (esterases). Two phases: Phase I (functionalization — introduces or exposes polar groups) and Phase II (conjugation — attaches endogenous polar group). Most drugs: Phase I → Phase II → Excretion. Some drugs skip Phase I (direct Phase II conjugation — e.g., Morphine → Morphine-6-glucuronide).
2Phase I Reactions: Introduce or expose functional groups (–OH, –NH₂, –SH, –COOH) → ↑polarity. (1) Oxidation (MOST COMMON): Cytochrome P450 (CYP450) mixed-function oxidase system — heme-containing enzymes in hepatic microsomes. Key CYPs: CYP3A4 (metabolizes ~50% of drugs), CYP2D6 (25%), CYP2C9, CYP1A2, CYP2C19. Reactions: aromatic hydroxylation, aliphatic hydroxylation, N-dealkylation, O-dealkylation, S-oxidation, epoxidation. Non-microsomal oxidation: MAO (monoamine oxidase), ADH (alcohol dehydrogenase), xanthine oxidase. (2) Reduction: nitro reduction, azo reduction, carbonyl reduction. (3) Hydrolysis: ester hydrolysis (esterases), amide hydrolysis.
3Phase II Reactions (Conjugation): Attach endogenous polar molecules to drug/Phase I metabolite → highly polar → readily excreted. (1) Glucuronidation (MOST COMMON Phase II): UDP-glucuronosyltransferase (UGT) → glucuronic acid conjugate. Occurs in liver microsomes. (2) Sulfation: sulfotransferases (SULT) → sulfate ester. (3) Acetylation: N-acetyltransferase (NAT) — fast and slow acetylators (genetic polymorphism). INH, Hydralazine, Procainamide (slow acetylators → ↑drug toxicity). (4) Glutathione conjugation: Glutathione S-transferase (GST) → mercapturic acid. Protective — detoxifies reactive intermediates (Paracetamol NAPQI). (5) Methylation: COMT (catechol-O-methyltransferase) — catecholamines. (6) Amino acid conjugation: Glycine conjugation (benzoic acid → hippuric acid).
4Renal Excretion of Drugs: Three processes: (1) Glomerular Filtration: passive, non-selective for small molecules. GFR ≈ 125 mL/min. Only FREE (unbound) drug is filtered — protein-bound drug stays behind. Measured by inulin or creatinine clearance. (2) Active Tubular Secretion: active transport in proximal tubule. Carrier-mediated (saturable, competitive). Separate carriers for organic acids (OAT: penicillin, uric acid, NSAIDs) and organic bases (OCT: cimetidine, morphine). Probenecid blocks OAT → ↓penicillin secretion → ↑penicillin t½. (3) Passive Tubular Reabsorption: in distal tubule/collecting duct. Unionized (lipophilic) drug reabsorbed back into blood. Manipulated by urinary pH (alkaline diuresis for aspirin poisoning — ionize the drug → ↓reabsorption → ↑excretion).
5Renal Clearance: CLR = rate of excretion / plasma concentration = (Cu × Vu) / Cp. Where Cu = urine drug concentration, Vu = urine flow rate, Cp = plasma concentration. Interpretation: CLR = GFR (125 mL/min) → filtration only (inulin). CLR > GFR → net tubular secretion occurring (e.g., PAH — renal clearance ≈ 600 mL/min). CLR < GFR → net tubular reabsorption occurring (e.g., glucose — normal CLR = 0). Factors affecting renal excretion: urine pH (affects reabsorption), urine flow rate (↑flow → ↓reabsorption time → ↑excretion), protein binding (only free drug filtered), renal blood flow, renal disease (↓GFR → dose adjustment — Cockcroft-Gault equation).
6Non-Renal Excretion: Biliary excretion: liver secretes drug/metabolite into bile → GI tract → feces. For large MW (>500 Da), polar compounds, glucuronide conjugates. Enterohepatic circulation: glucuronide conjugate excreted in bile → gut bacteria hydrolyze conjugate → free drug reabsorbed → ↑t½ and ↑duration of action (e.g., oral contraceptives, Morphine, Digoxin). Antibiotics (destroying gut flora) can interrupt this cycle → ↓drug levels. Pulmonary excretion: volatile/gaseous drugs (anesthetic gases, alcohol — breathalyzer). Salivary excretion: passive diffusion → therapeutic drug monitoring via saliva. Breast milk excretion: basic drugs concentrate in acidic milk (pH 6.8 vs plasma 7.4) → ion trapping. Sweat, tears.
7Bioavailability – Definition: Bioavailability (BA) = rate and extent to which the active drug reaches systemic circulation from a dosage form. For IV bolus: BA = 100% (by definition — entire dose in blood). For oral: BA < 100% (due to incomplete absorption, first-pass metabolism, gut wall metabolism, efflux by P-gp). Measured by: AUC (area under the plasma concentration-time curve — reflects EXTENT), Cmax (peak concentration — reflects RATE), Tmax (time to Cmax — reflects RATE).
8Absolute & Relative Bioavailability: Absolute BA (F) = (AUC_oral / Dose_oral) × (Dose_IV / AUC_IV) × 100%. Compares oral formulation to IV (reference = 100%). Example: F = (AUC_oral/AUC_IV) × (Dose_IV/Dose_oral). Relative BA = (AUC_test / Dose_test) × (Dose_reference / AUC_reference) × 100%. Compares two non-IV formulations (test vs reference product). Used in bioequivalence studies. If unequal doses: correct for dose. If equal doses: simply AUC_test/AUC_reference × 100%.
9In-Vitro Dissolution & IVIVC: Dissolution testing: measures drug release from dosage form in vitro. USP Apparatus: Type I (basket — capsules), Type II (paddle — tablets), Type III (reciprocating cylinder), Type IV (flow-through cell). Dissolution media: pH 1.2 (gastric), 4.5 (intermediate), 6.8 (intestinal), with/without surfactant. IVIVC (In-Vitro In-Vivo Correlation): relationship between in-vitro dissolution and in-vivo bioavailability. Levels: Level A (point-to-point, most predictive — dissolution rate = absorption rate at each time point), Level B (statistical moment comparison), Level C (single point — e.g., t₅₀% dissolution vs Cmax). BCS-based biowaiver: Class I drugs with rapid dissolution → in-vivo study may be waived.
10Bioequivalence Studies: Two products are bioequivalent if they show similar rate and extent of absorption (no clinically significant difference). Study design: randomized, crossover (each subject gets both formulations with washout period), single-dose, fasting. Endpoints: AUC₀₋ₜ, AUC₀₋∞, Cmax. Statistical: 90% confidence interval of geometric mean test/reference ratio must fall within 80-125% for AUC and Cmax. Regulatory requirement: ANDA (Abbreviated New Drug Application) for generic drugs must prove bioequivalence to reference listed drug (RLD). CDSCO guidelines in India similar.
11Methods to Enhance Bioavailability: (1) Salt formation: ↑dissolution (Na/K salts of acids, HCl salts of bases). (2) Particle size reduction: micronization, nanonization → ↑surface area → ↑dissolution. (3) Solid dispersions: drug dispersed in hydrophilic carrier (PEG, PVP, HPMC) → amorphous form → ↑dissolution. (4) Complexation: cyclodextrin inclusion complexes → ↑solubility. (5) Prodrug approach: modify drug to ↑absorption (Enalapril → Enalaprilat). (6) Lipid-based formulations: SEDDS/SMEDDS → self-emulsifying → ↑solubility of lipophilic drugs (Cyclosporine — Neoral®). (7) Nanoparticles, liposomes, solid lipid nanoparticles. (8) Cocrystals: drug + coformer → new crystal with improved solubility. (9) Amorphous solid: higher energy state → ↑solubility (but stability concern).

Learning Objectives

Phase I vs II: Compare Phase I and Phase II metabolic reactions with examples and their significance in drug inactivation/activation.

Renal Excretion: Describe the three renal processes (filtration, secretion, reabsorption) and how each can be clinically manipulated.

Bioavailability Calculation: Calculate absolute and relative bioavailability from given AUC and dose data.

Bioequivalence Criteria: State the FDA/CDSCO criteria for bioequivalence (80-125% rule for 90% CI).

Enhancing BA: List and explain five methods to improve bioavailability of BCS Class II drugs.

Exam Prep Questions

Q1. What is the First-Pass Effect?

The First-Pass Effect (or presystemic metabolism) is the phenomenon where the concentration of a drug is greatly reduced before it reaches the systemic circulation. When a drug is administered orally, it is absorbed from the gastrointestinal tract and enters the portal vein. Before reaching the general circulation, it must pass through the liver, where significant metabolism can occur by hepatic enzymes. If a drug is highly susceptible to the first-pass effect, only a small fraction of the administered dose reaches the target tissues.

Q2. What is the 80-125% rule in bioequivalence?

The 80-125% rule is the standard regulatory criterion used to determine if a generic drug is “bioequivalent” to a brand-name reference drug.

Definition: Two products are considered bioequivalent if the 90% confidence interval for the ratio of their geometric mean values (usually $AUC$ and $C_{max}$ ) falls entirely within the range of 80.0% to 125.0%.
Significance: This does not mean the generic can be 20% different in its effect; rather, it ensures that any difference in the rate and extent of absorption is statistically small enough that it is clinically insignificant for the patient.

Q3. What is Enterohepatic Circulation?

Enterohepatic circulation is a process that involves the transit of a drug (or its metabolites) from the liver to the bile, then to the intestine, and finally its reabsorption back into the bloodstream.

Liver: The drug is excreted into the bile.
Bile: The drug travels into the small intestine.
Intestine: The drug may be reabsorbed across the intestinal wall back into the portal circulation.
Loop: This creates a recycling effect that can significantly prolong the half-life ( $t_{1/2}$ ) of the drug within the body, as it keeps the drug circulating instead of being excreted immediately via feces.