Pharmaceutics: Dosage Forms, Stability, and Biopharmaceutics
Core concepts
Pharmaceutics questions test whether you can connect a formulation property to a patient-relevant outcome. Know the classification of dosage forms (solid, semisolid, liquid, sterile), the excipient roles (diluent, binder, disintegrant, lubricant, glidant, coating, preservative, antioxidant, suspending agent), and the manufacturing consequences of each. For oral solids, understand that dissolution is usually rate-limiting for poorly soluble drugs, while for immediate-release highly soluble drugs disintegration and gastric emptying dominate. For sterile products, distinguish sterilization (terminal) from aseptic processing, and know the parameters: moist heat 121 °C/15 min/15 psi, dry heat 160–170 °C/2 h, ethylene oxide with humidity and temperature control, and filtration for heat-labile solutions (0.22 µm for sterility).
Stability and kinetics
Zero-order vs first-order degradation is a classic trap. First-order half-life t½ = 0.693/k; shelf life t90 = 0.105/k. Zero-order t90 = 0.1·C0/k. Arrhenius analysis predicts stability at lower temperatures from accelerated data. Know the ICH climatic zones and long-term/accelerated conditions (25 °C/60% RH and 40 °C/75% RH), and that photostability testing is separate.
Biopharmaceutics
BCS Class I (high solubility/high permeability), II (low solubility/high permeability), III (high solubility/low permeability), IV (both low). Class II often benefits from particle-size reduction or amorphous solid dispersion; Class III from permeation enhancers. Bioavailability = fraction absorbed × fraction escaping first-pass; bioequivalence uses AUC and Cmax with 90% CI within 80–125%.
Traps and rule of thumb
Questions often swap Cmax (rate) with AUC (extent), or ask which excipient is incompatible with a drug (e.g., reducing sugars with amines via Maillard reaction; calcium salts with tetracyclines). Rule of thumb: if a question says "rate of absorption," look for Cmax/Tmax; if it says "extent," look for AUC.