Pharmaceutics and Pharmaceutical Technology
From dosage form to plasma
Pharmaceutics links formulation to bioavailability. Key concepts: dissolution rate (Noyes–Whitney, surface area and solubility dependent), the Biopharmaceutics Classification System (Class I high solubility/high permeability, Class II low solubility/high permeability = dissolution-rate limited, Class III high solubility/low permeability = permeability limited, Class IV both low), excipient function (diluent, binder, disintegrant, glidant, lubricant, coating), and the difference between immediate-, delayed- and extended-release systems.
Stability and shelf life
Drug degradation is usually first-order, so shelf life is the time to 90% potency (t90 = 0.105/k). Arrhenius behaviour means a small temperature rise accelerates degradation; hence refrigerated storage for reconstituted products and light protection for photolabile drugs. Solid-state form matters too: polymorphs differ in solubility and stability, and the metastable form can convert.
Traps to expect
- Confusing bioavailability (rate and extent reaching systemic circulation) with bioequivalence (comparison of two products' rate and extent within acceptance limits).
- Assuming an enteric coat is for taste only - it protects acid-labile drug or the gastric mucosa and must not be crushed.
- Forgetting that extended-release products generally should not be split unless scored and labelled.
- Mixing up suspension uniformity (shake well) with solution homogeneity.
Worked example
If k = 0.02 per month at 25 °C, t90 = 0.105 ÷ 0.02 ≈ 5.3 months. A rule of thumb: roughly 10% loss equals about 0.105/k, and doubling the rate constant halves the shelf life.