Preview PCN prep

Not a catalogue of equipment. It tests whether a preparation will deliver a known amount of drug to the right place, still within its labeled strength, at the point of use. Every question resolves to one of four failures: the drug does not dissolve, the dose degrades, the patient cannot take the form, or the calculation is wrong. The first three are design decisions the pharmaceutist made years earlier; the last is made at the counter in ninety seconds — which is why the registration examination sets a calculation with an inference in it. So read each dosage form as a trade. Aqueous solution: fastest onset and easiest dose accuracy, but hydrolysis-prone and bulky to carry. Oral solid: stable and portable, but dependent on dissolution and on the patient's ability to swallow. Suspension: the answer for a drug insoluble in a palatable medium, bought at the price of redispersibility and a dose that separates if the patient does not shake. Injection: certainty of delivery, and the highest cost in sterility assurance. Topical, rectal and inhaled forms trade systemic exposure for local effect or a shorter route. Being able to say what a form sacrifices is the professional answer; naming the form is only the first half.

Three properties decide most of the design. Solubility sets the maximum concentration available for absorption and drives the choice of salt form, co-solvent, surfactant or particle-size reduction. pKa relative to the pH of the environment sets the fraction unionised: a weak acid with pKa 4.4 in gastric fluid at pH 1.4 is 99% unionised and, on the pH-partition view, well absorbed from the stomach — the same molecule in plasma at pH 7.4 is ionised and held there, which is the ion-trap explanation examiners expect for aspirin in overdose. Partition coefficient governs how readily that unionised fraction crosses a membrane, and it is why a very lipophilic drug accumulates in fat and a very hydrophilic one is confined to plasma and is dialysable. Formulation then manipulates those properties rather than arguing with them. Salt selection raises aqueous solubility; a surfactant lowers the interfacial barrier for a poorly wetted powder; a co-solvent or cyclodextrin carries a molecule that will not dissolve in water alone; micronisation shortens the diffusion distance; a polymorph with higher free energy dissolves faster and may revert on storage. And for every one of those there is a price in stability, in irritation at the site, or in a re-dispersibility problem in the bottle. The dissolution rate of a solid dose is what a bioavailability claim is really about — which is why a generic's demonstration of bioequivalence is a statement about dissolution and not about the tablet's colour or shape.