drug-excipient compatibility

A tablet or capsule is rarely just the drug — it is the drug mixed with fillers, binders, disintegrants and lubricants. Drug-excipient compatibility asks a basic but vital question before those ingredients are combined: will any of them react with or destabilize the drug? It is the screening done in preformulation to spot bad chemical marriages early.

Incompatibilities can be chemical or physical. Classic chemical examples include the Maillard reaction between amine drugs and reducing sugars like lactose, hydrolysis catalyzed by acidic or basic excipients, oxidation promoted by peroxide impurities in polymers like povidone or PEG, and metal-ion catalysis from impurities. Physical issues include moisture transfer, polymorph conversion, adsorption onto excipient surfaces, or eutectic melting that softens the blend.

Compatibility is screened by storing binary or simple blends of drug with each candidate excipient under stress conditions (elevated temperature and humidity), then looking for degradation by chromatography and for thermal or solid-state changes by differential scanning calorimetry. The caveat is that simple binary mixtures can over- or under-predict real behavior, so suspicious findings are confirmed in representative prototype formulations rather than taken at face value.