In-process crystallization of acidic drugs in acrylic microparticle systems: influence of physical factors and drug-polymer interactions.

Emulsion-solvent evaporation is an established method to fabricate amorphous drug-loaded microparticles. In some cases, however, the encapsulated drug is present in its crystalline form, which can affect drug release and negatively impact on other characteristics of the final product. This work aimed to investigate the factors that are responsible for the formation (and inhibition) of drug crystals in modified-release microparticles. Five acidic drugs were encapsulated into Eudragit S or Eudragit L microparticles. Drug crystallinity was observed when indometacin and naproxen were encapsulated, while crystallization was not observed in the case of ketoprofen, salicylic acid, or paracetamol (acetaminophen). All drug-loaded microparticles had single glass transition temperature (T(g) ) intermediate between the T(g) of the drug and that of the polymer. The drop in T(g) in the case of the paracetamol-loaded particles was higher than predicted from the Gordon-Taylor equation, indicating that paracetamol was acting as a plasticizer in this system. After melt quenching in the presence of the Eudragit polymers, the crystallization of paracetamol was inhibited. The ratio of drug to polymer in the microparticles was the major determinant of drug crystallization, as was the solubility of the drug in the processing solvent. This work confirms that drug crystallization is a complex phenomenon, and that drug-polymer molecular interactions play a role in the inhibition of drug crystallization.