Glass Forming Properties of Benzodiazepines and Co-evaporate Systems with Poly(hydroxyethyl Methacrylate)

Abstract

In the present study, we report on the thermal properties of a series of benzodiazepines. The heat of fusion varied between approximately 25 and 40 kJ mol⁻¹, except for oxazepam and lorazepam where dimerization in the solid state increased the heat of fusion to 78.54(±0.37) and 77.03 (±0.84)kJ mol⁻¹, respectively. Heating alprazolam at a low rate (0.5 K min⁻¹) showed that polymorphs I and II are an enantiotropic pair with a solid-solid transition at 481.4 K It was shown that all benzodiazepines could be transformed to the glassy state by cooling fused samples, irrespective of the cooling rate. The size of the relaxation endotherm accompanying the glass transition increased by heating the glassy drugs at a higher rate through T_g or by cooling the fused samples at a slower rate. The time dependence of the glass to liquid transition can be described to a good approximation as a first order transformation. The Gordon-Taylor equation was used to predict T_g of a binary mixture of temazepam, diazepam or prazepam with polyHEMA. It was shown that the predictability was acceptable as long as the drug concentration was below 10%w/w; at higher concentration, specific drug-polymer interactions causing changes in free volume of the system could not be ignored.