Investigation of the capacity of low glass transition temperature excipients to minimise amorphisation of sulfadimidine on co-milling

The coprocessing of active pharmaceutical ingredient (API) with an excipient which has a high glass transition temperature (Tg) is a recognized strategy to stabilize the amorphous form of a drug. This work investigates whether coprocessing a model API, sulfadimidine (SDM) with a series of low Tg excipients, prevents or reduces amorphization of the crystalline drug. It was hypothesized that these excipients could exert a Tg lowering effect, resulting in composite Tg values lower than that of the API alone and promote crystallization of the drug. Milled SDM and comilled SDM with glutaric acid (GA), adipic acid (AA), succinic acid (SA), and malic acid (MA) were characterized with respect to their thermal, X-ray diffraction, spectroscopic, and vapor sorption properties. SDM was predominantly amorphous when milled alone, with an amorphous content of 82%. No amorphous content was detected by dynamic vapor sorption (DVS) on comilling SDM with 50% w/w GA, and amorphous content of the API was reduced by almost 30%, relative to the API milled alone, on comilling with 50% w/w AA. In contrast, amorphization of SDM was promoted on comilling with 50% w/w SA and MA, as indicated by near-infrared (NIR) spectroscopy. Results indicated that the API was completely amorphized in the SDM:MA comilled composite. The saturated solubility of GA and AA in the amorphous API was estimated by thermal methods. It was observed that the Tg of the comelt quenched composites reached a minimum and leveled out at this solubility concentration. Maximum crystallinity of API on comilling was reached at excipient concentrations comparable to the saturated concentration solubility of excipient in the API. Moreover, the closer the Hildebrand solubility parameter of the excipient to the API, the greater the inhibition of API amorphization on comilling. The results reported here indicate that an excipient with a low Tg coupled with high solubility in the API can prevent or reduce the generation of an amorphous phase on comilling