A study on polymorphic forms of rifampicin for inhaled high dose delivery in tuberculosis treatment

Rifampicin is a first-line, highly effective drug currently used orally as a part of a lengthy multi-drug regimen against tuberculosis (TB). Despite the potential of inhaled therapy as an effective approach for TB treatment, an inhalable formulation of rifampicin has not yet been developed for clinical use. In order to do so, it is necessary to evaluate its solid-state properties, which regulate important characteristics like solubility, dissolution, aerosolization, stability and bioavailability. In this study, a crystallization technique and spray drying were used to prepare inhalable rifampicin formulations. Spray drying yielded amorphous formulation of rifampicin while crystalline dihydrate and pentahydrate formulations were obtained by crystallization. The powders were evaluated for their solid-state properties, in vitro aerosolization and aerosolization stability for three months when stored at different relative humidity conditions. All formulations had a mean particle size smaller than 3.8 µm and had a fine particle fraction (FPF) higher than 58.0%. Amorphous and crystalline dihydrate formulations showed no change in aerosolization parameters (emitted dose and FPF) upon storage for three months. The amorphous and pentahydrate formulations were found to undergo oxidative degradation upon storage, and a decrease in their drug content was observed. Among the formulations prepared, rifampicin dihydrate formulation showed the least degradation over the three months period. The inhalable rifampicin formulations prepared in this study, being excipient free, have the potential to be delivered as inhaled dry powder high-dose rifampicin to the lungs for effective treatment of TB.