The 3'-keto-diol equilibrium of trospectomycin sulfate bulk drug and freeze-dried formulation: solid-state carbon-13 cross-polarization magic angle spinning (CP/MAS) and high-resolution carbon-13 nuclear magnetic resonance (NMR) spectroscopy studies.

Understanding how moisture interacts with a drug or formulation is a critical component of product development. This study demonstrates how water affects the 3'-gem-diol<==>3'-keto equilibrium in trospectomycin sulfate bulk drug and freeze-dried formulation, as probed by solid-state carbon-13 cross-polarization magic angle spinning (CP/MAS) and high-resolution nuclear magnetic resonance (NMR) spectroscopy. Drying the bulk drug or formulation to low water levels dehydrates trospectomycin sulfate from the diol to the keto form. Carbon-13 CP/MAS NMR spectroscopy measures the keto drug concentration in solid samples directly. The bulk drug, which contains approximately 16% water, is more than 90% in the 3'-diol form. Oven drying to < 3% water converts approximately 75% of the drug to the 3'-keto form. The drug is formulated as a freeze-dried, sterile powder that can contain up to 12% water depending on the freeze-drying conditions. These studies show that the 3'-keto concentration rises uniformly (up to 75%) with decreasing residual water in the freeze-dried cake. The keto-diol equilibrium was also studied in solution by high-resolution carbon-13 NMR experiments, and it was found that raising the temperature or using dimethyl sulfoxide (DMSO) as a solvent also dehydrates the drug. For example, in aqueous solution at 25 degrees C, nearly all (> 95%) of the drug is in the 3'-diol form. After equilibration at 60 degrees C, however, the 3'-keto content increases to 7%, and in d6-DMSO solvent at 25 degrees C the drug is mostly (60%) in the 3'-keto form.