Development of a Green and Sustainable Manufacturing Process for Gefapixant Citrate (MK-7264). Part 6: Development of an Improved Commercial Salt Formation Process

The development of a sustainable commercial salt formation process for gefapixant citrate (MK-7264), an investigational new P2X3 antagonist for the treatment of chronic cough, is described. Due to the low solubility of the gefapixant free base, the first-generation process for citrate salt formation was a slurry-to-slurry process with poor quality control, wherein impurities were not well rejected and unreacted free base often persisted in the citrate produced. The development of a controlled crystallization from a homogeneous solution, which overcame these deficiencies, was complicated by solubility constraints and a daunting solid form landscape. Herein, we report a novel solution to this problem where the free base is transiently converted to a highly soluble glycolate salt enabling complete dissolution, from which direct crystallization of the final citrate salt occurs in a high yield through salt metathesis. Robust crystallization control was ensured by conducting a comprehensive polymorph screen on the glycolate salt and demonstrating its metastability compared to the desired citrate salt. In addition, process-relevant solvates of the citrate salt were discovered and derisked via a thorough understanding of their stability regions. With this information, a second-generation process salt formation with robust crystalline form and purity control was achieved, utilizing a salt metathesis co-feed process that greatly reduces the amount of solvent required, the overall manufacturing time, and the energy consumption compared to the first-generation conditions.