Process Development for a TLR-7 Immunostimulatory Drug-Linker: Salt Screening and Precipitation for Drug-Linker Intermediates

Interest is growing in Toll-like receptor agonist-based immune-stimulating antibody conjugates (ISACs) due to their potential to enhance efficacy and lower systemic toxicity. This report describes the development work toward a scalable process for producing a TLR-7 immunostimulatory drug-linker (aka linker-payload) for use in an ISAC. The synthesis involves bromination to provide the linker fragment, quaternization with the payload, global deprotection, and installation of conjugation handle. The global deprotection step was optimized by controlling critical impurities to provide the crude penultimate intermediate in high purity. This was followed by development of a precipitation process for the structurally flexible and hydrophilic penultimate intermediate using a salt-screening workflow guided by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis. The reaction conditions for the final step using the modified salt form were optimized to ensure consistent properties of the drug-linker. In addition, stability and purification studies were carried out to facilitate a comprehensive understanding of the impurity control necessary for successful scale-up. Overall, two expensive and time-consuming column purifications were removed, and a robust and scalable process was developed to seamlessly deliver a high-quality drug-linker.