Synthesis and Development of Highly Selective Pyrrolo[2,3-d]pyrimidine CSF1R Inhibitors Targeting the Autoinhibited Form

[Image: see text] Colony-stimulating factor-1 receptor (CSF1R) is a receptor tyrosine kinase that controls the differentiation and maintenance of most tissue-resident macrophages, and the inhibition of CSF1R has been suggested as a possible therapy for a range of human disorders. Herein, we present the synthesis, development, and structure–activity relationship of a series of highly selective pyrrolo[2,3-d]pyrimidines, showing subnanomolar enzymatic inhibition of this receptor and with excellent selectivity toward other kinases in the platelet-derived growth factor receptor (PDGFR) family. The crystal structure of the protein and 23 revealed that the binding conformation of the protein is DFG-out-like. The most promising compounds in this series were profiled for cellular potency and subjected to pharmacokinetic profiling and in vivo stability, indicating that this compound class could be relevant in a potential disease setting. Additionally, these compounds inhibited primarily the autoinhibited form of the receptor, contrasting the behavior of pexidartinib, which could explain the exquisite selectivity of these structures.