Discovery of an Orally Bioavailable and Selective PKMYT1 Inhibitor, RP-6306

PKMYT1 is an important regulator of CDK1 phosphorylation and is a compelling therapeutic target for the treatment of certain types of DNA damage response cancers due to its established synthetic lethal relationship with CCNE1 amplification. To date, no selective inhibitors have been reported for this kinase that would allow for investigation of the pharmacological role of PKMYT1 in the treatment of cancer. To address this need we conducted a focused screening effort that identified compound 1 as a weak PKMYT1 inhibitor. Introduction of a dimethylphenol dramatically increased potency on PKMYT1. These dimethylphenol analogs were found to exist as Type III atropisomers that could be separated and profiled as single enantiomers. Structure-based drug design aided by co-crystal structures of several analogs enabled optimization of cell-based potency and kinase selectivity. Parallel optimization of ADME properties led to the identification of potent and selective inhibitors of PKMYT1 with favorable pharmacokinetics. RP-6306 inhibits the phosphorylation of CDK1 Thr14 in vivo in tumor tissue and inhibits CCNE1-amplified tumor cell growth in several preclinical xenograft models. The first-in-class clinical candidate RP-6306 is currently being evaluated in Phase 1 clinical trials (NCT04855656) for treatment of various solid tumors.