Lead Optimization of an Aminopyrazole Scaffold to EXEL-1855, a Highly Selective PLK4 Inhibitor with Potent Antitumor Efficacy.

Polo-like kinase 4 (PLK4) is a serine/threonine kinase that has an essential role in regulating centriole duplication and centrosome biogenesis during cell-cycle progression and a potential therapeutic target in various human cancers. Structure-guided lead optimization of an aminopyrazole scaffold at three key vectors led to the identification of EXEL-1855, which demonstrated a significant improvement in cellular potency, increased selectivity over related-kinases and an improved pharmacokinetic profile. Targeting the non-conserved Met91 residue in the PLK4 lower hinge region and optimizing this interaction, enabled EXEL-1855 to be most selective PLK4 inhibitor reported to date. In a preclinical CHP-134 neuroblastoma mouse xenograft model, EXEL-1855 showed robust efficacy and was well-tolerated at all dose levels. Taken together, our medicinal chemistry campaign resulted in a highly optimized, orally bioavailable, and efficacious inhibitor of PLK4, with the potential to advance as an oral single-agent for the treatment of neuroblastoma with a projected human dose of 540 mg QD.