Cell cycle intervention beyond Palbociclib; discovery of a CDK2/4/6 inhibitor

Phosphorylation of the retinoblastoma protein (RB1) by cyclin-dependent kinases (CDK) 4 and 6 is a critical checkpoint for G1/S cell cycle progression and commitment to cellular proliferation. Palbociclib(IBRANCE®), a highly selective inhibitor of CDK4/6, significantly improves progression free survival, and is approved for prescription use, in hormone receptor positive (HR+) HER2 negative breast cancer when combined with an anti-hormonal agent. In anticipation of intrinsic and acquired therapy-refractory disease we sought to discover CDK4/6 inhibitor resistance mechanisms in preclinical models of cancer which coalesced on the induction of Myc oncogene and Cylin E/CDK2 activity. We propose that targeting the G1kinases CDK2, -4 and -6 with a single small molecule inhibitor will overcome CDK4/6 inhibitor resistance. CDK1 and CDK9 anti-targets share several physical structure characteristics with the G1kinases making chemical design iterations particularly challenging. Relying heavily on advanced structure-based drug design techniques, PF-06873600, a pyridopyrimidine with potent biochemical activity on CDK2/4/6 and >40 fold selectivity over anti-targets, was characterized in models of CDK2 and CDK4/6 driven tumor growth for pharmacodynamic and efficacy response. Further, Myc driven molecular correlates of response were identified for PF-06873600 across multiple tumor lineages. Finally, we find that even under drug pressure, where the G1 CDKs are inhibited, the tumor specific immune response that occurs during checkpoint blockade remains intact in syngeneic models of cancer. We anticipate that PF-06873600 will offer a novel therapeutic option to cancer patients where CDK4/6 inhibition is not or is no longer beneficial to blocking disease progression.