CDK2 inhibition enhances CDK4/6 inhibitor antitumor activity in comprehensive breast cancer PDX model screen.

Aberrant cyclin-dependent kinase 2 (CDK2) activity is implicated as a resistance mechanism to CDK4/6 inhibitors (CDK4/6i) in hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) breast cancer. Using preclinical patient-derived xenograft models, the CDK2i + CDK4/6i combination was active broadly across CDK4/6i-resistant and -naïve HR+ and triple-negative breast cancer models. A novel, weighted mRNA expression signature involving CCND1, CCNE1, RB1, and CDKN2A (p16) predicted response to combined inhibition of CDK2 and CDK4/6. Addition of endocrine therapy significantly enhanced antitumor activity in HR+ models, providing preclinical proof-of-concept for the broad antitumor activity of the triple combination. Early clinical data demonstrated activity of BLU-222, a potent and selective CDK2 inhibitor, both as monotherapy (CCNE1 amplified) and in combination with ribociclib and fulvestrant in patients with HR+/HER2- breast cancer. These findings provide evidence that CDK2i combined with CDK4/6i can address multiple known mechanisms of resistance to CDK4/6i, enhancing antitumor responses in preclinical breast cancer models.