Small-molecule inhibitors of the CDK4/6 cell-cycle kinases have shown clinical efficacy in estrogen receptor (ER)-positive metastatic breast cancer, although their cytostatic effects are limited by primary and acquired resistance. Here we report that ER-positive breast cancer cells can adapt quickly to CDK4/6 inhibition and evade cytostasis, in part, via noncanonical cyclin D1-CDK2–mediated S-phase entry. This adaptation was prevented by cotreatment with hormone therapies or PI3K inhibitors, which reduced the levels of cyclin D1 (CCND1) and other G1–S cyclins, abolished pRb phosphorylation, and inhibited activation of S-phase transcriptional programs. Combined targeting of both CDK4/6 and PI3K triggered cancer cell apoptosis in vitro and in patient-derived tumor xenograft (PDX) models, resulting in tumor regression and improved disease control. Furthermore, a triple combination of endocrine therapy, CDK4/6, and PI3K inhibition was more effective than paired combinations, provoking rapid tumor regressions in a PDX model. Mechanistic investigations showed that acquired resistance to CDK4/6 inhibition resulted from bypass of cyclin D1–CDK4/6 dependency through selection of CCNE1 amplification or RB1 loss. Notably, although PI3K inhibitors could prevent resistance to CDK4/6 inhibitors, they failed to resensitize cells once resistance had been acquired. However, we found that cells acquiring resistance to CDK4/6 inhibitors due to CCNE1 amplification could be resensitized by targeting CDK2. Overall, our results illustrate convergent mechanisms of early adaptation and acquired resistance to CDK4/6 inhibitors that enable alternate means of S-phase entry, highlighting strategies to prevent the acquisition of therapeutic resistance to these agents.

Purpose: PARP1/2 inhibitors are a class of anticancer agents that target tumor-specific defects in DNA repair. Here, we describe BMN 673, a novel, highly potent PARP1/2 inhibitor with favorable metabolic stability, oral bioavailability, and pharmacokinetic properties.Experimental Design: Potency and selectivity of BMN 673 was determined by biochemical assays. Anticancer activity either as a single-agent or in combination with other antitumor agents was evaluated both in vitro and in xenograft cancer models.Results: BMN 673 is a potent PARP1/2 inhibitor (PARP1 IC 50 ¼ 0.57 nmol/L), but it does not inhibit other enzymes that we have tested. BMN 673 exhibits selective antitumor cytotoxicity and elicits DNA repair biomarkers at much lower concentrations than earlier generation PARP1/2 inhibitors (such as olaparib, rucaparib, and veliparib). In vitro, BMN 673 selectively targeted tumor cells with BRCA1, BRCA2, or PTEN gene defects with 20-to more than 200-fold greater potency than existing PARP1/2 inhibitors. BMN 673 is readily orally bioavailable, with more than 40% absolute oral bioavailability in rats when dosed in carboxylmethyl cellulose. Oral administration of BMN 673 elicited remarkable antitumor activity in vivo; xenografted tumors that carry defects in DNA repair due to BRCA mutations or PTEN deficiency were profoundly sensitive to oral BMN 673 treatment at well-tolerated doses in mice. Synergistic or additive antitumor effects were also found when BMN 673 was combined with temozolomide, SN38, or platinum drugs.Conclusion: BMN 673 is currently in early-phase clinical development and represents a promising PARP1/2 inhibitor with potentially advantageous features in its drug class.

Inhibitors of poly (ADP-ribose)-polymerase-1 (PARP) are highly lethal to cells with deficiencies in BRCA1, BRCA2 or other components of the homologous recombination pathway. This has led to PARP inhibitors entering clinical trials as a potential therapy for cancer in carriers of BRCA1 and BRCA2 mutations. To discover new determinants of sensitivity to these drugs, we performed a PARP-inhibitor synthetic lethal short interfering RNA (siRNA) screen. We identified a number of kinases whose silencing strongly sensitised to PARP inhibitor, including cyclindependent kinase 5 (CDK5), MAPK12, PLK3, PNKP, STK22c and STK36. How CDK5 silencing mediates sensitivity was investigated. Previously, CDK5 has been suggested to be active only in a neuronal context, but here we show that CDK5 is required in non-neuronal cells for the DNA-damage response and, in particular, intra-S and G 2 /M cell-cycle checkpoints. These results highlight the potential of synthetic lethal siRNA screens with chemical inhibitors to define new determinants of sensitivity and potential therapeutic targets.