Development of a novel selective CDK4 inhibitor for HR+/HER2- breast cancer

Targeting cell cycle-dependent kinases 4 and 6 (CDK4/6) has been proven to be a successful strategy for blocking oncogenic cell cycle progression in breast cancer cells. FDA-approved CDK4/6 inhibitors have delivered significant clinical benefits in patients with HR+/HER2- breast cancer. However, despite remarkable clinical and commercial success, dual CDK4/6 inhibitors are associated with severe on-target toxicity, leading to serious adverse events (SAEs), such as neutropenia in clinical settings. The mechanism underlying these hematologic adverse events was attributed to CDK6, rather than CDK4, for its direct mediation of myeloid cell differentiation when CDK6 expression is upregulated. To avoid CDK6 toxicity, we sought to develop selective and potent CDK4 inhibitors that in ideal cases only inhibit CDK4 activity with no impact on CDK6 or other CDKs in HR+/HER2- breast cancer cells to improve the hematologic safety of the therapeutic approach targeting the cancer cell cycle. TYK Medicines has been making continuous efforts to develop better CDK4 inhibitors with higher selectivity, potency, and lower hematologic toxicity. The candidate CDK4 inhibitor discussed in this abstract exhibits strong selectivity for CDK4 over CDK6 compared to palbociclib, confirming its identity as a selective CDK4 inhibitor rather than a dual CDK4/6 inhibitor. The compound confers a dose-dependent reduction in phosphorylated RB (Ser780) in HR+/HER2-breast cancer cell lines following a 48-hour treatment with the cells. The antiproliferative effects of the candidate compounds were evaluated via CTG cell viability assays after 5-day incubation in three cell lines: MCF7, JEKO-1, and MOLM13. Palbociclib potently inhibited MCF7/T47D cells because of its dual CDK4/6 kinase activity. In contrast, the selective CDK4 inhibitor strongly inhibited JEKO-1 cells and had no inhibitory effect on MOLM13 cells expressing high levels of CDK6. In a MCF7-xenograft mouse model, the compound significantly suppressed tumor growth in a dose-dependent manner. It also exhibited synergistic antitumor effects when combined with fulvestrant, with good tolerability evidenced by stable animal body weight throughout the study. Given the genetic diversity of HR+/HER2-breast cancer patients, targeting CDK4 may present a keystone strategy in a spectrum of combinatorial therapies. Preliminary in vitro combination studies revealed that pairing the selective CDK4 inhibitor with a CDK2 inhibitor or a PI3K inhibitor produced significant synergistic inhibitory effects on MCF7 and palbociclib-resistant MCF7 cells. Optimization of the lead compound is ongoing with the goal of identifying a preclinical candidate (PCC) for IND-enabling studies. # Apeng Liang and Meihua Li contributed equally to this work. * Correspondence authors.