Abstract LB348: Small-molecule targeting of p27 enables coordinated CDK2/4/6 inhibition in CDK4/6i-resistant breast cancer

Abstract Cyclin-dependent kinases 4 and 6 (CDK4/6) regulate G1-S cell-cycle progression through trimeric complexes with cyclin D1 and the CDK interactor p27Kip1. CDK4/6 inhibitors (CDK4/6i), including palbociclib, are standard-of-care for ER-positive metastatic breast cancer; however, resistance commonly emerges through CDK2 activation, enabling bypass of CDK4/6 inhibition. Although CDK4/6i initially suppress all G1 CDKs by releasing p27 to inhibit CDK2, tumors restore proliferation via mechanisms such as cyclin E or CDK6 overexpression and loss of RB or FAT1. We report a differentiated therapeutic strategy that directly targets p27, a master regulator of CDK2/4/6 activity. Phosphorylation of p27 at Y88 displaces its C-terminal tail from the CDK active site, activating CDKs and promoting p27 degradation. Inhibition of p27 Y88 phosphorylation maintains CDKs in an inactive state and stabilizes p27. In treatment-naïve and palbociclib-resistant breast cancer models, blockade of p27 Y88 phosphorylation using a therapeutic peptide significantly reduced tumor growth in xenografts, with evidence of CDK2 inhibition, tumor necrosis, and improved survival. Repeat dosing (IV; TIWx2) was well tolerated in mice. Building on this foundation, Concarlo has identified a first-in-class oral, small-molecule p27 molecular glue stabilizer, Conc142, that preferentially binds the trimeric CDK-cyclin-p27 complex to prevent p27 Y88 phosphorylation and inhibit CDK2, CDK4, and CDK6. Conc142 increases the thermal stability of the p27-Cyclin D-CDK4 complex and binds to p27 with low nM affinity in cells. Conc142 suppress proliferation in both treatment-naïve and CDK4/6i-resistant breast cancer cells while sparing normal mammary cells. Cellular target engagement studies show reduced phosphorylation of RB, CDK2, and Cdc6, along with increased total p27 levels, consistent with reduced p27 degradation and sustained CDK2 inhibition. Conc142 also potentiate the activity of approved CDK4/6 inhibitors. This compound demonstrates favorable in vitro ADME properties, minimal off-target interactions, and encouraging pharmacokinetics in vivo. Building on Conc142, we have continued structure-activity relationship (SAR) efforts and have employed AI-driven scaffold optimization. Several optimized analogs showed improved cellular potency and target engagement in CDK4/6 inhibitor-resistant breast cancer models, supporting further development of this series. Clinical data show that all three targets (CDK2/4/6) must be inhibited to yield durable response and minimize drug resistance, but approaches at targeting CDK2/4/6 directly failed due to toxicity, and combinations of CDK4i and CDK2i monotherapies are currently being met with similar dose-limiting toxicity issues. Direct modulation of p27 represents a novel therapeutic paradigm that overcomes limitations of ATP-competitive CDK inhibitors and offers potential applicability across tumors driven by dysregulated p27, CDK2/4/6, or RAS/MAPK signaling. This approach has distinct advantages over small molecule CDK inhibitors, in that p27 is selective for CDK2/4/6 and has the potential to avoid the off-target toxicity seen with the CDK inhibitor class. Citation Format: Grace Chen, Carolina Guido, Charles H. Reynolds, Krishna P. Allamneni, Stacy W. Blain. Small-molecule targeting of p27 enables coordinated CDK2/4/6 inhibition in CDK4/6i-resistant breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86 (8Suppl): Abstract nr LB348.