Abstract 6914: p27 small molecule inhibitors for CDK4/6i-resistant metastatic breast cancer (mBC)

Abstract Cyclin-dependent kinases 4/6 (CDKs) are key cell cycle regulators that form trimeric complexes with cyclin D1 and the master regulator p27Kip1, acting downstream of proliferation-driving pathways. CDK4/6 inhibitors (CDK4/6i) like palbociclib have been pivotal for the treatment of ER+ mBC but resistance to these drugs is common. This resistance is due to CDK2 activation, which bypasses CDK4/6 inhibition, restoring cell proliferation and suggesting that effective targeting requires simultaneous inhibition of CDK4/6, and CDK2. CDK4/6i work by dissolving the trimeric CDK4/6-cyclin D1 complex, freeing p27 to inhibit CDK2, initially blocking the activity of all three G1 CDKs, but patients become resistant by reactivating CDK2 by a variety of mechanisms including overexpression of CDK6 and cyclin E or loss of RB and FAT1. Concarlo’s unique approach directly targets the master CDK regulator, p27, to simultaneously inhibit CDK2/4/6 in concert, leveraging p27’s ability to turn G1 CDKs ON and OFF during proliferation and quiescence, respectively. Phosphorylation of p27 at Y88 causes p27’s C-terminal tail to leave CDK active site, turning the kinases on; without phosphorylation, p27's C-terminal tail remains in the CDK active site, keeping the kinases off. When p27 is phosphorylated at Y88, it becomes a target for ubiquitin-mediated degradation, reducing p27 levels, and reinforcing the unconstrained activity of CDK2. Our studies demonstrated the feasibility and tolerability of targeting p27 in breast cancer in treatment naïve and palbociclib resistant models. Blocking p27 Y-phos with a therapeutic peptide significantly reduced tumor growth in xenograft models, with evidence of CDK2 inactivation, tumor necrosis and increased survival. Moreover, p27 inhibition was well-tolerated during repeat dosing in mice. Concarlo has identified first-in-class small molecule p27 inhibitors (SMp27i) that act as molecular stabilizers or glues, binding and inactivating the trimeric CDK complex. They block p27 Y-phos and inhibit CDK2/4/6. SMp27i inhibit cell proliferation in treatment-naïve and palbociclib-resistant BC cells but have little effect on normal mammary cells. Target engagement in treated cells showed that Rb, CDK2, and Cdc6 phosphorylation were all inhibited. Total p27 levels increase demonstrating that when p27 Y88-phos is blocked, p27 degradation is reduced and more p27 is present to inhibit CDK2. When combined with approved CDK4/6i, SMp27i enhance their potency. SMp27i have been favorably profiled in in vitro ADME screens, off-target binding screens, and have encouraging in vivo mouse PK. Direct targeting of p27 presents a new treatment paradigm bypassing the challenges of ATP-competitive CDKis, with potential to expand into tumor types with aberrant p27, CDK2/4/6 or RAS/MAPK activity. Citation Format: Grace Chen, Carolina Guido, Natalia Zisman, Krishna Allamneni, Stacy Wister Blain. p27 small molecule inhibitors for CDK4/6i-resistant metastatic breast cancer (mBC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6914.