CDK Inhibition Primes for Anti-PD-L1 Treatment in Triple-Negative Breast Cancer Models

SIMPLE SUMMARY: The cyclin E/CDK2 complex may present a promising target axis for the treatment of triple-negative breast cancers (TNBC); however, therapeutically relevant doses of CDK2 inhibitors have been associated with toxicities. Here, we report that the suboptimal dosing of the CDK 2, 7 and 9 inhibitor SNS-032 reduced the viability of TNBC cells and upregulated the checkpoint ligand PD-L1 expression in surviving cancer cells in vitro and in human orthotopic MDA-MB-231 TNBC xenografts grown in immunodeficient mice. Moreover, in immunodeficient, TNBC xenograft-bearing mice engrafted with human immune cells, SNS-032 treatment was associated with the infiltration of CD45(+) human immune cells in tumors. In these orthotopic MDA-MB-231 TNBC-bearing mice, suboptimal SNS-032 doses given sequentially ahead of dosing with the anti-PD-L1 antibody avelumab significantly restricted tumor growth compared with monotherapy. These findings suggest that surviving cancer cells following suboptimal CDK inhibitor treatment may be responsive to checkpoint immunotherapy. ABSTRACT: Triple-negative breast cancers (TNBC) expressing PD-L1 qualify for checkpoint inhibitor immunotherapy. Cyclin E/CDK2 is a potential target axis in TNBC; however, small-molecule drugs at efficacious doses may be associated with toxicity, and treatment alongside immunotherapy requires investigation. We evaluated CDK inhibition at suboptimal levels and its anti-tumor and immunomodulatory effects. Transcriptomic analyses of primary breast cancers confirmed higher cyclin E/CDK2 expression in TNBC compared with non-TNBC. Out of the three CDK2-targeting inhibitors tested, the CDK 2, 7 and 9 inhibitor SNS-032 was the most potent in reducing TNBC cell viability and exerted cytotoxicity against all eight TNBC cell lines evaluated in vitro. Suboptimal SNS-032 dosing elevated cell surface PD-L1 expression in surviving TNBC cells. In mice engrafted with human immune cells and challenged with human MDA-MB-231 TNBC xenografts in mammary fat pads, suboptimal SNS-032 dosing partially restricted tumor growth, enhanced the tumor infiltration of human CD45(+) immune cells and elevated cell surface PD-L1 expression in surviving cancer cells. In tumor-bearing mice engrafted with human immune cells, the anti-PD-L1 antibody avelumab, given sequentially following suboptimal SNS-032 dosing, reduced tumor growth compared with SNS-032 alone or with avelumab without prior SNS-032 priming. CDK inhibition at suboptimal doses promotes immune cell recruitment to tumors, PD-L1 expression by surviving TNBC cells and may complement immunotherapy.