NK cell therapy enhances the efficacy of CDK4/6i in breast cancer

Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) are routinely used for patients with estrogen receptor-positive metastatic breast cancer. Abemaciclib is a CDK4/6 inhibitor that blocks cancer cell proliferation by preventing progression through the cell cycle. However, these drugs primarily inhibit cancer cell growth and do not typically induce complete tumor elimination. Combining CDK4/6i with a synergistic therapeutic modality could enable regression of metastatic breast cancer tumors and dramatically improve patients' outcomes. Adoptive cell therapy is a type of cancer therapy where effector immune cells, such as T and NK cells, are transferred into the patient's body to eliminate malignant cells. We found that CDK4/6i-treated tumor cells secrete NK cell-attracting chemokines and express stress ligands on their surface, which are known to trigger NK cell cytotoxic activity. This indicates that pre-CDK4/6i treatment increases tumor vulnerability to NK cells. Therefore, we hypothesized that CDK4/6i efficacy could be improved by combining it with NK cell therapy, resulting in the elimination of tumor cells. Methods: We used human MCF7 and mouse PYMT-B6 cell line to evaluate the effect of CDK4/6 inhibition on chemokine production in tumors and NK cell migration toward the tumor. We used spectral cytometry to test the surface expression of NK cell-activating stress ligands on breast cancer cells and patient-derived organoids (PDO). PDOs were co-cultured with NK cells to investigate patient-to-patient differences in CDK4/6 inhibitor-mediated NK cell recruitment and tumor recognition. We performed patient-derived xenograft (PDX) implantation in huIL-15 mouse to test Abema and NK combination treatment in vivo . Results and conclusions: We found that abemaciclib induced the expression of NK cell-attracting chemokines, including CCL5 and CXCL10, by tumor cells. We also found that therapeutic NK cells expressed the corresponding chemokine receptors, suggesting enhanced chemotactic potential. Pre-treatment of tumor cells with abemaciclib significantly increased the expression of stress ligands on the surface of breast cancer cells and PDOs. This stress ligands are recognized by NKG2D receptors on the NK cells that trigger their cytotoxicity. In NK&PDO co-culture assay, pre-treatment with CDK4/6i promoted the killing and infiltration of human NK cells to PDOs. Additionally, NK cells co-cultured with pre-treated tumor cells displaced activated functional phenotype with increased cytotoxicity and cytokine secretion. We conducted patient-derived xenograft (PDX) implantation in mice and observed slower tumor growth and improved survival following treatment with CDK4/6 inhibitors and NK cell infusion. Significance: This research can provide a pre-clinical basis for the further development of CDK4/6i and immunotherapy combinations that could improve the outcomes of metastatic breast cancer treatment.