Background: CDK4/6 inhibitors (CDKi) have improved disease control in hormone-receptor-positive, HER2-negative metastatic breast cancer, but most patients develop progressive disease. Methods: We asked whether host stromal senescence after CDK4/6 inhibition affects metastatic seeding and growth of CDKi-resistant mammary cancer cells by using the p16-INK-ATTAC mouse model of inducible senolysis. Results: Palbociclib pretreatment of naïve mice increased lung seeding of CDKi-resistant syngeneic mammary cancer cells, and this effect was reversed by depletion of host senescent cells. RNA sequencing analyses of lungs from non-tumor-bearing p16-INK-ATTAC mice identified that palbociclib downregulates immune-related gene sets and gene expression related to leukocyte migration. Concomitant senolysis reversed a portion of these effects, including pathway-level enrichment of TGF-β- and senescence-related signaling. CIBERSORTx analysis revealed that palbociclib alters intra-lung macrophage/monocyte populations. Notably, lung metastases from palbociclib-pretreated mice revealed senescent endothelial cells. Palbociclib-treated endothelial cells exhibit hallmark senescent features in vitro, upregulate genes involved with the senescence-associated secretory phenotype, leukocyte migration, and TGF-β-mediated paracrine senescence and induce tumor cell migration and monocyte trans-endothelial invasion in co-culture. Conclusions: These studies shed light on how stromal senescence induced by palbociclib affects lung metastasis, and they describe palbociclib-induced gene expression changes in the normal lung and endothelial cell models that correlate with changes in the tumor microenvironment in the lung metastatic niche.

Patients with hormone receptor-positive/HER2-negative (HR+/HER2-) metastatic breast cancer have benefitted from treatment with palbociclib, a cyclin-dependent kinase (CDK) 4/6 inhibitor capable of ...

CDK4/6 inhibitors are used in the treatment of advanced estrogen receptor (ER)(+) breast cancer. Their efficacy in ER(−) and early-stage breast cancer is currently under investigation. Here, we show that palbociclib, a CDK4/6 inhibitor, can inhibit both progression of ductal carcinoma in situ (DCIS) and growth of invasive disease in both an ER(−) basal breast cancer model (MCFDCIS) and an ER(+) luminal model (MCF7 intraductal injection). In MCFDCIS cells, palbociclib repressed cell-cycle gene expression, inhibited proliferation, induced senescence, and normalized tumorspheres formed in Matrigel while the formation of acini by normal mammary epithelial cells (MCF10A) was not affected. Palbociclib treatment of mice with MCFDCIS tumors inhibited their malignant progression and reduced proliferation of invasive lesions. Transcriptomic analysis of the tumor and stromal cell compartments showed that cell cycle and senescence genes, and MUC16, an ovarian cancer biomarker gene, were repressed during treatment. Knockdown of MUC16 in MCFDCIS cells inhibited proliferation of invasive lesions but not progression of DCIS. After cessation of palbociclib treatment genes associated with differentiation, for example, P63, inflammation, IFNγ response, and antigen processing and presentation remained suppressed in the tumor and surrounding stroma. We conclude that palbociclib can prevent progression of DCIS and is antiproliferative in ER(−) invasive disease mediated in part via MUC16. Lasting effects of CDK4/6 inhibition after drug withdrawal on differentiation and the immune response could impact the approach to treatment of early-stage ER(−) breast cancer.

Background. Ductal carcinoma in situ (DCIS) is a non-invasive disease of the breast. This study assessed the impact of the CDK4/6 inhibitor palbociclib on halting cell cycle progression in DCIS between diagnostic biopsy and surgical resection of the lesion. Methods. A dual-arm study was conducted involving 17 participants with DCIS who were given either 100 mg of palbociclib PO daily for twelve days or no treatment before surgery. FFPE tissues from the diagnostic biopsy and from surgical specimens were analyzed to assess changes due to treatment. Results. Palbociclib treatment significantly reduced Ki67 staining in surgical tissues. RNA-seq analysis comparing the diagnostic biopsy and surgical specimens showed significant downregulation of cellular proliferation pathways after treatment. A cellular deconvolution analysis demonstrated a non-significant trend of a treatment-induced decrease in the abundance of mature luminal epithelial cells. Conclusions. Short-term palbociclib monotherapy before surgical resection was safe and well tolerated in participants. Tissue analysis showed a significant reduction in proliferation markers and the downregulation of cell cycle progression genes within DCIS. These results support further investigation of palbociclib in patients with DCIS. Trial registration. ClinicalTrials.gov NCT03535506.