Targeting CDK4 and CDK6 in cancer

Cellular senescence is a cell state implicated in various physiological processes and a wide spectrum of age-related diseases. Recently, interest in therapeutically targeting senescence to improve healthy aging and age-related disease, otherwise known as senotherapy, has been growing rapidly. Thus, the accurate detection of senescent cells, especially in vivo, is essential. Here, we present a consensus from the International Cell Senescence Association (ICSA), defining and discussing key cellular and molecular features of senescence and offering recommendations on how to use them as biomarkers. We also present a resource tool to facilitate the identification of genes linked with senescence, SeneQuest (available at http://Senequest.net). Lastly, we propose an algorithm to accurately assess and quantify senescence, both in cultured cells and in vivo.

Background A phase 2 study showed that progression-free survival was longer with palbociclib plus letrozole than with letrozole alone in the initial treatment of postmenopausal women with estrogen-receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer. We performed a phase 3 study that was designed to confirm and expand the efficacy and safety data for palbociclib plus letrozole for this indication. Methods In this double-blind study, we randomly assigned, in a 2:1 ratio, 666 postmenopausal women with ER-positive, HER2-negative breast cancer, who had not had prior treatment for advanced disease, to receive palbociclib plus letrozole or placebo plus letrozole. The primary end point was progression-free survival, as assessed by the investigators; secondary end points were overall survival, objective response, clinical benefit response, patient-reported outcomes, pharmacokinetic effects, and safety. Results The median progression-free survival was 24.8 months (95% confidence interval [CI], 22.1 to not estimable) in the palbociclib-letrozole group, as compared with 14.5 months (95% CI, 12.9 to 17.1) in the placebo-letrozole group (hazard ratio for disease progression or death, 0.58; 95% CI, 0.46 to 0.72; P<0.001). The most common grade 3 or 4 adverse events were neutropenia (occurring in 66.4% of the patients in the palbociclib-letrozole group vs. 1.4% in the placebo-letrozole group), leukopenia (24.8% vs. 0%), anemia (5.4% vs. 1.8%), and fatigue (1.8% vs. 0.5%). Febrile neutropenia was reported in 1.8% of patients in the palbociclib-letrozole group and in none of the patients in the placebo-letrozole group. Permanent discontinuation of any study treatment as a result of adverse events occurred in 43 patients (9.7%) in the palbociclib-letrozole group and in 13 patients (5.9%) in the placebo-letrozole group. Conclusions Among patients with previously untreated ER-positive, HER2-negative advanced breast cancer, palbociclib combined with letrozole resulted in significantly longer progression-free survival than that with letrozole alone, although the rates of myelotoxic effects were higher with palbociclib-letrozole. (Funded by Pfizer; PALOMA-2 ClinicalTrials.gov number, NCT01740427 .).

Cyclin-dependent kinases 4 and 6 (CDK4/6) are fundamental drivers of the cell cycle and are required for the initiation and progression of various malignancies 1,2 . Pharmacologic inhibitors of CDK4/6 have shown significant activity against several solid tumors 3,4 . Their primary mechanism of action is thought to be the inhibition of phosphorylation of the retinoblastoma (RB) tumor suppressor, inducing G1 cell cycle arrest in tumor cells 5 . Here, we use murine models of breast carcinoma and other solid tumors to show that selective CDK4/6 inhibitors not only induce tumor cell cycle arrest, but also promote anti-tumor immunity. We confirm this phenomenon through transcriptomic analysis of serial biopsies from a clinical trial of CDK4/6 inhibitor treatment for breast cancer. The enhanced anti-tumor immune response has two underpinnings. First, CDK4/6 inhibitors activate tumor cell expression of endogenous retroviral elements, thus increasing intracellular levels of double-stranded RNA. This in turn stimulates production of type III interferons and hence enhances tumor antigen presentation. Second, CDK4/6 inhibitors markedly suppress the proliferation of regulatory T cells (Tregs). Mechanistically, the effects of CDK4/6 inhibitors on both tumor cells and Tregs are associated with reduced activity of the E2F target, DNA methyltransferase 1. Ultimately, these events promote cytotoxic T cell-mediated clearance of tumor cells, which is further enhanced by the addition of immune checkpoint blockade. Our findings indicate that CDK4/6 inhibitors increase tumor immunogenicity and provide rationale for new combination regimens comprising CDK4/6 inhibitors and immunotherapies as anti-cancer treatment.