abemaciclib + letrozole + nivolumab

This is a combination therapy consisting of three agents: - **Abemaciclib** is an oral, small molecule inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/6), which blocks cell cycle progression in cancer cells. - **Letrozole** is a nonsteroidal aromatase inhibitor that reduces estrogen production, thereby inhibiting the growth of hormone receptor-positive breast cancer cells. - **Nivolumab** is a monoclonal antibody immune checkpoint inhibitor targeting programmed cell death protein 1 (PD-1), enhancing T-cell mediated anti-tumor immunity. This combination has been studied as first-line or second-line treatment for hormone receptor-positive, HER2-negative metastatic breast cancer. The rationale for combining these agents stems from preclinical evidence suggesting synergy between CDK4/6 inhibition and immune checkpoint blockade. However, clinical studies have reported that while the regimen shows antitumor activity, it also results in severe and prolonged immune-related adverse events likely due to enhanced proinflammatory cytokine production and regulatory T cell suppression[1][2].