PALBOCIB-125 Capsules PALBOCIB-125 contains Palbociclib, a selective inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/6). It works by blocking the phosphorylation of retinoblastoma (Rb) protein, arresting the G1-to-S phase cell cycle progression and inhibiting tumor cell proliferation. It is indicated for the treatment of hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced or metastatic breast cancer, used in combination with an aromatase inhibitor as initial endocrine-based therapy or with fulvestrant in disease progression.