Abemaciclib metabolite M20 is primarily involved in the pharmacological activity of abemaciclib, a CDK4/6 inhibitor used in cancer therapy. This metabolite retains significant inhibitory activity against CDK4 and CDK6, contributing to the overall antitumor effect observed in the treatment of hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) breast cancer. Due to its ability to cross the blood-brain barrier, M20 may play a role in controlling central nervous system metastases, which are challenging to treat with many conventional therapies. Its sustained activity supports the continuous dosing regimen of abemaciclib, enhancing suppression of tumor cell proliferation. The presence and activity of M20 are considered in pharmacokinetic and pharmacodynamic assessments during clinical use and drug development.