palbociclib + topotecan + cyclophosphamide

**Palbociclib + topotecan + cyclophosphamide** is a triple combination regimen investigated in pediatric and young adult patients with recurrent or refractory solid tumors, notably neuroblastoma. Palbociclib is a small molecule, oral, highly selective inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/6), which play a role in cell cycle progression from G1 to S phase. Topotecan is a topoisomerase I inhibitor (a cytotoxic agent interfering with DNA replication and transcription), and cyclophosphamide is an alkylating agent that crosslinks DNA, leading to cell death. The combination is studied to assess antitumor activity, particularly to enhance efficacy beyond that seen with standard chemotherapy regimens. Most current research is pre-approval/experimental, sponsored primarily by Pfizer, and includes phase 1/2 trials in relapsed/refractory pediatric solid tumors—especially neuroblastoma and Ewing sarcoma[2][4][5][6][7][10].