Trilaciclib

Trilaciclib is inhibits cyclin-dependant kinase 4 (CDK4) at a concentration of 1 nmol/L and cyclin-dependent kinase 5 (CDK5) at 4 nmol/L. Inhibition of CDK2, CDK5, and CDK7 is over 1000-fold less at these concentrations and inhibition of CDK9 is 50-fold less.

CDK4 and CDK5 are expressed in hematopoietic stem cells and progenitor cells. They are capable of phosphorylating and inactivating the retinoblastoma protien; a tumor suppressor. When trilaciclib is given to patients with retinoblastoma protein-null small cell lung cancer, it does not interfere with the intended chemotherapy induced cytotoxicity of cancer cells. Inhibition of CDK4 and CDK5 leads to a reversible pause in the cell cycle in the G1 phase for approximately 16 hours. The temporary cell cycle arrest prevents chemotherapy induced DNA damage in healthy cells, reducing the activity of caspases 3 and 7, which reduces apoptosis of healthy cells.

Other studies have shown inhibitors of CDK4 and CDK6 enhance T-cell activation, upregulating major histocompatibility complex (MHC) class I and II, and stabilize programmed death-ligand 1 (PD-L1). Together these activities increase T-cell activity, increase antigen presentation, and sensitize cells to immune checkpoint inhibitors.