CDK4 and breast cancer: Indeed, combination of CDK4/CDK6 inhibition with the HSP90 inhibitor, Pimitespib has demonstrated additive anti-cancer effect in several solid tumor preclinical models, leading to the development of a Phase I trial of Palbociclib and Pimitespib in advanced breast cancer progressing on Palbociclib alone and other treatment-refractory solid tumors [50].