TBCRC 035: randomized phase II pharmacodynamic study of standard and reduced-dose palbociclib with endocrine therapy in hormone receptor (HR)-positive previously treated metastatic breast cancer

Palbociclib 100 mg was associated with a reduced incidence of ≥grade 3 neutropenia. While PFS was numerically lower at 100 mg, the difference was not statistically significant and was limited by sample size and complicated by varying degrees of pre-treatment. Analysis of skin and tumor pRb or Ki-67 demonstrated robust molecular response at both doses. PIK3CA and TP53 mutations and higher baseline Ki-67 were associated with inferior clinical outcome.