LAUSR

251Background: Palbociclib, a CDK4/6 inhibitor, blocked proliferation and promoted G1 arrest in an Rb and Cyclin D dependent manner in preclinical models of HSPC. Alterations in this pathway contribute to the development of CRPC. We hypothesized that co-targeting AR (ADT) and cell cycle (palbociclib) would improve outcomes including PSA RR at 28 weeks in mHSPC pts. Methods: mHSPC pts with Rb intact tumors based on IHC of metastatic biopsy were stratified and randomized (1:2) to Arm A: ADT or Arm B: ADT+ palbociclib (125mg 3 weeks on, 1 week off). Primary endpoint is confirmed PSA RR (≤ 4 ng/mL) after 28 weeks of therapy. With 20 patients randomized to ADT and 40 to ADT + palbociclib there is a 64.2% power to detect a 20% difference in proportions with a one-sided type I error of 0.10 using the mid p-value method of the Fisher’s exact test. Secondary endpoints include safety/tolerability, biochemical and clinical PFS, PSA and radiographic RR and exploratory biomarkers (circulating DNA, tumor cells and tran...