Background: Most patients (pts) with mHSPC treated with androgen deprivation therapy (ADT) will progress to castration resistance. Androgens drive PC cell proliferation via upregulation of cyclin D which complexes with the cyclin-dependent protein kinases 4/6 (CDK4/6) resulting in phosphorylation of retinoblastoma (Rb) tumor suppressor protein and G1/S progression. Alterations in this pathway contribute to progression to castration-resistance (CR). In preclinical PC models, palbociclib, a novel specific CDK4/ 6 inhibitor, blocked proliferation and promoted G1 arrest in an Rb and Cyclin D dependent manner. Rb is lost in <10% of localized PC, but >30% of CR disease. Rb loss in mHSPC is estimated to be 10-20%. Based on this preclinical data, we designed a randomized phase II clinical trial (NCT02059213) to test the hypothesis that palbociclib + ADT in newly metastatic Rb-positive PC pts will significantly increase ADT efficacy. Methods: mHSPC pts with Rb intact tumors based on metastatic biopsy are stratified by disease extent and randomized (1:2) to ADT or ADT+ palbociclib. Primary endpoint is confirmed prostate specific antigen (PSA) response (≤ 4 ng/mL) after 28 weeks of therapy. With 20 patients randomized to ADT and 40 to ADT + palbociclib there will be 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: safety and tolerability, rate of undetectable PSA (≤ 0.2ng/mL), biochemical and clinical PFS, overall PSA and radiographic RR, therapeutic biomarkers (circulating DNA and tumor cells, tumor protein and transcriptome) and to establish a repository of mHSPC samples. Results: Since July 2014, 68 patients have been registered and underwent tumor metastatic disease biopsy (36 soft tissue, 32 bone). 60/68 (88%) had adequate tissue for Rb assessment. 58/60 (97%) tumors were Rb positive (IHC). To date, 58/60 patients have been randomized to ADT (19 patients) or ADT + palbociclib (39 patients). Conclusions: Biopsy driven studies are feasible in the mHSPC population. Rb expression was present in 58/60 (97%) patients. This study is ongoing. Support: Movember-PCF Challenge Award, Pfizer. Clinical trial information: NCT02059213