Background: Targeted therapies based on tumor molecular markers are not currently used in mHSPC. Palbociclib, a CDK4/6 inhibitor, blocked proliferation and promoted G1 arrest in a Rb-and Cyclin D-dependent manner in preclinical models of HSPC. We hypothesized that co-targeting AR (ADT) and cell cycle (palbociclib) would improve outcomes in mHSPC pts. Methods: mHSPC pts with Rb intact tumors based on IHC of metastatic tumor 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 was confirmed PSA RR (≤ 4 ng/mL) after 28 weeks of therapy. Secondary endpoints included safety/tolerability, PFS, PSA and radiographic RR. Metastatic biopsy and primary tumors were subjected to whole exome and transcriptomic sequencing where available. CTC’s were enumerated at various time points. Results: 72 eligible pts (median age 67 years, PSA 73ng/mL) with newly diagnosed mHSPC were registered and underwent biopsy. 97% retained RB expression (IHC). 62 pts were stratified by disease extent and early initiation of ADT, and randomized. 60 pts initiated therapy (Arm A: 20; Arm B: 40). Adverse events were reported previously. 80% of pts (Arm A: 16/20, Arm B: 32/40; p = 0.87) on both arms met primary PSA endpoint (≤4ng/mL at 28 weeks). PSA undetectable rate at 28 weeks was Arm A: 50% (10/20) and Arm B: 43% (17/40; p = 0.5). Measurable disease RR: Arm A: 89% and Arm B: 89%. 12-month biochemical PFS was Arm A 69% (95%CI: 44-85%), Arm B 74% (95%CI: 57-85%). 41 patients on trial underwent sequencing of metastatic biopsy and 10 patients had matched primary prostate tumor sequencing results. CCND1 amp, 8q gain, PTEN and KMT2C mutations were each observed in metastatic, but not paired prostate primary tumors. TP53, PIK3 pathway (PIK3CA, AKT1, PTEN) mutations and 8q gains were associated with reduced PSA PFS [HR (95%CI): 3.0 (1.2-7.2), p = 0.018; 3.2(1.03-10),p = 0.044; 4.96 (1.8-12), p = 0.001, respectively]. Pretreatment CTCs were associated with lower PSA CR (p = 0.04) and shorter PFS (12-month PFS: 58% vs. 86%, p = 0.031). Conclusions: A tissue based biomarker preselected trial is feasible in mHSPC. ADT + palbociclib did not impact outcomes. Pretreatment CTC counts, TP53 and PIK3 pathway mutations, and 8q gain may offer prognostic value in mHSPC. Support: Movember-PCF Challenge Award, Pfizer. Clinical trial information: NCT02059213.