Background: Palbociclib is a CDK4/6 inhibitor used to treat metastatic hormone receptor-positive (HR+) breast cancer (MBC) in combination with endocrine therapy. Tamoxifen is an effective treatment for HR+ MBC, with different toxicity profile compared with aromatase inhibitors (AI) and fulvestrant. Preclinical data demonstrated synergy for the combination of tamoxifen and palbociclib, being effective in a model of acquired tamoxifen resistance. Methods: We conducted an open-label, single-arm, multicenter phase II trial of palbociclib in combination with tamoxifen in patients with HR+/HER2 - advanced BC, with no prior therapy for MBC. Ovarian suppression was recommended for pre-menopausal women, but not required. Primary objective was progression free survival. Secondary objectives: objective response rate (CR or PR) based on RECIST 1.1 or MDA Criteria (for patients with bone only disease); safety and tolerability (using CTCAE v4); clinical benefit rate (CR, PR or SD lasting min 24 weeks); 2-year overall survival. Correlative objectives: proteomic analysis of plasma exosomes to identify mechanisms of primary and secondary resistance to tamoxifen/palbociclib. Results: Between 6/30/2016 and 7/02/2019, we enrolled 49 patients (47 evaluable): 23 pts with de-novo metastatic disease and 24 pts with recurrent BC (12 pts were on adjuvant treatment with AI at time of recurrence and 12 pts on surveillance). As of 1/5/21 data cut-off, 7 pts were still on treatment. Median follow-up time was 24 months (range 8-42). Median age was 60 (range 39-82). The median PFS was 14.6 months with 95% CI (7-41) for pts with de-novo MBC and 6 months (2-12) for pts with recurrent BC. The ORR was 30% overall, 39% for pts with de novo MBC, 21% for pts with recurrent BC. CBR was 64% overall, 78% for pts with de novo MBC and 50% for pts with recurrent BC. CBR was 65% for white pts and 55% for African American pts. Best response per RECIST1.1: 14 pts (34%) had PR, 18 pts (44%) had SD, 9 pts (22%) had PD. All 6 pts with bone only disease had SD. The most common drug related grade ≥ 3 AE was neutropenia (51%), transient and manageable by dose modifications, no cases of febrile neutropenia. Four patients developed thromboembolic events (1 grade 2, 2 grade 3, 1 grade 4). One patient died while on treatment from PD. Conclusions: The combination of palbociclib and tamoxifen showed tolerable, expected safety profile. This may be an alternative approach for selected patients in first line treatment of HR+ MBC, especially those who are intolerant to AI, although this small study indicates a lower PFS. Clinical trial information: 02668666.