Background: Among mechanisms of resistance to BRAF inhibitors (BRAFi), cell cycle effectors including CDK4 have been involved in ERK reactivation. In this phase I-II open label study, we aimed to establish the Maximum Tolerated Dose (MTD) of PD0332991, an inhibitor of CDK4/6, when added to vemurafenib (VM) in metastatic melanoma patients. Methods: Patients with BRAF^V600E/K mutated metastatic melanoma harbouring CDKN2A loss and RB1 expression were included. Patients were treated with a 14 days followed by 7 days rest daily dosing schedule of PD0332991 + continuous BID dosing of VM, and stratified into 2 groups according to previous BRAFi treatment (no group 1, yes group 2). Dose levels (PD0332991 (mg/QD)/VM (mg/BID) ranged from 25/720 to 200/960. The primary endpoint was the occurrence of a DLT within the first 2 cycles of therapy. Secondary endpoints included best response (RECIST), OS, PFS, pharmacokinetics parameters, tumour molecular profiling on baseline lesions using transcriptomic and NGS analysis. Results: Nineteen patients were enrolled, among them 16 (84%) in group 2, with 18.5 months median follow-up. Characteristics at baseline were: male 11 (58%), median age 54.4 years, unresectable stage IIIC 2 (11%), stage IV 17 (89%), M1C 12 (67%), high LDH 9 (47%), median time from advanced melanoma diagnosis to inclusion 26.8 months, ≥ 2 lines therapy 13 (68%). A DLT was observed for 1 and 5 patients in group 1 and 2 respectively, defining the MTD at PD0332991 25mg and VM 960mg in group 2. No significant evidence for drug-drug interaction between PD0332991 and VM was highlighted. In group 2, ORR was estimated to 4 (25%), SD to 8 (50%), median PFS to 9.3 months and median OS to 13.2 months. Baseline transcriptomic analysis revealed high alteration rate associated with clinical response and enrichment in genes related to MAPK, cell cycle and apoptosis pathways. Conclusions: While combination of fixed dose of PD0332991 + VM did not allow us to increase PD0332991 dosage above 25mg, significant clinical benefit was achieved in heavily pretreated patients; baseline molecular analysis revealed an association between transcriptomic data and clinical response. Clinical trial information: NCT02202200