Melanoma is the most aggressive form of skin cancer. Although progress has been made for advanced melanoma patients with 13 new FDA-approved therapies since 2011, resistance arises in most cases. Our focus is on melanomas that harbor activating BRAF mutations (~50% of patients). Most of these patients respond dramatically to combination therapy with a BRAF and MEK inhibitor (BRAFi/MEKi). However, four out of every five patients relapse within two years due to the persistence of therapy-resistant subpopulations of melanoma cells. This expanding BRAFi/MEKi-resistant patient cohort is the greatest challenge of the field; few experience durable benefit from immune therapy and no alternative effective therapies exist. Therefore, there is an unmet need to develop more effective strategies. We have characterized therapy-resistant subpopulations and identified common features; 1) existence prior to therapy, 2) a slow-growing state, 3) high metastatic potential and 4) stem cell-like molecular and biological properties that allow for high adaptability in stressful conditions including therapy. Shared gene signatures by stem cells and melanoma cells are poorly understood. In our initial studies, we identified a developmental receptor, LPAR1, as key for the survival of melanoma and stem cells. LPAR1 increases the proliferation of neuronal stem cells and aggressiveness of breast and lung cancer. We show LPAR1 expression increases with progression in melanoma patient tumor tissue relative to benign nevi. Further, a) LPAR1 expression is higher in BRAFi/MEKi resistant melanoma cells, b) hyperactivation of a down-stream LPAR1 effector, YAP1, increases the presence of resistant stem cell-like melanoma cells, and c) genetic or pharmacological targeting of LPAR1 kills BRAFi/MEKi resistant melanoma cells. This provides strong scientific rationale for investigating LPAR1 as a novel target to overcome BRAFi/MEKi resistance. We propose to validate LPAR1 as a clinically relevant target by using models that closely mimic the in vivo biology of melanoma. This includes 3D human skin-, spheroid-, and a collection of >500 patient-derived xenograft (PDX)-models where patient tumor material is inoculated directly into mice, including >200 patients that relapsed on BRAFi/MEKi. Towards this goal, we will define the molecular consequences of inhibiting LPAR1 on the survival and growth of stem cell-like melanoma cells and in BRAFi/MEKi resistance. We will identify the most potent LPAR1 inhibitor that can synergize with BRAFi/MEKi to eliminate all tumor cells without causing toxicity. As LPAR1 inhibitors are currently being clinically investigated, we expect our proposed studies will provide the scientific rationale to clinically test new therapeutic strategies that will increase the curative potential of BRAFi/MEKi and facilitate the development of future clinical trials.