The overall objective of this study is to determine the therapeutic efficacy of concomitant inhibition of PLK1 and NOTCH against melanoma progression and drug resistance as well as to identify novel signaling mechanisms associated with drug response using two human-relevant melanoma mouse models. The available therapeutic strategies against melanoma have either failed to achieve >25% response in patients, or the responses are short-lived with developing resistance to therapy. For example, BRAF inhibitors Vemurafenib and Dabrafenib were found to achieve significant improvement over chemotherapy and were FDA-approved for melanomas with BRAF-mutations. Even with a combination of Dabrafenib with MEK inhibitor Trametinib (also FDA approved), the patients develop acquired resistance. More recently advancements in immunotherapy have improved melanoma treatment outcomes. Despite the success of immune checkpoint inhibitors durable responses are not seen in all patients due to drug resistance. Therefore, novel mechanism- based combinatorial approaches are needed for an effective management of this neoplasm. Polo-like kinase 1 (PLK1) is a serine/threonine protein kinase that plays a key role in cell proliferation. We have previously reported that PLK1 is significantly overexpressed in melanoma and can be therapeutically targeted. Further, the NOTCH pathway, an evolutionally conserved pathway, which plays important roles in cell fate determination, proliferation, differentiation and survival, has been shown to regulate many aspects of melanomagenesis. NOTCH1 is considered a primary oncogenic factor in melanoma and activation of NOTCH1 and its target genes is linked with metastatic melanoma. Moreover, inhibition of PLK1 or NOTCH has been shown to modulate markers of epithelial mesenchymal transition (EMT) and metastasis. Interestingly, both PLK1 and NOTCH are also linked with drug resistance. Our recently published data suggest that PLK1 and NOTCH expressions have significant positive correlation in melanoma clinical tissues and simultaneous small molecule inhibition of PLK1 and NOTCH by BI 6727 (specific inhibitors of PLK1) and MK-0752 (γ-secretase inhibitor), respectively, caused a significant anti-proliferative response in multiple melanoma cell lines, warranting further pre-clinical testing in in vivo melanoma models. This data together with other published studies provide a strong scientific premise for our proposed hypothesis that combined inhibition of PLK1 and NOTCH will be therapeutically superior for the management of melanoma. We will challenge this hypothesis in two specific aims. In Aim 1, we will determine the in vivo therapeutic efficacy and mechanism of concomitant inhibition of PLK1 (by BI 6727 or PCM-075) and NOTCH (by MK-0752) on melanoma progression and metastasis in genetically engineered Braf-Pten melanoma mouse model, which recapitulates human disease progression from localized to metastatic disease. In Aim 2, we will determine therape...