Despite novel therapies for melanoma, drug resistance remains a significant hurdle to achieving optimal responses.,NRAS‐mutant melanoma is an archetype of therapeutic challenges in the field, which we used to test drug combinations to avert drug resistance.,We show that BET proteins are overexpressed in NRAS‐mutant melanoma and that high levels of the BET family member BRD4 are associated with poor patient survival.,Combining BET and MEK inhibitors synergistically curbed the growth of NRAS‐mutant melanoma and prolonged the survival of mice bearing tumors refractory to MAPK inhibitors and immunotherapy.,Transcriptomic and proteomic analysis revealed that combining BET and MEK inhibitors mitigates a MAPK and checkpoint inhibitor resistance transcriptional signature, downregulates the transcription factor TCF19, and induces apoptosis.,Our studies demonstrate that co‐targeting MEK and BET can offset therapy resistance, offering a salvage strategy for melanomas with no other therapeutic options, and possibly other treatment‐resistant tumor types.
The therapeutic landscape of melanoma is improving rapidly.,Targeted inhibitors show promising results, but drug resistance often limits durable clinical responses.,There is a need for in vivo systems that allow for mechanistic drug resistance studies and (combinatorial) treatment optimization.,Therefore, we established a large collection of patient-derived xenografts (PDXs), derived from BRAFV600E, NRASQ61, or BRAFWT/NRASWT melanoma metastases prior to treatment with BRAF inhibitor and after resistance had occurred.,Taking advantage of PDXs as a limitless source, we screened tumor lysates for resistance mechanisms.,We identified a BRAFV600E protein harboring a kinase domain duplication (BRAFV600E/DK) in ∼10% of the cases, both in PDXs and in an independent patient cohort.,While BRAFV600E/DK depletion restored sensitivity to BRAF inhibition, a pan-RAF dimerization inhibitor effectively eliminated BRAFV600E/DK-expressing cells.,These results illustrate the utility of this PDX platform and warrant clinical validation of BRAF dimerization inhibitors for this group of melanoma patients.,•Patient-derived xenograft (PDX) platform comprises 89 metastatic melanoma tumors•Platform includes several pre-vemurafenib and vemurafenib-resistant PDXs•Duplication of the BRAFV600E kinase domain is identified as a resistance mechanism•Pan-RAF dimerization inhibitor LY3009120 eliminates melanoma cells with this duplication,Patient-derived xenograft (PDX) platform comprises 89 metastatic melanoma tumors,Platform includes several pre-vemurafenib and vemurafenib-resistant PDXs,Duplication of the BRAFV600E kinase domain is identified as a resistance mechanism,Pan-RAF dimerization inhibitor LY3009120 eliminates melanoma cells with this duplication,Kemper et al. have built a platform composed of 89 metastatic melanoma xenografts.,Using this collection as a resource, they identified a BRAFV600E protein harboring a duplicated kinase domain.,A pan-RAF dimerization inhibitor suppresses expansion of PDXs expressing this BRAF mutant.