Abstract 1301: Evolution of heterogenous BRAFi/MEKi+CDK4/6i resistance mechanisms and targeting site-specific resistant populations in metastatic cutaneous melanoma

Intrapatient heterogeneity among different metastatic sites contributes to targeted therapy resistance. Determining the proteomic profile of metastatic melanoma lesions that develop within the same patient could help elucidate adaptive mechanisms that drive tumor progression during therapy. Tumors that persist through treatment and colonize distant organs adapt to and rely on organ-specific microenvironments. To assess how protein expression is altered in cutaneous melanoma derived from different metastatic sites in a single patient, we utilized patient-derived BRAFi/MEKi and BRAFi/MEKi+CDK4/6i-treated samples from the LOGIC2 clinical trial. Eight cell lines were established from patient biopsies at chronological time points throughout treatment and subsequent post-mortem resection of the bone, ovary, and brain metastatic sites. While the initial metastatic tumor in the breast was BRAFV600E mutant, NRASQ61 mutations were acquired after treatment with CDK4/6i in the bone and ovary tumors. RPPA was performed on six of the cell lines to characterize site-specific differences in protein expression. We detected upregulation of ErbB3 in the brain metastatic tumor. Since neuregulin 1, a ligand for ErbB3, is abundant in brain tissue, we tested its ability to promote resistance to BRAFi/MEKi in melanoma brain metastases. Neuregulin 1 rescued the growth of brain metastasis cells and restored phospho-AKT and phospho-ERK1/2 levels in the presence of BRAFi/MEKi. Therefore, the ErbB3-Neuregulin axis may be targeted, for example by using ErbB3 inhibitors to improve response to MAPK inhibitors in melanoma brain metastases. Further studies aim to investigate the evolution of metastatic cutaneous melanoma longitudinally in response to targeted therapies. Site-specific signaling mechanisms will be functionally examined as possible avenues for metastasis-specific targeted therapies to improve patient outcomes. Citation Format: Haley P. Wilson, Timothy J. Purwin, Claudia Capparelli, Phil F. Cheng, Mitch P. Levesque, Reinhard Dummer, Jessica Teh, Andrew E. Aplin. Evolution of heterogenous BRAFi/MEKi+CDK4/6i resistance mechanisms and targeting site-specific resistant populations in metastatic cutaneous melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1301.