1248P - Tumor-stroma interactions as a determinant of drug resistance in BRAF-mut melanoma

Abstract

Background: In BRAF-mut melanoma combined BRAF/MEK inhibition increases survival; however, pharmacological effects on the genetically “normal” tumor microenvironment (i.e. paradox MAPK activation) may set the stage for the development of drug resistance. Methods: GFP-labeled cutaneous fibroblasts (HFF) were co-cultured with melanoma cells, in the presence or absence of direct cell-cell contact, and response to Dabrafenib (D) and Trametinib (T), alone or combined, was monitored over time. SEMA6A and AXL were preliminarily evaluated as potential mediators of such interactions. Results: HFF significantly protected (60-100% protection at the lowest two drug concentrations) BRAF-mut M14 melanoma cells from the growth inhibitory activity of D and T, alone or combined; however, combined D+T at the highest concentrations overcame stroma-mediated protection and eliminated both cell populations. Thus, combined BRAF/MEK inhibition resulted in strongly synergistic interactions, as compared to single agent treatments, only under co-culture conditions (CI 0.6 and 0.2 for M14 and HFF cells, respectively). Protective melanoma/stroma interactions were mediated by direct cell-cell contact, as co-cultures in trans-well Boyden chambers or isolated cultures using conditioned medium (HFF-conditioned medium for M14; M14-conditioned medium for HFF), did not affect pharmacological response. As SEMA6A expression is tightly controlled by MAPK and AXL mediates resistance to MAPK inhibition in melanoma, we assessed their potential as mediators of stroma-mediated melanoma protection: interestingly, SEMA6A and AXL expression in a panel of melanoma cell lines were inversely correlated; moreover, in cell lines derived by primary and cutaneous metastases of the same patient, AXL expression was upregulated at the mRNA and protein level in cells derived from metastatic lesions. Conclusions: Tumor-stroma interactions protect BRAF-mut melanoma from MAPK inhibition; such functional protection is mediated by cell-cell contact. SEMA6A and AXL are possible mediators of this interaction and their reciprocal relationships are being studied in melanoma cell line models and clinical series. Legal entity responsible for the study: Regina Elena Cancer Institute- San Gallicano Dermatologic Institute Funding: AIRC (18622-14362-9979) Disclosure: All authors have declared no conflicts of interest.