Abstract 4747: Surviving skills in melanoma cells: An important consideration to both targeted and immunological approach in the treatment of melanoma

While immunotherapy with checkpoint inhibitor in the treatment of melanoma patients (pts) can produce about 40-50 % response rate and with durable response, but 50% of pts still do not respond and other form of treatments are needed. On the other hand, combination of BRAF and MEK inhibitor can produce a rapid response in pts whose tumors harbor BRAFV600E or BRAFV600K mutation, but over 70% of pts will relapse with the duration of response of approximately 12 months. Immunotherapy with checkpoint inhibitor in this group of pts often showed a variable rate of response ranging from 15- 35%. The underlying mechanism for the variable response is not known. We investigated the possible contributory factors to this variation. Five pairs of BRAF mutant and their BRAF and MEK resistant variants (BMR): A375/BMR, A2058/BMR, Mel1220/BMR, SK28/BMR, UACC2/BMR and one pair of A2058v/ BMR were used for the study. A2058v expressed argininosuccinate synthetase and can survive/proliferate in arginine-free media with citrulline supplement. All BMR cell lines were generated from stepwise increasing dosage of BRAF and MEK inhibitor similar to our previous publication in generating BRAF resistant cells (Oncotarget 2016:17665). All BMR cell lines exhibit 3 fold resistance to BRAF inhibitor and 1.8 fold to MEK inhibitor. This panel of cell lines was used to analyze for PD-L1, BCL-2, BCL-XL, MCL1, and NOXA, and AKT/MEK/ERK activation. Our results showed that three cell lines MEL1220, SKmel28 and UACC62 which have high levels of BCL2, do not have high PD-L1 levels both in parental cells and their BMR variants. In contrast, both A375 BMR and A2058BMR exhibit high PD-L1 ( >10 fold). The highest level (>50 fold) was found in A2058v and its BMR variant. All BMR cells have more AKT/MEK/ERK activation. Other anti- and pro-apoptotic expression do not differ significantly. These initial results suggest that the presence of antiapoptotic protein BCL2 may mitigate the necessity to express high PD-L1 and hence lessen the response to checkpoint inhibitor therapy. To further clarify this possibility, we have studied a panel of primary culture from melanoma pts, both treatment naive and those who failed BRAF/MEK inhibitor, as well as BRAF wild type pts . Eight BRAF mutant tumors and seven BRAF wild type were used. Similar correlation existed for both BRAF mutant and wild type primary culture. However, in BRAF wild type pts, two primary culture with high BCL2 still express low levels of PD-L1. Our data suggest that BcL-2 expression and possible other pro-and antiapoptotic proteins may be an additional useful parameter to predict response to checkpoint inhibitor therapy especially when they failed BRAF/MEK inhibitor. Combination with other inhibitors in the apoptotic pathway such as BCl-2 inhibitor may be also useful to improve the therapeutic outcome in these BMR pts. Supported by the VA Merit Review Award(1BX003328 to N. Savaraj) Citation Format: Niramol Savaraj, Ying-Ying Li, Chunjing Wu, Medhi Wangpaichitr, Seth Spector, Michael Sentome, Lynn G. Feun. Surviving skills in melanoma cells: An important consideration to both targeted and immunological approach in the treatment of melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4747.