Racha Cherradi

Background: Melanoma comprises approximately 5% of cancer incidence worldwide, with a 23% higher relative risk in individuals with BRAF mutations compared to those with wild-type BRAF¹. Genetic mutations in genes such as BRAF, CDKN2A, and CDK4 are implicated in melanoma, with BRAF mutations present in around 50% of cutaneous melanoma cases². Current treatments for BRAF mutant melanoma include BRAF/MEK inhibitors, which initially offer significant tumor shrinkage and enhanced survival³. However, the majority of patients experience relapse within a year of treatment due to frequent resistance development³, often due to reactivation of the PI3K/AKT pathway⁴. An alternative solution is to target the mTOR pathway alongside the BRAF/MEK pathway. In this experimental analysis of pharmacological therapies, rapamycin (mTORC1 inhibitor) and  NVP-BEZ235 (dual PI3K and mTOR inhibitor) were tested in combination with PLX (BRAF inhibitor) and PD (MEK inhibitor) to assess their efficacy in overcoming resistance in BRAF mutant melanoma cells⁵.

Objective: In this review, we explore the role of mTOR pathway inhibitors in combatting pharmacological resistance development in BRAF mutant melanoma.

Methods:  An online search in the PubMed database was conducted from 2018 to 2023 using the following keywords: “melanoma”, ” BRAF”, and ” resistance”.

Results: The effects of mTOR/PI3K inhibitors on the proliferation of combination-resistant BRAF mutant melanoma cells (CR cells) were evaluated using a 5-ethynyl-2′-deoxyuridine (EdU) cell proliferation assay. Three CR cell lines were tested: WM164-CR (wild-type PTEN), A2058-CR (homozygous PTEN loss), and UACC903-CR (homozygous PTEN loss). Rapamycin treatment significantly reduced the percentage of EdU-positive cells in WM164-CR (17.24% vs. 30.14%, P < 0.05), UACC903-CR (6.10% vs. 16.25%, P < 0.05), and A2058-CR (12.21% vs. 19.21%, P < 0.05). BEZ235 almost completely inhibited cell proliferation in all three cell lines. Subsequently, a cell cycle analysis was performed to investigate the distribution of CR cells across various cell cycle phases after treatment with mTOR/PI3K inhibitors. The analysis revealed that Rapamycin and BEZ235 increased the proportion of CR cells in the G1 phase, suggesting slowed cell division in all CR melanoma cell lines. Furthermore, BEZ235 treatment increased the proportion of CR cells in the sub-G1 phase, indicating the induction of apoptosis in all CR melanoma cell lines. To investigate the influence of mTOR/PI3K inhibitors on programmed cell death in CR cells, an Annexin-V and 7-AAD double staining assay was performed. The results showed that both Rapamycin and BEZ235 significantly induced late-stage apoptosis (P < 0.05) compared to PLX+PD treatment in all cell lines. These findings further confirm the pro-apoptotic effects of mTOR/PI3K inhibitors in CR melanoma cells⁵.

Conclusion:  Based on the experimental results, targeting the mTOR pathway with rapamycin and BEZ235 overcomes resistance to BRAF and MEK inhibitors in BRAF mutant melanoma cells.  These combinational therapies significantly suppress cell proliferation, induce G1 arrest, and promote apoptosis in BRAF mutant melanoma cells⁵. Future research should encompass long-term assessments of these therapies in animal models to simulate extended treatment periods.

Works cited:

[1] Ny L, Hernberg M, Nyakas M, et al. BRAF mutational status as a prognostic marker for survival in malignant melanoma: a systematic review and meta-analysis. Acta Oncol. 2020;59(7):833-844. doi:10.1080/0284186X.2020.1747636

[2] Abdo JF, Sharma A, Sharma R. Role of Heredity in Melanoma Susceptibility: A Primer for the Practicing Surgeon. Surg Clin North Am. 2020;100(1):13-28. doi:10.1016/j.suc.2019.09.006

[3] Arozarena I, Wellbrock C. Overcoming resistance to BRAF inhibitors. Ann Transl Med. 2017;5(19):387. doi:10.21037/atm.2017.06.09

[4] Arozarena, I., & Wellbrock, C. (2017). Overcoming resistance to BRAF inhibitors. Annals of translational medicine, 5(19), 387. https://doi.org/10.21037/atm.2017.06.09

[5] Wang B, Zhang W, Zhang G, et al. Targeting mTOR signaling overcomes acquired resistance to combined BRAF and MEK inhibition in BRAF-mutant melanoma. Oncogene. 2021;40(37):5590-5599. doi