639 Identification of experimental therapeutics overcoming NRAS-based BRAFi-resistant malignant melanoma targeting brain metastases in a bioluminescent murine model

Molecularly targeted therapeutics have revolutionized the treatment of BRAFV600E-driven malignant melanoma, but rapid development resistance to BRAF kinase inhibitors (BRAFi) presents a significant obstacle. Here, we describe identification experimental melanoma overcoming NRAS-based BRAFi-resistance employing stringent genetic model BRAFi-resistance, i.e. isogenic cell lines that differ only by NRAS mutational status (BRAFi-sensitive A375-BRAFV600E/NRASQ61 versus BRAFi-resistant A375 BRAFV600E/NRASQ61K). As result unbiased candidate screening, clinical antimalarial mefloquine (MQ) was apoptogenic agent causing death at low micromolar concentrations. Comparative gene expression-array analysis (A375-BRAFV600E/NRASQ61 A375-BRAFV600E/NRASQ61K) revealed MQ is dual inducer endoplasmic reticulum (ER) and redox stress responses precede MQ-induced loss viability. ER-trackerTM DPX fluorescence imaging electron microscopy indicated ER swelling, accompanied induction signaling (phospho-eIF2α, XBP-1s, ATF4). Fluo-4 AM-fluorescence occurrence cytosolic calcium overload observable within seconds exposure. In bioluminescent murine intracranial injection A375-Luc2 (BRAFV600E/NRASQ61K), an oral regimen efficiently antagonized growth brain metastases. Taken together, these data suggest feasibility identifying valid candidates for drug repurposing aiming chemotherapeutic elimination cells targeting BRAFV600E/NRASQ61K even if metastasized brain.