Induced cross-resistance of BRAFV600E melanoma cells to standard chemotherapeutic dacarbazine after chronic PLX4032 treatment

The maximum response and 10-year survival rate for metastatic melanoma patients treated with standardised chemotherapy is still less than 15% and 10%, respectively. In contrast, oncogene targeting was found a promising tool for killing of BRaf^V600 mutated melanoma cells. Nevertheless, despite improved response and survival rates, resistance acquisition remains an ongoing problem. In this context, the impact of chronic BRaf inhibition on the efficacy of commonly applied cytostatics is still unknown. In our study, human melanoma cells with BRaf^V600E mutation were treated with chemotherapeutics and a BRaf Inhibitor. Resistance patterns were analysed by microelectrode array-based impedance spectroscopy, XTT and flow cytometric Apoptosis/proliferation assay. BRaf^V600E melanoma cells acquired a time- and concentration-dependent desensitisation up to 100-fold towards oncogene-specific PLX4032 and chemotherapeutic dacarbazine after twelve months treatment. The impact of multiple drug insensitivity on molecular melanoma characteristics was elaborated via mRNA and protein quantification. Following BRaf^V600E targeting, melanoma cells developed an increasingly aggressive, dacarbazine-insensitive phenotype. Thereby, hyperactivated canonical alternative MAPK and bypass PI3K/Akt signalling caused cross-resistance of differently acting drugs. With these results, we are the first to show that long-term melanoma therapy with BRaf inhibitors can prevent further therapeutic success with dacarbazine due to acquisition of cross-resistance.