Vemurafenib, a small molecule inhibitor to mutant B-RafV600E protein, has been recently approved by the FDA (USA) for the treatment of B-Raf mutant metastatic melanomas. However, the majority of the patients treated develop progressive disease due to acquired drug resistance1. The main mechanisms of resistance reported result in the re-activation of the MAPK signalling pathway2.
We have assessed the ability of various Receptor Tyrosine Kinases (RTKs) activated by their respective exogenous ligands, or constitutively activated mutant forms that are detected in melanoma, for their ability to confer resistance to vemurafenib in the same B-RafV600E melanoma cell lines. RTKs varied in their ability to drive resistance. In particular the c-KIT RTK known to be expressed and mutated in melanoma, was not able to confer resistance to Vemurafenib in several vemurafenib sensitive BRAFV600E melanoma cell lines that EGFR was able to confer resistance in. Adaptation of KIT expressing cells to vemurafenib over 3 weeks resulted in selection of cells expressing lower levels of the KIT, supporting our observation that KIT was not able to confer any advantage to the melanoma cells in the context of drug treatment. The main signalling observation was the sustained activation of downstream phospho-MEK and phosphor-ERK in EGFR expressing lines but not KIT expressing lines following drug treatment. We also found that drug adaptation was able to select for resistance regardless of expression of RTK.
Interestingly, across a panel of over 25 BRAF mutant melanoma lines, expression of KIT was high in the cell lines sensitive to vemurafenib and low in those resistant. The reverse was observed with respect to EGFR expression levels. These results demonstrate that the various RTKs are able to act differently in melanoma cell lines particularly in terms of their association with vemurafenib sensitivity or resistance.