Abstract B111: Molecular and chemosensitivity profiling of melanoma cell lines with acquired resistance to BRAF inhibitor Vemurafenib.

Introduction: One of the main limitations of patient benefit from targeted therapies is the rapid development of resistance. The BRAF inhibitor Vemurafenib is approved for BRAF mutant melanoma. Here we generated four Vemurafenib resistant melanoma cell lines and found that each had a unique profile of receptor tyrosine kinase over or underexpression, compared to the parent cell lines. Methods: Cell lines used were the commercially available HT-144 and three Oncotest patient-derived xenograft-derived cell lines MEXF 276L, MEXF 394L and MEXF 520L, all sensitive to Vemurafenib and carrying the BRAF V600E or V600K mutation. Each was treated continuously with increasing Vemurafenib concentration up to 6 µM. Results: Upon continuous Vemurafenib treatment all four melanoma cell lines developed Vemurafenib resistance. The resistant cell lines had between 19 and 168 fold higher IC50 values compared to the mock-treated lines. These IC50 values are comparable with IC50 values for cell lines without BRAF mutations, meaning that these cell lines are truly resistant to Vemurafenib. Chemosensitivity profiling revealed cross-resistance to other BRAF inhibitors (n=3) as well as to MEK inhibitors (n=6), while for AKT, c-MET, Eg5, EGFR or PI3K inhibitors sensitivities were mostly similar to those of the parental cell lines. The cross-resistance with BRAF and MEK inhibitors suggests a mechanism of resistance either downstream of MEK or independent of the RAS/RAF/MEK pathway. Excess receptor tyrosine kinase (RTK) expression has been associated with resistance of BRAF mutant melanomas to Vemurafenib. Molecular analyses showed that each of the four resistant cell lines displayed an altered expression of RTKs with each showing a different combination of over- and under-expressed receptors. Downstream phosphorylation of Erk1/2, Akt and S6RP were also found to be altered in the resistant cell lines compared to their sensitive counterparts. Conclusions: Four BRAF-mutant melanoma cell lines acquired resistance to BRAF inhibitor Vemurafenib with cross- resistance to other BRAF and MEK inhibitors. However, the cross resistance seems to be the only similarity between the resistant cell lines, as changes in RTK expression and downstream Erk1/2, Akt and S6RP phosphorylation were unique to each cell line. This leads to the conclusion that each of the four resistant cell lines may represent a resistance mechanism present in a sub-group of the patient population. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B111. Citation Format: Rebekka Krumbach, Anne-Lise Peille, Torsten Giesemann, Vincent Vuaroqueaux, Heiner Fiebig, Thomas Metcalfe, Gerhard Kelter. Molecular and chemosensitivity profiling of melanoma cell lines with acquired resistance to BRAF inhibitor Vemurafenib. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B111.