A Stochastically Arising Subpopulation Of B-Raf(V600e)-Expressing Melanoma Continues Division In The Presence Of Vemurafenib

Patients with melanomas expressing B-Raf V600E benefit from treatment with B-Raf-targeted therapeutics, such as vemurafenib. Initial responses to these agents are occasionally profound, but the duration of response is variable from patient to patient, with some tumors recurring within a few weeks and others requiring months before tumors progress. The source of this variability of response is under intense investigation. A number of acquired resistance mechanisms have been described, but it is not known whether they pre-exist or develop after treatment initiation. To address this question, we tracked the cellular response to PLX4720 (the vemurafenib research compound) over 96 hours in the A375 and SK-MEL-5 cell line models of B-Raf V600E -expressing melanoma. Both cell lines exhibited a significant early reduction in cell number compared to control, but after 60 hours of treatment cells numbers began to increase even in the presence of 8 μM drug, albeit at a slow rate. This “rebound” proliferation effect was not due to drug decay since re-administration of the drug did not prevent it. We further examined the rebound effect at the single cell level using our newly described Fractional Proliferation assay (Tyson et al., Nat Methods, 2012, 9(9):923-928, doi:10.1038/nmeth.2138). The rebound corresponded to a dividing cell subpopulation whose size and rate of division depend on the concentration of drug. This subpopulation does not represent rare non-responding variants within the cell lines, but rather it arises stochastically as sibling cells do not always exhibit the same phenotypic response. Investigation of the molecular correlates of this dividing subpopulation, which may act as the reservoir from which the resistant cells arise, are ongoing. Citation Format: Darren R. Tyson, Chengwei Peng, Keisha N. Hardeman, Bishal Paudel, Vito Quaranta. A stochastically arising subpopulation of B-Raf V600E -expressing melanoma continues division in the presence of vemurafenib. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3403. doi:10.1158/1538-7445.AM2013-3403