Abstract LB-144: Modeling vemurafenib resistance in melanoma reveals a strategy to forestall drug resistance.
Authors: Das Thakur Meghna , Fisher Rosalie , Salangsang Fernando , Landman Allison , Sellers William , Pryer Nancy , Levesque Mitchell Paul , Dummer Reinhard , Gore Martin , Larkin James , McMahon Martin , Stuart Darrin 
Published: August 2014

Abstract
Mutational activation of BRAF is the most prevalent genetic alteration in human melanoma, with ≥50% of tumors expressing the BRAF (V600E) oncoprotein. Moreover, the impressive tumor regression and increased survival of late-stage BRAF-mutated melanoma patients in response to treatment with vemurafenib demonstrates the essential role of oncogenic BRAF in melanoma maintenance. Although vemurafenib therapy has transformed the treatment of late-stage melanoma patients, the durability of patient response to this drug is limited by the emergence of lethal drug resistant disease. Thus, understanding mechanisms and designing approaches to prevent the emergence of resistance are critical to providing improved therapy.
Here we model the emergence of vemurafenib resistance using two early passage primary human-patient-derived xenograft (PDX) BRAF (T1799A)-mutated melanoma models, which were continuously treated with vemurafinib in immunocompromised mice. In one of these models vemurafenib resistance is due to selection of cells with elevated expression of the BRAF (V600E) oncoprotein. Remarkably, vemurafenib resistant tumors are not only drug resistant but also develop drug dependency such that tumors require exposure to vemurafenib for their continued proliferation. Indeed, cessation of drug administration in these PDX models leads to inhibition of tumor growth and, in many cases, tumor regression. This fitness deficit upon cessation of drug administration is based on an adaptive response to levels of MAPK pathway signaling output. Melanoma cells appear to proliferate best at a defined intermediate level of RAF/MEK/ERK signaling output. Hence, when drug resistant tumor cells are deprived of drug, the level of signaling output is increased leading to decreased proliferation.
We sought to determine whether vemurafenib-resistant tumors in melanoma patients displayed similar evidence of drug dependency. In a small cohort of patients (n=42) with vemurafenib-resistant tumors for which CT scans were available following cessation of treatment (n=19), 14 out of 19 patients demonstrated evidence of reduced tumor growth velocity. Taken together, these data support the hypothesis that continuous treatment with vemurafenib can select for drug-resistant tumor cells that show a fitness deficit in the absence of the drug. Furthermore, we demonstrate in the preclinical setting that these observations can be leveraged therapeutically by using a discontinuous treatment regimen to forestall the emergence of resistance in two different PDX models. Consequently, these data may have important implications for the treatment of BRAF mutated melanoma patients with BRAF (V600E) inhibitors and suggest that altered dosing might delay or prevent the emergence of lethal drug resistant disease.
Citation Format: Meghna Das Thakur, Rosalie Fisher, Fernando Salangsang, Allison Landman, William Sellers, Nancy Pryer, Mitchell Paul Levesque, Reinhard Dummer, Martin Gore, James Larkin, Martin McMahon, Darrin Stuart. Modeling vemurafenib resistance in melanoma reveals a strategy to forestall drug resistance. [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 LB-144. doi:10.1158/1538-7445.AM2013-LB-144