Abstract 5592: Modeling the evolution of resistance to RAF inhibitors in clinically relevant melanoma models
Authors: Thakur Meghna Das , Salangsang Fernando , Pryer Nancy , Stuart Darrin , McMahon Martin 
Published: August 2012

Abstract
Small molecule RAF inhibitors such as Vemurafenib have provided proof-of-concept that BRAFV600E is a key driver of proliferation and survival in melanoma, as evidenced by tumor responses in this patient population. Unfortunately, in many patients the tumor response is short-lived, as resistance to the inhibitor rapidly develops. Understanding the mechanisms that underlie drug resistance will be critical to providing improved treatment options for these patients. Here we model the emergence of resistance to selective RAF inhibitors, utilizing a limited passage BRAFV600E primary human melanoma xenograft model. This model, is expected to retain genetic diversity in a physiologically relevant microenvironment. Mice bearing HMEX1906 tumors were treated with one of two selective RAF inhibitors, Vemurafenib or the novel RAF inhibitor NVP-LGX818, at clinically-relevant doses, providing an opportunity to model pharmacokinetics, and replicate the selective pressure observed in patients. Long-term continuous treatment of BRAFV600E primary melanoma xenogafts with selective RAF inhibitors resulted in the appearance of resistant tumors over the course of 4 to 6 weeks. Pharmacodynamic (PD) analysis of individual tumors indicated that the MAPK pathway is suppressed in resistant tumors, although the degree and duration of suppression is less than in tumors that remain sensitive to RAF inhibitors. Furthermore, there is heterogeneity in the PD response, with greater extent and duration of P-Erk inhibition in sensitive tumors than in those resistant to RAF inhibitors. Biochemical analyses indicate that multiple receptor tyrosine kinases are activated and modulation of negative feedback loops may contribute to resistance. In addition increased expression of BRAFV600E, was detected in a subset of resistant tumors. Increasing the dose of RAF inhibitor administered to mice bearing resistant tumors leads to a significant yet transient tumor response, followed by tumor progression. Taken together with the PD data, this supports tumor cell heterogeneity, and rapid adaptation of tumors to the selective pressure being applied. Both in vivo drug withdrawal studies as well as in vitro analyses indicate that the resistant cells have become dependant on the presence of the RAF inhibitor for their survival. Based on these observations it appears that resistant tumor cells are less fit than sensitive tumor cells in the absence of drug. To test this hypothesis, mice were dosed on an intermittent regimen(4 weeks on / 2 weeks off) of RAF inhibitors. This regimen prevented the evolution of resistance for the duration of the 4 month experiment. These data could have implications in optimizing dose schedules to prevent the emergence of clinical resistance to targeted therapy.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5592. doi:1538-7445.AM2012-5592