Abstract B131: Modeling the evolution of resistance to RAF inhibitors in clinically relevant melanoma models.
Authors: Das Thakur Meghna , Salangsang Fernando , Pryer Nancy , McMahon Martin , Stuart Darrin 
Published: June 2012

Abstract
Activating mutations in BRAF kinase are one of the most common genetic alterations in human melanoma, with over 50% of tumors expressing BRAFV600E. Small molecule RAF inhibitors such as vemurafenib have demonstrated proof-of-concept that BRAFV600E is a key driver of proliferation and survival in melanoma, as evidenced by tumor regression and prolonged survival in patients in late stage clinical trials. Unfortunately, in many patients the tumor response can be quite 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.
To model the emergence of resistance to vemurafenib, we have established an early passage primary human melanoma xenograft model. In contrast to using tumor cell lines, this in vivo system recapitulates a more appropriate microenvironment, genetic diversity, and selective pressure observed in the clinical setting. The selection of resistance in vivo also reflects the clinical setting by modeling drug pharmacokinetics in patients.
Treatment of these primary melanoma xenogafts with vemurafinib on a long term continuous dosing regimen at clinically relevant doses resulted in the appearance of resistant tumors over the course of 4 to 6 weeks. Pharmacodynamic (PD) analysis within individual tumors indicated that the RAF-MEK-ERK pathway is still suppressed by vemurafinib in resistant tumors, although the degree and duration of suppression is less than in sensitive tumors. Furthermore, the kinetics of pathway inhibition and recovery are different between each resistant tumor. While genetic analysis of resistant tumors is ongoing, biochemical analyses indicate that receptor tyrosine kinases (RTK) and modulation of negative feedback loops to RTK may be involved in resistance, as well as up-regulation of BRAFV600E expression in this model. Pharmacological evaluation of tumor response has provided insight into tumor cell populations and the evolution of resistance. We find that increasing the dose of drug 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 suggests there is a great deal of tumor cell heterogeneity, and that tumors are able to rapidly adapt to the selective pressure being applied. Further support for this was obtained by suspending drug treatment from mice with resistant tumors. Upon drug withdrawal, tumors initially regressed for several days to weeks, followed by re-growth. These data indicate that the adaptation which occurs within a tumor cell population under selective pressure make the cells less fit in the absence of drug and could have significant implications in optimizing dose schedules to prevent the emergence of resistance.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B131.