Abstract 2519: Efficacy of a potent and selective Raf inhibitor against human xenograft models displaying specific genetic mutations in the MAPK signaling pathway

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC Background: A significant percentage of human melanomas as well as colon, ovarian and thyroid carcinomas display missense mutations in B-Raf which can drive cellular transformation through constitutive activation of the MAPK signaling pathway. Thus, mutant B-Raf represents an attractive target to develop new oncology therapeutics. Here we describe the in vivo potency and efficacy of a novel Raf inhibitor (cmpd 1) against xenograft models displaying mutations in the MAPK signaling pathway. Methods: Established human xenograft models implanted in athymic female nude mice were used to examine the effects of cmpd 1 on P-ERK basal level and tumor growth. For examination of pharmacodynamic effect, cmpd 1 was dosed orally, once for 6-8 hours prior to collection of the xenograft and peripheral blood. Collected xenografts were lysed and levels of P-ERK analyzed by Meso Scale Detection Assay (MSD). Plasma concentrations of cmpd 1 were determined using Quantitative Liquid Chromatography-Tandem Mass Spectometry (LC-MS/MS). In order to study the effects of cmpd 1 on tumor growth inhibition (TGI), mice with established xenografts (∼ 200-250 cubic mm) were randomized into 4 groups (n=10) on day 0. Mice were dosed orally, once (QD) or twice (BID) per day starting on day 1 until completion of the experiment. Tumor volume and body weight were measured twice per week. Significant TGI was determined using repeated measures ANOVA (RMANOVA) followed by the Dunnett's test. Results: Models expressing B-Raf activating mutations (A375, WM-266 and Colo-205) showed significant decreased expression of P-ERK (>70%) and significant TGI, including complete stasis (100% TGI) and regression at 5 mg/kg QD (ED50 1-3 mg/kg). Models with activating NRAS mutations also showed sensitivity to cmpd 1 but higher doses were required to achieve significant reduction of P-ERK level and tumor growth inhibition (ED50 11 mg/kg). Models with activating KRAS mutations displayed responses ranging from stimulation of tumor growth (MiaPaCa-2) to lack of effect (A549) or 40% TGI (HCT-116). These suboptimal responses in KRAS mutant models were observed even when cmpd 1 was dosed at 10 mg/kg BID. Finally, BxPC-3, a xenograft model with a wild-type (WT) MAPK signaling pathway, displayed sensitivity to cmpd 1 albeit to a lesser extend than observed in B-Raf mutant models (ED50 6.0 mg/kg). In most models, TGI was directly correlated to the ability of cmpd 1 to reduce P-ERK levels in vivo. Plasma exposures of cmpd 1 were approximately proportional to the administered dose. Conclusion: These results show that inhibition of P-ERK by this Raf inhibitor can result in significant TGI in mutant B-Raf, NRAS and WT models while significant sensitivity can be lost in KRAS mutant models. In addition, the data also show that under certain circumstances, the inhibition of Raf in KRAS mutant cell lines can result in stimulation of tumor growth. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2519.