Abstract 1065: Growth factor-mediated resistance to BRAF/MEK inhibitors in BRAF mutant melanoma

BRAF and MEK inhibitors induce striking tumor regressions in BRAF V600E melanoma patients. However, relapse occurs in the majority of patients within several months. Reported resistance mechanisms include acquisition of NRAS mutations, PDGFR or IGF1R activation, Cot amplification, or truncated BRAF upregulation. We report here that hepatocyte growth factor (HGF), neuregulin-1 (NRG1), and fibroblast growth factor-2 (FGF2) cause robust resistance in vitro to BRAF or MEK inhibitors in BRAF mutant melanoma cell lines. NRAS mutant lines were similarly resistant to MEK inhibition in the presence of these growth factors. By contrast other mitogenic factors such as PDGF, EGF, NGF, BDNF, MSP, MDK, GRO, and IGF1 did not maintain cell viability or growth. HGF, NRG1 and FGF2 prevented growth arrest and cell death and in some cell lines maintained cells in progressive growth. In addition, HGF and NRG1 could prevent growth arrest after dual treatment with BRAF and MEK inhibitors. Western analysis revealed that HGF and NRG1 reactivated Erk signaling and activated S6RP and AKT phosphorylation. Combined pharmacological inhibition mTor signaling and Akt signaling was required to fully block growth factor-mediated proliferation. In addition, small molecule Met inhibitors and an anti-HGF antibody blocked HGF-mediated resistance, while lapatinib and an anti-NRG1 antibody blocked NRG1-mediated resistance. While combined Akt and mTor inhibition potently inhibited cell line growth independent of BRAF or MEK inhibition, lapatinib and Met inhibitors had minimal effect on growth, suggesting potential for less adverse events with receptor inhibitors. In vitro co-culture experiments revealed that primary cells present in normal skin can cause resistance, and we have identified growth factors produced by these cells that contribute to resistance. We are analyzing receptor activation (Met and Erbb3) and FGF2 expression in wildtype, NRAS, and BRAF mutant primary melanoma samples. In future experiments we plan to test clinical samples from patients treated with BRAF/MEK inhibitors, and we hypothesize that HGF, NRG1, and FGF2 signaling is elevated in patients with poor initial responses or relapse to BRAF or MEK inhibitors. 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 1065. doi:1538-7445.AM2012-1065