A high percentage of human cancers arise from mutations in the ras gene that encodes an abnormal Ras protein locked in a constitutively active GTP-bound state that promotes tumor cell proliferation, survival, and metastasis. These gain-in-function mutations in the ras gene or constitutive activation of tyrosine kinase receptors upstream of the Ras protein drive tumor cell growth by activating Raf/MAPK and PI3K/AKT signaling pathways. Ras has been an elusive drug target for which no inhibitors are available to treat Ras-driven cancers. Screening a library of indene derivatives in a differential phenotypic assay identified a novel compound class displaying high potency and selectivity to inhibit the growth of tumor cells harboring mutant Ras relative to tumor cells with wild type (WT) Ras. Lead optimization resulted in a drug development candidate (DC070-547) and several back-up analogs (e.g. ADT-006) with IC50 values in the low nanomolar range and selectivity indices of 100 fold or greater to inhibit the growth of tumor cells with constitutively activated Ras relative to tumor cells with low levels of activated Ras. Sensitivity among a large panel of tumor cell lines to this compound class strongly correlated with levels of activated Ras, but did not appear to be limited to a specific ras gene mutation or Ras protein isoform. Here, we report that DC070-547 and ADT-006 potently and selectively inhibit the growth of the human melanoma cell line, SK-MEL-2, harboring a mutation in the ras gene that encodes the constitutively active N-Ras protein with IC50 values of 7 and 25 nM, respectively. In addition, both compounds potently inhibited the growth of the murine melanoma cell line, B16-F10 with WT ras, but harboring a mutation in a tyrosine kinase receptor upstream of Ras, specifically PDGFRα, which results in high levels of active, GTP-bound Ras as confirmed by Ras-RBD pull-down assays. To determine if treatment can disrupt Ras signaling, B16-F10 cells were incubated with the compounds before being subjected to Western blotting for phosphorylated signaling molecules downstream of Ras. Treatment reduced levels of phosphorylated c-Raf and MEK at concentrations that inhibit tumor cell growth. These findings support further investigation of this novel class of Ras inhibitors for the treatment of Ras-driven melanoma.
Citation Format: Kate M. Saville, Kevin Lee, Tyler E. Mattox, Xi Chen, Jacob Valiyaveettil, Kristy Berry, Veronica Ramírez-Alcántara, Bing Zhu, Adam Keeton, Michael Boyd, Gary Piazza, Ashley S. Lindsey. Sensitivity of melanoma cells to a novel class of Ras inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5166. doi:10.1158/1538-7445.AM2017-5166