Abstract 3843: Rational selection of combinatorial therapeutic targets for melanoma identified by synthetic lethal screening with small molecule inhibitors
Authors: Roller Devin G. , Molhoek Kerrington R. , Slingluff Craig L. , Weber Michael J. , Gioeli Daniel 
Published: February 2011

Cellular signals are not necessarily transmitted via linear pathways, but through a dynamic interconnected network. Effective pharmacologic inhibition of cellular signaling in malignant cells will thus require identifying the key nodes, and intervening at more than one site within the network. To systematically identify functional interactions between signaling proteins within the cell signaling network we have screened nine melanoma cell lines of diverse genetic backgrounds for sensitivity to combinations of drugs targeting the major signaling pathways altered in cancer. Seven drugs representative of standard targets were designated as “primary drugs”. These seven primary targeted therapies were screened against each other and an additional 67 signal transduction inhibitors. The screen was performed robotically with a Biomek NX workstation in a 96-well format. Cell viability was measured with alamarBlue on a microplate reader. Various concentrations of the secondary drug were tested against a partial inhibitory concentration of the primary drug. Twelve percent of a total of 4672 drug dose and cell combinations tested displayed superadditivity in vitro. Nearly 10% of those hits demonstrated greater than 50% superadditivity according to first principles analysis; the actual growth inhibition of the combination was more than 50% greater than the sum of the growth inhibition produced by each drug alone. One intriguing drug combination showing superadditivity in most cell lines tested was sorafenib plus diclofenac. The degree of inhibition and concentration in which superadditivity was observed was cell line dependent, and was independent of known mutational status. The MEK inhibitor, PD325901, was able to substitute qualitatively for sorafenib indicating that Raf may be the major target for sorafenib when used in combination with diclofenac. Consistent with this, sorafenib inhibited MEK and ERK activity at doses that achieved superadditivity with diclofenac. However, the combination of PD325901 with diclofenac was less robust at inhibiting growth than sorafenib plus diclofenac suggesting that the alternate sorafenib targets may play a role in the observed superadditivity. Celecoxib, a COX2 inhibitor, or ibuprofen, a preferential COX1 inhibitor, substituted for diclofenac indicating that diclofenac was acting as a COX inhibitor, although the inhibition was considerably less robust, suggesting a role for a balanced inhibition of COX1 and COX2, or a role for off-target effects. Collectively, these results indicate an unexpected functional interaction between Raf and COX signaling in melanoma. Since both sorafenib and diclofenac are in clinical use, the therapeutic potential of these observations is striking and is being further explored in pre-clinical models.

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 3843.