Figure 2. Characterization of isothiocyanates and isoselenocyanates as inhibitors of melanoma.

A. Structures of isothiocyanates and selenium containing isoselenocyanates. Chemical structures of isothiocyanates containing 1(benzyl), 2 (phenethyl), 4 (phenylbutyl) and 6 (phenylhexyl) carbon spacers and corresponding isosteric selenium analogs in which sulfur was replaced with selenium. A 6-carbon selenocyanate (phenylhexyl selenocyanate, PHSC), served as a control to show compound structure and not selenium caused the inhibitory effect. B and C. Comparison of melanoma cell survival following exposure to isothiocyanates versus isoselenocyanates. Cell viability was measured using the MTS assay and IC₅₀ (μM) values plotted against carbon chain length. 5X10³ melanoma cells were plated in 96-well plates and allowed to attach for 24 h. Increasing concentrations of isothiocyanates and selenium analogs were added in culture medium. Values represent averages of percentage of control DMSO treated cells. API-2 and PHSC served as an Akt inhibitor and selenium control respectively. Results show that ISC-4 and ISC-6 were the most effective inhibitors. D. ISC-4 kills melanoma cells at 2–4 fold lower concentrations than normal cells. 5X10³ normal human fibroblasts (FF2441) or metastatic melanoma cells (UACC 903) were plated in 96-well plates in 100 μl DMEM media containing 10% FBS and grown for 72 and 36 h respectively. Exponentially growing cells were treated with increasing concentrations (2–100 μM) of ISC-4 or PBITC for 12 and 24 h and IC₅₀ (μM) values determined. ISC-4 was found to consistently inhibit melanoma cells growth at concentrations 2–4 fold lower than fibroblast cells (One way ANOVA ***P<0.001); error bars, S.E.M.