Abstract 920: Effective elimination of drug resistsant melanoma cells by α-phenylethyl isothiocyanate (PEITC)

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Drug resistance is a major problem in clinical treatment of malignant melanoma. A characteristic feature of melanoma is the abnormally high levels of basal free radicals produced in transformed melanosomes when compared to normal melanocytes. Reactive oxygen species (ROS) activate pro-oncogenic signaling pathways that promote cancer progression, metastasis and contribute to chemotherapeutic drug resistance. However, an excessive overload of ROS in cells beyond a certain tolerance limit may trigger oxidative damage and cell death. Thus cellular ROS modulation presents a valuable therapeutic strategy for melanoma. Based on the above observations, we hypothesize that causing ROS agglomeration in cells, either through depletion of antioxidants or by initiating uncontrolled ROS production, is a plausible strategy to selectively kill melanoma cells, and that α-phenylethyl isothiocyanate (PEITC), a natural compound known to disable cellular GSH antioxidant system, will be effective for this purpose. We showed that that PEITC exhibited potent inhibitory effect on malignant melanoma cells with an average IC50 value of approximately 3 µM. Interestingly, MEL624 and HS294T cells, which are resistant to existing antitumor agents used in melanoma treatment such as AZD6244 and Vemurafenib, were highly sensitive to PEITC. The cell death was primarily apoptotic; the effect of PEITC could be partially reversed using the pan-caspase inhibitor Z-VAD. Furthermore, treatment of cells with PEITC lead to an aberrant increase in reactive oxygen species levels, as evidenced by FACS analysis using the fluorescent ROS dyes DCF-DA, DAF-FM and dihyroethidium. ROS increase upon PEITC treatment occurred as early as one hour, indicating that seemed to a primary event leading to subsequent cell damage. This increase in ROS was associated with a decrease in the glutathione levels in cells, suggesting that depletion of the glutathione antioxidant could be the reason for the abnormal ROS elevation induced by PEITC. This was further verified by using PEITC in combination with the antioxidant N-acetyl cysteine, which completely rescued the cells from the toxic effect of PEITC. Importantly, normal melanocytes were minimally sensitive to PEITC. In addition, combination of PEITC with other standard melanoma drugs like carmustine and temozolomide exhibited synergistic effect, suggesting the possibility of using this compound to overcome drug resistance. This was further validated through in vivo studies in mice, where a combination of PEITC with BCNU seemed to reduce the tumor burden. The cytotoxic effect of PEITC was also validated in primary cancer cells isolated from melanoma specimens from patients. Thus PEITC proves to be an effective antimelanoma agent that warrants further investigation. 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 920. doi:1538-7445.AM2012-920