Nitric oxide-releasing selective cyclooxygenase-2 inhibitors as promising radiosensitizers in melanoma cells in vitro

Expression of cyclooxygenase-2 (COX-2) and subsequent higher availability of eicosanoids are important modulators of tumor radioresistance. Additionally, elevated COX-2 protein is closely associated with hypoxia, which itself is a key promoter of tumor radioresistance. In this regard, selective COX-2 inhibitors (coxibs) containing a nitric oxide (NO)-releasing moiety (NO-coxibs) are hypothesized to act as bifunctional radiosensitizers. Therefore, novel NO-coxibs with a (pyrazolyl)benzenesulfonamide lead were investigated in vitro. As model, two human melanoma cell lines were exposed to several doses of X-ray in presence or absence of the novel NO-coxibs or the corresponding coxib during irradiation. Cells were examined in clonogenic cell survival assays to determine radiosensitizing effects under both normoxic and hypoxic conditions. COX-2 protein expression of two melanoma cell lines with a dissimilar baseline COX-2 synthesis was increased by irradiation and, furthermore, by experimental hypoxia. Radiosensitivity of both cell lines was significantly enhanced by the novel NO-coxibs and, to a lesser extent, also by the corresponding coxib. Moreover, the most potent NO-coxib 5 significantly increased the radiosensitivity of both cell lines also under hypoxic conditions. By administration of 5 the required radiation dose for 10% survival could be reduced from 6.6 Gy (DMSO control) to 5.2 Gy (p<0.01) for the ‘COX-2-positive’ A2058 cells and from 4.2 Gy (DMSO control) to 3.2 Gy for the ‘COX-2-negative’ Mel-Juso cells. The results confirm the auspicious bifunctional approach of the novel NO-coxibs as potential adjuvant radiosensitizers under normoxic and hypoxic conditions in vitro. Further studies are necessary to confirm the promising findings in vivo.