DNAR-10. THE EFFICACY OF TEMOZOLOMIDE AND LOMUSTINE IN GLIOBLASTOMA CELL LINES MAY BE ENHANCED BY CONCOMITANT TREATMENT WITH TUMOR TREATING FIELDS (TTFIELDS)

Abstract

Abstract Glioblastoma (GBM) is the most common primary malignant brain tumor in adults. Temozolomide (TMZ) is part of the standard of care for patients with newly diagnosed GBM (ndGBM). Expression of the enzyme MGMT involved in TMZ-induced DNA damage repair is associated with irresponsiveness of some patients to TMZ. The addition of lomustine (CCNU) to TMZ demonstrated clinical benefit in patients with ndGBM. Tumor Treating Fields (TTFields) are electric fields that display anti-mitotic effects and lower expression levels of proteins from the FA-BRCA DNA repair pathway in cancerous cells. TTFields therapy concomitant with TMZ is approved for the treatment of ndGBM. The aim of the current study was to examine the effect of TTFields in conjunction with TMZ and CCNU in GBM cells. Human GBM cell lines with different MGMT expression levels (U-87 MG, LN229, U118, and LN18) were treated with TTFields (0.83 V/cm RMS, 200 kHz) concomitant with TMZ and/or CCNU. Cell counts, colony formation, and apoptosis were used to measure effectiveness. The effect of TTFields with concomitant TMZ was additive in all cell lines, regardless of MGMT expression levels. TTFields in conjunction with CCNU displayed an additive effect in cell lines with high MGMT expression, and tendency to synergism in cell lines with low or no MGMT expression. TTFields concurrent with both TMZ and CCNU further increased treatment efficacy. Mechanistically, TTFields elevated the levels of DNA double strand breaks and lowered expression of FA-BRCA pathway proteins, as detected by fluorescence microscopy and western blot, respectively. The involvement of the FA-BRCA pathway in repair of DNA damage induced by CCNU and the BRCAness state induced by TTFields rationalize the synergy seen for TTFields with CCNU, especially in cells absent of MGMT. The results suggest a potential benefit of TTFields therapy concomitant with TMZ/CCNU for the treatment of GBM.