P10.06.B APPLYING TUMOR TREATING FIELDS (TTFIELDS) FOR ENHANCEMENT OF TEMOZOLOMIDE AND LOMUSTINE EFFICACY IN GLIOBLASTOMA CELL LINES

Abstract

Abstract BACKGROUND Tumor Treating Fields (TTFields) are electric fields that disrupt cellular processes critical for cancer cell viability and tumor progression, ultimately leading to cell death. TTFields therapy concurrent with maintenance temozolomide (TMZ) is approved for treatment of newly-diagnosed glioblastoma (GBM). Recently, benefit was demonstrated in patients with O6-methylguanine DNA methyltransferase (MGMT) promoter methylation for the combination of TMZ with lomustine (CCNU). The addition of adjuvant TTFields to this treatment regimen further improved patient outcomes, leading to a CE mark. The current study aimed to elucidate the mechanism underlying the benefit of this treatment protocol by utilizing GBM cell lines with different MGMT expression status. MATERIAL AND METHODS Human GBM cell lines with different MGMT promoter methylation status [LN-18 (unmethylated MGMT promoter), U-118 MG (partially methylated MGMT promoter), and U-87 MG and LN-229 (methylated MGMT promoter)] were examined for MGMT expression levels. Next, the cells were treated with TTFields (intensity of 0.83 V/cm RMS; frequency of 200 kHz) using the inovitro system. Cell count and colony formation were employed to detect the effectiveness of TTFields concomitant with TMZ and/or CCNU. Western blot allowed the detection of expression levels of DNA repair proteins in control versus TTFields-treated cells. RESULTS The cell lines displayed comparable responses to TTFields and CCNU, while they differed in sensitivity to TMZ according to MGMT expression status. TTFields concomitant with TMZ displayed an additive effect, irrespective of MGMT expression levels. The effect of concomitant TTFields with CCNU was additive in MGMT-expressing cell lines, whereas it trended toward synergy in MGMT-non-expressing cell lines. Expression of proteins from the Fanconi Anemia (FA)-BRCA pathway for DNA repair was downregulated following TTFields application. TTFields concurrent with both TMZ and CCNU provided higher efficacy relative to TMZ plus CCNU or TTFields apart, with additivity in MGMT-expressing cell lines and a tendency toward synergy in MGMT-non-expressing cell lines. CONCLUSION The results support the clinical benefit demonstrated for concomitant TTFields with TMZ plus CCNU, showing that the efficacy of TMZ and CCNU may be enhanced by TTFields, with synergism seen for the latter case in cells not expressing MGMT. The BRCAness state induced by TTFields rationalizes this, as in the absence of MGMT DNA damage induced by CCNU requires the FA-BRCA pathway for repair.