Tumor Treating Fields (TTFields) Enhance the Efficacy of Temozolomide and Lomustine in Glioblastoma Cell Lines

Abstract

<h3>Purpose/Objective(s)</h3> Temozolomide (TMZ) is part of the standard of care treatment for newly diagnosed glioblastoma (ndGBM), the most common primary malignant brain tumor in adults. However, 50% of patients do not respond to TMZ due to expression of O6-methylguanine-DNA methyltransferase (MGMT), the enzyme involved in repair of TMZ-induced DNA damage. Tumor Treating Fields (TTFields) are alternating electric fields that display anti-mitotic effects on cancerous cells, and have also been shown to lower expression levels of proteins from the DNA repair BRCA pathway. Concurrent treatment with TMZ and TTFields demonstrated a major advance in treatment of patients with ndGBM, and was approved by the FDA in 2014. Recently, the addition of lomustine (CCNU) to TMZ demonstrated clinical benefit in ndGBM patients, with improved overall and progression free survival. The aim of the current study was to examine in GBM cells the effect of TTFields in conjunction with TMZ and CCNU. <h3>Materials/Methods</h3> U-87 MG, LN229, U118, and LN18 human GBM cell lines were treated with TTFields at an intensity of 0.83 V/cm RMS and frequency of 200 kHz using the inovitro system. Expression levels of MGMT in each cell line were confirmed by Western Blot. Effectiveness of TTFields concomitant with TMZ and/or CCNU was tested by measuring cell counts, colony formation, and apoptosis. Fluorescence microscopy detection of ɣH2AX was employed to measure levels of DNA double strand breaks. <h3>Results</h3> Application of TTFields with concurrent TMZ was additive in all cell lines, irrespective of MGMT expression levels. Effect of concomitant TTFields with CCNU was additive in cell lines with high MGMT expression, whereas it displayed tendency to synergism in cell lines with limited or null MGMT expression. TTFields also elevated the levels of DNA double strand breaks. TTFields in conjunction with both TMZ and CCNU further increased the efficacy of TMZ and CCNU or TTFields apart. <h3>Conclusion</h3> TTFields enhance the efficacy of TMZ and CCNU, with synergism seen for the later in cells with limited MGMT expression. The outcomes are in line with the BRCAness state induced by TTFields, as DNA damage induced by CCNU requires the BRCA pathway for repair, especially in the absence of MGMT. The results suggest a potential benefit of TTFields as concomitant treatment with TMZ/CCNU combination therapy in GBM.