The effects of tumor treating fields and temozolomide in MGMT expressing and non-expressing patient-derived glioblastoma cells

Abstract

A recent Phase 3 study of newly diagnosed glioblastoma (GBM) demonstrated the addition of Tumor Treating Fields (TTFields) to temozolomide (TMZ) after combined radiation/TMZ significantly increased survival and progression free survival. Preliminary data suggested benefit with both methylated and unmethylated O-6-methylguanine-DNA methyl-transferase (MGMT) promoter status. To date, however, there have been no studies to address the potential interactions of TTFields and TMZ. Thus, the effects of TTFields and TMZ were studied in vitro using patient-derived GBM stem-like cells (GSCs) including MGMT expressing (TMZ resistant:12.1 and 22 GSC) and non-MGMT expressing (TMZ sensitive:33 and 114 GSC) lines. Dose-response curves were constructed using cell proliferation and sphere-forming assays. Results demonstrated a ≥10-fold increase in TMZ resistance of MGMT-expressing (12.1 GSCs: IC50=160 μM; 22 GSCs: IC50=44 μM) compared to MGMT non-expressing (33 GSCs: IC50=1.5 μM; 114 GSCs: IC50=5.2 μM) lines. TTFields inhibited 12.1 GSC proliferation at all tested doses (50-500 kHz) with an optimal frequency of 200 kHz. At 200 kHz, TTFields inhibited proliferation and tumor sphere formation of both MGMT GSC subtypes at comparable levels (12.1 GSC: 74±2.9% and 38±3.2%, respectively; 22 GSC: 61±11% and 38±2.6%, respectively; 33 GSC: 56±9.5% and 60±7.1%, respectively; 114 GSC: 79± 3.5% and 41±4.3%, respectively). In combination, TTFields (200 kHz) and TMZ showed an additive anti-neoplastic effect with equal efficacy for TTFields in both cell types (i.e., +/- MGMT expression) with no effect on TMZ resistance. This is the first demonstration of the effects of TTFields on cancer stem cells. The expansion of such studies may have clinical implications.

PLK1 inhibitor facilitates the suppressing effect of temozolomide on human brain glioma stem cells

“…Temozolomide (TMZ) is a DNA‐alkylating agent that causes lethal DNA lesions in fast‐dividing cancer cells . TMZ is widely used to treat primary or metastatic brain cancer, which had efficacy on the recovery of patients . It was reported that BIS or HSF1 knockdown combined with TMZ treatment increased tumour apoptosis while cancer stem‐like properties was suppressed, such as SOX2 protein expression .…”

Tumor Treating Field Therapy in non-MGMT-Methylated Newly Diagnosed Glioblastoma: Is there a Role for Temozolomide?

“…The results of Clark et al [21] failing to demonstrate a synergistic effect between the alkylating agent TMZ, although significant, is not surprising. From a mechanistic standpoint, TTFields were shown to interfere with the spindle tubulin polymerization as a consequence of generating non-uniform intracellular fields that exert forces that move organelles and polar macromolecules, resulting in apoptotic cell death [11] .…”

“…Thus, to test the hypothesis that TTFields might exhibit a synergistic, or supra-additive interaction with TMZ, particularly in MGMT protein expressing GBM cells that are resistant to TMZ (modeling unmethylated MGMT GBM patients), investigators at the University of Wisconsin elected to address this question with an in vitro model (see figure 2). In view of the clinical relevance of these recently published results [21] , the review below summarizes these preclinical experiments:…”

“…Publication Summary [21] The effects of TTFields and TMZ were studied in vitro using 2 different sets of patient-derived GBM stem-like cells (GSCs) including MGMT expressing (TMZ resistant) GSC and non-MGMT expressing (TMZ sensitive) lines. Dose-response curves were constructed using cell proliferation and sphere-forming assays.…”

The Clinical Application of Tumor Treating Fields Therapy in Glioblastoma

“…In another randomized clinical trial of patients with glioblastoma who had received standard radiation and concomitant temozolomide chemotherapy, the addition of TTFields to maintenance temozolomide chemotherapy resulted in improved outcomes compared to those that received maintenance temozolomide alone 23 . Additionally, research has shown that TTFields work irrespective of patient MGMT promoter methylation status; therefore TTFields may constitute a clinical intervention that also works in patients with unmethylated MGMT status 24 . Taken together, these studies suggest broad implications on the effectiveness of TTFields for the treatment of glioblastomas.…”