J. K. Strebe5, P. A. Clark5, C. Pasch2, D. A. Demings2,3,4,8, H. I. Robins2,3,6,7,8, J. S. Kuo5 2University Of Wisconsin School Of Medicine And Public Health,Carbone Cancer Center,Madison, WI, USA 3University Of Wisconsin School Of Medicine And Public Health,Division Of Hematology And Oncology, Department Of Medicine,Madison, WI, USA 4University Of Wisconsin School Of Medicine And Public Health,McArdle Laboratory For Cancer Research, Department Of Oncology,Madison, WI, USA 5University Of Wisconsin School Of Medicine And Public Heath,Department Of Neurological Surgery,Madison, WI, USA 6University Of Wisconsin School Of Medicine And Public Health,Department Of Neurology,Madison, WI, USA 7University Of Wisconsin School Of Medicine And Public Health,Department Of Human Oncology,Madison, WI, USA 8William S Middleton Memorial Veterans Hospital,Madison, WI, USA
Introduction: Tumor Treating Fields (TTFs) are a novel antimitotic, non-invasive, externally applied cancer treatment being developed for use against many human solid cancers. Glioblastoma (GBM) is the most frequently diagnosed adult brain malignancy, with a median survival of less than 2 years after the standard therapeutic regimen of maximal surgery followed by radiation and temozolomide (TMZ) chemotherapy. In a Phase 3 clinical trial (NCT#00916409), addition of TTFs to current therapies significantly increased overall and progression free survival for newly diagnosed GBM. TTFs are FDA-approved for newly diagnosed and recurrent GBM. We report the first study of TTFs alone, or combined with TMZ, against GBM stem-like cells (GSC) expressing different levels of the therapy resistance DNA repair enzyme, O-6-methylguanine DNA methyl-transferase (MGMT), to test the hypothesis that TTF is effective against TMZ-resistant GSC.
Methods: Effects of TTFs and TMZ were studied in the patient-derived 22 GSC (MGMT-expressing, TMZ resistant) and 33 GSC (non-MGMT-expressing, TMZ sensitive) cell lines with continuous application of in vitro TTFs at varying frequencies to GSC cultures using the Inovitro system (Novocure Ltd). The effects of varying doses or frequencies of TMZ, TTFs, and combined TMZ+TTFs on GSC proliferation and sphere-forming ability were analyzed.
Results: We have previously determined in vitro that 200 kHz is the optimal TTF frequency to inhibit GSC proliferation, identical to the clinical trial frequency. At 200 kHz, TTFs significantly inhibited proliferation (22 GSC: 61±10.8%; 33 GSC 56±9.5%; p<0.05) and clonogenic tumor sphere formation (22 GSC: 38±2.6%; 33 GSC: 60±7.1%; p<0.05) in both TMZ-resistant and TMZ-sensitive GSC subtypes. In combination, TTFs and TMZ (at IC25, IC50, IC75 concentrations) showed an additive interaction.
Conclusion: This is the first study to directly demonstrate that TTFs can overcome TMZ-resistance in GBM cancer stem cells (GSC), and reports the effects of TTFs on GSC proliferation and clonogenic tumor sphere formation with equivalent effectiveness against both therapy resistant and sensitive GSC subtypes (+/- MGMT expression). The combination of TTFs and TMZ was additive, and supports the hypothesis that TTF acts via a mechanism independent of TMZ-mediated DNA alkylation. Further study of TTFs in additional human cancers and in potential combination with other therapeutics will be needed to optimize TTFs as a new therapy option for human cancer.