In vitro evaluation of combined temozolomide and radiotherapy using X rays and high-linear energy transfer radiation for glioblastoma

High-linear energy transfer radiation offers superiorbiophysical properties over conventional radiotherapy andmay have a great potential for treating radioresistanttumors, such as glioblastoma. However, very little preclinical data exists on the effects of high-LET radiation onglioblastoma cell lines and on the concomitant application ofchemotherapy. This study investigates the in vitro effects oftemozolomide in combination with low-energy protons and aparticles. Cell survival, DNA damage and repair, and cellgrowth were examined in four human glioblastoma cell lines(LN18, T98G, U87 and U373) after treatment with either Xrays, protons (LET 12.91 keV/lm), or a particles (LET 99.26keV/lm) with or without concurrent temozolomide atclinically-relevant doses of 25 and 50 lM. The relativebiological effectiveness at 10% survival (RBE10) increased asLET increased: 1.17 and 1.06 for protons, and 1.84 and 1.68for a particles in the LN18 and U87 cell lines, respectively.Temozolomide administration increased cell killing in the O6-methylguanine DNA methyltransferase-methylated U87 andU373 cell lines. In contrast, temozolomide provided notherapeutic enhancement in the methylguanine DNA methyltransferase-unmethylated LN18 and T98G cell lines. Inaddition, the residual number of c-H2AX foci at 24 h aftertreatment with radiation and concomitant temozolomide wasfound to be lower than or equal to that expected by DNAdamage with either of the individual treatments. Kinetics offoci disappearance after X-ray and proton irradiationfollowed similar time courses; whereas, loss of c-H2AX fociafter a particle irradiation occurred at a slower rate thanthat by low-LET radiation (half-life 12.51–16.87 h). Thecombination of temozolomide with different radiation typescauses additive rather than synergistic cytotoxicity. Nevertheless, particle therapy combined with chemotherapy mayoffer a promising alternative with the additional benefit ofsuperior biophysical properties. It is also possible that newfractionation schedules could be designed to exploit thechange in DNA repair kinetics when MGMT-methylated cellsrespond to high-LET radiation.