gms | German Medical Science

Background: High-grade gliomas grow rapidly and aggressively and despite the introduction of new multimodality treatment regimens, such as postoperative radiotherapy and radiochemotherapy, prognosis is still poor with only slight improvments. A new promising option is heavy ion therapy which may be of advantage in the hypoxic environment of malignant gliomas and with respect to the induction of complex DNA damage which resists repair. The better radiotherapy (photons) outcomes when combined with chemotherapy for many tumor entities (including high-grade glioma), neccessitates that also the possible interactions of heavy ion irradiations with chemotherapeutic exposures are evaluated in preclinical settings. Therefore, the purpose of the ongoing study is to test a panel of chemotherapy agents representative of different mechanisms of action for their combined efficacy with therapeutic carbon ion exposures in vitro and using different glioma cell lines. Here, first data obtained with the U87 cell line exposed to Paclitaxel or to Gemcitabine are reported.

Materials and methods: U87 cells from log-phase cultures were exposed for 4 hours to different concentrations of Paclitaxel (5 nM or 20 nM), Gemcitabine (10 nM or 30 nM), or mock-treated. Immediately following drug wash-out (medium change), cell cultures were irradiatiated with either 6 MV photons (2.5 Gy/min, up to 10 Gy) or exposed by raster scanning to extended Bragg-peak carbon ions (LET: 103 keV/µm, 0.5 Gy/min, 0.125, 0.5, 2, 3 Gy). The cytotoxic effects were assessed using the clonogenic survival assay run at least in triplicate (independent determinations) for each exposure condition.

Results: Monotherapy with radiation yielded survival curves that were analyzed by LQ-fitting and the respective sensitivity parameters for photons were a=0.29 Gy^-1 and b=0.0063 Gy^-2. Although the carbon ion survival curve displayed a slight biphasic shape with a somewhat steeper slope at the low doses (0.125 Gy and 0.5 Gy), monoexponential fitting was applied and resulted in a=1.15 Gy^-1. Accordingly, RBE (fitted) declined with decreasing survival level from 3.95 (low dose extrapolation) to 3.26 (at 3 Gy carbon ion dose). The chemotherapy exposures were adjusted to yield an only moderate single agent toxicity (Paclitaxel: 80% to 60% survival; Gemcitabine: 90% to 70% survival). When combined with photon irradiations, the respective survival data for both drugs exhibited only simple additive effects (independent toxicities). Additivity was also found in the combination experiments of Paclitaxel or of Gemcitabine with high LET carbon ions (at least ³ 0.5 Gy within error bars). The biphasic behaviour was qualitatively maintained but was enhanced at the lowest dose of 0.125 Gy).

Discussion: The present data do not justify to assume a combination effect of Paclitaxel or Gemcitabine and carbon ion exposures that would differ from the respective response after photon irradiation (independent toxicities) with these log-phase culture U87 cells. The unexpected, though moderate, deviation from a monoexponential dose-response of survival after carbon ion monotherapy could be taken to indicate an inhomogeneous dose delivery to the cells. However, with this scenario one would need to assume that the fraction of “overdosed” cells increased in response to chemotherapy pretreatment, which is not possible. Alternatively, the irradiated cell population could be inhomogeneous with respect to carbon ion radiation sensitivity, the more sensitive subpopulation being increased by chemotherapy pretreatment. But such speculations can only be substantiated when data precision is increased by additional experimentation.