Temozolomide enhances herpes simplex virus thymidine kinase/ganciclovir therapy of malignant glioma

Gene therapy for malignant glioma with the herpes simplex virus thymidine k inase/ganciclovir (HSV-tk/GCV) system is already in the stage of clinical t rials, but still needs major improvement to achieve greater clinical effica cy. The aim of this study was to determine whether combining HSV-tk/GCV gen e therapy with temozolomide (TMZ), an alkylating drug clinically proven to be efficient in recurrent high-grade gliomas, would result in enhanced anti tumor effect in malignant glioma in culture and in vivo. Human U87MG gliobl astoma (GBM) cells with or without expression of HSV-tk were treated with d ifferent concentrations of GCV, TMZ, or both drugs. Cell viability was acce ssed by an automated microplate assay (MTT). The isobologram method and the combination index (Cl) method of Chou-Talalay were used to measure the int eractions between the two drugs when applied simultaneously. U87-tk and con trol U87 cells (5 x 10(6) each) were implanted in the flanks of nude mice, and animals were treated with GCV or TMZ or with both drugs. All tumors wer e measured and weighed at specified time points. IC50 for GCV was 511 muM i n control U87 cells and 14.3 muM in U87-tk cells, resulting in 35.7-fold in crease of toxicity in the HSV-tk-expressing cells. TMZ had an IC50 of 20.2 mM in control cells and 2.35 mM in U87-tk cells, resulting in 8.6-fold incr ease in sensitivity of the HSV-tk-expressing cells. TMZ and HSV-tk/GCV acti ons were synergistic (Cl < 1) in both control and U87-tk cells with higher synergism in U87-tk cells at high effect levels. Tumors expressing HSV-tk a nd treated with TMZ and GCV were significantly smaller than those treated b y TMZ, but not by GCV. There was also a significant difference between the weight of HSV-tk expressing versus control tumors treated with TMZ, with GC V, or with both drugs. These data demonstrate synergism between HSV-tk/GCV and TMZ and higher sensitivity against TMZ in HSV-tk-expressing GBM cells. The potential importance for clinical studies combining both local tumor ge ne therapy and systemic chemotherapy should be explored further.