Gene therapy for malignant glioma with the herpes simplex virus thymidine kinase/ganciclovir (HSV-tk/GCV) system is already in the stage of clinical trials, but still needs major improvement to achieve greater clinical efficacy. The aim of this study was to determine whether combining HSV-tk/GCV gene therapy with temozolomide (TMZ), an alkylating drug clinically proven to be efficient in recurrent high-grade gliomas, would result in enhanced antitumor effect in malignant glioma in culture and in vivo. Human U87MG glioblastoma (GBM) cells with or without expression of HSV-tk were treated with different concentrations of GCV, TMZ, or both drugs. Cell viability was accessed by an automated microplate assay (MTT). The isobologram method and the combination index (CI) method of Chou-Talalay were used to measure the interactions between the two drugs when applied simultaneously. U87-tk and control U87 cells (5x10(6) each) were implanted in the flanks of nude mice, and animals were treated with GCV or TMZ or with both drugs. All tumors were measured and weighed at specified time points. IC(50) for GCV was 511 microM in control U87 cells and 14.3 microM in U87-tk cells, resulting in 35.7-fold increase of toxicity in the HSV-tk-expressing cells. TMZ had an IC(50) of 20.2 mM in control cells and 2.35 mM in U87-tk cells, resulting in 8.6-fold increase in sensitivity of the HSV-tk-expressing cells. TMZ and HSV-tk/GCV actions were synergistic (CI<1) in both control and U87-tk cells with higher synergism in U87-tk cells at high effect levels. Tumors expressing HSV-tk and treated with TMZ and GCV were significantly smaller than those treated by 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 GCV, 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 gene therapy and systemic chemotherapy should be explored further.