Drug interference of ifosfamide and sodium 2-mercaptoethanesulphonate (MESNA) was studied in three malignant glioma cell cultures (HTZ-17, HTZ-209B, and HTZ-243) by a recently developed in vitro method for evaluation of multimodal treatment interactions. Glioma cell cultures were treated in monolayer 96-well tissue-culture plates for 2 h each, with 4-hydroperoxyifosfamide and MESNA combined in both sequences, or alone. Concentrations ranged from 0.01 microM to 50 microM in single-modality exposures, and from 0.01 microM to 10 microM in combination exposures. After five population doubling times, DNA synthesis was determined by a standard [3H]Tdr-incorporation liquid-scintillation-counting protocol. Data points were evaluated for mono- and combined treatment dose effects (adapted with a probit function), and a model-free three-dimensional response surface was created that was compared to the theoretical additive, anticipated response surface. Local additivity was analysed for any ratio of combined treatment. No tumour effects were seen with MESNA in single-drug exposure, whereas ifosfamide resulted in more than 90% inhibition of tumour DNA synthesis. In combination experiments, MESNA could be confirmed to be inert: the anticipated theoretical combination response surfaces formed a three-dimensional extension of the single-drug ifosfamide dose/response curves--the experimental combination response surfaces displayed an identical appearance (P < or = 0.05). In conclusion, these results indicate no drug interference of MESNA and ifosfamide in malignant glioma cells.