Introduction Maximizing responses of malignant gliomas is hampered by resistance to temozolomide (TMZ). Increasing efficacy but not toxicity is a key issue when testing drug combinations. The antimyeloma agent bortezomib (BZ) has shown promising results in vitro and is currently being tested in glioblastoma (GBM) patients. In this study we investigate whether reduction of TMZ dosage is feasible without compromising the antitumor effect of TMZ-BZ combination. Material and methods U87 GBM cells were treated with increasing doses of TMZ (1, 10, 100, 1000 µM), BZ (0.001, 0.01, 0.1, 1) and the combination during a 48-hour period, and apoptotic or/and necrotic cell death was evaluated by flow cytometry. Results Bortezomib alone at a dose as low as 0.001 µM markedly induced cell death, particularly late apoptosis, to a level which was comparable with high TMZ dosage. For combination treatments, the dose of 0.1 µM BZ, which was more potent than the maximal dose of TMZ (1000 µM), was chosen to be added to increasing TMZ concentrations. The combination of 0.1 BZ µM BZ with low doses of TMZ (1, 10 µM) further increased the cell death rate in an additive manner, at levels higher than those induced by high doses of TMZ monotherapy (100, 1000 µM). Conclusions Efficacy of TMZ-BZ combination is feasible with low doses of TMZ in vitro.
