Metronomic treatment of temozolomide inhibits tumor cell growth through reduction of angiogenesis and augmentation of apoptosis in orthotopic models of gliomas

Glioblastoma is a highly angiogenic tumor with a dismal prognosis. Temozolomide (TMZ), a methylating agent is one of the most effective chemotherapeutic agents against glioblastoma. To overcome the problem that most of these tumors become resistant to chemotherapeutic regimens within a year, we investigated the antitumor efficacy of metronomic administration of low-dose TMZ in in vitro cell proliferation/ cytotoxicity assay and in vivo rat and nude mouse orthotopic glioma model. By in vitro assay, we elucidated that C6/LacZ rat glioma cells were more resistant to metronomic treatment of TMZ than U-87MG human glioblastoma cells and bEnd.3 mouse brain endothelial cells. Compared with the conventional chemotherapeutic regimen of TMZ, we found that frequent administration of TMZ at a low dose (metronomic treatment) markedly inhibited angiogenesis as well as tumor growth in a TMZ-resistant C6/LacZ rat glioma model. In addition, metronomic treatment of TMZ significantly augmented apoptosis of tumor cells in this model. For the TMZ-sensitive U-87MG cells, even with a very low dose of TMZ, which is not effective to reduce tumor mass, the metronomic treatment of TMZ reduced the microvessel density, i.e. angiogenesis, in a nude mouse orthotopic model. In conclusion, for both models, the metronomic treatment of TMZ decreased angiogenesis. Especially, in TMZ-resistant glioma cells, this regimen increased apoptosis of tumor cells and decreased tumor growth. The metronomic treatment of TMZ in orthotopic glioma models demonstrated a successful antiangiogenic effect which can overcome the chemoresistance in conventional TMZ chemotherapy.