Human glioblastoma and carcinoma xenograft tumors treated by combined radiation and imatinib (Gleevec).

Background and Purpose: Imatinib (Gleevec, Glivec) is an inhibitor of alpha- and beta-platelet-derived growth factor receptors and other tyrosine kinases, that are also associated with the function of growth factors. Imatinib has been approved for the treatment of chronic myelogenous leukemia and gastrointestinal stromal tumors and is under investigation for the therapy of several other malignant tumors. Since radiotherapy is an important treatment option in many tumors, combined effects of imatinib and radiation were analyzed here.
Material and Methods: In vitro, U87 cells (human glioblastoma), A431 cells (human epidermoid carcinoma), and HUVECs (human umbilical venous endothelial cells) were treated with imatinib alone and in combination with radiation. Clonogenic survival and cell proliferation were determined with and without additional radiation (0-10 Gy). In vivo, U87 and A431 cells (5 x 10(6)) were subcutaneously injected into hind limbs of balb c nu/u mice. Drug and radiation treatments started on day 0 when tumor volumes were approximately 400-500 mm(3). Tumors were treated with 5 x 5 Gy (U87) or 6 x 5 Gy (A431) on consecutive days from day 0. Imatinib was administered orally via the mouse diet starting on day 0 until the end of observation. Tumor growth and microvessel density (CD31 IHC) were analyzed.
Results: In vitro, imatinib increased radiosensitivity of U87 and A431 tumor cells as well as HUVECs in both clonogenic and cell number/proliferation assays. The enhancement of radiosensitivity in HUVECs was comparable to that observed in the tumor cells. In vivo, the concurrent and continuous administration of imatinib increased tumor growth delay of fractionated radiotherapy in the carcinoma and the glioblastoma models at reduced microvessel densities. No apparent additional toxicity by the combination of radiation and imatinib versus monotherapies was observed in terms of weight, skin, or general behavior.
Conclusion: Imatinib (Gleevec), a "molecular targeted" approved drug for human malignancies, can enhance the tumor growth reduction induced by fractionated radiotherapy in glioblastoma and carcinoma models. The data provides a rationale to further investigate the combination.