Validation of an amino-acid-based radionuclide therapy plus external beam radiotherapy in heterotopic glioblastoma models

Abstract

Malignant gliomas represent a major therapeutic challenge because no efficient treatment is currently available. p-[(131)I]iodo-L-phenylalanine ([(131)I]IPA) is a glioma avid radiopharmaceutical that demonstrated antiproliferative and tumoricidal effects in gliomas. The present study validated the therapeutic efficiency of [(131)I]IPA combined with external beam radiotherapy in experimental gliomas. Glioma cells derived from the primary human A1207, T5135, Tx3868 and M059K glioblastoma cell lines or rat F98 glioma cell line were treated with various doses of [(131)I]IPA, external photon irradiation (RT) or combined [(131)I]IPA/RT treatment. Responsiveness of glioma cells to the different therapy modalities was investigated at 24, 48 and 72 h after treatments by trypan blue, WST-1 assay, propidium iodide and bisbenzimide staining as well as by clonogenic assay. In addition, the therapy-induced DNA damage and repair were evaluated using phosphorylated histone H2AX (γ-H2AX). In vivo, the effectiveness of the combination treatment was validated in human Tx3868 and A1207 glioblastoma xenografts in CD1 nu/nu mice and RNU rats. In vitro, the combination treatment resulted in a greater than additive increase in cytotoxic effect in glioma cell lines. Cell survival rate following a treatment with 1.0 μCi (37 kBq) of [(131)I]IPA amounted to 70%±15% and 60%±10% after 48 and 72 h, respectively, and decreased under 20% after additional RT with 5 Gy. At higher RT doses, cell survival rate decreased below 5%. As a measure of DNA double-strand break, nuclear γ-H2AX foci were determined as a function of time. Within 24 h, the number of γ-H2AX foci per cell was significantly greater after combined modality compared with the individual treatments. In vivo, when combined with RT, the radionuclide therapy with [(131)I]IPA resulted in an extended tumor growth delay, a reduction of the initial tumor volume and an enhanced radiosensitivity in Tx3868 and A1207 glioblastoma xenografts in CD1 nu/nu mice and RNU rats. On day 90 after monotherapy with [(131)I]IPA (20 MBq) or RT (20 Gy), 35%-50% of the treated rats were still alive. In comparison, up to 70%-80% survival rates were registered after combined [(131)I]IPA/RT treatment on day 100 for all animal models. These preclinical data convincingly demonstrated that [(131)I]IPA plus external beam photon radiotherapy is a safe and highly effective treatment for experimental gliomas, which may merit a clinical trial to ascertain its potential as a therapeutic approach in patients. As only a low [(131)I]IPA activity and a low RT dose were applied, further optimization strategies should be pursued experimentally, including application of higher radiation doses and conventional fractionated regimens or use of methods aiming to increase target doses and maximize dose effects.