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Phosphatase and Tensin Homologue Deficiency in Glioblastoma Confers Resistance to Radiation and Temozolomide that Is Reversed by the Protease Inhibitor Nelfinavir

¹ Department of Radiation Oncology and ² Biostatistics and Epidemiology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania; ³ Department of Neuro-oncology and Department of Molecular Genetics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas; and ⁴ Departments of Radiation Oncology and Biology, Radiobiology Research Institute, Oxford University, Oxford, United Kingdom

Requests for reprints: Amit Maity, 195 John Morgan Building, University of Pennsylvania School of Medicine, 3620 Hamilton Walk, Philadelphia, PA 19104. Phone: 215-614-0078; Fax: 215-898-0090; E-mail: maity{at}xrt.upenn.edu.

Glioblastomas are malignant brain tumors that are very difficult to cure, even with aggressive therapy consisting of surgery, chemotherapy, and radiation. Glioblastomas frequently have loss of the phosphatase and tensin homologue (PTEN), leading to the activation of the phosphoinositide-3-kinase (PI3K)/Akt pathway. We examined whether PTEN deficiency leads to radioresistance and whether this can be reversed by nelfinavir, a protease inhibitor that decreases Akt signaling. Nelfinavir decreased Akt phosphorylation and enhanced radiosensitization in U251MG and U87MG glioblastoma cells, both of which are PTEN deficient. In the derivative line U251MG-PTEN, induction of wild-type PTEN with doxycycline decreased P-Akt expression and increased radiosensitivity to a similar extent as nelfinavir. Combining these two approaches had no greater effect on radiosensitivity than either alone. This epistasis-type analysis suggests that the nelfinavir acts along the Akt pathway to radiosensitize cells. However, nelfinavir neither decreased Akt phosphorylation in immortalized human astrocytes nor radiosensitized them. Radiosensitization was also assessed in vivo using a tumor regrowth delay assay in nude mice implanted with U87MG xenografts. The mean time to reach 1,000 mm³ in the radiation + nelfinavir group was 71 days, as compared with 41, 34, or 45 days for control, nelfinavir alone, or radiation alone groups, respectively. A significant synergistic effect on tumor regrowth was detected between radiation and nelfinavir. (P = 0.01). Nelfinavir also increased the sensitivity of U251MG cells to temozolomide. These results support the clinical investigation of nelfinavir in combination with radiation and temozolomide in future clinical trials for patients with glioblastomas. [Cancer Res 2007;67(9):4467–72]

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