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Differential p53-dependent Mechanism of Radiosensitization in Vitro and in Vivo by the Protein Kinase C-specific Inhibitor PKC412¹

Departments of Radiation Oncology [K. Z., S. R., H. R., C. G., S. B., M. P.] and Neuropathology [I. H.], University Hospital Zurich, CH-8091 Zurich; Department of Biochemistry, ETH Zurich, CH-8091 Zurich [S. R.]; and Novartis Pharma Inc. Department of Oncology Research, CH-4002 Basel [D. F.], Switzerland

The cellular response to ionizing radiation is governed by the DNA-damage recognition process but is also modulated by cytoplasmic signal transduction cascades that are part of the cellular stress response. Growth-promoting protein kinase C activity antagonizes irradiation-induced cell death, and, therefore, protein kinase C inhibitors might be potent radiosensitizers. The antiproliferative and radiosensitizing effect of the novel N-benzoylated staurosporine analogue PKC412 was tested in vitro against genetically defined p53-wild type (+/+) and p53-deficient (-/-) murine fibrosarcoma cells and in vivo against radioresistant p53-/- murine fibrosarcoma and human colon adenocarcinoma tumor xenograft (SW480, p53-mutated). PKC412 sensitized both p53+/+ and p53-/- tumor cells in vitro and in vivo for treatment with ionizing radiation but with a different mechanism of radiosensitization depending on the p53 status. In p53+/+, cells combined treatment with PKC412 and ionizing radiation drastically induced apoptotic cell death, whereas no apoptosis induction could be observed in p53-deficient cells in vitro and in histological tumor sections. Combined treatment resulted in an increased G₂ cell cycle distribution in p53-/- cells at PKC412 concentrations that did not alter cell cycle distribution when applied alone. In vivo, a minimal treatment regimen during 4 consecutive days of PKC412 (4 x 100 mg/kg) in combination with ionizing radiation (4 x 3 Gy) exerted a substantial tumor growth delay for both p53-disfunctional tumor xenografts and showed that the clinically relevant protein kinase C inhibitor PKC412 is a promising new radiosensitizer with a potentially broad therapeutic window.

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