POTENTIATION OF RADIATION THERAPY BY CARBOPLATIN

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A new platinum complex diammine(1,1-cyclobutanedicarboxylato)platinum(II) [Carboplatin or CBDCA] promies to replace or complement Cisplatin (cis-DDP) as an important anti-tumor agent based on clinical trials that have demonstrated efficacy without dose-limiting nephro- or gastrointestinal toxicity. Since the parent complex, cis-DDP, and several platinum analogs have been shown to potentiate radiation-induced cell kill in vitro and radiation therapy (RT) in animal tumors, this project proposes to test for two types of interaction when CBDCA is combined with RT and to compare the results with those obtained using cis-DPP. The study is divided into three parts. First, we will test for, and distinguish between, two interactions: the radiosensitization (RS) of hypoxic and euoxic cells, and (b)\radiation-induced enhanced chemotoxicity (EC) that results when platinum complexes are added after irradiation, which may result from the inhibition of radiation damage repair mechanisms. Second, we will determine whether both RS of hypoxic cells by platinum and post-radiation EC are demonstrable and important in human tumor cells treated as xenografts in nude mice. Third, we will measure the concentrations of CBDCA and cis-DDP in cells and in tumors and correlate total free and DNA-bound platinum levels, with the magnitude of interactions produced. The in vitro studies will employ two "normal" cells lines: the rodent line V79 and the human fetal lung fibroblast HFL. In addition, the human malignant melanoma cell line HX34 will be used along with two strains of a rodent tumor (Walker 256), one of which (WS) is more sensitive to platinum than the other (WR). The cell kill resulting from combined modalities will be assessed using colony forming unit (CFU) analysis on petri dish surfaces or in agar. The xenograft studies will employ the human malignant melanoma HX34, and cell survival will be evaluated using CFU analysis of cells explanted into agar following treatment in situ. Platinum levels will be measured using atomic absorption spectroscopy (AAS). These studies are designed to answer such questions as: (a) Does this second generation platinum complex interact more extensively with radiation than does Cisplatin? (b) Is this interaction greatest when cellular levels of total or free platinum are at their highest or when maximum platinum is bound to DNA?; and (c) Do these interactions occur in solid tumor? The results of this project will contribute directly to a rationale for combining CBDCA or cis-DDP with RT and are necessary to the formulation of appropriate designs for the clinical protocols that are being planned to exploit these interactions.