Local failure remains a major problem for radiotherapy of human solid tumors, and at least a part of tumor radioresistance is generally agreed to be linked to hypoxia. Clinical approaches to overcoming the problem of hypoxic tumor cells have generally been directed toward improving tumor oxygenation and developing better hypoxic cell radiosensitizers. In the laboratory, however, new considerations suggest that tumor hypoxia may present a therapeutic opportunity, and considerable effort is now directed toward developing agents with specific toxicity to hypoxic cells. While intriguing, the practicality of either reducing or exploiting tumor hypoxia nonetheless remains questionable for human disease, largely due to uncertainty regarding the nature of hypoxia in human tumors, and lack of availability of appropriate modifying agents suitable for clinical use. Our ongoing studies with spheroids in vitro, recently extended to human xenografts in vivo, have revealed that a number of agents capable of modifying tumor hypoxia are already in clinical use: common cancer chemotherapeutic drugs. Our intent in this project is to test selected drugs as radiation modifiers in human tumor xenografts in mice, by identifying time and dose schedules which produce radiosensitization, altered blood flow, metabolic perturbations leading to reoxygenation, and/or preferential cytotoxicity. The therapeutic potential of the combination treatments will be assessed using fluorescence-activated cell sorting techniques to isolate and differentially study hypoxic versus aerobic cell subpopulations from the xenografts. Concurrently, drug induced changes in blood flow will be quantified at the macroscopic level with laser Doppler flowmetry, and at the microregional level with image analysis techniques. Oxygen utilization rates will be measured for cells in situ using an innovative fluorescent staining procedure. Using conventional cancer chemotherapy drugs for the specific purpose of modifying the amount and degree of tumor hypoxia is, to the applicant's knowledge, unique to this laboratory. We consequently anticipate that our integrated results will, at the very least, provide novel information concerning the nature and role of hypoxia in the in vivo action and interaction of cisplatin, doxorubicin, mitomycin-C and CCNU with radiation; at best, new treatment schemes entirely feasible for immediate clinical testing will be suggested.