Gemcitabine (dFdCyd) is an analog of cytosine arabinoside with clinical activity against a variety of solid tumors. Based on our preclinical studies demonstrating that dFdCyd is a potent radiation (RT) sensitizer, we designed a novel clinical trial combining dFdCyd with RT for the treatment of patients with advanced head and neck cancers. This trial produced approximately 80% pathologic complete responses, but there was substantial toxicity. The long term goal of this proposal is to understand the mechanism of dFdCyd-mediated radiosensitization and to use this information, in combination with data from animal models and biopsies from patients' tumors, to increase the efficacy of dFdCyd as a radiation sensitizer. This goal will be addressed through 3 specific aims: Specific Aim 1 is to elucidate the mechanism by which dFdCyd increases radiation-induced apoptosis. Our preliminary data establish that apoptosis plays a critical role in maximizing sensitization. We propose to dissect the apoptotic pathways to determine the molecular mechanism by which sensitization is achieved. Specific Aim 2 is to assess the effect of radiation on dFdCyd metabolism, clearance, and efficacy in vivo. A unique strength of this aim is that the effect of radiation on the presence of dFdCyd and its metabolites will be measured in vivo using MRS. We hypothesize that dFdCyd metabolism within tumors and normal tissues is affected by radiation, and that an understanding of the changes in metabolism will influence the design of clinical trials. Specific Aim 3 is to carry out new clinical trials based on our laboratory and clinical experience using dFdCyd. In Specific Aim 3A, we will carry out a phase I trial using twice-weekly dFdCyd administered during the last 2 weeks of radiation. This trial is derived directly from our preclinical and clinical data. In Specific Aim 3B, we will carry out a phase I trial using intraarterial carotid dFdCyd in combination with radiation. This approach also has a high likelihood of increasing the therapeutic index by limiting the exposure of the contralateral tissues to dFdCyd and of being superior to IA administration of cisplatin with radiation (RADPLAT), which produces substantial systemic toxicity. These clinical trials will include, as have our past studies, biopsies to assess the content of dFdCyd metabolites. We feel our preliminary data, research team, and record for translating preclinical findings to the clinic suggest that this proposal is likely to generate data which will improve the outcome of treatment of patients with unresectable head and neck cancer.