The experimental and clinical rationale for the use of S-phase-specific radiosensitizers to overcome tumor cell repopulation.

Clinical and laboratory evidence suggests that several common human cancers contain populations of rapidly proliferating clonogens that may have a substantial impact on local control following conventional radiotherapy. Strategies to improve locoregional control include the use of S-phase-specific radiosensitizers, such as the halogenated pyrimidine analogues (5-iododeoxyuridine, 5-bromodeoxyuridine, fluorodeoxyuridine, 5-fluorouracil) and hydroxyurea. These drugs are taken up and metabolized only by cells synthesizing DNA so that increased tumor proliferation should result in increased radiosensitization. Although the initial clinical trials with these agents were inconclusive, several recent reports have rekindled interest in these radiosensitizers. Ongoing laboratory research has provided further insight into the basic mechanisms of radiosensitization. However, many questions remain unanswered. We will review the data that suggest rapid tumor proliferation, experimental studies with the S-phase-specific drugs, and the results of clinical trials. We will also consider the possible design of future trials based on our current understanding of tumor proliferation and the mechanisms of radiosensitization of S-phase-specific agents.