New and emerging radiosensitizers and radioprotectors

The combination of chemotherapy and radiation has led to clinical breakthroughs in several disease sites, and current work continues to define optimum combinations of proven chemotherapy as well as more recently available, noncytotoxic agents. Administration of systemic therapies allows modulation of radiation response to improve tumor control (radiosensitization) or to prevent normal tissue toxicity (radioprotection). Substantial progress has been made in identifying the targets of standard chemotherapeutic radiation sensitizers and protectors as well as in the introduction of a new generation of molecularly targeted therapies in combination with radiation. We have reviewed the most recent, predominantly early phase clinical trials combining systemic agents with radiation. Although the proof of an improved schedule ultimately needs to come from well-run Phase III trials, the search among schedules could be shortened by the use of surrogate endpoints such as presence of active drug metabolites in the tumor. This has been accomplished only in a few cases and needs to become a more standard part of radiation sensitizer and protector trials.