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An increase in the number of identified therapeutic cancer targets achieved through recent biomedical research has resulted in the generation of a large number of molecules that need to be tested further. Current development of (anticancer) drugs is a rather inefficient process that for an average new molecule takes around 10–15 years. It is also a challenging process as it is associated with high costs and a low rate of approval. It is known that less than 10% of new molecular entities entering clinical Phase I testing progress beyond the investigational programme and reach the market; this probability is even lower for anticancer agents. In 2003, the US Food and Drug Administration (US FDA) declared the urgent need for new toolkits to improve the critical development path that leads from scientific discovery to the patient. In this scenario, Phase 0 (zero) trials should allow an early evaluation in humans of pharmacokinetic and pharmacodynamic profiles of test compounds through administration of sub-pharmacological doses and for a short time period to a low number of humans. Typically, Phase 0 studies have no therapeutic or diagnostic intent. Owing to the low doses administered and the low risk of toxicity, shorter preclinical packages to support these studies are required. Phase 0 trials have been proposed to help in making an early selection of promising candidates for further evaluation in Phase I–III trials, providing a potentially useful instrument for drug discovery, particularly in the field of oncology. Phase 0 studies are expected to reduce costs of drug development, and to limit the preclinical in vitro and in vivo testing and the time period of drug development. However, there are also concerns about the utility and feasibility of Phase 0 studies. In January 2006, guidelines on exploratory investigational new drug studies in humans have been published by the US FDA, and currently a Phase 0 programme is ongoing at the National Cancer Institute to evaluate the impact (feasibility and utility) of Phase 0 studies on drug development. In Europe, a Position Paper produced by the Evaluation of Medicinal Products (EMEA) in 2004 raised the possibility of a reduced preclinical safety package to support early microdose clinical studies, and, as announced by a recent Concept Paper on medicinal products published by the committee for medicinal products for human use of the EMEA, EMEA's guidelines on Phase 0 studies are expected shortly. The true impact of Phase 0 studies on the drug development process as well as on the safety needs to be carefully explored.British Journal of Cancer (2007) 97, 577–581. doi:10.1038/sj.bjc.6603925 www.bjcancer.com Published online 28 August 2007 [ABSTRACT FROM AUTHOR]