By the time an IND application has been initiated and a drug reaches the stage of testing in humans, its pharmacokinetic, pharmacodynamic, and toxic properties have been evaluated in vivo in several species of animals in accordance with regulations and guidelines published by the FDA. Although the value of many requirements for preclinical testing is self-evident, such as those that screen for direct toxicity to organs and characterize dose-related effects, the value of others is controversial, particularly because of the well-known interspecies variation in the effects of drugs.
Trials of drugs in human beings in the U.S. generally are conducted in three phases, which must be completed before an NDA can be submitted to the FDA for review; these are outlined in Figure 5—6. Although assessment of risk is a major objective of such testing, this is far more difficult than is the determination of whether a drug is efficacious for a selected clinical condition. Usually about 2000—3000 carefully selected patients receive a new drug during phase 3 clinical trials. At most, only a few hundred are treated for more than 3-6 months regardless of the likely
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