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Small doses of radiolabeled drugs can be used for diagnostic purposes. Such microdoses are defined as 1/100th or less of the dose that would be expected to have a pharmacological effect.
In August 2018 FDA published final guidance on “Microdose Radiopharmaceutical Diagnostic Drugs: Nonclinical Study Recommendations” that provides recommendations for nonclinical studies of such drugs. The microdose evaluated during early clinical trials is less than or equal to 100 micrograms. Because such low doses are used, the guidance recommends an abbreviated nonclinical program. This guidance is intended to help sponsors facilitate the timely conduct of clinical trials, reduce the use of drug development resources, and reduce the use of animals by specifying study types for which in vitro methods may be used.