What chemical tests are needed? I would want to have reasonably well characterized material used in the toxicology studies and in me to ensure that no new toxic contaminant has appeared in my dose, but if we are doing only single-dose or short-term studies in animals there will be little need for stability studies, testing animal feeds containing the test drug, etc. Using labeled material early can be a major help before sophisticated assays are perfected.

If metabolism in the first human is unexpected, and significant metabolites are seen that have not been observed in animals, one might wish to return to other animal species and strains to find one in which the human metabolites can be evaluated. Perhaps this metabolite will cause dose-limiting toxicity. Perhaps knowing the toxicity of this metabolite in animals will make one wish to abort the project or alter plans about the tolerated dose.

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