Bioavailability of bound residues

Due to the use of radiolabeled drugs and feed additives in residue evaluation of preclearance approval of new animal drug applications (NADAs), metabolites including those which are covalently bound are detected in the edible tissues of target food animals. Isolation and characterization of these residues is often a difficult task. The approach taken is to couple safety assessment with development of the required residue information. An outline of data development is made in the context of toxicity assessment. Unless otherwise adjusted, the sponsor's method of residue analysis and drug withdrawal time reflect responsibility of all drug residues of toxicological concern found in edible tissues. The use of bioavailability studies and how they interact with safety evaluation is developed. Focus is brought to the specific problem of covalently bound residues and a framework for their assessment is presented.