VPT 421: Veterinary Toxicology (2+0)

TYPES OF RESIDUES

Three types of residues rapidly appear after drug administration.

• Total residues, determined by overall quantitative assessment of residual radioactivity after administration of the labeled compound. This value is often used but has the major disadvantage of expressing a nonspecific radioactivity. In this way, residues, the nature of which is unknown or different from that of the initial molecule are sometimes given an acceptable daily intake, which is relative to the initial compound. This practice is often disadvantageous.
• Extractable residues, a fraction that, can be extracted from biological tissues or fluids using various solvents (water at varying pH, organic solvents) before and after denaturation of macromolecules. This fraction includes all compounds, i.e., the parent compound and its metabolites, in free form or loosely bound to tissues.
• Non-extractable or bound residues, the fraction of radioactivity that persists in tissue extracts after the treatment. The nature of these residues can be determined only after almost complete breakage, particularly of proteins (enzymatic or acid hydrolysis for instance); it may or may not be related to that of their initial molecule.