Use of pharmacokinetics when dealing with the drug residue problem in food-producing animals

If the principal pharmacokinetic parameters of a veterinary drug-such as its elimination half-life from blood plasma, volume of distribution, plasma protein binding and metabolism-are known, then the presence and concentration of residues in animal tissue after administration of the drug can be predicted. This makes it possible to reduce the number of animals required by conventional residue testing and satisfies a legitimate demand of animal welfare groups. Using radioactively labelled drugs, the ratio of the parent substance to its metabolites can be established. Thus as a rule the determination of parent substance is sufficient for the routine determination of residues in foodstuffs. This is true in particular of sulfonamides. The pharmacokinetics of this class of substances are such that their main metabolites are eliminated from the animal's body faster than the parent substance. Other examples given are ceftriaxone, carprofen und climazolam.