there is evidence of genotoxicity in vitro; in addition, a dose–response relationship
or threshold dose for the induction of aplastic anaemia in humans treated with the
drug is lacking. In the case of the nitrofurans, there is evidence of genotoxicity and
carcinogenicity.
Establishing MRLs requires information relating to how each drug product is to
be used, including: the recommended dose and frequency of administration and
application; pharmacokinetic and metabolic studies in experimental and food-
producing animals; residue depletion studies with radiolabelled drugs in target
animals; a description of the method of analysis for detecting and quantifying the
residues, including the marker residue; and studies designed to assess the impact of
residues of antimicrobial agents on food processing.
The use pattern of a drug determines its pharmacokinetic behaviour relating to
absorption, distribution, metabolism and excretion (ADME), and therefore its
residue kinetics. Pharmacokinetic studies, which are usually conducted in healthy
animals and usually also of a similar age, weight and breed, should use the same
mode of administration, product formulation, and dose and frequency of adminis-
tration or application as proposed for the product’s intended use(s). Concentration-
time profiles of the parent drug and its metabolite(s) in tissues and body fluids are
examined between the time of drug administration and the time of slaughter for
human consumption. The residue data generated in these pharmacokinetic studies
form the basis of comparisons with the MRLs proposed by the sponsor.
Metabolism studies in the food-producing animals are required to identify and
quantify the residues before the safety of residues can be assessed. The design of
these studies simulates the conditions of intended use(s) as closely as is practical.
Typically, residue depletion studies use a radiolabelled drug to examine tissue
profiles from zero withdrawal time to periods extending beyond the proposed
withdrawal time. These studies define total residues, including free and bound
residues, and major residue components. Major residues are those substances that
contribute at least 10% of the total radioactivity or are present at a tissue concen-
tration of 0.1 mg/kg or greater.
Bound residues may arise from the incorporation of residues of the drug into
endogenous compounds, chemical reaction of the parent drug or its metabolites into
macromolecules, or physical encapsulation or integration of radioactive residues
into tissue matrices (WHO 1989 ). The incorporation of small fragments of the drug,
usually consisting of one or two carbon units, into naturally occurring molecules is
not of toxicological concern. In circumstances where bound residues are charac-
terised and shown to be of no toxicological concern, total residues may be dis-
counted by the equivalent amount of bound residues. This may be necessary when
the bound residue comprises a significant portion of the total residue, or when a
high concentration of bound residues prevents the total residue from depleting
below the residue of toxicological concern and thereby precluding the assignment
of a practicable withdrawal period. On this basis, the 70th meeting of the JECFA
discounted bound residues of triclabendazole in tissues from cattle 28 days after
treatment; total residues of triclabendazole accounted for 237% of the upper bound
of the ADI compared with 31% for bioavailable residues (Reeves and Swan 2009 ).
274 P.T. Reeves