Residues of veterinary drugs in eggs and their distribution between yolk and white.

Veterinary drugs and feed additives (especially some coccidiostats) can be absorbed by the digestive tract of laying hens and transferred to the egg. Physicochemical characteristics of these compounds determine their pharmacokinetic behavior and distribution to and within the egg. Traditionally the quite lipid soluble drugs and additives are expected to yield residues only in the fat-rich yolk. However, the quite lipid soluble drug doxycycline--as well as many other drugs--showed during long-term administration higher residues in white than in yolk. In a model study with 11 sulfonamides differing in pK(a) value and lipid solubility, their distribution in vivo between yolk and white was determined. Neither differences in pK(a) values nor those in lipid solubility could explain the distributions found. Binding to egg white macromolecules in vivo as an explanatory factor was tested with five sulfonamides, and no correlation between binding and the distribution of sulfonamides between white and yolk was found. Literature data on the distribution of drugs between egg white and yolk showed a reasonable consistency within drugs and a large variability among drugs (as could be expected). This larger database also did not provide a clue as to what factor determines the distribution of a drug between egg white and yolk when given to laying hens.