Partitioning of Drug Residues in Dairy Systems

Data under this agreement are needed to properly assess the impact of commercial dairy partitioning on veterinary drug residues in pasteurized milk. There are few available data on how drug residues bind to different proteins in milk and milk products. The concentration of potential veterinary drug residues in any finished dairy product depends on the composition of the dairy product, its processing history, and the properties of the veterinary drugs. Two major factors that affect final drug concentration in dairy products are the drug partitioning in the fat and aqueous phases and drug binding to milk proteins. Models of chemical partitioning typically include the octanol-water partitioning coefficient (log Kow). Data on milk products suggests that the log Kow for a drug alone may not provide a quantitative prediction for drug partitioning in these systems. Drug binding to proteins is often specific and complex; very few studies have investigated drug binding to milk proteins and these studies have been limited to only a few veterinary drugs. These proposed experiments will be the first to systematically and comprehensively study the partitioning and protein binding specific to milk and milk products for a wide range of veterinary drugs of interest. This Interagency agreement will experimentally generate data to support the evaluation of the potential impact of partitioning on drug residues, if present, in milk. The data will help evaluate the impact of selective binding of drug residues to the whey and casein fractions of milk, and hydrophobic-based binding to milk fat. Data on the drug residue-fat/protein-binding characteristics will be used to address the following two questions in a risk assessment under development within the FDA: I. If drug residues get into the bulk-tank milk, what is the fate of these residues during processing/manufacturing of various milk products (i.e., in what milk products would these drug residues be found)? II. If present in milk, and commercially processed milk products, which drug residues have the potential for concentration in dairy products? This Interagency agreement will allow the FDA and the USDA-ARS to work collaboratively to develop the experimental data necessary to evaluate these two questions.