Drug-protein interactions in plasma play an important role in the distribution, excretion and metabolism of drugs within the body. It is of great interest to study the potential binding sites on transporting proteins for a drug candidate while it is circulating in plasma. A limited digestion method followed by stable isotope labeling was developed to identify such binding sites on the protein human serum albumin. A protein control sample and a sample containing a drug plus the protein were both tested by this method and analyzed by LC/MS. This approach made it possible to identify the binding sites on the protein by comparing the digestion pattern for both the protein control and drug-protein samples. Warfarin, indole-3-propionic acid and carbamazepine were all examined using this method to study their binding sites on human serum albumin. On the other hand, an important topic in drug discovery and development is figuring out the roles of different serum proteins in this drug binding and distribution process.
The use of immobilized drugs (or hormones) in microaffinity columns was also developed for use in trapping columns for quickly extracting drug binding proteins from plasma. This strategy used affinity microcolumns that were packed with a solid support that contained an immobilized analog of the drug or hormone of interest. Those proteins that had strong binding to the drug (or hormones) were then captured by the column and later identified by LC/MS after digestion.
