Chapter 6.5 Assaying protein-unbound drugs using microdialysis techniques

This article provides a concise overview of available methodological approaches for sampling protein-unbound drugs using microdialysis. Microdialysis is a technique for protein-unbound drug sampling without withdrawal of biological fluids, and thus involving minimal disturbance of physiological functions. Conventional total drug sampling includes both protein-bound and -unbound drugs. However, only the unbound fraction of a drug is available for absorption, distribution, metabolism, and elimination, as well as for delivery to the target sites for pharmacological action. Although several techniques have been used to determine protein-unbound drugs from biological fluids, including ultrafiltration, equilibrium dialysis, and microdialysis, only microdialysis allows simultaneous in vivo online sampling of protein-unbound chemicals from blood, tissues, and body fluids, such as bile juice and cerebral spinal fluid for pharmacokinetic and pharmacodynamic studies. This chapter describes the technique of microdialysis and its application in pharmacokinetic studies. Furthermore, the advantages and limitations of microdialysis are discussed, including the detailed surgical techniques for animal experiments with rat blood, brain, muscle, subcutaneous adipose tissue, liver, and bile duct to evaluate hepatobiliary excretion and enterohepatic circulation of unbound drug sampling and in vitro cell culture applications.