Impact of protein binding on drug sampling by microdialysis and open flow microperfusion

Microdialysis (MD) and open flow microperfusion (OFM) are promising minimally invasive technologies for continuously sampling local drug concentrations in the interstitial fluid (ISF) of target tissues. However, translating the measured drug concentrations to actual tissue concentrations remains challenging, particularly for highly protein-bound drugs. This study investigated the impact of drug-protein binding, perfusate composition, and diffusion resistance on the measured concentrations of four drugs with different binding affinities to human serum albumin (HSA). MD and OFM were used in in vitro solutions and agar gels with varying HSA concentrations and in ex vivo dermal tissue setups with perfusates containing 0–4 % HSA. Our data showed that in vitro relative recoveries (RRs) were overestimated (>100 %) and strongly influenced by the perfusate's composition for highly protein-bound drugs, with e.g., a fivefold increase observed for diclofenac when comparing a 0 % to a 4 % HSA perfusate. A similar dependence of the RR on the perfusate's composition was observed in gel setups, although to a lesser extent, with the lower RRs reflecting increased diffusion resistance in agar gels compared to solutions. In ex vivo setups, dermal diffusion rather than perfusate composition predominantly influenced sampling efficiency. Unlike the fivefold increase observed in vitro, no significant change in MD RR was found ex vivo, while OFM showed the highest RR with perfusates that matched actual ISF protein concentrations. Our results emphasize that quantitative pharmacokinetic studies, particularly for highly protein-bound drugs, can be considerably optimized by selecting an appropriate combination of sampling technology and perfusate composition, while considering tissue-specific factors.