Comparison of cerebral Open Flow Microperfusion and Microdialysis when sampling small lipophilic and small hydrophilic substances

Abstract Background Assessment of drug concentration in the brain interstitial fluid (ISF) is crucial for development of brain active drugs, which are mainly small, lipophilic substances able to cross the blood-brain barrier (BBB). We aimed to compare the applicability of cerebral Open Flow Microperfusion (cOFM) and Microdialysis (MD) to sample the lipophilic substance amitriptyline (AMI), its metabolites Hydroxyamitriptyline (HYA), Nortriptyline (NOR), Amitriptyline -N- Oxide (ANO), deuterated water (D 2 O) and the hydrophilic substance sodium fluorescein (Naf) in brain ISF. New method cOFM has been refined to yield increased spatial resolution and performance. Comparison of cOFM and MD and results Performance of cOFM and MD was assessed by in vivo AUC ratios of probe samples (AUC COFM /AUC MD ) and the in vivo relative recovery of D 2 O (RR vv,D2O ). Adsorption of AMI and Naf to MD and cOFM was assessed by the in vitro relative recovery (RR vt ) prior to the in vivo experiments. The in vivo AUC ratio of AMI and RR vv,D2O was about two times higher for cOFM than for MD (AUC OFM /AUC MD  = 2.0, RR vv,D2O (cOFM)/RR vv,D2O (MD) = 2.1). cOFM detected all investigated AMI metabolites except NOR. MD did not detect HYA, NOR, ANO and Naf. In vitro adsorption of AMI and Naf to the MD membrane was strong (RR vt,AMI  = 4.4%, RR vt,Naf  = 1.5%) but unspecific adsorption to cOFM was negligibly small (RR vt,AMI  = 98% and RR vt,Naf  = 98%). Conclusions cOFM showed better performance when sampling AMI and its metabolites, Naf and D 2 O, and had an about two times higher RR vv,D2O than MD. MD did not detect HYA, NOR, ANO and Naf, most likely due to membrane adsorption.