Application of microdialysis to study the in vitro metabolism of drugs in liver microsomes

Time-resolved microdialysis for in vivo neurochemical measurements and other applications

Monitoring changes in chemical concentrations over time in complex environments is typically performed using sensors and spectroscopic techniques. Another approach is to couple sampling methods, such as microdialysis, with chromatographic, electrophoretic, or enzymatic assays. Recent advances of such coupling have enabled improvements in temporal resolution, multianalyte capability, and automation. In a sampling and analysis method, the temporal resolution is set by the mass sensitivity of the analytical method, analysis time, and zone dispersion during sampling. Coupling methods with high speed and mass sensitivity to microdialysis sampling help to reduce some of these contributions to yield methods with temporal resolution of seconds. These advances have been primarily used in monitoring neurotransmitters in vivo. This review covers the problems associated with chemical monitoring in the brain, recent advances in using microdialysis for time-resolved in vivo measurements, sample applications, and other potential applications of the technology such as determining reaction kinetics and process monitoring.

An in vitro microdialysis methodology to study 11β-hydroxysteroid dehydrogenase type 1 enzyme activity in liver microsomes

Microdialysis sampling coupled with liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS/MS) was used to observe in vitro 11beta-hydroxysteroid dehydrogenase type 1 (HSD1) enzyme-catalyzed conversion of stable-isotope-labeled cortisone to cortisol in liver microsomes from dog, monkey, and human. Experimental conditions that would affect the microdialysis sampling approach including probe length, perfusion fluid flow rate, extraction efficiency (E(d)), substrate concentration, and enzyme reaction conditions were evaluated. Dialysates containing high salt concentrations (>150 mM) were directly assayed using LC/MS/MS without additional sample cleanup. The sensitivity (with lower level of quantitation at 0.1 ng/mL) and selectivity of this assay allowed detection of the enzyme reactants at physiologically relevant levels. The interconversion from M+4 cortisone to M+4 cortisol was detected in dog, human, and monkey liver microsomes. Results show species-specific reaction profiles, with a five times higher conversion rate in dog liver microsomes than in human and monkey liver microsomes. Based on M+4 cortisol production rate obtained using a microdialysis infusion of M+4 cortisone to the microsomes coincubated with a proprietary 11beta-HSD1 inhibitor of different concentrations, the degrees of enzyme inhibition were found to be 40 and 85%, consistent with values obtained by a traditional in vitro incubation method. The microdialysis sampling methodology with LC/MS/MS provided extensive information about 11beta-HSD1 activities in microsomes from different mammalian species.

Binding study of drug with bovine serum album using a combined technique of microdialysis with flow-injection chemiluminescent detection

Microdialysis coupled with flow-injection chemiluminescence (FI-CL) has been developed to determine the binding parameters of a drug binding to protein by using antibiotic tetracycline hydrochloride binding to bovine serum albumin as a model system. The drug and protein were mixed in different molar ratios in 0.067 M phosphate buffer, pH 7.4, and incubated at 37 degrees C in a water bath. The microdialysis probe was utilized to sample the mixed solution at a perfusion rate of 5 microL/min. The concentration of unbound tetracycline hydrochloride in the microdialysate was determined by FIA-CL. In vitro recovery of tetracycline hydrochloride under experimental conditions was 30.0%. The data obtained by the present microdialysis-FI-CL system was analyzed using the Scatchard analysis and Klotz plot. The results show that the Scatchard plot and Klotz plot are linear with good correlation coefficient, indicating a good agreement of the experimental data and to the theoretical equation. The FIA chemiluminescence system combined with microdialysis developed in this work demonstrated its use for determination of interaction between drug and protein by using relatively simple instrument.

Analytical considerations for microdialysis sampling

Adaptations in microdialysis probe designs have made it possible to obtain samples from the extracellular fluid of a variety of tissues with high temporal resolution. The resulting small volume samples, often with low concentration of the analyte(s) of interest, present a particular challenge to the analytical system. Rapid separations can be coupled on-line with microdialysis to provide near real-time data. By combining microdialysis sampling with a liquid chromatographic or capillary electrophoretic separation and a highly sensitive detection method, a separation-based sensor can be developed. Such sensors have been applied to the investigation of drug entities as well as to study endogenous analytes.

Investigation of stereoselective metabolism of amphetamine in rat liver microsomes by microdialysis and liquid chromatography with precolumn chiral derivatization

The utility of microdialysis as a quantitative sampling technique for in vitro drug metabolism studies was demonstrated by investigating the stereoselective metabolism of D-, L- and DL-amphetamine by the cytochrome P-450 enzymes. Microdialysates containing the isomers of amphetamine and its metabolite were derivatized with the fluorescent chiral derivatizing agent, (-)-fluorenylethyl chloroformate. The diastereoisomers were isocratically separated by liquid chromatography (LC) on a reversed-phase C18, 3-micron (100 x 3.2 mm) column. The intra- and inter-assay relative standard deviation (R.S.D.) was below 10%. Michaelis-Menten parameters, K(m) and Vmax were obtained for the formation of both D- and L-hydroxyamphetamine from D-, L- and DL-amphetamine in the concentration range of 10-350 microM.