Plasma serotonin monitoring by blood microdialysis coupled to high-performance liquid chromatography with electrochemical detection in humans

The Measurement of Platelet-Poor Plasma Serotonin: A Systematic Review of Prior Reports and Recommendations for Improved Analysis

BACKGROUND

Recent reports of new and important roles for serotonin (5-hydroxytryptamine, 5-HT) in the periphery have substantially increased interest in measuring peripheral serotonin. Nearly all circulating serotonin is found within platelets and this pool has been assessed by measuring serotonin in whole blood or in platelet-rich plasma. Measurement of the much smaller but potentially critically important pool of human free plasma serotonin in platelet-poor plasma (PPP) has proven much more difficult, with a wide range of reference values reported.

CONTENT

To characterize the available data we carried out a systematic literature search of previous reports of PPP serotonin and attempted to determine the best estimate of true PPP serotonin concentration in humans. A total of 101 published reports that included PPP serotonin values in healthy controls were found and included in the summary statistical analyses. The distribution of PPP serotonin values demonstrated high skewness (+1.98), and the reported values ranged from 0.6 to 179 nmol/L, with a mean of 31.6 nmol/L, an SD of 38.9 nmol/L, and a median of 14.8 nmol/L.

SUMMARY

Reported concentrations for human PPP or free plasma serotonin were highly discrepant, with most reports giving erroneously high values that should be disregarded. Inherent difficulties in selectively measuring the extremely low concentrations of serotonin present in PPP and in preparing PPP without contamination from platelet-derived serotonin contributed to the problem, as did the failure of researchers to compare their results with those from prior studies. There is a clear and pressing need for reference materials for the measurement of plasma (PPP) serotonin.

Electrochemical Behavior of Biologically Important Indole Derivatives

Voltammetric techniques are most suitable to investigate the redox properties of a new drug. Use of electrochemistry is an important approach in drug discovery and research as well as quality control, drug stability, and determination of physiological activity. The indole nucleus is an essential element of a number of natural and synthetic products with significant biological activity. Indole derivatives are the well-known electroactive compounds that are readily oxidized at carbon-based electrodes, and thus analytical procedures, such as electrochemical detection and voltammetry, have been developed for the determination of biologically important indoles. This paper explains some of the relevant and recent achievements in the electrochemistry processes and parameters mainly related to biologically important indole derivatives in view of drug discovery and analysis.

Plasma serotonin levels are normal in pulmonary arterial hypertension

Serotonin (5-HT) has been implicated in the pathogenesis of pulmonary arterial hypertension (PAH). We hypothesized that plasma 5-HT would be elevated in PAH related to associated conditions. We performed a prospective cohort study of 21 patients with PAH undergoing initial right heart catheterization and 6 healthy controls. Platelet-free plasma 5-HT levels were similar in patients with idiopathic PAH, PAH related to associated conditions, and healthy controls. Higher 5-HT levels correlated with increased six-minute walk distance (r=0.55, p=0.04). These data suggest that plasma 5-HT levels are normal in PAH. Plasma 5-HT may therefore not be a useful biomarker in this condition.

In vivo microdialysis for PK and PD studies of anticancer drugs

In vivo microdialysis technique has become one of the major tools to sample endogenous and exogenous substances in extracellular spaces. As a well-validated sampling technique, microdialysis has been frequently employed for quantifying drug disposition at the desired target in both preclinical and clinical settings. This review addresses general methodological considerations critical to performing microdialysis in tumors, highlights selected preclinical and clinical studies that characterized drug disposition in tumors by the use of microdialysis, and illustrates the potential application of microdialysis in the assessment of tumor response to cancer treatment.

Development of an intravenous microdialysis method for pharmacokinetic investigations in humans

INTRODUCTION

Limited blood volume is a major problem in pharmacokinetic investigations in specific populations, e.g. children. Intravenous microdialysis might help to obtain improved data sets as it is already successfully done in small animals. Since quantification of drugs is crucial in microdialysis, we developed an in vitro method to produce a workable intravenous microdialysis for human use.

METHODS

A specifically designed microdialysis cell consisting of glass was heated to 37 degrees C. The cell was filled with Ringer's solution, plasma or whole blood. A microdialysis probe was inserted into the cell and perfused with Ringer's solution with addition of 4% dextran. The beta-receptor blocker sotalol served as a test drug. The stepwise in vitro evaluation process addressed issues of loss of dialysate, calibration by retrodialysis and relative recovery. These conditions were then applied in an in vivo pilot study to one single healthy volunteer after written informed consent.

RESULTS

To address loss of perfusion fluid 4% of dextran was added and high and constant amounts of dialysate were achieved. To account for changes in the relative recovery a continuous use of retrodialysis by the calibrator atenolol was introduced. The recovery of atenolol was comparable to sotalol. The pharmacokinetic analysis revealed that sotalol concentrations from microdialysates were not different from conventional plasma samples (100+/-11%, n=33) resulting in subsequent comparable pharmacokinetic parameters.

DISCUSSION

This stepwise approach using an in vitro device enabled us to demonstrate the determination of pharmacokinetic parameters of sotalol. The most important evaluation step is represented by the continuous use of retrodialysis by the calibrator atenolol because it can account for changes in the relative recovery of the drug. This approach should be a starting point to simplify pharmacokinetic studies in special populations, e.g. in small children, to improve drug treatment.

Assaying protein unbound drugs using microdialysis techniques

Compared with traditional sampling methods, microdialysis is a technique for protein unbound drug sampling without withdrawal of biological fluids and involving minimal disturbance of physiological function. Conventional total drug sample consists of unbound drugs and protein bound drugs, which are loosely bound to plasma proteins such as albumin and alpha-1 acid glycoprotein, forming an equilibrium ratio between bound and unbound drugs. However, only the unbound fraction of drug is available for absorption, distribution, metabolism and elimination, and delivery to the target sites for pharmacodynamic actions. Although several techniques have been used to determine protein unbound drugs from biological fluids, including ultrafiltration, equilibrium dialysis and microdialysis, only microdialysis allows simultaneous sampling of protein unbound chemicals from plasma, tissues and body fluids such as the bile juice and cerebral spinal fluid for pharmacokinetic and pharmacodynamic studies. This review article 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 in animal experiments from rat blood, brain, liver, bile duct and in vitro cell culture for unbound drug analysis.

Biomedical applications of capillary electrophoresis with laser-induced fluorescence detection

Capillary electrophoresis (CE) is a high-efficiency analytical technique that has had a great impact as a tool in biomedical research, clinical and forensic practice in the last ten years. Only in one of the applications, the DNA analysis, it has had an explosive exponential growth in the last few years. This impact is expressed in an enormous amount of CE articles and many reviews. The CE advantages with respect to other analytical techniques: the required very small sample volume, rapid analysis, great resolution power and low costs, have made this technique ideal for the analysis of a numerous endogenous and exogenous substances present in biological fluids. The different modes of CE have been coupled to different detection techniques such as UV-absorbance, electrochemical, mass spectrometry and laser-induced fluorescence detection (LIFD) to detect different nature and molecular size separated analytes. This review focuses mostly on the applications of CE-LIFD, to measure drugs and endogenous neuroactive substances such as amino acids and monoamines, especially in microdialysis samples from experimental animals and humans. CE-LIFD trends are discussed: automated faster analysis with capillary array systems, resolution power improvement, higher detection sensitivity, and CE systems miniaturization for extremely small sample volume, in order to make CE easier and affordable to the lab bench or the clinical bed.

Clinical chemistry of serotonin and metabolites

Analyses of serotonin and other 5-hydroxyindoles, such as its precursor 5-hydroxytryptophan and major metabolite 5-hydroxyindoleacetic acid (5-HIAA), are indispensable for the elucidation of their (patho)physiological roles. In clinical chemistry attention is mainly focused on the diagnosis and follow-up of carcinoid tumours. For this most laboratories routinely measure urinary 5-HIAA. More recently, measurements of serotonin in platelets and urine have been advocated. Platelet serotonin may be the most sensitive indole marker for the detection of carcinoid tumours that secrete only small amounts of serotonin and/or its precursor 5-hydroxytryptophan. Although several chromatographic techniques have emerged for the analysis of tryptophan-related indoles, HPLC with either electrochemical or fluorometric detection have become the methods of choice for their quantification. HPLC-based methods combine selectivity, sensitivity and high precision, and enable the simultaneous investigation of several metabolically related indoles. This review aims to place the analysis of indoles in biological matrices in a biochemical, physiological and clinical perspective and highlights several important steps in their chromatographic analysis and quantification.

Neutral amino acids monitoring in phenylketonuric plasma microdialysates using micellar electrokinetic chromatography and laser-induced fluorescence detection

Neutral and non-polar amino acids such as phenylalanine (Phe), valine (Val), tyrosine (Tyr), threonine (Thre) and GABA are hard to resolve by capillary zone electrophoresis (CZE). Their separation is possible by adding a surfactant to the mobile phase. This method is called micellar electrokinetic chromatography (MEKC). We used MEKC with laser-induced fluorescence detection (LIFD) to separate and quantitate these amino acids in plasma microdialysates of patients with phenylketonuria (PKU). This disease is an inborn enzymatic defect with decreased conversion of Phe to Tyr that causes severe neurological damage and mental deterioration, which is diagnosed by measuring plasma Phe and Phe/Tyr ratio. The amino acids tested had linear concentration-signal relation. PKU patients had significantly higher Phe, lower Tyr, 21 times higher Phe/Tyr ratio and decreased values of Val and Thre than controls. These results show that microdialysis of biological fluids coupled with MEKC-LIFD is a convenient technique to measure neutral amino acids in clinical disorders such as PKU.

Monitoring gamma-aminobutyric acid in human brain and plasma microdialysates using micellar electrokinetic chromatography and laser-induced fluorescence detection

Due to its low electrophoretic mobility, few studies have been able to measure gamma aminobutyric acid (GABA) in biological samples by means of capillary zone electrophoresis. Nevertheless, in micellar electrokinetic chromatography (MEKC) by adding a surfactant to the mobile phase separation can be carried out on the basis of the partition coefficient of the molecules rather than their electrophoretic mobility. In the present study microdialysis coupled to MEKC with laser induced fluorescence detection was used to successfully monitor GABA from cerebrospinal fluid and plasma dialysates. Moreover, we monitored changes in extracellular GABA from a human brain. Microdialysis samples were collected from a Parkinson's disease patient undergoing a thallamotomy as part of her treatment. Significant decreases in extracellular GABA were detected during high frequency electrical stimulation and following a thermolesion of the thalamus. These results demonstrate the feasibility of MEKC coupled to laser-induced fluorescence detection in resolving neutral amino acids, specifically GABA, from different human body fluids.