Microdialysis studies of the middle ear distribution kinetics of amoxicillin in the awake chinchilla.

This work was performed to develop an experimental animal model for the study of antibiotic drug distribution into middle ear fluid (MEF) and to evaluate its relevance and significance to the clinical treatment of otitis media (OM). Chinchillas were assigned to normal or infected ear groups after Eustachian tube obstruction (ETO) or direct trans-bullar inoculation with type 3 Streptococcus pneumoniae. Following survival surgery to implant microdialysis (MD) probes in the jugular vein and middle ear (ME), amoxicillin was given intravenously (iv) as a bolus or infusion. Drug concentrations in blood and MEF were continuously monitored by microdialysis. The measured concentrations were corrected for probe recovery by simultaneous retrodialysis. Multiple MEF and blood sampling was also performed to validate the animal model and MD sampling technique. Bacterial infection was successfully induced 3-7 days after the inoculation, whereas the control group gave negative bacterial culture results. The beta-lactam antibiotic, amoxicillin, exhibited an elimination half-life of 0.33+/-0.23 h (n = 9) in chinchilla blood, 1.46+/-0.50 h (n = 5) and 1.75+/-0.84 h (n = 4) in MEF of normal and infected ears (p = 0.6), respectively. MEF-to-blood amoxicillin concentration ratios at steady state following iv infusion were 0.26+/-0.06 (n = 5) and 0.28+/-0.11 (n = 4) for normal and infected ears (p = 0.7), respectively. MD allows continuous monitoring of drug concentration-time profiles in blood and MEF in an awake chinchilla model. The concentrations measured by MD were validated by direct sampling. The ratio of the area under the curve (AUC) of drug concentration in MEF versus time to that in blood after iv bolus doses was less than unity, as was the steady-state concentration ratio following constant-rate iv infusion, suggesting an active transport mechanism was involved in the efflux of amoxicillin from the ME of chinchilla. The results of studies involving infected ears were not significantly different from those in normal ears in terms of amoxicillin distribution across the ME mucosal membrane after systemic administration.     

Experimental Animal Models for Studying Antimicrobial Pharmacokinetics in Otitis Media

Antimicrobial treatment of otitis media, especially drug dosing considerations, is largely empiric, with few reported pharmacologic studies of drug distribution into the middle ear. A chinchilla animal model of serous and purulent otitis media has been used for some time to investigate mechanisms of disease pathogenesis. This model was adapted to investigate the penetration of amoxicillin, trimethoprim, and sulfamethoxazole into middle ear effusion. Purulent otitis media was produced by direct middle ear inoculation with type 7F Streptococcus pneumoniae. Serous otitis media was produced by eustachian tube obstruction using silastic sponge or Coeflex cement, but the Coeflex caused an undesirable local inflammatory response. The three antibiotics were administered to chinchillas with serous and purulent middle ear effusion. Plasma and ear fluid drug concentrations were measured by liquid chromatography and demonstrated the value of this model in assessing antibiotic penetration.     

Simultaneous intravenous and intramiddle-ear dosing to determine cefditoren influx and efflux clearances in middle ear fluid in freely moving chinchillas

This study was conducted to determine cefditoren (CDTR) transport kinetics between plasma and middle ear fluid (MEF) by characterizing influx (CLin) and efflux (CLout) clearances expressed in terms of unbound concentrations and their ratio. Simultaneous intravenous bolus and intramiddle-ear dose were administered to two groups of chinchillas: normal control and infected. In vivo microdialysis was employed to determine protein-unbound CDTR levels in MEF. Compartmental and noncompartmental analysis was performed. Parameters determined in both groups were compared to assess the effect of infection and inflammation on CDTR distribution kinetics. CLin and CLout estimates obtained by compartmental and noncompartmental analysis agreed well. The calculated CLin/CLout ratio was 0.76 +/- 0.23 and 0.56 +/- 0.25 in normal (n = 9) and infected (n = 6) animals, respectively. The 95% confidence interval of this ratio in both groups does not include unity. Statistical tests showed no difference (p > 0.05) in CLin, CLout, and their ratio between the two groups. In conclusion, middle ear infection and inflammation does not affect CDTR distribution. The CLin/CLout ratio determined in chinchillas compares well with values estimated from data in pediatric patients. An active efflux mechanism in middle ear mucosa may be involved in CDTR distribution in MEF.     

Tissue penetration of cefpodoxime and cefixime in healthy subjects

Microdialysis is a technique that allows the measurement of free antibiotic concentrations in different tissues, which are responsible for the antibacterial activity at the infection site. In an open, randomized, 2-way crossover study in healthy volunteers, the muscle penetration of orally administered cefpodoxime (400 mg) and cefixime (400 mg) was compared using microdialysis. The results show that the total plasma concentration-time profiles of each antibiotic were similar; the area under the curve for cefpodoxime was 22.4 +/- 8.7 versus 25.6 +/- 8.5 mg/L*h for cefixime. However, tissue penetration was twice as high for cefpodoxime (area under the curve 15.4 +/- 5.1 mg/L*h) as for cefixime (area under the curve 7.3 mg/L*h). This degree of tissue distribution is consistent with their protein binding of 21% for cefpodoxime and 65% for cefixime. After equilibration, the unbound tissue concentrations of both antibiotics were similar to their unbound plasma concentrations. Pharmacokinetic modeling was applied to describe the pharmacokinetic profiles in plasma and muscle. The study demonstrates that cefpodoxime shows greater tissue penetration than cefixime.     

An Experimental Model for Measuring Middle Ear Antimicrobial Drug Penetration in Otitis Media

Bacteria are an important cause of acute otitis media and successful treatment depends on achieving inhibitory or bacteriacidal antimicrobial drug concentrations in the middle ear. To evaluate further otitis media treatment success and failure, we developed a chinchilla model to study antimicrobial drug penetration through the middle ear mucosa. Using quantitative histomorphometry, we measured the middle ear space in 10 chinchillas and found a mean ±SD volume of 2.09 ± 0.08 ml and a mean SD surface area of 14.41 ± 1.48 cm². To measure the apparent rate constant (K _e) of antibiotic elimination from the middle ear, through the middle ear mucosa, an antibiotic solution was inoculated into the middle ear cavity, and samples were aspirated between 1 and 8 hr later. In normal ears, the mean K _e ±SD for amoxicillin was 0.118 ± 0.013 hr^–1, that for a trimethoprim 0.461 ± 0.090 hr^–1, and that for sulfamethoxazole 0.265 ± 0.062 hr^–1. In ears inoculated with type 7F Streptococcus pneumoniae to induce acute otitis media, the K _e ±SD increased for all three drugs (P < 0.05): amoxicillin to 0.286 ± 0.089 hr^–1, trimethoprim to 0.662 ± 0.118 hr^–1, and sulfamethoxazole to 0.411 ± 0.056 hr^–1. These values demonstrate that amoxicillin had the lowest apparent penetration rate constant of the three antibiotics but the greatest increase from normal to infected mucosa (142%). Trimethoprim had the highest apparent penetration rate constant of the three antibiotics but the smallest increase from normal to infected mucosa (44%), while the sulfamethoxazone apparent penetration rate constant increased from normal to infected mucosa by 55%. The K _e for amoxicillin was the same for inoculation volumes of 0.8 and 1.6 ml (P = 0.557) and the same for sampling intervals of 4 and 8 hr (P = 0.054). All three antimicrobial drug concentration–time curves were log-linear, as predicted by Fick's first law of diffusion. In conclusion, this model overcomes the technical limitations of previous models and permits investigation of the many factors that can influence antibiotic penetration into the middle ear and reduce otitis media treatment efficacy.     

Microdialysis sampling of carbamazepine,phenytoin and phenobarbital in subcutaneous extracellular fluid and subdural cerebrospinal fluid in humans: an in vitro and in vivo study of adsorption to the sampling device

The purpose of the study was to determine if binding of the drugs to the sampling equipment during microdialysis would influence the results for carbamazepine, phenytoin and phenobarbital. In vitro experiments with microdialysis catheters and separate parts of catheters were performed to estimate the degree of drug binding to the dialysis equipment. A mathematical model to calculate drug binding and recovery is proposed. In vivo protein unbound carbamazepine concentrations in subcutaneous extracellular fluid at different flow rates (6 patients), unbound carbamazepine (1 patient) and unbound phenobarbital (I patient) in subdural cerebrospinal fluid and subcutaneous extracellular fluid were estimated and the in vivo data were compared to the in vitro results and data generated by the mathematical model. Binding to the soft outlet polyurethane tubing was extensive and variable for phenytoin, which precluded in vivo testing, but limited and more predictable for carbamazepine and phenobarbital. None of the three compounds bound to the hard internaltubing. Phenytoin and phenobarbital did not bind to the dialysis membrane, while a small degree of binding may be present for carbamazepine. In vivo estimates of carbamazepine protein unbound subcutaneous extracellular concentrations by microdialysis, adjusted for binding to the plastic tubing, were 81% of protein unbound plasma concentrations. In single case studies, subdural cerebrospinal fluid and subcutaneous extracellular levels of carbamazepine and phenobarbital were similar and when corrected for binding to the plastic tubings they were also close to protein unbound plasma concentrations. Microdialysis can be used for reliable estimations of protein unbound carbamazepine and possibly phenobarbital concentrations when drug binding to the plastic tubing is considered. Reliable estimation of unbound phenytoin is not possible at present.     

Microdialysis Sampling of Carbamazepine,Phenytoin and Phenobarbital in Subcutaneous Extracellular Fluid and Subdural Cerebrospinal Fluid in Humans: An in vitro and in vivo Study of Adsorption to the Sampling Device

Abstract: The purpose of the study was to determine if binding of the drugs to the sampling equipment during microdialysis would influence the results for carbamazepine, phenytoin and phenobarbital. In vitro experiments with microdialysis catheters and separate parts of catheters were performed to estimate the degree of drug binding to the dialysis equipment. A mathematical model to calculate drug binding and recovery is proposed. In vivo protein unbound carbamazepine concentrations in subcutaneous extracellular fluid at different flow rates (6 patients), unbound carbamazepine (1 patient) and unbound phenobarbital (1 patient) in subdural cerebrospinal fluid and subcutaneous extracellular fluid were estimated and the in vivo data were compared to the in vitro results and data generated by the mathematical model. Binding to the soft outlet polyurethane tubing was extensive and variable for phenytoin, which precluded in vivo testing, but limited and more predictable for carbamazepine and phenobarbital. None of the three compounds bound to the hard internal tubing. Phenytoin and phenobarbital did not bind to the dialysis membrane, while a small degree of binding may be present for carbamazepine. In vivo estimates of carbamazepine protein unbound subcutaneous extracellular concentrations by microdialysis, adjusted for binding to the plastic tubing, were 81% of protein unbound plasma concentrations. In single case studies, subdural cerebrospinal fluid and subcutaneous extracellular levels of carbamazepine and phenobarbital were similar and when corrected for binding to the plastic tubings they were also close to protein unbound plasma concentrations. Microdialysis can be used for reliable estimations of protein unbound carbamazepine and possibly phenobarbital concentrations when drug binding to the plastic tubing is considered. Reliable estimation of unbound phenytoin is not possible at present.     

Effect of Streptococcus pneumoniae and Influenza A Virus on Middle Ear Antimicrobial Pharmacokinetics in Experimental Otitis Media

Antimicrobial treatment failures in children with acute otitis media and concomitant viral respiratory tract infection prompted us to study the effects of influenza A virus infection on middle ear antimicrobial drug penetration. Using a chinchilla model of Streptococcus pneumoniae we compared middle ear elimination rates in 4 groups of chinchillas: (1) control, (2) influenza A virus inoculation alone intranasally, (3) both influenza A and S. pneumoniae inoculation directly into the middle ear, and (4) S. pneumoniae inoculation alone into the middle ear. After infection was established, a solution containing amoxicillin, sulfamethoxazole, and trimethoprim was instilled into the middle ear and removed 4 hours later. The rate constant of elimination and half-life were calculated from measured drug concentrations initially and at 4 hours. S. pneumoniae infection alone significantly shortened the middle ear elimination half-life compared with the control group: amoxicillin, 2.65 ± 0.73 vs. 6.63 ± 2.55 hr; sulfamethoxazole, 1.75 ± 0.28 vs. 2.74 ± 0.6 hr; and trimethoprim, 1.06 ± 0.14 vs. 1.56 ± 0.34 hr (n = 16 ears, p values all <0.01). The combined influenza virus and S. pneumoniae infection significantly lengthened the half-life compared with the S. pneumoniae infection alone: amoxicillin, 5.65 ± 6.44 vs. 2.65 ± 0.73 hr; sulfamethoxazole, 2.5 ± 0.85 vs. 1.75 ± 0.28 hr; and trimethoprim, 1.26 ± 0.42 vs. 1.06 ± 0.14 hr (n = 16 ears, p values all <0.01). Influenza virus produced the longest half-lives for all 3 antimicrobials: amoxicillin 25.52 ± 14.96 hr; sulfamethoxazole, 5.46 ± 0.87 hr; and trimethoprim, 2.57 ± 0.75 hr. These effects demonstrate that influenza and S. pneumoniae infections alone and together affect middle ear antimicrobial penetration. The decreased penetration of antimicrobials that occurred with the combined viral and bacterial infection vs. the bacteria alone supports the clinical observation that patients with infections caused by both organisms may have decreased middle ear antimicrobial concentrations, producing treatment failures.     

Determination of norfloxacin free interstitial levels in skeletal muscle by microdialysis

Tissue penetration and distribution of antibiotics are important issues when establishing antibiotic therapies. Free concentrations of antibiotics at the infection site are responsible for bacteria killing effect. The knowledge of the correlation between blood levels and tissue concentrations can be helpful for adequate dosing of these drugs. It was the aim of this study to investigate norfloxacin pharmacokinetics in rats to predict free interstitial levels of the drug, determined by microdialysis, using pharmacokinetic parameters derived from total plasma data. Norfloxacin free tissue and total plasma levels were determined in Wistar rats after administering 5 and 10 mg/kg i.v. bolus doses. Plasma and microdialysis samples were analyzed by high-performance liquid chromatography. Norfloxacin plasma pharmacokinetics was consistent with a two compartments model. A simultaneous fitting of plasma and tissue concentrations was performed using a proportionality factor because norfloxacin free tissue levels determined by microdialysis were lower than those predicted using plasma data. A similar proportionality (f(T)) factor was calculated by the computer program Scientist((R)) for both doses (0.25 +/- 0.08). It can be concluded that it is possible to predict concentration time profiles of norfloxacin in the peripheral compartment based on plasma data using the adequate tissue penetration factor.     

Penetration of Cefaclor Into the Interstitial Space Fluid of Skeletal Muscle and Lung Tissue in Rats

Purpose. To measure and compare the penetration of cefaclor from the plasma compartment into the interstitial space of lung and skeletal muscle in rats and to integrate the data in a pharmacokinetic model. Methods. Unbound interstitial concentrations in muscle and lung were measured by in vivo microdialysis following i.v. bolus doses of 50 and 75 mg/kg cefaclor. Unbound muscle concentrations were also measured after a primed, continuous i.v. infusion at an infusion rate of 0.3 mg/kg/min. Results. The cefaclor half-life in plasma, muscle and lung was approximately 1 h. Unbound cefaclor concentrations in muscle and lung were found to be virtually identical. A 2-compartment body model was fitted to the data with a tissue penetration factor (AUC_{tissue(unbound)}/AUC_{plasma(unbound)}) of approximately 0.26 independent of dose, tissue and mode of administration. Conclusions. Unbound concentrations of cefaclor in the interstitial space fluid of lung and skeletal muscle are of similar magnitude and lower than those in plasma. Using total plasma concentrations would overestimate the antibacterial activity of the drug and therefore its clinical efficacy. Instead, therapeutically active levels of cefaclor at the site of action should be taken into account. Microdialysis allows direct measurement of these unbound concentrations.     

Microdialysis for in vivo pharmacokinetic/pharmacodynamic characterization of anti-infective drugs

Inadequate tissue penetration of antibiotics can lead to therapeutic failure and bacterial resistance. Pharmacokinetic evaluation of antibiotics should therefore be based on tissue rather than serum concentrations. Over several years, tissue concentration data obtained by methods such as tissue biopsies have flawed the correct interpretation of antibiotic tissue distribution. Microdialysis--a semi-invasive catheter-based sampling technique--has been employed for the in vivo measurement of antibiotic tissue pharmacokinetics. Owing to selective access to the target site for most anti-infective drugs, microdialysis satisfies regulatory requirements for pharmacokinetic distribution studies and might become a reference technique for tissue distribution studies in the near future. Furthermore, microdialysis might contribute to the definition of meaningful surrogate markers for antibiotic efficiency during drug development.     

Microdialysis: current applications in clinical pharmacokinetic studies and its potential role in the future

Microdialysis is a probe-based sampling method, which, if linked to analytical devices, allows for the measurement of drug concentration profiles in selected tissues. During the last two decades, microdialysis has become increasingly popular for preclinical and clinical pharmacokinetic studies. The advantage of in vivo microdialysis over traditional methods relates to its ability to continuously sample the unbound drug fraction in the interstitial space fluid (ISF). This is of particular importance because the ISF may be regarded as the actual target compartment for many drugs, e.g. antimicrobial agents or other drugs mediating their action through surface receptors. In contrast, plasma concentrations are increasingly recognised as inadequately predicting tissue drug concentrations and therapeutic success in many patient populations. Thus, the minimally invasive microdialysis technique has evolved into an important tool for the direct assessment of drug concentrations at the site of drug delivery in virtually all tissues. In particular, concentrations of transdermally applied drugs, neurotransmitters, antibacterials, cytotoxic agents, hormones, large molecules such as cytokines and proteins, and many other compounds were described by means of microdialysis. The combined use of microdialysis with non-invasive imaging methods such as positron emission tomography and single photon emission tomography opened the window to exactly explore and describe the fate and pharmacokinetics of a drug in the body. Linking pharmacokinetic data from the ISF to pharmacodynamic information appears to be a straightforward approach to predicting drug action and therapeutic success, and may be used for decision making for adequate drug administration and dosing regimens. Hence, microdialysis is nowadays used in clinical studies to test new drug candidates that are in the pharmaceutical industry drug development pipeline.     

Pharmacokinetics of cefprozil in plasma and middle ear fluid: in children undergoing treatment for acute otitis media

BACKGROUND: Despite the wide-scale use of cefprozil for acute otitis media (AOM), there are only limited data available regarding the pharmacokinetic profile of this agent in the pediatric population. OBJECTIVE: To characterize the plasma and middle ear fluid (MEF) pharmacokinetic profile of cefprozil in pediatric patients with AOM. METHODS: Pharmacokinetic sampling was obtained as part of a phase IV, multicenter, open-label study of children with AOM receiving cefprozil suspension 15 mg/kg twice daily. A single blood sample was obtained 4-6 days after the initiation of cefprozil therapy and a simultaneous MEF sample was obtained by tympanocentesis when clinically indicated. Cefprozil concentrations in both matrices were determined using a validated high-performance liquid chromatography methodology. A composite profile of cefprozil concentration data in each matrix was constructed and values for the pharmacokinetic parameters were obtained using conventional modeling techniques. RESULTS: Plasma concentrations were obtained in 53 children aged 6-48 months. In this population the maximum concentration (C(max)) in plasma was 9.18 microg/mL, the time to C(max) (t(max)) was 1.5 hours, and the terminal elimination half-life (t((1/2))(beta)) was 0.98 hours. Simultaneous plasma and MEF concentration data were available in 22 children. In this subset the C(max) in plasma was 8.2 microg/mL, the t(max) was 1.9 hours, and the t((1/2))(beta) was 1.02 hours; the corresponding MEF C(max) was 2.4 microg/mL, the t(max) was 3.5 hours, and the t((1/2))(beta) was 1.23 hours. Cefprozil MEF penetration as assessed using the ratio of the area under the concentration-time curves from the two matrices was 28%. Moreover, concentrations in MEF approximated 1 microg/mL 6 hours' post-dose. CONCLUSIONS: The plasma profile of cefprozil in the current analysis is consistent with previously reported values in children receiving the 15 mg/kg twice daily dose. MEF penetration and the duration of drug exposure at the site of infection support the clinical utility of this agent for organisms with minimum inhibitory concentrations (MIC) of < or =1 microg/mL. However, these results also predict higher clinical failure when using this dose of cefprozil against penicillin-non-susceptible Streptococcus pneumoniae or Haemophilus influenzae because of typically higher MIC values for these organisms.     

Skin and soft tissue concentrations of tedizolid (formerly torezolid), a novel oxazolidinone, following a single oral dose in healthy volunteers

Plasma concentrations of antimicrobial drugs have long been used to correlate exposure with effect, yet one cannot always assume that unbound plasma and tissue concentrations are similar. Knowledge about unbound tissue concentrations is important in the development of antimicrobial drugs, since most infections are localised in tissues. Therefore, a clinical microdialysis study was conducted to evaluate the distribution of tedizolid (TR-700), the active moiety of the antimicrobial prodrug tedizolid phosphate (TR-701), into interstitial fluid (ISF) of subcutaneous adipose and skeletal muscle tissues following a single oral 600 mg dose of tedizolid phosphate in fasting conditions. Twelve healthy adult subjects were enrolled. Two microdialysis probes were implanted into the thigh of each subject, one into the vastus medialis muscle and one into subcutaneous adipose tissue. Probes were calibrated using retrodialysis. Dialysate samples were collected every 20 min for 12h following a single oral dose of 600 mg tedizolid phosphate, and blood samples were drawn over 24h. Unbound tedizolid levels in plasma were similar to those in muscle and adipose tissue. The ratios of unbound (free) AUC in tissues over unbound AUC in plasma (fAUC(tissue)/fAUC(plasma)) were 1.1 ± 0.2 and 1.2 ± 0.2 for adipose and muscle tissue, respectively. The median half-life was 8.1, 9.2 and 9.6h for plasma, adipose tissue and muscle tissue, respectively. Mean protein binding was 87.2 ± 1.8%. The study drug was very well tolerated. The results of this study show that tedizolid distributes well into ISF of adipose and muscle tissues. Unbound levels of tedizolid in plasma, adipose tissue and muscle tissue were well correlated. Free plasma levels are indicative of unbound levels in the ISF of muscle and adipose tissues.     

RIA-linked microdialysis sampling in the awake rat: Application to free-drug pharmacokinetics of hydrocortisone

The purpose of this research was to combine microdialysis sampling techniques with a highly sensitive radioimmunoassay (RIA) to study the in vivo kinetic response of pharmacologically important substances. This technique allowed for a dense sampling regimen from an awake, free-roaming experimental animal with no loss of blood and with rapid analysis of the dialysate. An important methodological criterion for accurate quantitation of a test drug in the extracellular space was knowledge of the relative recovery of the sampling system at the time of experimentation. Accordingly, the factors which influenced the recovery of drug during dense in vivo microdialysis sampling were examined and an analytical technique was developed to measure the instantaneous recovery of drug from the extracellular space. This information was applied to in vivo (iv) sampling experiments on anaesthetized and awake, free-roaming rats following bolus and multiple long-term iv administrations of the highly protein bound steroid (i.e. greater than 90%), hydrocortisone-21-phosphate. These studies indicated that unbound hydrocortisone levels as determined by the RIA-linked microdialysis (RIALM) technique fluctuated rapidly between each 2-min sampling interval, but nevertheless decreased to predose endogenous concentrations in a first-order fashion (t1/2 = 17-29 min). The rapid fluctuations of unbound hydrocortisone may reflect real pharmacokinetic or pharmacodynamic phenomena, attributed, perhaps, to reequilibration of the unbound drug pool with proteins and tissues in the blood.     

Microdialysis in the study of drug transporters in the CNS

Quantitative microdialysis in the central nervous system (CNS) has recently provided evidence for the existence of transporters as they relate to the brain distribution of a variety of drugs. Support for the existence of drug transporters in the blood-brain barrier (or in the blood-CSF barrier) comes from investigations that have found: unbound drug concentrations in brain fluids that are lower than corresponding levels in plasma; saturability of transport clearances across the blood-brain barrier and; the regulation of transport by putative inhibitors. Additional confirmatory evidence for the existence of active transport or carrier-mediated processes has also been derived from models that relate observed drug levels in the CNS with those in plasma or blood. The conclusion that reduced drug levels in brain fluids generally indicate the existence of active efflux transport is questioned. In the case of relatively polar compounds with modest blood-brain barrier permeability, lower unbound concentrations in brain may be a consequence of dilution by turnover of brain fluids. This review summarizes recent reports (grouped by class of compounds) where investigators have used microdialysis to examine the distribution of therapeutic agents to the CNS, and have reached conclusions regarding the functional presence of drug transporters in the brain.     

Microdialysis: An Alternative for in Vitro and in Vivo Protein Binding Studies

The aim of the present study was to compare the performance of conventional equilibrium dialysis method with a microdialysis method in studying drug protein binding. The two methods were assessed by comparing the measured mean unbound drug fraction in different plasma species in vitro in plasma of four different species and at two concentrations of the non-indolic melatonin analog S 20098. For the microdialysis study, the unbound drug fraction was calculated after correction for membrane recovery. Plasma protein binding of S 20098 ranged from 75 to 95%. In humans, rabbits and rats (10 ng/ml), equal unbound percentages were found between equilibrium dialysis and microdialysis. Microdialysis gave slightly but significantly higher values in rat (2000 ng/ml), and in monkey plasma independent of the drug concentration. Microdialysis was also performed in vivo in freely moving rats under steady-state conditions, yielding similar unbound fraction values (26.0 ± 0.9%) to those obtained using microdialysis probes in rat plasma in vitro (24.4 ± 1.6%). These results support the use of in vivo microdialysis in pharmacokinetic studies in freely moving animals.     

Microdialysis in peripheral tissues

The objective of this review is to survey the recent literature regarding the applications of microdialysis in pharmacokinetic studies and facilitating many other studies in peripheral tissues such as muscle, subcutaneous adipose tissue, heart, lung, etc. It has been reported extensively that microdialysis is a useful technique for monitoring free concentrations of compounds in extracellular fluid (ECF), and it is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses the use of microdialysis technique for ECF sampling in peripheral tissues in animal studies. The second part of the review describes the use of microdialysis for ECF sampling in peripheral tissues in human studies. Microdialysis has been applied extensively to measure both endogenous and exogenous compounds in ECF. Of particular benefit is the fact that microdialysis measures the unbound concentrations in the peripheral tissue fluid which have been shown to be responsible for the pharmacological effects. With the increasing number of applications of microdialysis, it is obvious that this method will have an important place in studying drug pharmacokinetics and pharmacodynamics.