The binding of cyclosporin A to human plasma: an in vitro microdialysis study

Zebrafish (Danio rerio) larva as an in vivo vertebrate model to study renal function

There is an increasing interest in using zebrafish (Danio rerio) larva as a vertebrate screening model to study drug disposition. As the pronephric kidney of zebrafish larvae shares high similarity with the anatomy of nephrons in higher vertebrates including humans, we explored in this study whether 3- to 4-day-old zebrafish larvae have a fully functional pronephron. Intravenous injection of fluorescent polyethylene glycol and dextran derivatives of different molecular weight revealed a cutoff of 4.4-7.6 nm in hydrodynamic diameter for passive glomerular filtration, which is in agreement with corresponding values in rodents and humans. Distal tubular reabsorption of a FITC-folate conjugate, covalently modified with PEG2000, via folate receptor 1 was shown. Transport experiments of fluorescent substrates were assessed in the presence and absence of specific inhibitors in the blood systems. Thereby, functional expression in the proximal tubule of organic anion transporter oat (slc22) multidrug resistance-associated protein mrp1 (abcc1), mrp2 (abcc2), mrp4 (abcc4), and zebrafish larva p-glycoprotein analog abcb4 was shown. In addition, nonrenal clearance of fluorescent substrates and plasma protein binding characteristics were assessed in vivo. The results of transporter experiments were confirmed by extrapolation to ex vivo experiments in killifish (Fundulus heteroclitus) proximal kidney tubules. We conclude that the zebrafish larva has a fully functional pronephron at 96 h postfertilization and is therefore an attractive translational vertebrate screening model to bridge the gap between cell culture-based test systems and pharmacokinetic experiments in higher vertebrates.NEW & NOTEWORTHY The study of renal function remains a challenge. In vitro cell-based assays are approved to study, e.g., ABC/SLC-mediated drug transport but do not cover other renal functions such as glomerular filtration. Here, in vivo studies combined with in vitro assays are needed, which are time consuming and expensive. In view of these limitations, our proof-of-concept study demonstrates that the zebrafish larva is a translational in vivo test model that allows for mechanistic investigations to study renal function.

Immunosuppressant quantification in intravenous microdialysate - towards novel quasi-continuous therapeutic drug monitoring in transplanted patients

OBJECTIVES

Therapeutic drug monitoring (TDM) plays a crucial role in personalized medicine. It helps clinicians to tailor drug dosage for optimized therapy through understanding the underlying complex pharmacokinetics and pharmacodynamics. Conventional, non-continuous TDM fails to provide real-time information, which is particularly important for the initial phase of immunosuppressant therapy, e.g., with cyclosporine (CsA) and mycophenolic acid (MPA).

METHODS

We analyzed the time course over 8 h of total and free of immunosuppressive drug (CsA and MPA) concentrations measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) in 16 kidney transplant patients. Besides repeated blood sampling, intravenous microdialysis was used for continuous sampling. Free drug concentrations were determined from ultracentrifuged EDTA-plasma (UC) and compared with the drug concentrations in the respective microdialysate (µD). µDs were additionally analyzed for free CsA using a novel immunosensor chip integrated into a fluorescence detection platform. The potential of microdialysis coupled with an optical immunosensor for the TDM of immunosuppressants was assessed.

RESULTS

Using LC-MS/MS, the free concentrations of CsA (fCsA) and MPA (fMPA) were detectable and the time courses of total and free CsA comparable. fCsA and fMPA and area-under-the-curves (AUCs) in µDs correlated well with those determined in UCs (r≥0.79 and r≥0.88, respectively). Moreover, fCsA in µDs measured with the immunosensor correlated clearly with those determined by LC-MS/MS (r=0.82).

CONCLUSIONS

The new microdialysis-supported immunosensor allows real-time analysis of immunosuppressants and tailor-made dosing according to the AUC concept. It readily lends itself to future applications as minimally invasive and continuous near-patient TDM.

Role of P-glycoprotein in the brain disposition of seletalisib: Evaluation of the potential for drug-drug interactions

Seletalisib is an orally bioavailable selective inhibitor of phosphoinositide 3-kinase delta (PI3Kδ) in clinical development for the treatment of immune-mediated inflammatory diseases. The present study investigated the role of P-gp in seletalisib disposition, especially brain distribution, and the associated risks of interactions. Seletalisib was found to be actively transported by rodent and human P-gp in vitro (transfected LLC-PK1 cells; K_m of ca. 20 µM), with minimal or no affinity for the other tested transporters. A distribution study in knockout rats (single oral dosing at 750 mg kg^-1) showed that P-gp restricts the brain disposition of seletalisib while having minimal effect on its intestinal absorption. Restricted brain penetration was also observed in cynomolgus monkeys (single oral dosing at 30 mg kg^-1) using brain microdialysis and cerebrospinal fluid sampling (K_p,uu of 0.09 and 0.24, respectively). These findings opened the question of potential pharmacokinetic interaction between seletalisib and P-gp inhibitors. In vitro, CsA inhibited the active transport of seletalisib with an IC₅₀ of 0.13 µM. In rats, co-administration of high doses of CsA (bolus iv followed by continuous infusion) increased the brain distribution of seletalisib (single oral dosing at 5 mg kg^-1). The observed data were found aligned with those predicted by in vitro-in vivo extrapolation. Based on the same extrapolation method combined with literature data, only very few P-gp inhibitors (i.e. CsA, quinine, quinidine) were predicted to increase the brain disposition of seletalisib in the clinical setting (maximal 3-fold changes).

Quantification of [(11)C]yohimbine binding to α2 adrenoceptors in rat brain in vivo

We quantified the binding potentials (BPND) of [(11)C]yohimbine binding in rat brain to alpha-2 adrenoceptors to evaluate [(11)C]yohimbine as an in vivo marker of noradrenergic neurotransmission and to examine its sensitivity to the level of noradrenaline. Dual [(11)C]yohimbine dynamic positron emission tomography (PET) recordings were applied to five Sprague Dawley rats at baseline, followed by acute amphetamine administration (2 mg/kg) to induce elevation of the endogenous level of noradrenaline. The volume of distribution (VT) of [(11)C]yohimbine was obtained using Logan plot with arterial plasma input. Because alpha-2 adrenoceptors are distributed throughout the brain, the estimation of the BPND is complicated by the absence of an anatomic region of no displaceable binding. We used the Inhibition plot to acquire the reference volume, VND, from which we calculated the BPND. Acute pharmacological challenge with amphetamine induced a significant decline of [(11)C]yohimbine BPND of ~38% in all volumes of interest. The BPND was greatest in the thalamus and striatum, followed in descending order by, frontal cortex, pons, and cerebellum. The experimental data demonstrate that [(11)C]yohimbine binding is sensitive to a challenge known to increase the extracellular level of noradrenaline, which can benefit future PET investigations of pathologic conditions related to disrupted noradrenergic neurotransmission.

Measurement of plasma protein and lipoprotein binding of pyrethroids

INTRODUCTION

A simple, reliable procedure was developed to measure binding of pyrethroid insecticides to total proteins and lipoproteins of rat and human plasma.

METHODS

The extent of binding of (14)C-labeled deltamethrin (DLM), cis-permethrin (CIS) and trans-permethrin (TRANS) was quantified by a 3-step organic solvent extraction technique. Rat and human plasma samples, containing NaF to inhibit esterases, were spiked with a range of concentrations of each radiolabeled pyrethroid. Protein binding reached equilibrium within ~1h of incubation at 37°C. The samples were extracted in turn with: isooctane to collect the unbound fraction; 2-octanol to extract the lipoprotein-bound fraction; and acetonitrile to obtain the protein-bound fraction.

RESULTS

Absolute recoveries of DLM, CIS and TRANS ranged from 86 to 95%. Adherence of these very lipophilic chemicals to glass and plastic was minimized by using silanized glass vials and LoBind® plastic pipettes. The method's ability to distinguish lipoprotein from protein binding was confirmed by experiments with diazepam and cyclosporine, drugs that bind selectively to albumin and lipoproteins, respectively.

DISCUSSION

This procedure was effectively utilized for studies of the species-dependence of plasma protein and lipoprotein binding of three pyrethroids for inclusion in physiologically-based pharmacokinetic models of pyrethroids for use in health risk assessments of the insecticides in children and adults.

Interspecies scaling in pharmacokinetics: a novel whole-body physiologically based modeling framework to discover drug biodistribution mechanisms in vivo

Drug approval processes require extensive testing and have recently put more emphasis on understanding mechanistic drug action in the body including toxicity and safety.1 Consequently, there is an urgent need in the pharmaceutical industry to develop mechanistic pharmacokinetic (PK) models able to both expedite knowledge gain from experimental trials and, simultaneously, address safety concerns. We previously developed a first principles based whole-body PK model, which incorporated physiological dimensions and drug mass transport. In this follow-up article, we demonstrate how the first principles model in combination with novel physiological scaling laws yields more reliable interspecies and intraspecies extrapolation of drug biodistribution. We show how experimental dose-response data in rats for immunosuppressant cyclosporin are sufficient for predicting the biodistribution of this drug in pigs, monkeys, and humans. The predicted drug concentrations extrapolated by interspecies scaling laws match well with the experimental measurements. These promising results demonstrate that the whole-body PK modeling approach not only elucidates drug mechanisms from a biochemical standpoint, but offers better scaling precision. Better models can substantially accelerate the introduction of drug leads to clinical trials and eventually to the market by offering more understanding of the drug mechanisms, aiding in therapy design, and serving as an accurate dosing tool.

Molecular aspects of cyclophilins mediating therapeutic actions of their ligands

Cyclosporine A (CsA) is an immunosuppressive cyclic peptide that binds with a high affinity to 18 kDa human cyclophilin-A (hCyPA). CsA and its several natural derivatives have some pharmacological potential in treatment of diverse immune disorders. More than 20 paralogues of CyPA are expressed in the human body while expression levels and functions of numerous ORFs encoding cyclophilin-like sequences remain unknown. Certain derivatives of CsA devoid of immunosuppressive activity may have some potential in treatments of Alzheimer diseases, Hepatitis C and HIV infections, amyotrophic lateral sclerosis, congenital muscular dystrophy, asthma and various parasitic infections. Here, we discuss structural and functional aspects of the human cyclophilins and their interaction with various intra-cellular targets that can be under the control of CsA or its complexes with diverse cyclophilins that are selectively expressed in different cellular compartments. Some molecular aspects of the cyclophilins expressed in parasites invading humans and causing diseases were also analyzed.

Analysis of interaction property of bioactive components in Danggui Buxue Decoction with protein by microdialysis coupled with HPLC–DAD–MS

Interaction of a commonly used combined prescription of Danggui Buxue Decoction (CPDBD) with protein was studied by microdialysis coupled with HPLC-DAD-MS. Nine compounds in CPDBD were unequivocally identified by comparing with their t(R), MS data and UV spectra with those of reference compounds, and simultaneously quantified. Microdialysis recoveries and binding degrees of 20 compounds in CPDBD with bovine serum albumin (BSA) were determined. Recoveries of microdialysis sampling ranged from 66.9 to 91.5% with RSD below 3.0%, and the binding degrees of those to BSA ranged from 6.3 to 59.8% (0.3mM BSA) and from 6.9 to 86.6% (0.6mM BSA). The results were determined at pH 7.4 and the influence of different pH value was investigated. Furthermore, the binding degrees of eight reference compounds were determined separately under the same conditions, indicating a significant effect of the interaction of compounds with each other on their binding degrees to BSA. By comparing their binding degrees with BSA with those of proven active compounds in CPDBD, i.e. chlorogenic acid (3), ferulic acid (6), ononin (12) and calycosin (16), other five compounds were found to possess potential activities, which were tentatively identified as calycosin-7-O-beta-D-glucoside-6-O-malonate (9), senkyunolide I or H (10), formononetin-7-O-beta-D-glucoside-6-O-malonate (17), and two unknown compounds.

Effect of cyclosporine and tacrolimus on cytochrome p450 activities in human liver microsomes

The effects of cyclosporine and tacrolimus on cytochrome P450 (CYP) 1A2-mediated 7-ethoxyresorufin O-deethylation, CYP2C9-mediated tolbutamide hydroxylation, CYP2C19-mediated S-mephenytoin 4'-hydroxylation, CYP2D6-mediated debrisoquine 4-hydroxylation, CYP2E1-mediated chlorzoxazone 6-hydroxylation, CYP3A4-mediated nifedipine oxidation, and CYP3A4-mediated testosterone 6beta-hydroxylation activities in human liver microsomes were compared. Cyclosporine and tacrolimus, at concentrations of 0.2 or 2 muM, neither inhibited nor stimulated any of the metabolic activities except for those of CYP3A4. On the other hand, cyclosporine and tacrolimus competitively inhibited CYP3A4-mediated nifedipine oxidation activity, with inhibition constants (K(i)) of 1.42 and 0.36 muM, respectively. In addition, 20 muM cyclosporine inhibited CYP2C19 and CYP2D6 activities by 29% and 30%, respectively. These results suggest that tacrolimus would not cause clinically significant interactions with other drugs, which are metabolized by CYPs, via the inhibition of hepatic metabolism and that the reason why cyclosporine, but not tacrolimus, has a pharmacokinetic inhibitory effect might be that the dosage and/or the unbound concentrations around its metabolic enzymes are higher than those of tacrolimus, rather than the differences in the inhibition potential. Obvious substrate-dependent effects on CYP3A4-inhibition potential were not observed.

The role of transporters in drug interactions

Transport proteins play an important role in the adsorption, distribution and elimination of a wide variety of drugs. Therefore, it is not surprising that transporter-based drug interactions can occur in the clinic. These interactions can lead to changes in toxicity and/or efficacy of the affected drug. Here, we review such interactions and ask if these interactions could have been predicted from in vitro data. Conducting such in vitro-in vivo correlation is important for predicting future transporter-based drug interactions.

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.

Determination of cyclosporine in saliva using liquid chromatography-tandem mass spectrometry

Saliva may offer an alternative specimen for the therapeutic monitoring of cyclosporine (CsA) in children and patients with difficult venous access. For a highly protein-bound drug such as CsA, saliva may also provide a practical approach for measuring the unbound concentration. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is ideally suited for the measurement of drugs in saliva. A solid-phase extraction technique, analytic liquid chromatography over an Aqua Perfect C18 column, maintained at 65 degrees C, and electrospray tandem mass spectrometry were used to quantify CsA in saliva. The method used cyclosporine C (CsC) as the internal standard. Mobile phase comprised of a 97:3 vol/vol mixture of methanol and 30 mmol ammonium acetate at a flow rate of 0.5 mL/min. Chromatograms using mass transitions of m/z 1219.9 --> m/z 1202.9 for CsA and m/z 1235.9 --> m/z 1218.9 for CsC were obtained. The calibration curve was linear from 1 to 300 microg/L with correlation coefficient values ranging from 0.9732 to 0.9968). The lower limit of quantification was 1 microg/L, and limit of detection was 0.6 microg/L with an average extraction recovery of 84.7 +/- 2.6% for CsA and 93.7 +/- 4.4% for CsC from the saliva matrix. The accuracy of the method ranged from 92% to 104.7%, and the intra- and interrun coefficients of variation were 6.9-12.2% and 8.3-12.1%, respectively. The correlation coefficient value between the CsA concentration measurements in 15 paired blood-saliva samples from kidney transplant recipients was 0.695 (P = 0.006). The noninvasive and simple method of saliva collection coupled with the LC-MS/MS quantification technique for CsA analysis would generate novel data that could benefit patients undergoing CsA therapy.

Microdialysis for evaluating the entrapment and release of a lipophilic drug from nanoparticles

The aim of the study was to evaluate the microdialysis (MD) as a tool to determine entrapment efficiency and drug release of a lipophilic drug model, diclofenac (DIC), from nanocapsules, nanospheres, and nanoemulsions. Factors that could interfere with the MD probe recovery were investigated: perfusion fluid composition, concentration and form of the drug in the perfusate, and recovery method. DIC entrapment efficiency to nanoparticles and the drug release in phosphate buffer pH 7.4 after different dilutions were evaluated by MD and ultrafiltration/centrifugation (UC). DIC recovery for the 5 microL/min flux was concentration and pH dependent. DIC sodium was used for the recoveries determination since it did not differ from the DIC acid recovery for the same media. DIC entrapment efficiency determined applying both techniques were equivalent and close to 100% for all nanoparticles. In pH 7.4 DIC release from the nanoparticles was partial for the dilution rate 1:1 (v/v), around 50-60%. A complete release was observed from 1:10 (v/v) dilution. Only nanocapsules presented a incomplete release for 1:5 (v/v) dilution, around 86%. MD and UC techniques were equivalent for the evaluation of DIC entrapment efficiency and drug release from the nanoparticles.

Thermally responsive core-shell nanoparticles self-assembled from cholesteryl end-capped and grafted polyacrylamides:

The thermally responsive cholesteryl end-capped poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) and cholesteryl grafted poly[N-isopropylacrylamide-co-N-(hydroxymethyl) acrylamide] amphiphilic polymers were synthesized and utilized to encapsulate cyclosporin A (CyA) and indomethacin (IND) within core-shell nanoparticles by a membrane dialysis method. The blank and drug-loaded nanoparticles were characterized using various analytical tools. The blank nanoparticles had a mean diameter less than 100 nm, whereas the drug-loaded nanoparticles were between 100 and 200 nm in diameter. The CAC value of cholesteryl end-capped and grafted polymers in PBS (pH 7.4) was estimated to be 16 and 8.5mg/l, respectively. The LCST value for both nanoparticle systems in PBS (pH 7.4) was determined to be 33.4 degrees C and 38.3 degrees C, respectively. The presence of proteins in PBS reduced the LCST. The core-shell nanoparticles provided great capacity for drug loading. In particular, the cholesteryl grafted polymer yielded a higher encapsulation efficiency for drugs. Compared to CyA, better entrapment was observed for IDN. A reduced fabrication temperature provided greater drug encapsulation efficiency. An increase in the initial drug content yielded lower drug encapsulation efficiencies at 10 degrees C and 15 degrees C. Increasing the polymer concentration increased drug encapsulation efficiency. The drug-loading process was analyzed to understand the effect of various fabrication parameters on drug encapsulation efficiency. IND release from the nanoparticles was responsive to temperature changes, being faster at a temperature around the LCST than below the LCST.

Flow-injection chemiluminescence detection for studying protein binding of terbutaline sulfate with on-line microdialysis sampling

The binding of terbutaline sulfate to bovine serum albumin was studied in vitro using the technique of microdialysis sampling combined with flow-injection chemiluminescence analysis (FIA-CL). In the presence of formaldehyde, terbutaline sulfate can be oxidized by KMnO(4) to produce high chemiluminescence emission in sulfate acid media. The concentration of terbutaline sulfate is proportional with the CL intensity in the range of 1 x 10(-7)-2 x 10(-5) mol l(-1) with a detection limit of 3 x 10(-8) mol l(-1). The drug and protein were mixed in different molar ratios in 0.067 mol l(-1) 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(-1) and the dialytic efficiency of terbutaline sulfate under the experimental conditions was 26.3%. The data obtained by proposed microdialysis flow-injection chemiluminescence method was analyzed with Scrathard analysis and Klotz plot. The estimated association constant (K) and the number of the binding site (n) on one molecule of BSA by Scrathard analysis were 4.11 x 10(4) l mol(-1) and 1.06, respectively. The proposed system proved that FIA-CL coupled with on-line microdialysis sampling is a simple and reliable technique for the study of drug-protein interaction.

Distribution of cyclosporin in organ transplant recipients

Cyclosporin is an immunosuppressive agent with a narrow therapeutic index. The total concentration of cyclosporin in blood is usually monitored to guide dosage adjustment and to compensate for substantial interindividual and intraindividual variability in cyclosporin pharmacokinetics. Cyclosporin is a highly lipophilic molecule and widely distributes into blood, plasma and tissue components. It mainly accumulates in fat-rich organs, including adipose tissue and liver. In blood, it binds to erythrocytes in a saturable fashion that is dependent on haematocrit, temperature and the concentration of plasma proteins. In plasma, it binds primarily to lipoproteins, including high-density, low-density and very-low-density lipoprotein, and, to a lesser extent, albumin. The unbound fraction of cyclosporin in plasma (CsA(fu)) expressed as a percentage is approximately 2%. It has been shown that both the pharmacokinetic and pharmacodynamic properties of cyclosporin are related to its binding characteristics in plasma. Furthermore, there is some evidence to indicate that the unbound concentration of cyclosporin (CsA(U)) has a closer association with both kidney and heart allograft rejection than the total (bound + unbound) concentration. However, the measurement of CsA(fu) is inherently complex and cannot easily be performed in a clinical setting. Mathematical models that calculate CsA(fu), and hence CsA(U), from the concentration of plasma lipoproteins may be a more practical option, and should provide a more accurate correlate of effectiveness and toxicity of this drug in transplant recipients than do conventional monitoring procedures. In conclusion, the distribution characteristics of cyclosporin in blood, plasma and various tissues are clinically important. Further investigations are needed to verify whether determination of CsA(U) improves the clinical management of transplant recipients.

Validation of subcutaneous microdialysis sampling for pharmacokinetic studies of flurbiprofen in the rat

The objective of this study was to validate subcutaneous (sc) microdialysis sampling to study flurbiprofen pharmacokinetics and plasma protein binding in the awake freely moving rat. A linear microdialysis probe was manufactured using a Hemophane hollow fiber which was tested in vitro and in vivo for the recovery of flurbiprofen and naproxen used as retrodialysis marker. Flurbiprofen was administered intraperitoneally and intravenously at a dose of 20 mg/kg in rats. In both cases, conventional blood sampling and sc microdialysis sampling were simultaneously performed. The microdialysates were analyzed on-line by high-pressure liquid chromatography. Naproxen, which was shown to have a similar in vivo loss by retrodialysis as flurbiprofen (71.5 +/- 0.9% and 71.0 +/- 0.8% respectively, n = 3), was used to continuously monitor probe recovery. Concentration-dependent protein binding of flurbiprofen was demonstrated in vivo based on experiments with a simultaneous sc microdialysis and blood sampling. Values of unbound fraction were similar to those reported previously by intravenous microdialysis sampling, demonstrating that the sc unbound concentrations are very similar to those in the central compartment. There was no significant difference among pharmacokinetic parameters (AUC, CL, t(1/2z), Vd) for total or unbound flurbiprofen determined after intraperitoneal and intravenous administration. Subcutaneous microdialysis is a simple yet powerful tool to study the pharmacokinetics and the in vivo plasma protein binding of flurbiprofen in the awake unrestrained rat.

Can oral midazolam predict oral cyclosporine disposition?

Effective cyclosporine therapy is confounded by large interindividual differences in oral bioavailability and a narrow therapeutic window. Because cytochrome P450 (CYP) 3A-mediated first-pass metabolism contributes to this unpredictable bioavailability, an in vivo oral CYP3A phenotyping probe could be a valuable tool in optimizing cyclosporine therapy. Based on similarities in the metabolic kinetics of cyclosporine and midazolam by the liver and intestinal mucosa, we evaluated whether midazolam oral clearance would predict cyclosporine oral clearance when the two drugs were administered to 20 medically stable kidney transplant recipients. Despite earlier findings in liver transplant recipients who displayed a strong correlation between the systemic clearances of midazolam and cyclosporine, there was a weak correlation between their oral clearances in the current group of subjects (r(s)=0.50, P=0.03). Differing extents of intestinal first-pass metabolic extraction between the two drugs, inhibition of midazolam metabolism by cyclosporine at the level of the intestine, and/or P-glycoprotein-mediated intestinal efflux of cyclosporine (but not midazolam) may account for this poor correlation. We conclude that although oral midazolam is unlikely to be clinically useful as a probe for cyclosporine disposition, its utility in the prediction of other orally administered CYP3A substrates cannot be out ruled.

Development of an in vitro dissolution method using microdialysis sampling technique for implantable drug delivery systems

The major challenge faced during the development of implantable dosage forms for site-specific delivery is monitoring the local concentration of the drug at or around the site of action. The tissue concentration at the site is generally measured by either sacrificing the animal at different points in time or by determining the amount of drug left in the implants at various time intervals. Unfortunately, there are no official in vitro dissolution methods available to study the release characteristics of drugs from this drug delivery system. The objective of this investigation was to develop a simple method using microdialysis sampling technique to serve as an in vitro dissolution method for implantable drug delivery systems. Ciprofloxacin implants were prepared by compressing ciprofloxacin microcapsules in poly(lactic acid) (PLA) and poly(lactic-glycolic acid) (PLGA). A sensitive HPLC method was developed and validated for the assay of Ciprofloxacin. An in vitro dissolution method was developed to study the release characteristics of drug from these implants. The method used a microdialysis sampling technique and a small sample volume of release medium. The various advantages and disadvantages of this method over other USP methods are discussed.

Determination of unbound drug concentration and protein-drug binding fraction in plasma

The aim of this work was to provide a short overview of existing methods for the determination of free drug concentration and protein-drug binding fraction in plasma. Various methods have been described in terms of principles, evaluation of methods, and applications in recent years, with an emphasis on the chromatographic method, i.e. high-performance frontal analysis (HPFA).

Analysis of recombinant cytokines in human body fluids by immunoaffinity capillary electrophoresis

An immunoaffinity capillary electrophoresis (ICE) system for rapidly quantifying recombinant cytokines in human body fluids has been developed. Cytokines within biological fluids were labeled with a red light emitting fluorochrome and injected into the capillary. Selected cytokines were captured by immobilized antibodies on the internal surface of the capillary, and held while unbound materials were purged. The cytokines were then eluted electrophoretically in acidic buffer. Individual cytokine peaks were detected by on-line laser-induced fluorescence detection coupled to a computerized fiber-optic spectrometer, and analyzed by integration of the eluted peaks. The comparison of the results of ICE to routine assays used for these cytokines demonstrates that ICE provides a fast and accurate procedure for defining these cytokines in complex biological samples. Immunoaffinity separations can be used for any material to which a specific antibody can be raised, making this procedure applicable to a wide range of molecules of biomedical interest.

Application of microdialysis in pharmacokinetic studies

The objective of this review is to survey the recent literature regarding the various applications of microdialysis in pharmacokinetics. Microdialysis is a relatively new technique for sampling tissue extracellular fluid that is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses various aspects of the technique with regard to its use in pharmacokinetic studies, such as: quantitation of the microdialysis probe relative recovery, interfacing the sampling technique with analytical instrumentation, and consideration of repeated procedures using the microdialysis probe. The remainder of the review is devoted to a survey of the recent literature concerning pharmacokinetic studies that apply the microdialysis sampling technique. While the majority of the pharmacokinetic studies that have utilized microdialysis have been done in the central nervous system, a growing number of applications are being found in a variety of peripheral tissue types, e.g. skin, muscle, adipose, eye, lung, liver, and blood, and these are considered as well. Given the rising interest in this technique, and the ongoing attempts to adapt it to pharmacokinetic studies, it is clear that microdialysis sampling will have an important place in studying drug disposition and metabolism.