Evaluation of microdialysis sampling of aqueous humor for in vivo models of ocular absorption and disposition

Aqueous humor dynamics in human eye: A lattice Boltzmann study

This paper presents a lattice Boltzmann model to simulate the aqueous humor (AH) dynamics in the human eye by involving incompressible Navier-Stokes flow, heat convection and diffusion, and Darcy seepage flow. Verifying simulations indicate that the model is stable, convergent and robust. Further investigations were carried out, including the effects of heat convection and buoyancy, AH production rate, permeability of trabecular meshwork, viscosity of AH and anterior chamber angle on intraocular pressure (IOP). The heat convection and diffusion can significantly affect the flow patterns in the healthy eye, and the IOP can be controlled by increasing the anterior chamber angle or decreasing the secretion rate, the drainage resistance and viscosity of AH. However, the IOP is insensitive to the viscosity of AH, which may be one of the causes that the viscosity would not have been considered as a factor for controlling the IOP. It's interesting that all these factors have more significant influences on the IOP in pathologic eye than healthy one. The temperature difference and the eye-orientation have obvious influence on the cornea and iris wall shear stresses. The present model and simulation results are expected to provide an alternative tool and theoretical reference for the study of AH dynamics.

A Study on Acetylglutamine Pharmacokinetics in Rat Blood and Brain Based on Liquid Chromatography-Tandem Mass Spectrometry and Microdialysis Technique

Acetylglutamine (NAG) is the derivative of glutamine, which is the richest free amino acid in the human body. In this work, a novel reliable method of the combination of liquid chromatography-tandem mass spectrometry (LC-MS/MS) and microdialysis (MD) technique for the evaluation of NAG and its metabolites γ-aminobutyric acid (GABA) and glutamic acid (Glu) in rat blood and brain was proposed. A Zorbax SB-C18 column (2.1 × 100 mm, 3.5 μM) was applied to separate the analytes. The mobile phase was acetonitrile-water (70:30, v/v) containing 5 mM ammonium acetate and the flow rate was 0.3 ml/min. Based on the multiple reaction monitoring (MRM) mode of positive ion, the precursors of product ions chosen for NAG, Glu, GABA, and N-carbamyl-L-glutamic (NCG, IS) were (m/z) 189.1→130.0, 148.0→84.1, 104→87.1, and 191.0→130.1, respectively. All the validation data, including precision, accuracy, inter-day repeatability, matrix effect, and stability, were within the acceptable ranges according to the reference of Bioanalytical Method Validation Guidance for Industry (2018). Rats with microdialysis probes inserted into jugular vein and hippocampus were administered the low (75 mg/kg, NAG-L), medium (150 mg/kg, NAG-M), and high (300 mg/kg, NAG-H) doses of NAG and 10 ml/kg Guhong injection (GHI) by tail vein, respectively. In the blood test, the Cmax values of NAG-L group were markedly lower (P < 0.01) than those of NAG-M, NAG-H, and GHI groups, respectively. No differences were observed between NAG-M and GHI groups, while the Cmax values in GHI group were significantly upgraded compared with NAG-H group. There were notable differences in the Cmax values of NAG in brain dialysate after administration of NAG and GHI. The drug distribution coefficients of NAG, Glu, GABA in brain and blood at low, medium, high doses of NAG and GHI groups were 13.99, 27.43, 34.81, 31.37; 11.04, 59.07, 21.69, 2.69%; 212.88, 234.92, 157.59, and 102.65%, respectively. Our investigation demonstrates that NAG and its related metabolites in rat blood and brain can be simultaneously measured according to the above proposed method. Meanwhile, NAG has easy and dose-dependently access to the blood-brain barrier and exhibits a medium retention time in rat.

Targeted Drug Delivery Systems for the Treatment of Glaucoma: Most Advanced Systems Review

Each year, new glaucoma drug delivery systems are developed. Due to the chronic nature of the disease, it requires the inconvenient daily administration of medications. As a result of their elution from the eye surface and penetration to the bloodstream through undesired permeation routes, the bioavailability of active compounds is low, and systemic side effects occur. Despite numerous publications on glaucoma drug carriers of controlled drug release kinetics, only part of them consider drug permeation routes and, thus, carriers' location, as an important factor affecting drug delivery. In this paper, we try to demonstrate the importance of the delivery proximal to glaucoma drug targets. The targeted delivery can significantly improve drug bioavailability, reduce side effects, and increase patients' compliance compared to both commercial and scientifically developed formulations that can spread over the eye surface or stay in contact with conjunctival sac. We present a selection of glaucoma drug carriers intended to be placed on cornea or injected into the aqueous humor and that have been made by advanced materials using hi-tech forming methods, allowing for effective and convenient sustained antiglaucoma drug delivery.

Nanosponge-Mediated Drug Delivery Lowers Intraocular Pressure

PURPOSE

We examined the efficacy of an extended-release drug delivery system, nanosponge (NS) encapsulated compounds, administered intravitreally to lower intraocular pressure (IOP) in mice.

METHODS

Bilateral ocular hypertension was induced in mice by injecting microbeads into the anterior chamber. Hypertensive mice received NS loaded with ocular hypotensive drugs via intravitreal injection and IOP was monitored. Retinal deposition and retinal ganglion cell (RGC) uptake of Neuro-DiO were examined following intravitreal injection of Neuro-DiO-NS using confocal microscopy.

RESULTS

Brimonidine-loaded NS lowered IOP 12% to 30% for up to 6 days (P < 0.02), whereas travoprost-NS lowered IOP 19% to 29% for up to 4 days (P < 0.02) compared to saline injection. Three bimatoprost NS were tested: a 400-nm NS and two 700-nm NS with amorphous (A-NS) or amorphous/crystalline (AC-NS) crosslinkers. A single injection of 400 nm NS lowered IOP 24% to 33% for up to 17 days compared to saline, while A-NS and AC-NS lowered IOP 22% to 32% and 18% to 26%, respectively, for up to 32 days (P < 0.046). Over time retinal deposition of Neuro-DiO increased from 19% to 71%; Neuro-DiO released from NS was internalized by RGCs.

CONCLUSIONS

A single injection of NS can effectively deliver ocular hypotensive drugs in a linear and continuous manner for up to 32 days. Also, NS may be effective at targeting RGCs, the neurons that degenerate in glaucoma.

TRANSLATIONAL RELEVANCE

Patient compliance is a major issue in glaucoma. The use of NS to deliver a controlled, sustained release of therapeutics could drastically reduce the number of patients that progress to vision loss in this disease.

Natural borneol enhances geniposide ophthalmic absorption in rabbits

The purpose of this study was to investigate the effects of natural borneol (NB) on the pharmacokinetics and bioavailability of ophthalmic administered geniposide (Ge) in rabbits. In vitro permeability characteristics of Ge in excised rabbit corneas were evaluated using Franz-type cells. The effect of NB on Ge pharmacokinetic profiles in vivo was studied with microdialysis. Concentrations of Ge were determined with reversed-phase high performance liquid chromatography (HPLC) following ophthalmic administration of Ge alone or with NB (0.01%, 0.02%, and 0.04%) or 0.5% ethylendiaminetetraacetic acid (EDTA). Ocular irritation was evaluated using the Draize method and histological examination. Ge solution alone (control solution) had limited corneal permeability. The ratio of the apparent permeability coefficient (Papp) with respect to the control solution significantly increased by approximately 1.6-, 2.0-, and 2.4-fold at NB concentrations of 0.01, 0.02, and 0.04%, respectively. The Papp for Ge with 0.5% EDTA (positive control) was approximately 1.7-fold higher than that for control solution. Compared to control solution, Ge exhibited a 1.46-, 2.16-, and 2.47-fold greater AUC0-6h, and 2.0-, 3.5-, and 4.4-fold greater Cmax, with 0.01, 0.02, and 0.04% NB, respectively, while Tmax remained unchanged. In conclusion, the ocular bioavailability of Ge significantly increased in the presence of NB.

Recent advances in ophthalmic drug delivery

Topical ocular drug bioavailability is notoriously poor, in the order of 5% or less. This is a consequence of effective multiple barriers to drug entry, comprising nasolacrimal drainage, epithelial drug transport barriers and clearance from the vasculature in the conjunctiva. While sustained drug delivery to the back of the eye is now feasible with intravitreal implants such as Vitrasert (-6 months), Retisert (-3 years) and Iluvien (-3 years), currently there are no marketed delivery systems for long-term drug delivery to the anterior segment of the eye. The purpose of this article is to summarize the resurgence in interest to prolong and improve drug entry from topical administration. These approaches include mucoadhesives, viscous polymer vehicles, transporter-targeted prodrug design, receptor-targeted functionalized nanoparticles, iontophoresis, punctal plug and contact lens delivery systems. A few of these delivery systems might be useful in treating diseases affecting the back of the eye. Their effectiveness will be compared against intravitreal implants (upper bound of effectiveness) and trans-scleral systems (lower bound of effectiveness). Refining the animal model by incorporating the latest advances in microdialysis and imaging technology is key to expanding the knowledge central to the design, testing and evaluation of the next generation of innovative ocular drug delivery systems.

Effects of poly(amidoamine) dendrimers on ocular absorption of puerarin using microdialysis

PURPOSE

The purpose of this study was to investigate the effects of poly(amidoamine) (PAMAM) dendrimers on ocular absorption of puerarin.

METHODS

The samples of rabbits' aqueous humor were collected by in vivo microdialysis. The ocular pharmacokinetic properties of puerarin were measured to evaluate the effects of PAMAM dendrimers on ocular absorption of puerarin.

RESULTS

In the pharmacokinetic studies, the AUC(0∼∞) values of puerarin solution with 0.2% (w/v) PAMAM dendrimers (G3.0, G4.0, G5.0) were greater than those in the control group by 2.3-, 3.5-, and 2.1-folds, respectively. The C(max) values of puerarin solution with 0.2% (w/v) PAMAM dendrimers were 1.5, 2.5, and 1.3 times the values of the control group and the t(1/2) values were significantly longer than that of control group.

CONCLUSIONS

Using PAMAM dendrimers might be a promising strategy to enhance the absorption of puerarin. The generation of PAMAM dendrimers may play a key role in increasing the ocular absorption of puerarin.

Ocular microdialysis: a continuous sampling technique to study pharmacokinetics and pharmacodynamics in the eye

The unique anatomy and physiology of the eye present many challenges to the successful development and delivery of ophthalmic drugs. Any therapeutic strategy developed to control the progression of anterior and posterior segment diseases requires continuous monitoring of effective drug concentrations in the relevant ocular tissues and fluids. Ocular microdialysis has gained popularity in recent years due to its ability to continuously monitor drug concentrations and substantially reduce the number of animals needed. The intrusive nature of ocular microdialysis experimentation has restricted these studies to animal models. This review article intends to highlight various aspects of ocular microdialysis and its relevance in examining the disposition of drugs in the anterior and posterior segments.

Vitreal pharmacokinetics of peptide-transporter-targeted prodrugs of ganciclovir in conscious animals

PURPOSE

To delineate the vitreal pharmacokinetics of dipeptide monoester prodrugs of ganciclovir (GCV) with conscious rabbit model using ocular microdialysis and to compare with published results from anesthetized model.

METHODS

New Zealand albino male rabbit was selected as the animal model. Conscious animal ocular microdialysis technique with permanently implanted probes was employed to delineate the pharmacokinetics of GCV, L-valine-GCV (Val-GCV), and dipeptide monoester GCV prodrugs [val-val and L-glycine-val (Gly-Val)] after intravitreal administration.

RESULTS

This work employs conscious model to evaluate vitreal pharmacokinetic parameters and compares the results with previously published data from anesthetized animal, thereby demonstrating the effect of anesthesia on the vitreal disposition of dipeptide prodrugs of GCV. Results have revealed that area under curve (AUC), clearance, and last measured plasma concentration (C(last)) for all 4 compounds were significantly altered in a conscious animal relative to the anesthetized model, while mean residence time (MRT) was significantly reduced. However, the AUCs of regenerated Val-GCV and GCV from Gly-Val-GCV and Val-Val-GCV were found to be unchanged, suggesting higher ocular metabolism in conscious animals.

CONCLUSION

This study for the first time delineates the vitreal pharmacokinetics of a GCV prodrug in conscious animals and compares the data with anesthetized animals. Lower vitreal exposure levels were obtained in case of conscious animal model; however, the elimination rates were not influenced by anesthesia.

Novel vehicle based on cubosomes for ophthalmic delivery of flurbiprofen with low irritancy and high bioavailability

AIM

To develop a novel vehicle based on cubosomes as an ophthalmic drug delivery system for flurbiprofen (FB) to reduce ocular irritancy and improve bioavailability.

METHODS

FB-loaded cubosomes were prepared using hot and high-pressure homogenization. Cubosomes were then characterized by particle size, zeta potential, encapsulation efficiency, particle morphology, inner cubic structure and in vitro release. Corneal permeation was evaluated using modified Franz-type cells. Ocular irritation was then evaluated using both the Draize method and histological examination. The ocular pharmacokinetics of FB was determined using microdialysis.

RESULTS

The particle size of each cubosome formulation was about 150 nm. A bicontinuous cubic phase of cubic P-type was determined using cryo-transmission electron microscopy (cryo-TEM) observation and small angle X-ray scattering (SAXS) analysis. In vitro corneal permeation study revealed that FB formulated in cubosomes exhibited 2.5-fold (F1) and 2.0-fold (F2) increase in P(app) compared with FB PBS. In the ocular irritation test, irritation scores for each group were less than 2, indicating that all formulations exhibited excellent ocular tolerance. Histological examination revealed that neither the structure nor the integrity of the cornea was visibly affected after incubation with FB cubosomes. The AUC of FB administered as FB cubosome F2 was 486.36+/-38.93 ng.mL(-1).min.microg(-1), which was significantly higher than that of FB Na eye drops (P<0.01). Compared with FB Na eye drops, the T(max) of FB cubosome F2 was about 1.6-fold higher and the MRT was also significantly longer (P<0.001).

CONCLUSION

This novel low-irritant vehicle based on cubosomes might be a promising system for effective ocular drug delivery.

Improved ocular absorption kinetics of timolol maleate loaded into a bioadhesive niosomal delivery system

BACKGROUND

Application of timolol maleate (TM) in a conventional dosage form (solution) into the eye results in almost 80% of the instilled dose being lost either through spillage or due to drainage into the nasolacrimal duct. Later results in systemic side-effects especially in patients suffering from heart diseases or bronchial asthma thus limiting the usefulness of TM for the control of glaucoma. Earlier we had reported on the development of a mucoadhesive coated niosomal system for TM (TM REV(bio)) containing 0.25% TM. Presently we establish and report the pharmacokinetic and pharmacodynamic superiority of the developed ocular formulation of TM.

METHODS

Aqueous humor concentration of TM in male albino rabbits, after instillation of one drop of TM solution (TMS) or TMREV(bio) was measured using the microdialysis method.

RESULTS

Peak concentration of drug in aqueous humor from TMREV(bio) (12.46 microg/ml achieved at 60 min) was almost 1.7 times that of the control drug solution (TMS, 0.25%; 7.2 microg/ml). An important observation was that the high drug concentrations achieved upon TMREV(bio) administration were maintained for up to 2 h. AUC for TMREV(bio) formulation was 2.34 times that of the TMS.

CONCLUSIONS

This study confirms a sustained and controlled effect of the developed formulation.

Ocular ciprofloxacin hydrochloride mucoadhesive chitosan-coated liposomes

The aim of this work is to improve the ocular bioavailability of ciprofloxacin hydrochloride (CPX) through the preparation of ocular mucoadhesive chitosan (CS)-coated liposomes. Liposomes were prepared by the thin film hydration technique, using different molar ratios of L-α-phosphatidylcholine (PC), cholesterol (CH), stearylamine (SA) and dicetyl phosphate (DP). CS was used to coat the optimal liposomal formulae. The prepared formulae were characterized regarding encapsulation efficiency (%EE), particle size, physical morphology and in vitro drug release. The in vivo characterization of the prepared formulae was performed through evaluating the level of CPX in the external eye tissue of nine albino rabbits. Results showed an alteration in release rate and %EE of CPX from liposomal formulae upon varying the molar ratios of the lipid bilayer composition. The optimal liposomal formulae F1 (10:0, PC:CH), F12 (10:0:0.5, PC:CH:SA) and F15 (10:0:1, PC:CH:DP), showed % EE of 38.5 ± 2.10, 39.65 ± 1.85 and 30.05 ± 0.75 and % in vitro release after 8 hours (Q(8h)) of 78.15 ± 2.4, 54.07 ± 2.3 and 62.14 ± 2.9, respectively. In vitro drug release and in vivo results confirmed that CS-coated liposomal formulae have exhibited a higher retention of CPX. Consequently, CS-coated liposomes could be a promising approach to increase the ocular bioavailability of CPX.

Study on the ocular pharmacokinetics of ion-activated in situ gelling ophthalmic delivery system for gatifloxacin by microdialysis

The poor bioavailability and therapeutic response exhibited by conventional ophthalmic solutions due to rapid precorneal elimination of the drug may be overcome by the use of gel system. The present work was conducted to evaluate the relative bioavailability of ion-activated in situ ophthalmic gel of gatifloxacin by microdialysis. The conventional ophthalmic solution of gatifloxacin was used as reference. The AUC of test group is 3.8-fold vs. the reference group (1.4316 +/- 0.1327 microg.mL(-1).h vs. 0.3756 +/- 0.0380 microg.mL(-1).hr) (P < 0.05), and the C(max) of test group vs. the control group is 3.0-fold (0.3363 +/- 0.0634 microg.mL(-1) vs. 0.1112 +/- 0.0151 microg.mL(-1)) (P < 0.05). The T(max) of test group is longer than that of reference group (2.0 +/- 0.67 hr vs. 0.667 +/- 0.17 hr) (P < 0.1), and K(e) of test group is lower than that of reference group. The developed formulation has a higher bioavailability and longer residence time in aqueous humor than conventional ophthalmic solutions. The developed system is a viable alternative to conventional eye drops.

Poly(N-isopropylacrylamide)–chitosan as thermosensitive in situ gel-forming system for ocular drug delivery

A novel copolymer, poly(N-isopropylacrylamide)-chitosan (PNIPAAm-CS), was investigated for its thermosensitive in situ gel-forming properties and potential utilization for ocular drug delivery. The thermal sensitivity and low critical solution temperature (LCST) were determined by the cloud point method. PNIPAAm-CS had a LCST of 32 degrees C, which is close to the surface temperature of the eye. The in vivo ocular pharmacokinetics of timolol maleate in PNIPAAm-CS solution were evaluated and compared to that in conventional eye drop solution by using rabbits according to the microdialysis method. The C(max) of timolol maleate in aqueous fluid for the PNIPAAm-CS solution was 11.2 microg/ml, which is two-fold higher than that of the conventional eye drop, along with greater AUC. Furthermore, the PNIPAAm-CS gel-forming solution of timolol maleate had a stronger capacity to reduce the intra-ocular pressure (IOP) than that of the conventional eye drop of same concentration over a period of 12 h. In addition, the MTT assay showed that there is little cytotoxicity of PNIPAAm-CS at concentration range of 0.5-400 microg/ml. These results suggest that PNIPAAm-CS is a potential thermosensitive in situ gel-forming material for ocular drug delivery, and it may improve the bio-availability, efficacy, and compliance of some eye drugs.

Study of the extent of ocular absorption of acetazolamide from a developed niosomal formulation, by microdialysis sampling of aqueous humor

Acetazolamide, a carbonic anhydrase inhibitor is used orally (no topical formulation being available in the market) for the reduction of intraocular pressure (IOP) in patients suffering from glaucoma. Two major reasons responsible for the failure to develop a topically effective formulation of acetazolamide are its low solubility (0.7mg/ml) and its low permeability coefficient. It is generally recognized that topical acetazolamide formulation possessing efficacy similar to that achieved upon oral administration would be a significant advancement in the treatment of glaucoma. In order to enhance the bioavailability of acetazolamide by topical route and to improve the corneal permeability of the drug, the niosomes of acetazolamide were prepared (by reverse phase evaporation method) and coated with Carbopol for the latter's bioadhesive effect. The pharmacodynamic studies showed 33% fall in IOP with the developed formulation, and the effect was sustained for 6h after instillation. The effect compared well with a four times higher concentration of dorzolamide (Dorzox, a topical CAI available in the market. In the present study, the aqueous humor disposition of the drug from the developed bioadhesive coated niosomal formulation (ACZREVbio) is compared with the aqueous suspension of the drug (containing 1% (w/v) Tween 80 as a dispersing agent) at similar concentrations. The concentration of acetazolamide absorbed in the aqueous humor at various times from the control suspension and from ACZREVbio was determined by microdialysis in male albino rabbits. Microdialysis provides a complete concentration versus time profile and hence is an important advance to the regional sampling of tissues. The peak concentration of drug absorbed in the aqueous humor from the ACZREVbio formulation (14.94microg/ml) was almost two times of that obtained with the equivalent amount of acetazolamide control suspension (6.93microg). The results show a significant broadening of peak from 80 to 120min with the concentration of more than 13microg being maintained at these times, for the bioadhesive coated niosomal formulation (ACZREVbio). An important observation was the fact that a high drug concentration of 12.02microg reached immediately, i.e., 20min after instillation of ACZREVbio indicating a high penetration being achieved, while a meagre concentration of only 3.53microg is obtained at 60min after instillation of the control suspension. The aqueous humor disposition indicates peaks and troughs in drug concentration which may be related to the decrease in aqueous humor formation, such that the drug concentration/volume increased at these points.

Microdialysis—theoretical background and recent implementation in applied life-sciences

In the past decade microdialysis has become a method of choice in the study of unbound tissue concentrations of both endogenous and exogenous substances. Microdialysis has been shown to offer information about substances directly at the site of action while being well tolerable and safe. The large variety of its field of application has been demonstrated. However, a few challenges have to be met to make this method generally applicable in routine applications. This review will provide an overview over theoretical aspects that have to be considered during the implementation of microdialysis. Moreover, a comparison between microdialysis and other tissue sampling techniques will demonstrate advantages and limitations of the methods mentioned. Subsequently, it will present a critical synopsis of a variety of scientific/biomedical applications of this method with emphasis on the most recent literature, focussing on target tissues while giving examples of substances examined. It is concluded that microdialysis will be of great value in future investigations of pharmacokinetics, pharmacodynamics and in monitoring of disease status and progression.

Pharmacokinetics of timolol in aqueous humor sampled by microdialysis after topical administration of thermosetting gels

In order to develop a thermosetting gel-based formulation, the ocular pharmacokinetics of timolol was studied utilizing microdialysis sampling technique after topical administration. A linear microdialysis probe was characterized and implanted in the anterior chamber of a rabbit. Dialysate samples collected from the aqueous humor (AH) were directly injected into the HPLC system without any pre-treatment and no interference was observed in the blank sample. The measured in vitro recovery of the probe was 57.67%; however, the in vivo recovery significantly decreased to 16.78% when assessed by the retrodialysis method, which was used to calculate the timolol concentration in AH. Although in the initial 15 min the drug concentrations in AH were comparable to that of the timolol solution, increased Cmax and significantly improved ocular bioavailability were obtained for the gel. When sodium deoxycholate (DC) was incorporated in the gel as a penetration enhancer, a 2-fold increment in the ocular bioavailability was achieved with an increased Cmax and significantly suspended Tmax. The results demonstrated that microdialysis coupled to HPLC is a powerful tool to investigate the ocular pharmacokinetic, and hence facilitates the design of ophthalmic formulations.

Estimation of perilymph concentration of agents applied to the round window membrane by microdialysis

OBJECTIVE

The round window membrane (RWM) is known to be permeable to various biological substances. Application of biological substances to the RWM has been shown to affect inner ear fluid composition and damage hair cells, resulting in functional changes RWM instillation of gentamicin, a preferentially vestibulotoxic aminoglycoside, is used as a therapeutic treatment for patients with intractable vertigo and is gaining acceptance as a chemical vestibular ablation agent, despite considerable variations in the incidence and severity of hearing loss associated with gentamicin. Clearly, the susceptibility of vestibular and auditory hair cells to the ototoxic effects of gentamicin is not well understood. The aim of this study was to understand the kinetics of urea and methylene blue instilled into the inner ear space through the RWM and to establish a method for determining the optimal dosage for the treatment of inner ear disorders.

MATERIAL AND METHODS

We used inner ear microdialysis to quantify changes in perilymph concentration of low molecular weight agents applied to the RWM in a chinchilla model.

RESULTS

Preliminary results after placement of a microdialysis probe and application of a low molecular weight marker (urea) to the RWM were extrapolated from a time versus concentration plot from dialysates sampled over a 3-min interval using modifications of standard microdialysis equations for estimation of in vivo recovery. Our data suggest that inner ear microdialysis can be used to measure the pharmacokinetics of a low molecular weight agent within the perilymphatic space without the need for repeated direct sampling.

CONCLUSION

Inner ear microdialysis may be a useful method for establishing a therapeutic dosage for ototoxic agents used in the treatment of inner ear disorders.

A review of membrane sampling from biological tissues with applications in pharmacokinetics, metabolism and pharmacodynamics

This review provides an overview of membrane sampling techniques, microdialysis and ultrafiltration, and cites illustrations of their applications in pharmacokinetics, metabolism and/or pharmacodynamics. The review organizes applications by target tissue and general type of information gleaned. It focuses on recently published microdialysis studies (1999 to this writing) and offers the first review of ultrafiltration sampling studies. The advantages and limitations of using microdialysis and ultrafiltration sampling as tools for obtaining pharmacokinetic and metabolism data are discussed. Numerous examples are described including studies in which several types of data are collected simultaneously. Reports that study local metabolism of drug delivered through the probe are also presented.

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.

Microdialysis and drug delivery to the eye

The eye presents unique challenges in both the development of tools for elucidating drug disposition as well as for the development of modes of drug delivery for treatment of ocular diseases. In this paper, we present a discussion of the anatomical and physiological characteristics and limitations present in the eye for microdialysis sampling of endogenous substrates and xenobiotics. To date, over twenty papers describing microdialysis approaches for assessment of ocular drug delivery and endogenous substrate characterization have been published. Although the majority of papers describe sampling of vitreous humor, recent efforts have been directed towards ocular anterior segment sampling using microdialysis. With this approach, an appreciable reduction in animal use has been realized. In addition, simultaneous examination of administered drug and endogenous substrates modulated by the drug is possible with this approach, facilitating construction of ocular pharmacokinetic/pharmacodynamic relationships through use of relevant surrogate markers.

Blood microdialysis in pharmacokinetic and drug metabolism studies

Microdialysis is a sampling technique allowing measurement of endogenous and exogenous substances in the extracellular fluid surrounding the probe. In vivo microdialysis sampling offers several advantages over conventional methods of studying the pharmacokinetics and metabolism of xenobiotics, both in experimental animals and humans. In the first part of this review article various practical aspects related to blood microdialysis will be discussed, such as: probe design, surgical implantation techniques, methods to determine the in vivo relative recovery of the analyte of interest by the probe, special analytical considerations related to small volume microdialysate samples, and pharmacokinetic calculations based on microdialysis data. In the second part of this review a few selected applications of in vivo microdialysis sampling to investigate pharmacokinetic processes are briefly discussed: determination of in vivo plasma protein binding in small laboratory animals, distribution of drugs across the blood-brain barrier, the use of microdialysis sampling to study biliary excretion and enterohepatic cycling, blood microdialysis sampling in man and in the mouse, and in vivo drug metabolism studies.

Pharmacodynamics of beta-blocker modulation of aqueous humor production

A conscious rabbit model with microdialysis sampling of endogenous aqueous humor ascorbate was developed in order to assess the pharmacodynamics of beta-blocker modulation of aqueous humor production. CMA/20 microdialysis probes were implanted in the anterior chamber of each eye of rabbits (n = 6). After a 2 week recovery period, an i.v. bolus of 14C-ascorbate (20 microCi) was administered. Blood samples and aqueous humor microdialysis probe effluent were collected and analysed for endogenous and 14C-ascorbate to estimate the basal rate of ascorbate blood to aqueous humor secretion (Ro). After a 1 hr washout, each rabbit received a series of three doses of 3H-propranolol (750-3000 microg, 16.5 microCi mg(-1)) every 60 min into the lower cul-de-sac of each eye. Probe effluent was analysed for endogenous ascorbate and 3H-propranolol; ascorbate and propranolol in the iris/ciliary body, vitreous and aqueous was determined at the end of the experiment. Nonlinear least-squares regression analysis of the concentration-time profiles for aqueous humor ascorbate was performed to estimate the change in aqueous humor flow. The average basal aqueous humor ascorbate secretion rate was approximately 48/microg hr(-1). Propranolol (1500 microg) produced significant increases in aqueous humor ascorbate, this observation is consistent with a reduction in aqueous humor production (approximately 47%). Analysis of intraocular tissue ascorbate indicated that propranolol inhibited ascorbate secretion at the 3000 microg dose, the highest dose examined in this study; this inhibition was not observed at the 750 microg or 1500 microg doses. Changes in aqueous humor production precipitated by the administration of beta-adrenergic antagonists can be estimated by measuring changes in aqueous humor ascorbate concentrations in the conscious rabbit. Microdialysis sampling of aqueous humor for endogenous ascorbate provides a relevant analytic tool to estimate modulatory effects of anti-glaucoma drugs on aqueous humor production.

In vivo microdialysis sampling: theory and applications

During the last two decades, a number of methods have been developed for in vivo collection, separation and characterization of biological samples and analytes. The capability and reliability of the microdialysis technique for measuring endogenous substances (such as neurotransmitters and their metabolites) as well as exogenous therapeutic agents in various tissue systems have brought it to the forefront of the in vivo tissue sampling methods. The usability of this technique is demonstrated by its application as reported in almost 3600 scientific papers (as of January 1998). This paper describes the general aspects and various applications of this fast growing technique. Emphasis has been given to analytical considerations with regards to microdialysis probe recovery and newer HPLC techniques.

Microdialysis evaluation of the ocular pharmacokinetics of propranolol in the conscious rabbit

PURPOSE

This study was conducted to assess the effects of anesthesia and aqueous humor protein concentrations on ocular disposition of propranolol.

METHODS

Rabbits were anesthetized and a microdialysis probe was inserted into the anterior chamber of one eye; the contralateral eye served as a control. At timed intervals after probe placement, a 100-microl sample of aqueous humor was aspirated from each eye to determine protein concentration. In vitro protein binding parameters were used to simulate the impact of protein concentration on propranolol disposition. To assess the influence of anesthesia, probes were implanted in the anterior chamber of each eye. After >5-day stabilization, conscious and anesthetized rabbits (n = 3/group) received a 200-microg topical dose of [3H]DL-propranolol in each eye; propranolol was assayed in probe effluent.

RESULTS

Changes in aqueous humor protein concentrations were observed following probe insertion. Simulations demonstrated that the unbound propranolol AUC (approximately 2.4-fold) in aqueous humor should be reduced due to protein influx. Intraocular propranolol exposure in anesthetized rabbits was approximately 8-fold higher than in conscious rabbits, and approximately 1.9-fold higher than in rabbits without a post-surgical recovery period.

CONCLUSIONS

Anesthesia and time-dependent aqueous humor protein concentrations may alter ocular pharmacokinetics, and must be taken into account in the design of microdialysis experiments.