Selective properties of the vitreous barrier

A drug delivery analysis of large molecules in ocular vitreous chamber: Dependency on saccadic movements after intravitreal injection

The purpose of this study is to investigate the effect of vitreous sloshing induced by saccades on the intravitreal delivery of large molecule drugs. The vitreous body was considered in its age-related liquefaction condition. Fluid dynamics and large molecule distribution were described by the coupling of mass conservation's and Fick's laws with continuity and momentum equations for a Newtonian incompressible fluid in a 3D unsteady analysis. Two injection sites were analyzed, in both the mixing effect of a 50° periodic saccade leads to uniform drug distribution in 30 s of simulation, the initial bolus site being left after 3 s of simulation. In absence of saccadic movements, the dominant transport contribution is the diffusive one and large molecules hardly reach their uniform distribution inside the vitreous cavity. A model describing the intravitreal distribution of large molecules in presence of saccades was developed, improving the understanding of drug transport mechanism after an intravitreal injection and highlighting how advection contribution enhances its distribution in the vitreous chamber.

Endothelial Cells, First Target of Drug Delivery Using Microbubble-Assisted Ultrasound

Microbubble-assisted ultrasound has emerged as a promising method for local drug delivery. Microbubbles are intravenously injected and locally activated by ultrasound, thus increasing the permeability of vascular endothelium for facilitating extravasation and drug uptake into the treated tissue. Thereby, endothelial cells are the first target of the effects of ultrasound-driven microbubbles. In this review, the in vitro and in vivo bioeffects of this method on endothelial cells are described and discussed, including aspects on the permeabilization of biologic barriers (endothelial cell plasma membranes and endothelial barriers), the restoration of their integrity, the molecular and cellular mechanisms involved in both these processes, and the resulting intracellular and intercellular consequences. Finally, the influence of the acoustic settings, microbubble parameters, treatment schedules and flow parameters on these bioeffects are also reviewed.

Targeted and reversible blood-retinal barrier disruption via focused ultrasound and microbubbles

The blood-retinal barrier (BRB) prevents most systemically-administered drugs from reaching the retina. This study investigated whether burst ultrasound applied with a circulating microbubble agent can disrupt the BRB, providing a noninvasive method for the targeted delivery of systemically administered drugs to the retina. To demonstrate the efficacy and reversibility of such a procedure, five overlapping targets around the optic nerve head were sonicated through the cornea and lens in 20 healthy male Sprague-Dawley rats using a 690 kHz focused ultrasound transducer. For BRB disruption, 10 ms bursts were applied at 1 Hz for 60 s with different peak rarefactional pressure amplitudes (0.81, 0.88 and 1.1 MPa). Each sonication was combined with an IV injection of a microbubble ultrasound contrast agent (Definity). To evaluate BRB disruption, an MRI contrast agent (Magnevist) was injected IV immediately after the last sonication, and serial T1-weighted MR images were acquired up to 30 minutes. MRI contrast enhancement into the vitreous humor near targeted area was observed for all tested pressure amplitudes, with more signal enhancement evident at the highest pressure amplitude. At 0.81 MPa, BRB disruption was not detected 3 h post sonication, after an additional MRI contrast injection. A day after sonication, the eyes were processed for histology of the retina. At the two lower exposure levels (0.81 and 0.88 MPa), most of the sonicated regions were indistinguishable from the control eyes, although a few tiny clusters of extravasated erythrocytes (petechaie) were observed. More severe retinal damage was observed at 1.1 MPa. These results demonstrate that focused ultrasound and microbubbles can offer a noninvasive and targeted means to transiently disrupt the BRB for ocular drug delivery.

Computer modeling of drug delivery to the posterior eye: effect of active transport and loss to choroidal blood flow

PURPOSE

The direct penetration route following transscleral drug administration presents several barrier and clearance mechanisms-including loss to choroidal blood flow, active transport by the retinal pigment epithelium (RPE), and loss to the conjunctival lymphatics and episcleral blood vessels. The objective of this research was to quantify the role of choroidal and episcleral losses.

MATERIALS AND METHODS

A finite element model was created for drug distribution in the posterior human eye. The volumetric choroidal loss constant, active transport component and mass transfer from the scleral surface were unknown parameters in the model. The model was used to simulate drug distribution from a systemic source, and the results were compared to existing experimental results to obtain values for the parameters.

RESULTS

The volumetric choroidal loss constant, mass transfer coefficient from the scleral surface and active transport component were evaluated to be (2.0 +/- 0.6) x 10(-5) s(-1), (2.0 +/- 0.35) x 10(-5) cm/s and 8.54 x 10(-6) cm/s respectively.

CONCLUSION

Loss to the choroidal circulation was small compared to loss from the scleral surface. Active transport was predicted to induce periscleral movement of the drug, resulting in more rapid distribution and elevated drug concentrations in the choroid and sclera.

Evaluation of coupled convective-diffusive transport of drugs administered by intravitreal injection and controlled release implant

A 3-dimensional finite element model was developed to simulate pharmacokinetics in the eye following drug administration by intravitreal injection and implant for the treatment of retinal disease. The contributions of (1) convection to the transport of drug through the vitreous and aqueous humor and (2) diffusion of drug in the vitreous were varied to study the drug elimination from a normal and diseased eye. Drug distribution achieved by intravitreal injection was compared to that for the same dose released at a constant rate over 15 h from an implant. The model was constructed for a rabbit eye and validated with experimental data for intravitreal injection of fluorescein. The implant reduced peak concentration by 43% and increased residence time by 71% for the baseline (6x10(-6) cm2/s drug diffusivity in vitreous and 0.1 microL/min vitreous outflow), when compared with that of intravitreal injection. Therefore, the implant could be beneficial in reducing the peak concentration and sustaining release of the drug for a longer duration. Convection has a relatively small influence in the normal eye for high diffusivity drugs (1x10(-5) cm2/s), but could have a significant effect for low diffusivity drugs (1x10(-7) cm2/s) in pathophysiologically elevated fluid outflow across the retina. By interpolating the results of this benchmark study, one could estimate the distributions for drugs of different molecular weight, and assess the effect of variable vitreous outflows associated with different pathophysiological conditions.

Computer simulation of convective and diffusive transport of controlled-release drugs in the vitreous humor

PURPOSE

Biodistribution of drugs in the eye is central to the efficacy of pharmaceutical ocular therapies. Of particular interest to us is the effect of intravitreal transport on distribution of controlled-released drugs within the vitreous.

METHODS

A computer model was developed to describe the three-dimensional convective-diffusive transport of drug released from an intravitreal controlled release source. Unlike previous studies, this work includes flow of aqueous from the anterior to the posterior of the vitreous. The release profile was based on in vitro release of gentamicin from poly(L-lactic acid) microspheres into vitreous.

RESULTS

For small drugs, convection plays a small role, but for large (slower diffusing) drugs, convection becomes more important. For the cases studied, the predicted ratio of drug reaching the retina to drug cleared by the aqueous humor was 2.4 for a small molecule but 13 for a large molecule. Transport in neonatal mouse eye, in contrast, was dominated by diffusion, and the ratio decreased to 0.39.

CONCLUSIONS

The interaction among convection, diffusion, and geometry causes significant differences in biodistribution between large and small molecules or across species. These differences should be considered in the design of delivery strategies or animal studies.

Direct analysis of anticonvulsant drugs in vitreous humour by HPLC using a column switching technique

A method is described for the rapid HPLC analysis of a number of anticonvulsant drugs in vitreous humour by injection directly onto a preconcentration column without the need for prior extraction. The conditions for optimum precision and accuracy were investigated and the method was subsequently applied to the analysis of phenytoin in a number of overdose cases. Sensitivity was 0.05 microgram/ml on a 0.5 ml vitreous sample. The mean blood/vitreous ratio for phenytoin was found to be 2.8 (range: 1.4-5.1). The relationship between blood and vitreous levels had a correlation coefficient of 0.85. Vitreous was found to be a very clean, stable sample, ideally suited to this technique.

Clinical biochemistry of tears

The composition of tear fluid as it exists in the conjunctival fornix and in the precorneal tear film is of a complicated nature. The precorneal tear film is a physically inhomogenous system, produced by the lacrimal glands, the accessory lacrimal glands and the goblet cells of the conjunctiva and the Meibomian glands of the lid margin. The method of collection is crucial for the quantity and concentration of a great number of different compounds that have been demonstrated in tears. A survey is given of the literature concerning tears as they are composed of proteins, enzymes, lipids, metabolites, electrolytes and drugs, the latter secreted during therapy. Clinical applications of the determination of several compounds for diagnosis or for monitoring drug therapy are summarized.

Cloxacillin distribution in the rabbit eye after intravenous injection

Distribution of isotopically labelled and intravenously injected cloxacillin was studied in rabbit eye. The antibiotic concentration determined by liquid scintillation counting proved to be a reliable measure of the total antibiotic concentration when controlled by microbiological assay. In the rabbit eye after an intravenous injection of 50 mg/kg of cloxacillin sodium, longlasting antibiotic concentration regarded as therapeutic against penicillinase producing staphylococci was obtained in all vascularized ocular structures and in the cornea. The antibiotic present in the iris and ciliary body, and in the retina and choroid preparations, proved to be partly intravascular, whereas it penetrated better into the extravascular tissue compartment of the sclera and limbal area. Cloxacillin failed to achieve a therapeutic antibiotic concentration in the vitreous body and in the lens. Administration of probenecid had an enhancing effect on ocular cloxacillin concentration allowing improved drug diffusion into the eye by means of an elevated plasma concentration. No specific ocular effect of probenecid was noticed. Therapeutic concentration of cloxacillin in the aqueous humour, otherwise barely achieved, was more satisfactorily obtained with a previous injection of probenecid.

Glycogen concentration changes in retina, vitreous body and other eye tissues caused by disturbances of blood circulation

The glycogen content in the individual eye tissues is strongly correlated to blood supply. Our investigations on the retina of bovines, which have not been fully evaluated, show that the time interval between interruption of blood supply and preparation of the retina is of special importance. Pressure ischemia affects a decrease in glycogen content in the retina and vitreous of rabbits, which is, however, less distinct in the vitreous. Decrease of glycogen with ischemia also takes place in the cornea and, to a lesser degree, in iris and choroid. In contrast, there is no decrease in the glycogen content of the lens. Changes in glycogen content of the rabbit retina after ligation of the A. carotis communis is less distinct than with pressure ischemia. In the vitreous, changes in glycogen content could not be observed. Values measured in both tissues of the ligated eye decrease with additional pressure ischemia.