Ocular penetration and bioconversion of prostaglandin F2alpha prodrugs in rabbit cornea and conjunctiva

Ocular Drug Delivery

Although the eye is an accessible organ for direct drug application, ocular drug delivery remains a major challenge due to multiple barriers within the eye. Key barriers include static barriers imposed by the cornea, conjunctiva, and retinal pigment epithelium and dynamic barriers including tear turnover and blood and lymphatic clearance mechanisms. Systemic administration by oral and parenteral routes is limited by static blood-tissue barriers that include epithelial and endothelial layers, in addition to rapid vascular clearance mechanisms. Together, the static and dynamic barriers limit the rate and extent of drug delivery to the eye. Thus, there is an ongoing need to identify novel delivery systems and approaches to enhance and sustain ocular drug delivery. This chapter summarizes current and recent experimental approaches for drug delivery to the anterior and posterior segments of the eye.

Computational modeling of drug transport across the in vitro cornea

A novel quasi-3D (Q3D) modeling approach was developed to model networks of one dimensional structures like tubes and vessels common in human anatomy such as vascular and lymphatic systems, neural networks, and respiratory airways. Instead of a branching network of the same tissue type, this approach was extended to model an interconnected stack of different corneal tissue layers with membrane junction conditions assigned between the tissues. The multi-laminate structure of the cornea presents a unique barrier design and opportunity for investigation using Q3D modeling. A Q3D model of an in vitro rabbit cornea was created to simulate the drug transport across the cornea, accounting for transcellular and paracellular pathways of passive and convective drug transport as well as physicochemistry of lipophilic partitioning and protein binding. Lipophilic Rhodamine B and hydrophilic fluorescein were used as drug analogs. The model predictions for both hydrophilic and lipophilic tracers were able to match the experimental measurements along with the sharp discontinuities at the epithelium-stroma and stroma-endothelium interfaces. This new modeling approach was successfully applied towards pharmacokinetic modeling for use in topical ophthalmic drug design.

Prostanoid Receptor Antagonist Effects on Intraocular Pressure, Supported by Ocular Biodisposition Experiments

PURPOSE

Since all prostanoid receptors affect intraocular pressure (IOP) and endogenous prostanoids are found in ocular tissues, the pressor effects of prostanoid antagonists were comprehensively evaluated. The absence of effects of most of these antagonists was not entirely anticipated. To ensure no false-negative results, ocular biodisposition studies were conducted.

METHODS

Monkeys with laser-induced ocular hypertension were used to study antagonist effects on IOP. Ocular biodisposition of each antagonist was assessed in rabbits, with LC/MS/MS analyses of tissue extracts and blood.

RESULTS

EP₁, EP₂, EP₃, EP₄, FP, IP, and TP prostanoid receptor antagonists did not affect IOP, even at a high 1% dose. These studies were followed by ocular biodisposition studies. Striking differences in ocular tissue bioavailability were observed, which were independent of solubility. Only the EP₁ antagonist SC-51322 failed to penetrate sufficiently to be bioavailable in the aqueous humor and ciliary body/iris. This obliged testing an alternative EP₁ antagonist, namely ONO-8713, to reliably conclude that an EP₁ antagonist does not alter IOP.

CONCLUSIONS

These antagonist studies provided no evidence for individual endogenous prostanoids exerting a meaningful role in regulating IOP. They do reaffirm the critical importance of studying ocular bioavailability for confirming negative data. Large differences among the antagonists in anterior segment and even ocular surface tissue biodisposition were observed in rabbits. It appears from these monkey studies, supported by rabbit ocular bioavailability data, that an absence of drug effect in the eye cannot be adequately substantiated without determination of ocular pharmacokinetics.

2,3,4-Trihydroxybenzyl-hydrazide analogues as novel potent coxsackievirus B3 3C protease inhibitors

Human coxsackievirus B3 (CVB3) 3C protease plays an essential role in the viral replication of CVB3, which is a non-enveloped and positive single-stranded RNA virus belonging to Picornaviridae family, causing acute viral myocarditis mainly in children. During optimization based on SAR studies of benserazide (3), which was reported as a novel anti-CVB3 3C(pro) agent from a screening of compound libraries, the 2,3,4-trihydroxybenzyl moiety of 3 was identified as a key pharmacophore for inhibitory activity against CVB3 3C(pro). Further optimization was performed by the introduction of various aryl-alkyl substituted hydrazide moieties instead of the serine moiety of 3. Among the optimized compounds, 11Q, a 4-hydroxyphenylpentanehydrazide derivative, showed the most potent inhibitory activity (IC50 = 0.07 μM). Enzyme kinetics studies indicated that 11Q exhibited a mixed inhibitory mechanism of action. The antiviral activity against CVB3 was confirmed using the further derived analogue (14b) with more cell permeable valeryl ester group at the 2,3,4-trihydroxy moiety.

Fast-dissolving ocular films of riboflavin acetate conjugate for treatment of keratoconus in UVA-CXL procedure: ex vivo permeation, hemolytic toxicity and apoptosis detection

BACKGROUND

Attempts to facilitate corneal epithelial penetration of riboflavin (Rb) without de-epithelization, so far, include the use of penetration enhancers, to devitalize corneal epithelium in order to disturb tight epithelial interjunctional complexes and zonulae occludentes. Though such approaches result in sufficient epithelial permeability of Rb to guarantee efficacy of CXL procedure, they lack the evidences of safety. Prodrug with improved lipophilicity targeted toward esterases and amidases has proven to be an effective and promising approach to overcome lipophilic corneal epithelial barrier.

OBJECTIVES

Fast-dissolving ocular films of newly synthesized and characterized riboflavin lipid conjugate (RbLDC) were developed to overcome corneal epithelial barrier resistance for treatment of keratoconus. The safety concern of the film was assessed by in vitro hemolytic toxicity and in vitro apoptosis detection for its safe clinical use.

RESULTS

The optimized film was tough, flexible and dissolved rapidly within 36.86 s in simulated tear fluid, pH 7.4. FE-SEM/EDX showed smooth surfaces of films and evidenced the quantitative elemental similarity, indicating drug homogeneity. The permeation profile of F18 demonstrated 13.28-fold increased permeation of RbLDC relative to Rb solution across intact cornea. Safety was confirmed by 3.74% hemolysis and 10% apoptosis.

CONCLUSION

Safe and efficient RbLDC fast-dissolving ocular films capable of overcoming corneal epithelial barrier resistance to avoid surgical intervention of corneal epithelial debridement were developed.

Novel strategies for anterior segment ocular drug delivery

Research advancements in pharmaceutical sciences have led to the development of new strategies in drug delivery to anterior segment. Designing a new delivery system that can efficiently target the diseased anterior ocular tissue, generate high drug levels, and maintain prolonged and effective concentrations with no or minimal side effects is the major focus of current research. Drug delivery by traditional method of administration via topical dosing is impeded by ocular static and dynamic barriers. Various products have been introduced into the market that prolong drug retention in the precorneal pocket and to improve bioavailability. However, there is a need of a delivery system that can provide controlled release to treat chronic ocular diseases with a reduced dosing frequency without causing any visual disturbances. This review provides an overview of anterior ocular barriers along with strategies to overcome these ocular barriers and deliver therapeutic agents to the affected anterior ocular tissue with a special emphasis on nanotechnology-based drug delivery approaches.

Penetration of fluorescein across the rabbit cornea from the endothelial surface

PURPOSE

To model the kinetics of penetration of fluorescein across the cornea from the endothelial surface.

METHODS

Rabbit corneas mounted in vitro were exposed to fluorescein at their endothelial surface. Trans-corneal fluorescence were acquired periodically for 6 h using a custom-built confocal microfluorometer. The profiles were then employed to fit a kinetic model for calculation of permeability and diffusion coefficients across the cellular layers and stroma, respectively.

RESULTS

At the endothelium-stroma and stroma-epithelium interfaces, the fluorescence profile exhibited sudden jumps. In each case, the fluorescence was higher at the stroma, indicating reduced partitioning of the dye into the lipid-rich cellular layers. The stroma did not swell significantly until 180 min of perfusion. The fluorescence profiles reached a pseudo-steady state at ~6 h. A transport model, which included convective and diffusive fluxes into the stroma, showed a good fit to the trans-corneal profiles at different time points. The estimated permeability coefficients for the cellular layers were close to the values reported previously, but the diffusion coefficient of fluorescein in the stroma was found to be smaller than the values obtained previously using Ussing chambers.

CONCLUSIONS

The penetration of fluorescein could be modeled accurately by a combination of diffusion and convection.

Molecular design for enhancement of ocular penetration

Over the past two decades, many oral drugs have been designed in consideration of physicochemical properties to attain optimal pharmacokinetic properties. This strategy significantly reduced attrition in drug development owing to inadequate pharmacokinetics during the last decade. On the other hand, most ophthalmic drugs are generated from reformulation of other therapeutic dosage forms. Therefore, the modification of formulations has been used mainly as the approach to improve ocular pharmacokinetics. However, to maximize ocular pharmacokinetic properties, a specific molecular design for ocular drug is preferable. Passive diffusion of drugs across the cornea membranes requires appropriate lipophilicity and aqueous solubility. Improvement of such physicochemical properties has been achieved by structure optimization or prodrug approaches. This review discusses the current knowledge about ophthalmic drugs adapted from systemic drugs and molecular design for ocular drugs. I propose the approaches for molecular design to obtain the optimal ocular penetration into anterior segment based on published studies to date.

A randomized, comparative open-label study on the efficacy of latanoprost and timolol in steroid induced ocular hypertension after photorefractive keratectomy

PURPOSE

To evaluate the effect of 0.005% latanoprost and 0.50% timolol for the treatment of steroid-induced ocular hypertension (SIOH) after excimer laser photorefractive keratectomy (PRK).

METHODS

In this comparative, open-label study we enrolled 29 patients who received steroid therapy after PRK and developed intraocular pressure (IOP) elevation within 30 days of treatment. Fifteen were randomized to 0.005% latanoprost (group A) and 14 to 0.50% timolol (group B). IOP measurements were scheduled at 1, 3, 7, 15, 30, 60, 90 and 120 days of therapy. RESULTS; We did not find any real differences between latanoprost and timolol except at the 7-day and 15-day timepoints, when latanoprost reduced IOP significantly more than timolol (p=0.033, 0.035, respectively). After 7 days of therapy two of the 14 timolol-treated patients had high IOP (24 and 26 mmHg) but these promptly returned to normal when latanoprost was added. No significant differences were observed in the ocular side effects considered.

CONCLUSIONS

0.005% latanoprost is as safe and effective as 0.50% timolol in the treatment of SIOH after PRK. Both drugs provide a significant and stable IOP reduction in the majority of patients after short-term treatment. These findings are encouraging for the use of latanoprost in the management of SIOH after PRK, although further trials are necessary to consider it as a primary treatment.