Drug release from an ophthalmic insert of a beta-blocker as an ocular drug delivery system

Promising therapeutic drug delivery systems for glaucoma: a comprehensive review

The delivery of ophthalmic drugs is challenging despite easy accessibility via the ocular surface. Topical instillation of eye drops is a relatively easy and most commonly used as a conduit for drug delivery for treating a myriad of ocular morbidities, particularly involving the anterior segment, and has an additional benefit of avoiding the first-pass metabolism while passing through the systemic circulation. The primary challenges of drug administration through traditional methods include-inadequate patient education for proper drug instillation technique, compliance, adherence, and persistence. Various dynamic (choroidal and conjunctival blood flow, lymphatic clearance, and tear dilution) and static (namely, different layers of cornea, sclera, and retina including blood aqueous and blood-retinal barriers) ocular barriers limit drug delivery to the target ocular tissues. The maintenance of the therapeutic drug levels on the ocular surface for a prolonged duration is an added challenge, thus preventing persistent delivery for longer durations. These factors result in inadequate management, leading to poor prognosis in vision loss in as many as 27% of the patients diagnosed with glaucoma. We have reviewed the research and advancements in the development of novel and well-tolerated drug delivery systems with the common goal of overcoming the factors limiting adequate drug delivery to the target tissues in glaucomatous patients with traditional techniques. In the recent past, multiple research groups have successfully designed noninvasive, sustained drug delivery systems, promoting the efficacy as well as the feasibility of delivering topical drugs to the anterior segment.

Formulation and In Vitro Evaluation of Cyclosporine-A Inserts Prepared Using Hydroxypropyl Methylcellulose for Treating Dry Eye Disease

PURPOSE

The aim of this study was to develop and characterize a novel sustained-release drug delivery system of cyclosporine-A (CsA) using hydroxypropyl methylcellulose (HPMC) and xanthan gum (XG) for treating dry eye disease (DED).

METHODS

Polymeric inserts of CsA were prepared using the solvent casting technique with a 2(3) full factorial design to evaluate the effect of HPMC and XG ratios and drug content on thickness, folding endurance, wettability, and in vitro drug release. Inserts were also evaluated for drug content, moisture absorption and loss, and surface pH. Inserts with an optimized ratio of HPMC and XG were sterilized with UV light and evaluated for morphology, thermal analysis, Fourier transform infrared spectroscopy, stability at 4°C, 25°C, and 40°C, cytotoxicity in cultured bovine corneal endothelial cells, and anti-inflammatory effect in Jurkat T cells.

RESULTS

The addition of XG increased the CsA release duration and enhanced the folding endurance of films. All films showed uniformity in drug content and thickness. Formulation F4 composed of 1% HPMC and 0.25% XG exhibited good folding endurance and sustained CsA release for up to 20 h. Sterility testing of F4 using plate and direct inoculation confirmed the formulation sterility and validated the sterilization method. The formulation was stable for at least 3 months at 4°C, 25°C, and 40°C. No cytotoxicity was observed in cultured bovine corneal endothelial cells for up to 24 h. The anti-inflammatory effect of CsA was intact in ophthalmic inserts.

CONCLUSION

In conclusion, combination therapy with HPMC and CsA can be a potential once-a-day formulation for treating DED.

Penetration of β-Blockers through Ocular Membranes in A1bino Rabbits

The purpose of this study was to investigate the barrier properties of ocular membranes for controlling the extent and pathway of ocular absorption of instilled β-blockers.

The penetration of β-blockers was measured across the isolated corneal, conjunctival and scleral membranes of the albino rabbit using a two-chamber glass diffusion cell. β-Blockers tested were atenolol, carteolol, tilisolol, timolol and befunolol.

Corneal penetration of befunolol was much higher than that of atenolol. Scraping the epithelium increased corneal penetration of β-blockers. Conjunctival membranes showed higher permeability than corneal and scleral membranes. The penetration parameters were estimated according to Fick's equation. The corneal permeability coefficient showed an apparent linear relationship with penetrant lipophilicity. The lipophilic character of the corneal barrier was determined by the partition coefficient of drug to corneal surface, not by the diffusion coefficient. Conjunctival and scleral permeability coefficients were not determined by the lipophilicity of β-blockers.

These results indicate that the conjunctiva, sclera and cornea of the rabbit eye are sufficiently different in permeation character to control the extent and pathway for ocular absorption.

One-side-coated insert as a unique ophthalmic drug delivery system

We newly prepared a unique one-side-coated insert that releases drug from only uncoated side. The purpose of this study is to determine whether ocular and systemic absorption of ophthalmic drug could be altered by an inserting direction of the insert in rabbit eyes. One-side-coated insert was prepared by attaching a polypropylene tape on the one side of the polymer disc of poly(2-hydroxypropyl methacrylate) (HPM) containing tilisolol as a model ophthalmic drug. The insert was applied in the lower conjunctival cul-de-sac of albino rabbits with the uncoated side facing bulbar conjunctiva/sclera (SC insert) or palpebral conjunctiva (CJ insert). At the adequate intervals, the tear fluid, plasma, aqueous humor, conjunctiva, and sclera were collected and the drug concentrations were determined by an HPLC. A release of tilisolol from the one-side-coated insert was twice slower than from the uncoated insert. Ocular application of the one-side-coated insert produced the constant concentrations of tilisolol in the tear fluid over 180 min. SC insert showed higher drug concentrations in the aqueous humor and sclera, and lower drug concentrations in the plasma and conjunctiva than CJ insert.The one-side-coated insert can alter the ocular and systemic absorption of drug by an inserting direction.

Soft contact lenses capable of sustained delivery of timolol

The aim of this work was to evaluate the influence of the composition and the application of an imprinting technique on the loading capability of weakly crosslinked hydroxyethyl methacrylate (HEMA) hydrogels, with a view to their use as reloadable soft contact lenses for administration of timolol. Hydrogels were prepared by dissolution of ethylene glycol dimethacrylate (EGDMA, 10 mM) in HEMA with or without methacrylic acid (MAA) or methyl methacrylate (MMA; 100-400 mM) and with or without timolol maleate (10 mg/mL), initiation of polymerization by addition of 2,2'-azo-bis(isobutyronitrile) (AIBN, 10 mM), injection in molds, and curing in an oven at 50-70 degrees C. Unreacted reagents were removed by boiling. The dry hydrogels were clear and fully polymerized with smooth, poreless surfaces and presented optimal mechanical properties. The hydrogels were then characterized by determination of their swelling and timolol release kinetics in 0.9% NaCl, phosphate buffer (pH 7.4) and artificial lacrimal fluid, and of the timolol loading capacity of both nonimprinted hydrogels and de-timololized imprinted hydrogels at various pHs. Both water uptake and timolol release exhibited Fickian kinetics, except in the case of hydrogels made with 400 mM MAA. Timolol diffusion into 0.9% NaCl from HEMA or HEMA/MMA was slow; release from HEMA/MAA into phosphate buffer or lacrimal fluid was faster and increased with the MAA content of the polymer. Timolol loading was significant for HEMA/MAA hydrogels (imprinted or not) at pH 5.5-7.5, and specially for imprinted hydrogels containing 100 mM MAA, which absorb 12 mg timolol/g dry hydrogel. The results indicate that the incorporation of MAA as comonomer increases the timolol loading capacity to therapeutically useful levels while retaining appropriate release characteristics.

Ocular preparations: the formulation approach

The main aim of pharmacotherapeutics is the attainment of an effective drug concentration at the intended site of action for a sufficient period of time to elicit the response. A major problem being faced in ocular therapeutics is the attainment of an optimal concentration at the site of action. Poor bioavailability of drugs from ocular dosage forms is mainly due to the tear production, non-productive absorption, transient residence time, and impermeability of corneal epithelium. This article reviews: (1) the barriers that decrease the bioavailability of an ophthalmic drug; (2) the objectives to be considered in producing optimal formulations; and (3) the approaches being used to improve the corneal penetration of a drug molecule and delay its elimination from the eye. The focus of this review is on the recent developments in topical ocular drug delivery systems, the rationale for their use, their drug release mechanism, and the characteristic advantages and limitations of each system. In addition, the review attempts to give various analytical procedures including the animal models and other models required for bioavailability and pharmacokinetic studies. The latter can aid in the design and predictive evaluation of newer delivery systems. The dosage forms are divided into the ones which affect the precorneal parameters, and those that provide a controlled and continuous delivery to the pre- and intraocular tissues. The systems discussed include: (a) the commonly used dosage forms such as gels, viscosity imparting agents, ointments, and aqueous suspensions; (b) the newer concept of penetration enhancers, phase transition systems, use of cyclodextrins to increase solubility of various drugs, vesicular systems, and chemical delivery systems such as the prodrugs; (c) the developed and under-development controlled/continuous drug delivery systems including ocular inserts, collagen shields, ocular films, disposable contact lenses, and other new ophthalmic drug delivery systems; and (d) the newer trends directed towards a combination of drug delivery technologies for improving the therapeutic response of a non-efficacious drug. The fruitful resolution of the above-mentioned technological suggestions can result in a superior dosage form for both topical and intraocular ophthalmic application.

Review on the systemic delivery of insulin via the ocular route

Systemic drug absorption from the ocular route is well known. Although there is some absorption from the conjunctival sac, the nasal meatus is the site where the majority of systemic absorption of instilled drug takes place. This article reviews the principles of systemic absorption of insulin applied topically to the eye. The physiological and pharmaceutical considerations for formulation development and the strategy of improving the systemic absorption and bioavailability of insulin are also discussed.

Ocular absorption behavior of palmitoyl tilisolol, an amphiphilic prodrug of tilisolol, for ocular drug delivery

The objective of this study was to examine the ocular absorption behavior of an amphiphilic prodrug after instillation onto the cornea of rabbits. A micellar solution of O-palmitoyl tilisolol (PalTL), an amphiphilic prodrug, was prepared. After instillation of tilisolol (TL) and PalTL, the drug concentrations in the tear fluid, cornea, aqueous humor, iris-ciliary body, vitreous body, and blood were measured. In addition, in situ ocular absorption behavior was also evaluated. After instillation of TL, the concentration of TL in the tear fluid quickly decreased. After instillation of PalTL, prolonged retention and high concentrations of PalTL in tear fluid and the cornea were observed. In addition, more prolonged retention of the TL concentration after instillation of PalTL than after instillation of TL was observed in the cornea, aqueous humor, and iris-ciliary body. In situ experiments demonstrated that PalTL was mainly absorbed by the corneal route and the improvement effects of PalTL under in vivo conditions was due to an enhanced transit time of PalTL in ocular tissues. PalTL, an amphiphilic prodrug, exhibited increased retention in the precorneal area compared with the parent drug, TL, resulted in improved ocular absorption of the parent drug.

Controlled release and ocular absorption of tilisolol utilizing ophthalmic insert-incorporated lipophilic prodrugs

To control ocular drug delivery, the O-butyryl ester prodrug of tilisolol (BUTL) and the O-palmitoyl ester prodrug of tilisolol (PalTL) were incorporated into an ophthalmic insert. The released TL from BUTL inserts and PalTL inserts in pH 7.4 phosphate-buffered saline until 5 h were approximately 25% and 3% of that from TL inserts, respectively. In addition, BUTL was also released from BUTL inserts. However, PalTL was not released from the PalTL insert. The release of drugs from TL inserts and BUTL inserts was little affected by the addition of bovine serum albumin (BSA) in pH 7.4 phosphate-buffered saline. In contrast, the release of drugs from PalTL inserts were enhanced by the addition of BSA. After application of TL, BUTL, and PalTL inserts to the rabbit eye, the aqueous humor concentration of TL was prolonged compared with TL instillation, and the plasma concentration of TL was much lower than that of TL instillation. The ratios of the area under the TL concentration-time curve (AUC) in the aqueous humor to AUC in the plasma (AUC(aqueous)/AUC(plasma)) after application of BUTL until 8 h were 3.1-fold and 3.8-fold higher than those of the TL insert and PalTL insert, respectively.

Feasibility of an in vitro microbiological model as an alternative to the rabbit eye model

Ocular inserts of gentamicin sulfate with polyvinyl alcohol (PVA) 1.5%, 2.0%, and 2.5% and a combination of methyl cellulose 2% and Eudragit NE 30D 30%, 35%, and 40% w/w of methyl cellulose were fabricated by a casting technique. The inserts were sterilized by gamma radiation at 25 kGy and tested for sterility. The microbiological efficacy of the ocular inserts against Staphylococcus aureus ATCC 6538P and Pseudomonas aeruginosa NCIM 2200 was evaluated by developing an in vitro microbiological model and an in vivo noninvasive rabbit eye model. Parameters of the in vitro microbiological model were varied, and the results correlated with a noninvasive rabbit eye model. The in vitro model proved to be a viable alternative to the rabbit eye model in evaluating the microbiological efficacy of gentamicin sulfate ocular inserts.

Ocular devices for the controlled systemic delivery of insulin: in vitro and in vivo dissolution

Both in vitro flow-through and in vivo device removal methods were utilized to determine the dissolution rate of insulin from a Gelfoam(R) based eye device. The dissolution profiles generated by these two methods are comparable. The in vivo data suggests that there is a direct relationship between blood glucose lowering and the rate of release of insulin from the device. The in vitro dissolution results indicate that the release of insulin from the device is flow-rate dependent. The prolonged activity of the insulin is due to the gradual release of insulin from the device which results from the lachrymal system's slow and constant tear production.

Ocular drug delivery in veterinary medicine

This paper provides a comprehensive overview of the various approaches currently used in the development of ocular drug delivery systems for the treatment of ocular diseases in animals. It is obvious from the literature that most of the products that are currently available are derived from human medicine without consideration given to the differences which exist between the anatomy and physiology of the eye of various animal species which ultimately affect product design and performance. As a result, many of the products for animal use seem in many circumstances inappropriate for animal care. The article deals with some aspects of eye anatomy and physiology of different animals, and then provides an overview of the most commonly encountered pathologies. The paper then discusses the currently available drug products and finally reviews new delivery concepts. Several hundred references are included in the paper and provide access to further information on the subject.

In-situ ocular absorption of ophthalmic beta-blockers through ocular membranes in albino rabbits

Ocular membranes have been characterized by in-situ absorption of the ophthalmic beta-blockers carteolol (hydrophilic) and timolol and befunolol (lipophilic) using a cylindrical cell. After introduction of drug solution into the cell on the cornea, sclera (bulbar conjunctival and scleral layer) or palpebral conjunctiva, the disappearance of the drug from the cell was determined as in-situ absorption. The ophthalmic drugs disappeared from the conjunctival and scleral membranes although disappearance from the cornea was hardly observed. The conjunctival membrane showed the highest permeability. Lipophilic drugs were more permeable than hydrophilic. In-situ apparent permeability coefficients of the ophthalmic drugs through the conjunctiva and sclera correlated with the lipophilicity of drugs. A high drug concentration in the aqueous humor was observed after corneal application. There is a relationship between concentration in the aqueous humor was observed after corneal application. There is a relationship between concentrations of drugs in the aqueous humor and previously reported in-vitro apparent permeability coefficients of the drugs in the cornea. This in-situ method using a cylindrical cell is a useful method of investigating the ocular absorption of ophthalmic drugs.

The formulation of drug for ocular administration

The different barriers that slow the penetration of active ingredients administered by the ocular route are described, and some novel dosage forms designed for this route are discussed. Both precorneal and corneal factors considerably restrict ocular penetration. The low bioavailability of classical ophthalmic dosage forms can be improved by several approaches, particularly by increasing the time the active ingredients remain in contact with the eye tissues. The new dosage forms are reviewed according to their type and their drug release mechanisms. The characteristics, advantages, and limitations of each are outlined. The potential of these dosage forms can be expected to enhance development. They offer prolonged effectiveness, reproducibility, fewer unwanted side effects, and improved tolerance.

Ophthalmic preservatives as absorption promoters for ocular drug delivery

The effects of ophthalmic preservatives on the drug permeability through isolated ocular membranes of albino rabbits were investigated using a two-chamber glass diffusion cell. Tilisolol and fluorescein isothiocyanate (FITC)-dextrans (average molecular weights 4400 and 9400 Da; FD-4 and FD-10, respectively) were used as model penetrants of ophthalmic beta-blockers and peptide drugs. Preservatives significantly enhanced the corneal penetration of not only tilisolol but also FITC-dextrans. Especially, benzalkonium chloride increased the corneal permeability of FD-4 and FD-10 by 28.8 and 37.1 times, respectively. These results indicate the usefulness of ophthalmic preservatives as absorption promoters for the ocular delivery of beta-blockers and hydrophilic macromolecules. Preservatives also enhanced the conjunctival permeability of tilisolol, FD-4 and FD-10. The promoting effect of preservatives on the conjunctival drug penetration was smaller than that on the corneal one. Preservative increased the ratio of corneal to conjunctival permeability of tilisolol, FD-4 and FD-10. The different responses of corneal and conjunctival drug penetrations to ophthalmic preservatives may be useful to control the extent and pathway for the ocular and systemic absorptions of instilled drugs.

Intestinal permeability of ophthalmic beta-blockers for predicting ocular permeability

The purpose of this study was to investigate the intestinal permeability of ophthalmic beta-blockers and evaluate the utility of intestinal membrane for predicting the ocular permeability. The penetrations of beta-blockers were measured across the isolated jejunum and colon of the albino rabbit using a two-chamber glass diffusion cell. beta-Blockers tested include atenolol, carteolol, tilisolol, timolol, and befunolol. Colonic membrane showed lower permeability of hydrophilic drugs than jejunal membrane. Scraping the entire cell monolayer of jejunum increased the drug permeability. There was a significant correlation between colonic permeability coefficients and lipophilicities of beta-blockers. The permeability coefficients through jejunum and scraped jejunum were not susceptible to drug lipophilicities. Jejunum, scraped jejunum, and colon showed permeability coefficients almost equal to those of sclera, conjunctiva, and cornea, respectively. There was a significant correlation between permeability coefficients through colon and cornea. These results indicate that the steady-state permeability of ophthalmic beta-blockers through ocular membranes may be predicted by measuring the permeability through certain intestinal membranes. However, the analyses of intestinal permeability using Fick's equation showed the functional difference of intestinal permeability from ocular permeability of ophthalmic beta-blockers.