Influence of drug loading and type of ointment base on the in vitro performance of acyclovir ophthalmic ointment

Recent updates on ocular disease management with ophthalmic ointments

Ophthalmic diseases can result in permanent vision loss and blindness. Convenient topical and systemic treatments are preferred to address these sight-threatening conditions. However, the unique anatomy of the eye presents challenges for drug delivery. Various ophthalmic ointment formulations have been developed to enhance bioavailability in the eye to prolong residence time and improve corneal permeability. This article explores a wide range of ocular diseases affecting individuals globally and how ointments are used to manage them. From eye to ocular barriers, this review focuses on published scientific research and formulation strategies for severe ocular complications using conventional topical ointments. Additionally, it delves through patented technologies and marketed formulations supporting the use of ointments in ocular drug delivery.

Equivalence assessment of creams with quali-quantitative differences in light of the EMA and FDA regulatory framework

EMA and FDA are upgrading guidelines on assessing the quality and the equivalence of topically applied drug products for developing copies of originator products and supporting post-marketing variations. For topical products having remarkably similar composition, both EMA and FDA accept the equivalence on the bases of the comparison of rheological properties and in vitro drug release constant (k) and skin permeation flux (J) values, instead of clinical studies. This work aims to evaluate the feasibility to expand this approach to variations of the composition of complex semi-solid preparations. Ibuprofen (IB) creams at two different strengths (i.e., 1 % and 10 %) were used as a model formulation. Two formulative changes were performed: (a) the addition of the humectant to simulate a minor post-marketing variation; (b) the substitution of the emulsifying system to simulate a major one. These variations impacted only in 1 % IB formulations where both the equivalences of rheological data and J-values failed. At the highest concentration, the presence of IB crystals broke down the differences in rheological patterns and lead the IB thermodynamic activity at the maximum figuring out an overlapping of the J-values. Such data suggest the combination of these studies, which are thought mainly for the development of copies, could be also applied to the management of post-marketing variations that involve product composition.

Impact of Apparatus and Adapter on In vitro Drug Release of Ophthalmic Semisolid Drug Products

PURPOSE

In vitro release testing (IVRT) is a widely used tool for evaluating the quality and performance of drug products. However, standardized sample adaptors or drug release apparatus setups for IVRT studies are still lacking for ophthalmic ointments. The aim of this study was to provide a better understanding of the impact of apparatus and sample adaptor setups on IVRT of ophthalmic ointments.

METHODS

Dexamethasone (DEX), a steroidal ingredient commonly used in ophthalmic drug products, was selected as a model drug. Ointments were prepared by mixing DEX in white petrolatum using a high shear mixer. A novel two-sided adapter was developed to increase the drug release surface area. DEX ointment was placed in one-sided or two-sided release adaptors coupled with 1.2 μm polyethersulfone membrane, and the drug release was studied in different USP apparatuses (I, II, and IV).

RESULTS

The sample adaptor setups had a minimal impact on cumulative drug release amount per area or release rate while USP IV apparatus with agitated flow enhanced drug release rates. The USP apparatus I with a two-sided semisolid adapter, which uses membranes on both sides, showed dramatically higher cumulative drug release and discriminative release profiles when evaluating ophthalmic formulations.

CONCLUSIONS

USP apparatuses and sample adaptors are critical considerations for IVRT. Two-sided semisolid adapter provides higher cumulative release, facilitating the discrimination between low drug content ophthalmic ointment formulations with good sensitivity and repeatability without affecting the drug release rate.

Formulation and Characterization of Epalrestat-Loaded Polysorbate 60 Cationic Niosomes for Ocular Delivery

The aim of this work was to develop niosomes for the ocular delivery of epalrestat, a drug that inhibits the polyol pathway and protects diabetic eyes from damage linked to sorbitol production and accumulation. Cationic niosomes were made using polysorbate 60, cholesterol, and 1,2-di-O-octadecenyl-3-trimethylammonium propane. The niosomes were characterized using dynamic light scattering, zeta-potential, and transmission electron microscopy to determine their size (80 nm; polydispersity index 0.3 to 0.5), charge (-23 to +40 mV), and shape (spherical). The encapsulation efficiency (99.76%) and the release (75% drug release over 20 days) were measured with dialysis. The ocular irritability potential (non-irritating) was measured using the Hen's Egg Test on the Chorioallantoic Membrane model, and the blood glucose levels (on par with positive control) were measured using the gluc-HET model. The toxicity of the niosomes (non-toxic) was monitored using a zebrafish embryo model. Finally, corneal and scleral permeation was assessed with the help of Franz diffusion cells and confirmed with Raman spectroscopy. Niosomal permeation was higher than an unencapsulated drug in the sclera, and accumulation in tissues was confirmed with Raman. The prepared niosomes show promise to encapsulate and carry epalrestat through the eye to meet the need for controlled drug systems to treat the diabetic eye.

Interpenetrating polymeric network (IPNs) in ophthalmic drug delivery: Breaking the barriers

To maintain the therapeutic drug concentration for a prolonged period of time in aqueous and vitreous humor is primary challenge for ophthalmic drug delivery. Majority of the locally administered drug into the eye is lost as to natural reflexes like blinking and lacrimation resulting in the short span of drug residence. Consequently, less than 5% of the applied drug penetrate through the cornea and reaches the intraocular tissues. The major targets for optimal ophthalmic drug delivery are increasing drug residence time in cul-de-sac of the eye, prolonging intraocular exposure, modulating drug release from the delivery system, and minimizing pre-corneal drug loss. Development of in situ gel, contact lens, intraocular lens, inserts, artificial cornea, scaffold, etc., for ophthalmic drug delivery are few approaches to achieve these major targeted objectives for delivering the drug optimally. Interpenetrating polymeric network (IPN) or smart hydrogels or stimuli sensitive hydrogels are the class of polymers that can help to achieve the targets in ophthalmic drug delivery due to their versatility, biocompatibility and biodegradability. These novel ''smart" materials can alter their molecular configuration and result in volume phase transition in response to environmental stimuli, such as temperature, pH, ionic strength, electric and magnetic field. Hydrogel and tissue interaction, mechanical/tensile properties, pore size and surface chemistry of IPNs can also be modulated for tuning the drug release kinetics. Stimuli sensitive IPNs has been widely exploited to prepare in situ gelling formulations for ophthalmic drug delivery. Low refractive index hydrogel biomaterials with high water content, soft tissue-like physical properties, wettability, oxygen, glucose permeability and desired biocompatibility makes IPNs versatile candidate for contact lenses and corneal implants. This review article focuses on the exploration of these smart polymeric networks/IPNs for therapeutically improved ophthalmic drug delivery that has unfastened novel arenas in ophthalmic drug delivery.

Contact lenses for pravastatin delivery to eye segments: Design and in vitro-in vivo correlations

Oral administration of cholesterol-lowering statins, HMG-CoA reductase inhibitors, is associated with beneficial effects on eye conditions. This work aims to design contact lenses (CLs) that can sustainedly deliver pravastatin and thus improve the ocular efficacy while avoiding systemic side effects of statins. Bioinspired hydrogels were prepared with monomers that resemble hydrophobic (ethylene glycol phenyl ether methacrylate) and amino (2-aminoethyl methacrylamide hydrochloride) functionalities of the active site of HMG-CoA. Best performing CLs loaded >6 mg/g, in vitro fulfilled the release demands for daily wearing, and showed anti-inflammatory activity (lowering TNF-α). High hydrostatic pressure sterilization preserved the stability of both the drug and the hydrogel network. Ex vivo tests revealed the ability of pravastatin to accumulate in cornea and sclera and to penetrate through transscleral route. In vivo tests (rabbits) confirmed that, compared to eye drops and for the same dose, CLs provided significantly higher pravastatin levels in tear fluid within 1 to 7 h of wearing. Moreover, after 8 h wearing pravastatin was present in cornea, sclera, aqueous humour and vitreous humour. Strong correlations between percentages of drug released in vitro and in vivo were found. Effects of volume and proteins on release rate and Levy plots were identified.

Asymmetry in Drug Permeability through the Cornea

The permeability through the cornea determines the ability of a drug or any topically applied compound to cross the tissue and reach the intraocular area. Most of the permeability values found in the literature are obtained considering topical drug formulations, and therefore, refer to the drug permeability inward the eye. However, due to the asymmetry of the corneal tissue, outward drug permeability constitutes a more meaningful parameter when dealing with intraocular drug-delivery systems (i.e., drug-loaded intraocular lenses, intraocular implants or injections). Herein, the permeability coefficients of two commonly administered anti-inflammatory drugs (i.e., bromfenac sodium and dexamethasone sodium) were determined ex vivo using Franz diffusion cells and porcine corneas in both inward and outward configurations. A significantly higher drug accumulation in the cornea was detected in the outward direction, which is consistent with the different characteristics of the corneal layers. Coherently, a higher permeability coefficient was obtained for bromfenac sodium in the outward direction, but no differences were detected for dexamethasone sodium in the two directions. Drug accumulation in the cornea can prolong the therapeutic effect of intraocular drug-release systems.

Advancement on Sustained Antiviral Ocular Drug Delivery for Herpes Simplex Virus Keratitis: Recent Update on Potential Investigation

The eyes are the window to the world and the key to communication, but they are vulnerable to multitudes of ailments. More serious than is thought, corneal infection by herpes simplex viruses (HSVs) is a prevalent yet silent cause of blindness in both the paediatric and adult population, especially if immunodeficient. Globally, there are 1.5 million new cases and forty thousand visual impairment cases reported yearly. The Herpetic Eye Disease Study recommends topical antiviral as the front-line therapy for HSV keratitis. Ironically, topical eye solutions undergo rapid nasolacrimal clearance, which necessitates oral drugs but there is a catch of systemic toxicity. The hurdle of antiviral penetration to reach an effective concentration is further complicated by drugs’ poor permeability and complex layers of ocular barriers. In this current review, novel delivery approaches for ocular herpetic infection, including nanocarriers, prodrugs, and peptides are widely investigated, with special focus on advantages, challenges, and recent updates on in situ gelling systems of ocular HSV infections. In general congruence, the novel drug delivery systems play a vital role in prolonging the ocular drug residence time to achieve controlled release of therapeutic agents at the application site, thus allowing superior ocular bioavailability yet fewer systemic side effects. Moreover, in situ gel functions synergistically with nanocarriers, prodrugs, and peptides. The findings support that novel drug delivery systems have potential in ophthalmic drug delivery of antiviral agents, and improve patient convenience when prolonged and chronic topical ocular deliveries are intended.

Ocular Physiologically Based Pharmacokinetic Modeling for Ointment Formulations

Purpose

The purpose of this study is to show how the Ocular Compartmental Absorption & Transit (OCAT™) model in GastroPlus^® can be used to characterize ocular drug pharmacokinetic performance in rabbits for ointment formulations.

Methods

A newly OCAT™ model developed for fluorometholone, as well as a previously verified model for dexamethasone, were used to characterize the aqueous humor (AH) concentration following the administration of multiple ointment formulations to rabbit. The model uses the following parameters: application surface area (SA), a fitted application time, and the fitted Higuchi release constant to characterize the rate of passage of the active pharmaceutical ingredient from the ointment formulations into the tears in vivo.

Results

Parameter sensitivity analysis was performed to understand the impact of ointment formulation changes on ocular exposure. While application time was found to have a significant impact on the time of maximal concentration in AH, both the application SA and the Higuchi release constant significantly influenced both the maximum concentration and the ocular exposure.

Conclusions

This initial model for ointment ophthalmic formulations is a first step to better understand the interplay between physiological factors and ophthalmic formulation physicochemical properties and their impact on in vivo ocular drug pharmacokinetic performance in rabbits.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11095-020-02965-y.

Diving into Batch-to-Batch Variability of Topical Products-a Regulatory Bottleneck

PURPOSE

Following the recent European Medicine Agency (EMA) draft guideline on quality and equivalence of topical products, a modular framework for bioequivalence assessment is proposed, wherein the qualitative, quantitative, microstructure and product performance sameness is demanded to support generic applications. Strict regulatory limits are now imposed, but, the suitability of these limits has been subject of intense debate. In this context, this paper aims to address these issues by characterizing a panel of 8 reference blockbuster semisolid topical products.

METHODS

For each product, three batches were selected and, whenever possible, batches retrieved from different manufacturing sites were considered. Product microstructure was evaluated in terms of globule size, pH, rheological attributes and, if required, the thermal behaviour was also assessed. Performance was evaluated through in vitro release testing (IVRT). Finally, an integrated multivariate analysis was performed to highlight the features that most contribute for product variability.

RESULTS

Marked differences were registered within reference products. Statistical analysis demonstrated that if EMA criteria are applied, none of the same product batches can be considered as equivalent. Rheological parameters as well as IVRT indicators account for the majority of batch-to-batch differences.

CONCLUSIONS

Semisolid dosage forms exhibit intrinsic variability. This calls for the attention to the need of establishing reasonable equivalence criteria applied to generic drug products. Graphical abstract.

In Vitro Release Testing of Acyclovir Topical Formulations Using Immersion Cells

The objective of the study was to reinforce the applicability of the immersion cells for the in vitro release testing (IVRT) of topical formulations by using marketed acyclovir 5% cream formulation (Cream 1) as a model. The method employing the immersion cells was optimized by studying the effect of variables, such as membrane type, media temperature and volume, agitation speed, and cell size, on acyclovir release from the formulation. The in-house formulation similar to the qualitative and quantitative composition of Cream 1 and the other trial formulations with variable compositions were prepared and studied by using the immersion cells. Various other brands of acyclovir topical formulations available in the Indian market were also subjected to IVRT by using the optimized method. An increase in the media temperature from 32°C to 37°C and the stirring speed from 50 to 100 to 150 rpm led to an increase in the drug release. As the immersion cell size increased (0.5, 2 and 4 cm² surface area), the release rate also increased. Nitrocellulose membrane showed the highest drug release and Fluoropore^™ the least. The optimized IVRT method could establish the differences in the drug release rates among the formulations with the altered compositions. The method could also prove its discriminatory potential for various marketed formulations. The immersion cell method could serve as a simpler, facile, and reliable aid during product development and also as a quality control tool in assessing stability, aging, and batch-to-batch uniformity of semisolid formulations.

Imprinted Contact Lenses for Ocular Administration of Antiviral Drugs

A variety of ocular diseases are caused by viruses, and most treatments rely on the use of systemic formulations and eye drops. The efficient ocular barriers that oppose antiviral drug penetration have prompted the development of improved topical delivery platforms. The aim was to design hydrogel contact lenses endowed with an affinity for acyclovir (ACV) and its prodrug valacyclovir (VACV), first-choice drugs against herpes simplex virus (HSV) ocular keratitis, and that can sustain the release of therapeutic doses during daily wearing. Functional monomers suitable for interaction with these drugs were screened using computational modeling. Imprinted and non-imprinted hydrogels were prepared with various contents in the functional monomer methacrylic acid (MAA) and characterized in terms of swelling, transmittance, mechanical properties, and ocular compatibility (hen’s egg test on chorioallantoic membrane (HET-CAM) assay). The values were in the range typical of soft contact lenses. Compared to ACV, the capability to load VACV was remarkably higher due to stronger electrostatic interactions with MAA. The advantages of the imprinting technology were evidenced for VACV. Stability of VACV loading solution/hydrogels under steam heat sterilization and subsequent drug release was investigated. Permeability studies through bovine and porcine cornea and sclera of the drug released from the hydrogels revealed that VACV accumulates in the cornea and can easily cross the sclera, which may facilitate the treatment of both anterior and posterior eye segments diseases.

Cytosine-functionalized bioinspired hydrogels for ocular delivery of antioxidant transferulic acid

Contact lenses (CLs) are being pointed out as feasible platforms for controlled delivery of ophthalmic drugs. Bioinspired strategies may endow CLs with affinity for a given drug by mimicking its physiological receptor using adequate functional monomers and tuning their conformation in the space through the molecular imprinting technology. However, there are some active substances, such as efficient antioxidant agents, that cannot be used as templates because they degrade during polymerization or even hinder the polymerization itself. Therefore, the development of CLs able to sustain the release of antioxidants for the prevention and/or treatment of several age-related and light-induced eye diseases has not been explored yet. Searching for an alternative bioinspired strategy, the present work relies on the fact that some drugs owe their therapeutic action to their ability to interact with nucleotides that build up DNA and RNA. Thus, the aim of this work was to design hydrogels functionalized with the nitrogenous base cytosine for the controlled uptake and release of transferulic acid (TA) having a complementary chemical structure in terms of hydrogen bonding and π-π stacking ability. Hydrogels were prepared from mixtures of 2-hydroxyethyl methacrylate (HEMA), glycidyl methacrylate (GMA) and ethyleneglycolphenylether methacrylate (EGPEM). GMA was used as a bridge to immobilize cytosine after hydrogel synthesis, while EGPEM was added to reinforce hydrophobic interactions with TA. The hydrogels were characterized in terms of suitability to be used as CLs (swelling, light transmission, mechanical properties, biocompatibility) and capability to host TA and sustain its release in lachrymal fluid while maintaining the antioxidant activity. Relevantly, the bioinspired CLs favored TA accumulation in cornea and sclera tissues.

Quality and equivalence of topical products: A critical appraisal

The approval of topical generic products is essentially governed by clinical endpoint studies. Is this the most efficient approach to document bioequivalence in these particular dosage forms? This issue has sparked multiple discussions among different stakeholders - academia, industry and several regulatory agencies - in the active pursuit for new and robust surrogate methodologies. This mini review attempts to critically discuss this topic in light of the recently issued European regulatory requirements within the proposed modular framework for bioequivalence assessment.

aQbD as a platform for IVRT method development - A regulatory oriented approach

The EMA draft guideline on quality and equivalence of topical products and the FDA non-binding product specific guidances release has encouraged the establishment of a regulatory background for in vitro release testing (IVRT). Herein, a novel framework applicable to the development of a discriminatory IVRT method is described, according to analytical quality by design (aQbD) principles. A commercially available diclofenac emulgel formulation was used as model product. Through the definition of IVRT analytical target profile, a risk assessment analysis was carried out, in which the critical analytical attributes (in vitro release rate, cumulative amount released at an initial/final point and dose depletion) and critical method variables (medium, membrane and dosage regimen) were identified. Based on this information, a 3 × 2 × 3 full factorial design was performed. Statistical modeling and system desirability assessment enabled the selection of the most suitable IVRT parameters, which were fully validated according with new EMA requirements. These consisted of PBS:Ethanol (80:20, pH = 7.4), Tuffryn membranes and 300 mg of applied product. aQbD provided a comprehensive framework for developing a reliable and effective IVRT method. A thorough analysis of the new EMA draft guideline requirements revealed that some of the established criteria may be challenging to attain.

Nanostructuring lipid carriers using Ridolfia segetum (L.) Moris essential oil

The therapeutic potential of essential oils is widely recognized since antiquity, due to their antibacterial, antifungal, anti-inflammatory and immuno-modulatory properties. In particular, their physicochemical characteristics, such as lipophilicity and permeation enhancement effect have sparked attention for the development of innovative lipid nanosystems. The present work aimed at developing a differentiated nanostructured lipid carrier (NLC) based formulation for topical application, using the Ridolfia segetum essential oil (REO), isolated by hydrodistillation from this Portuguese aromatic plant, with a dual key function, as active and simultaneously nanostructuring component of the nanoparticles. The incorporation of the essential oil in the solid lipid matrix, followed by the respective hot high-pressure homogenization, led to particles with a size of 143 ± 5 nm, along with a polydispersity index of 0.21, a zeta potential of -16.3 ± 0.6 mV, encapsulation efficacy of ca. 100% and loading capacity of 1.4%. A comprehensive physicochemical characterization of the lipid nanosystem, including morphology, structural, thermal and accelerated stability analysis confirmed its nanostructured nature. REO-NLC was further jellified for designing an appropriate semisolid topical dosage form. In vitro release, permeation and skin retention studies evidenced a sustained release behaviour and a reservoir-like effect, suitable for a prolonged topical delivery. Cytotoxicity studies, performed in fibroblasts and keratinocytes, revealed the biocompatibility of the developed formulations. This work highlights the critical role of REO as a multiaddressable compound, both as active pharmaceutical ingredient and nanostructuring agent, able to tailor the permeation enhancement profile of nanoparticles towards topical delivery purposes and concomitantly presenting a safety profile for cosmetic and/or pharmaceutical purposes.

Bioequivalence of topical generic products. Part 2. Paving the way to a tailored regulatory system

Hitherto, for the approval of a topical generic drug product, the majority of the regulatory agencies require clinical endpoint studies to prove its therapeutic equivalence in relation to a reference product. Pharmacodynamic studies are also available to support bioequivalence, however, these are solely applicable for corticosteroids. The first strategy is considered the "gold standard", since it can be applied to all drug products. Nevertheless, the high variability intrinsic to topical drug delivery makes this analysis relatively insensitive, costly, time-consuming, besides requiring a large number of subjects. There are, however, alternative methods capable of providing a more rigorous analysis and requiring a lower cost. Amongst them, in vitro methods have sparked considerable attention, not only in the academic field, but also in the pharmaceutical industry and regulatory agencies. In this context, this review attempts to discuss the main regulatory constraints and the recent advances in the regulatory science of topical generic drugs bioequivalence assessment. Initiatives, such as the Strawman decision tree and the topical drug classification system are specially referred, since these highlight the importance of establishing a broader concept of pharmaceutical equivalence for topical generic drugs, similar to the one already set for orally administered conventional dosage forms, such as tablets and capsules. Finally, the FDA Product-Specific Guidances for Generic Drug Development released for topical products in recent years and particular European Public Assessment Reports are presented and discussed, to illustrate the change of paradigm which is occurring in this regulatory field.

A Tutorial for Developing a Topical Cream Formulation Based on the Quality by Design Approach

The pharmaceutical industry has entered in a new era, as there is a growing interest in increasing the quality standards of dosage forms, through the implementation of more structured development and manufacturing approaches. For many decades, the manufacturing of drug products was controlled by a regulatory framework to guarantee the quality of the final product through a fixed process and exhaustive testing. Limitations related to the Quality by Test system have been widely acknowledged. The emergence of Quality by Design (QbD) as a systematic and risk-based approach introduced a new quality concept based on a good understanding of how raw materials and process parameters influence the final quality profile. Although the QbD system has been recognized as a revolutionary approach to product development and manufacturing, its full implementation in the pharmaceutical field is still limited. This is particularly evident in the case of semisolid complex formulation development. The present review aims at establishing a practical QbD framework to describe all stages comprised in the pharmaceutical development of a conventional cream in a comprehensible manner.

Ophthalmic Drug Delivery Systems for Antibiotherapy-A Review

The last fifty years, ophthalmic drug delivery research has made much progress, challenging scientists about the advantages and limitations of this drug delivery approach. Topical eye drops are the most commonly used formulation in ocular drug delivery. Despite the good tolerance for patients, this topical administration is only focus on the anterior ocular diseases and had a high precorneal loss of drugs due to the tears production and ocular barriers. Antibiotics are popularly used in solution or in ointment for the ophthalmic route. However, their local bioavailability needs to be improved in order to decrease the frequency of administrations and the side effects and to increase their therapeutic efficiency. For this purpose, sustained release forms for ophthalmic delivery of antibiotics were developed. This review briefly describes the ocular administration with the ocular barriers and the currently topical forms. It focuses on experimental results to bypass the limitations of ocular antibiotic delivery with new ocular technology as colloidal and in situ gelling systems or with the improvement of existing forms as implants and contact lenses. Nanotechnology is presently a promising drug delivery way to provide protection of antibiotics and improve pathway through ocular barriers and deliver drugs to specific target sites.

A comprehensive approach to qualify and validate the essential parameters of an in vitro release test (IVRT) method for acyclovir cream, 5

The rate of release of an active pharmaceutical ingredient (API) from a topical semisolid dosage form can be influenced by its physical and structural properties. An In Vitro Release Test (IVRT) is an established method to characterize this rate of API release and compare the underlying sameness in product quality characteristics. The purpose of this work was to validate an IVRT method to compare acyclovir cream, 5% products. However, despite widespread use of the IVRT since 1997, there has been no established approach to validate an IVRT method. Our approach included: 1) qualification of the diffusion cell apparatus, 2) qualification of the laboratory, 3) validation of the HPLC analytical method, and 4) validation of numerous critical parameters of the IVRT method, itself, and resulted in a comprehensive and successful IVRT method validation. Subsequent to the IVRT validation work described here, the U.S. Food and Drug Administration (FDA) drafted a guidance on the development and validation of an IVRT method for acyclovir cream, 5%. Although there are notable differences between our approach and the approach in that guidance, this report illustrates how many of the same essential qualification parameters and validation concepts were considered and systematically addressed in our approach to IVRT validation.

Analysis of steady state and non-steady state corneal permeation of diclofenac

The present study was undertaken for characterization of the steady state and non steady state corneal permeation kinetics of diclofenac potassium (DCP) using statistical moment theory.