New Ophthalmic Drug Delivery Systems

Progress in Ocular Drug Delivery: Challenges and Constraints

The eye has several dynamic and static barriers in place to limit the entry of foreign substances including therapeutics. As such, efficient drug delivery, especially to posterior segment tissues, has been challenging. This chapter describes the anatomical and physiological challenges associated with ocular drug delivery before discussing constraints with regard to formulation parameters. Finally, it gives an overview of advanced drug delivery technologies with a specific focus on recently marketed and late-stage clinical trial products.

Advanced nanodelivery platforms for topical ophthalmic drug delivery

Conventional eye drops have several limitations, including the need for multiple applications per dose, hourly based dosage regiments, and suboptimal ocular bioavailability (<5%). The efficacy of topical ophthalmic medications can be significantly improved by controlling their contact time with the adherent mucin layer and by inducing sustained release properties, thus allowing for a prolonged contact time of the drug with the ocular tissues, which eventually will lead to improved drug bioavailability and a significant decrease in the frequency of eyedrop instillation. In this review, we critically highlight recent and innovative nanodrug delivery platforms, with a primary focus on the integration of nanotechnology, biomaterials, and polymer chemistry to facilitate precise spatial and temporal control over sustained drug release to the cornea.

Terminalia arjuna gum/alginate in situ gel system with prolonged retention time for ophthalmic drug delivery

Poor availability is the major barrier to accept the new smart gel system as a preferred ophthalmic solution for various eye problems. Smart gel system especially derived from natural source allows the rapid transition of ocular solution into gel form upon contact to tear solution. The present experimental scheme was intended to prepare and characterize a pH triggered in situ gelling system using moxifloxacin HCl (MOX-HCl). Gum was extracted from Terminalia arjuna bark resin and used as gelling agent in blend with sodium alginate. Sterilized formulations were developed and characterized for their physicochemical attributes. These were further investigated for microbiological testing and eye irritation studies. Drug loaded in situ gel was appeared as clear sol that converted into gel phase in presence of tear solution. Optimized formulation was stable, therapeutically efficacious, non-irritant and has a sustained release of the drug for twelve hours period. Instillation of MOX-HCl loaded in situ gel did not cause any type of irritation symptoms like redness, inflammation and excessive tear production in rabbits as compared to control. MOX-HCl loaded in situ gel can be appraised as a substitute for conventional eye drops for extended precorneal retention, improved corneal permeability along with better ocular bioavailability.

Principles of pharmacology in the eye

The eye is a highly specialized organ that is subject to a huge range of pathology. Both local and systemic disease may affect different anatomical regions of the eye. The least invasive routes for ocular drug administration are topical (e.g. eye drops) and systemic (e.g. tablets) formulations. Barriers that subserve as protection against pathogen entry also restrict drug permeation. Topically administered drugs often display limited bioavailability due to many physical and biochemical barriers including the pre-corneal tear film, the structure and biophysiological properties of the cornea, the limited volume that can be accommodated by the cul-de-sac, the lacrimal drainage system and reflex tearing. The tissue layers of the cornea and conjunctiva are further key factors that act to restrict drug delivery. Using carriers that enhance viscosity or bind to the ocular surface increases bioavailability. Matching the pH and polarity of drug molecules to the tissue layers allows greater penetration. Drug delivery to the posterior segment is a greater challenge and, currently, the standard route is via intravitreal injection, notwithstanding the risks of endophthalmitis and retinal detachment with frequent injections. Intraocular implants that allow sustained drug release are at different stages of development. Novel exciting therapeutic approaches include methods for promoting transscleral delivery, sustained release devices, nanotechnology and gene therapy.

Protection and viability of fruit seeds oils by nanostructured lipid carrier (NLC) nanosuspensions

In this paper, we focused on the development of nanostructured lipid carriers (NLCs) for dermal application. The NLC matrix was designed as a protective reservoir of biological active compounds that naturally occur in domestic fruit seed oils. Over the years, emulsions, as a popular physicochemical form of personal care products, were refined in order to obtain the best possible penetration into the skin of any bioactive compound introduced in the formulation, such as polyunsaturated fatty acids (PUFAs). In fact, the bioactive components are useful only if they are able to penetrate the skin unchanged. Therefore, an alternate way to deliver naturally occurring PUFAs is needed. NLCs present a novel delivery and protection system for the PUFAs. The cold pressed fruit seed oils obtained from waste material were used in this paper: blackcurrant, blackberry, raspberry, strawberry and plum. Thermodynamic (DSC) and structural techniques ((1)H NMR) were applied in order to characterize the obtained systems in terms of seed oil incorporation into the NLC, and oxidative stability tests were used to confirm the protective quality of the systems. During the formulation optimization process the most stable nanosuspension with the best seed oil incorporation was a mixture of 4% nonionic emulsifiers, 88% water and 6% lipids with a ratio of 6:2, wax:oil. The oxidative stability tests showed that the NLC was an effective method of protection of the PUFAs.

Polymers for mucoadhesive drug delivery system: a current status

To overcome the relatively short gastrointestinal (GI) time and improve localization for oral controlled or sustained release drug delivery systems, bioadhesive polymers that adhere to the mucin/epithelial surface are effective and lead to significant improvement in oral drug delivery. Improvements are also expected for other mucus-covered sites of drug administration. Bioadhesive polymers find application in the eye, nose, and vaginal cavity as well as in the GI tract, including the buccal cavity and rectum. This article lays emphasis mainly on mucoadhesive polymers, their properties, and their applications in buccal, ocular, nasal, and vaginal drug delivery systems with its evaluation methods.

Vesicular systems in ocular drug delivery: an overview

The main aim of pharmacotherapeutics is the attainment of effective drug concentration at the intended site of action for a sufficient period of time to elicit a response. Poor bioavailability of drugs from ocular dosage form is mainly due to the tear production, non-productive absorption, transient residence time, and impermeability of corneal epithelium. Though the topical and localized application are still an acceptable and preferred way to achieve therapeutic level of drugs used to treat ocular disorders but the primitive ophthalmic solution, suspension, and ointment dosage form are no longer sufficient to combat various ocular diseases. This article reviews the constraints with conventional ocular therapy and explores various novel approaches, in general, to improve ocular bioavailability of the drugs, advantages of vesicular approach over these and the future challenges to render the vesicular system more effective.

A novel D2-dopaminergic and alpha2-adrenoceptor receptor agonist induces substantial and prolonged IOP decrease in normotensive rabbits

The effects of a novel and selective D2-dopaminergic/alpha2-adrenoceptor agonist, CHF1035, and its metabolite CHF1024 on intraocular pressure (IOP) were determined in rabbits. Because CHF1035 is a mixture of two enantiomers, CHF1800 (+) and CHF1810 (-), pure enantiomers were also studied to determine possible differences in IOP-decreasing ability depending on the stereochemistry of the molecule. CHF1035, CHF1800 (+), CHF1810 (-), CHF1024, brimonidine and 0.9% NaCl were administered topically to rabbits and IOP was then measured at fixed time intervals. The dose-response profile (0.01-1.0% w/v) was determined for CHF1035. CHF1035 and its metabolite CHF1024 significantly lowered IOP in the treated eyes. CHF1035 showed a maximum IOP decrease (7.6 +/- 1.5 mmHg) 5 h post-dosing, whereas the metabolite CHF1024 showed a maximum decrease in IOP (7.0 +/- 0.8 mmHg) 3 h post-dosing. The maximum IOP decrease produced by CHF1035 in the treated eye was comparable with that produced by brimonidine (7.8 +/- 0.9 mmHg), but CHF1035 had a significantly longer duration of action. Unlike brimonidine, CHF1035 and CHF1024 did not decrease IOP in the untreated eye. CHF1810 (-) lowered the IOP more than CHF1800 (+). No irritation, evaluated as eyelid closure, was observed after topical administration of any of the compounds. Only in the case of CHF1035 1% solution, two rabbits out of six closed the eye for 30-45 s. In conclusion, CHF1035 and its metabolite CHF1024 significantly decreased the IOP in rabbits, and are potential novel IOP lowering agents. Especially, CHF1035 produced a substantial decrease in IOP for a prolonged period of time, and thus may prove useful in glaucoma therapy.

Rheological and mucoadhesive characterization of polymeric systems composed of poly(methylvinylether-co-maleic anhydride) and poly(vinylpyrrolidone), designed as platforms for topical drug delivery

This study examined the rheological and mucoadhesive properties of polymeric systems composed of the bioadhesive copolymer poly(methylvinylether-co-maleic anhydride), PMVE/MA and poly(vinylpyrrolidone, PVP) as candidate platforms for improved drug delivery to the oral cavity. Aqueous, neutral formulations were prepared containing PMVE/MA (10 or 15% w/w) and PVP (0-9% w/w). The rheological properties were examined at 20 +/- 0.1 degrees C using both oscillatory rheometry (0.01 to 1.0 Hz at a constant strain of 1.0 x 10(-3) rad) and controlled stress flow rheometry using a Carri-Med CSL(2)-100 rheometer. The mechanical properties were examined using texture profile analysis (15-mm depth of insertion, 9 mm s(-1) rate of insertion, 15 s delay period between the end of the first and beginning of the second compressions). The interaction of the various formulations with mucin was examined using oscillatory rheometry. All formulations exhibited pseudoplastic flow with minimal thixotropy. Increasing the concentration of each polymeric component increased the zero-rate viscosity (calculated using the Cross model), hardness, compressibility, the storage (G') and loss (G") moduli, and decreased the loss tangent. These observations may be due to greater polymeric entanglement between adjacent polymer chains. Specifically, rheological synergy was observed between PMVE/MA and PVP, evidence of complexation between these two polymers. From the relationship between the viscoelastic properties and oscillatory frequency, and the mathematical comparison of steady and oscillatory shear response of formulations using the Cox-Merz equation, it may be concluded that formulations containing 15% w/w PMVE/MA and either 6 or 9% w/w PVP exhibited rheological properties that were consistent with gel behavior. All other formulations exhibited rheological properties that resembled polymeric liquids. Increasing the concentrations of PMVE/MA and PVP significantly increased the interaction with mucin, the rheological synergy between the polymeric formulations and mucin and, in addition, the adhesiveness of the formulations. This is evidence of the muco/bioadhesive properties of the formulations. Conversely, the time of contact between each formulation and mucin did not affect their resultant interaction. In conclusion, this study has uniquely described the rheological and mucoadhesive properties of aqueous systems composed of PMVE/MA and PVP. In particular, the concentrations required for gel formation have been defined and the interaction between these two polymeric components illustrated using rheological methods. Furthermore, the simultaneous analysis of rheological, mechanical, and mucoadhesive data provided an insight into their potential utility as platforms for drug delivery systems for application to the oral cavity.

Influence of hydroxypropyl beta-cyclodextrin on the corneal permeation of pilocarpine

The influence of hydroxypropyl beta-cyclodextrin (HPbetaCD) on the corneal permeation of pilocarpine nitrate was investigated by an in vitro permeability study using isolated rabbit cornea. Pupillary-response pattern to pilocarpine nitrate with and without HPbetaCD was examined in rabbit eye. Corneal permeation of pilocarpine nitrate was found to be four times higher after adding HPbetaCD into the formulation. The reduction of pupil diameter (miosis) by pilocarpine nitrate was significantly increased as a result of HPbetaCD addition into the simple aqueous solution of the active substance. The highest miotic response was obtained with the formulation prepared in a vehicle of Carbopol 940. It is suggested that ocular bioavailability of pilocarpine nitrate could be improved by the addition of HPbetaCD.

Examination of the flow rheological and textural properties of polymer gels composed of poly(methylvinylether-co-maleic anhydride) and poly(vinylpyrrolidone): rheological and mathematical interpretation of textural parameters

The purpose of this study was to mathematically characterize the effects of defined experimental parameters (probe speed and the ratio of the probe diameter to the diameter of sample container) on the textural/mechanical properties of model gel systems. In addition, this study examined the applicability of dimensional analysis for the rheological interpretation of textural data in terms of shear stress and rate of shear. Aqueous gels (pH 7) were prepared containing 15% w/w poly(methylvinylether-co-maleic anhydride) and poly(vinylpyrrolidone) (PVP) (0, 3, 6, or 9% w/w). Texture profile analysis (TPA) was performed using a Stable Micro Systems texture analyzer (model TA-XT 2; Surrey, UK) in which an analytical probe was twice compressed into each formulation to a defined depth (15 mm) and at defined rates (1, 3, 5, 8, and 10 mm s(-1)), allowing a delay period (15 s) between the end of the first and beginning of the second compressions. Flow rheograms were performed using a Carri-Med CSL(2)-100 rheometer (TA Instruments, Surrey, UK) with parallel plate geometry under controlled shearing stresses at 20.0 degrees +/- 0.1 degrees C. All formulations exhibited pseudoplastic flow with no thixotropy. Increasing concentrations of PVP significantly increased formulation hardness, compressibility, adhesiveness, and consistency. Increased hardness, compressibility, and consistency were ascribed to enhanced polymeric entanglements, thereby increasing the resistance to deformation. Increasing probe speed increased formulation hardness in a linear manner, because of the effects of probe speed on probe displacement and surface area. The relationship between formulation hardness and probe displacement was linear and was dependent on probe speed. Furthermore, the proportionality constant (gel strength) increased as a function of PVP concentration. The relationship between formulation hardness and diameter ratio was biphasic and was statistically defined by two linear relationships relating to diameter ratios from 0 to 0.4 and from 0.4 to 0.563. The dramatically increased hardness, associated with diameter ratios in excess of 0.4, was accredited to boundary effects, that is, the effect of the container wall on product flow. Using dimensional analysis, the hardness and probe displacement in TPA were mathematically transformed into corresponding rheological parameters, namely shearing stress and rate of shear, thereby allowing the application of the power law (eta = kgamma(n)) to textural data. Importantly, the consistencies (k) of the formulations, calculated using transformed textural data, were statistically similar to those obtained using flow rheometry. In conclusion, this study has, firstly, characterized the relationships between textural data and two key instrumental parameters in TPA and, secondly, described a method by which rheological information may be derived using this technique. This will enable a greater application of TPA for the rheological characterization of pharmaceutical gels and, in addition, will enable efficient interpretation of textural data under different experimental parameters.

The use of xanthan gum in an ophthalmic liquid dosage form: rheological characterization of the interaction with mucin

The development of an ocular dosage form containing xanthan gum and capable of interacting with mucin in the precorneal area is a challenge. The polymer concentration that can be applied is restricted because of the limited patient acceptability of highly viscous preparations. The precorneal mucin concentration is low and the high ionic strength of the lachrymal fluid forces xanthan gum in an ordered structure, less capable of interacting through heterotypic junctions. Intrinsic viscosity measurements and shear rheometry are used to investigate the effect of several factors (polymer concentration, additional boiling or sonication step to the preparation procedure) on the physicochemical properties of xanthan gum and the degree of interaction with a low (8%, w/v) and high (16.0%, w/v) concentrated mucin dispersion. Independent of the preparation procedure applied, a xanthan gum concentration of 1.0% (w/v) is required to obtain a measurable interaction with mucin. If an extra boiling or sonication step is added to the standard preparation procedure, the minimum mucin concentration necessary to achieve formation of heterotypic junctions is decreased. Only by sonication of the highly concentrated xanthan gum dispersion is the viscosity decreased to a level that is tolerable and comfortable to the patient. The findings of the present study clearly demonstrate that a significant interaction between a tolerable and comfortable ocular dosage form containing xanthan gum, and mucin 8% (w/v), is feasible after sonication of a highly concentrated polymer dispersion.

Evaluation of a mucoadhesive tablet for ocular use

The present study investigates the use of a polymer mixture containing Carbopol 974P and drum dried waxy maize starch to obtain prolonged drug release to the anterior eye segment. Two dosage forms with this composition are compared: a hydrated polymer dispersion and a minitablet. A model fluorescent tracer is used to study the ocular release and diffusion from the two dosage forms in humans. To evaluate the prolongation in the cornea/tearfilm compartment, the Apparent Fluorescein TurnOver (%/min) is calculated. The parameters Cmax, tmax, and C9h are used to characterize the pharmacokinetics of Na-fluorescein in the anterior chamber. Furthermore, the swelling behavior of the minitablet is evaluated macroscopically, while the degree of interaction with mucin is characterized by rheological measurements. Calculation of an acceptability score and a slug irritation potential is performed to evaluate user acceptability. In contrast to the hydrated dispersion, the minitablet significantly decreases the Apparent Fluorescein TurnOver (%/min) (P<0.05) and increases the apparent fluorescence in the anterior chamber 9 h after application of the preparation. Rheological data demonstrate the presence of elastic interactions between the polymer and mucin. The dry core of the minitablet becomes fully hydrated after approximately 2 h and is subsequently transformed into a highly concentrated gel. The acceptability of the minitablet is comparable to that of the polymer dispersion. Prolonging the release of Na-fluorescein to the anterior eye segment is only feasible with the dry preparation.

Bioadhesive-based dosage forms: the next generation

Prolonged contact time of a drug with a body tissue, through the use of a bioadhesive polymer, can significantly improve the performance of many drugs. These improvements range from better treatment of local pathologies to improved drug bioavailability and controlled release to enhanced patient compliance. There are abundant examples in the literature over the past 15 years of these improvements using first generation or "off-the-shelf" bioadhesive polymers. The present mini-review will remind us of the success achieved with these first-generation polymers and focus on proposals for the next-generation polymers and attendant benefits likely to occur with these improved polymeric systems.

Biomaterials used in the posterior segment of the eye

The treatment of posterior segment eye disease and related conditions has improved greatly in recent years with the advent of new therapies, materials and devices. Vitreoretinal conditions, however, remain significant causes of blindness in the developed world. Biomaterials play a major role in the treatment of many of these disorders and the success rate of vitreoretinal surgery, especially in the repair of retinal detachment and related conditions, would increase with the introduction of new and improved materials. This review, which focuses on disorders that feature retinal detachment, briefly describes the anatomy of the eye and the nature and treatment of posterior segment eye disorders. The roles, required properties and suitability of the materials used in vitreoretinal surgery as scleral buckles, tamponade agents or drug delivery devices, are reviewed. Experimental approaches are discussed, along with the methods used for their evaluation, and future directions for biomaterial research in the posterior segment of the eye are considered.

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.

Surface studies of albumin immobilized onto PE and PVC films

The aim of this study is to evaluate the thrombogenic behaviour of the low density polyethylene and poly(vinyl chloride) modified by radiation-grafting technique. After copolymerization with acrylic acid by gamma-rays from a 60Co source, BSA was immobilized onto functionalized graft copolymers. The biological interaction between these materials and blood was studies by in vitro methods. The BSA immobilization effectively suppressed the adhesion and activation of platelets when it contacted whole blood.

Impregnation of collagen corneal shields with liposomes: uptake and release of hydrophilic and lipophilic marker substances

PURPOSE

Liposomes and collagen corneal shields (CCS) have been used as ophthalmic drug delivery devices. With regard to a possibly combined application, we studied the effects of surface charge and bilayer fluidity of liposomes on their uptake and release by CCS.

METHODS

12-hours-CCS were soaked in large unilamellar liposomes, which had been labelled with 4,5-carboxyfluorescein (CF) and N-(lissamine rhodamine B sulfonyl)-diacyl-phosphatidylethanolamine (PE-RhB) in the aqueous space and in the liposome bilayer, respectively. Released fluorophores were determined fluorometrically in the elution buffer at intervals from 1 to 240 min after immersion.

RESULTS

The CF concentration in the CCS soaked in a CF solution was two to seven times higher than immersion in the liposome suspensions. Among those, the negatively charged, cholesterol-containing preparation led to the highest CF concentration in the CCS. The PE-RhB concentration was highest after soaking the CCS in neutral, cholesterol-free liposomes. All types of liposomes were found inside the CCS by freeze fracture electron microscopy. The release kinetics data indicate a first order release. More than 90% of CF was released by the CCS within the first 30 min. This was equal after soaking the CCS in the CF solution or in liposomes. With DOPC-liposomes, the maximal release was already attained after 10 min. In general, the differences in the release kinetics of both hydrophilic and lipophilic markers, obtained by the various liposome types were small.

CONCLUSIONS

Our results indicate that surface charge and bilayer fluidity are of minor importance for the interaction with collagen corneal shields. However, since the release kinetics of a liposome-encapsulated hydrophilic or lipophilic substance are similar to the release of a non-encapsulated drug, the combination of liposomes with collagen shields may be useful mainly with respect to the encapsulation of drugs which do not penetrate the ocular surface as well as to prolong corneal contact time of the liposomes.

The unusual routes of administration

A wide range of administration routes can be drawn on to optimise drug absorption. Though the oral route remains the favourite one for most drugs in many disease states, other routes are routinely used. Each has its strengths and weaknesses and needs to be selected carefully with full consideration of the drug, its target and the release pattern required. Advances in pharmacology and biopharmaceutics have led to exciting developments in the ways drugs can now be administered. Optimising drug administration means finding answers to a number of questions. These questions include: what, how, when and where to deliver, and how to retain the drug long enough for it to be fully effective. These issues are linked; route of administration, drug and therapeutic systems are interrelated; thus the choice of what to deliver influences how, when and where to deliver. Various definitions of 'usual/unusual' administration routes are presented, depending on the factors that determine the fate of the active ingredients, on the delivery device, and on the therapeutic objectives to be met. The various dosage forms designed for these routes, and the administration strategies developed to achieve the desired effects are described.