Ocular drug bioavailability from topically applied liposomes

Challenges and strategies for ocular posterior diseases therapy via non-invasive advanced drug delivery

Posterior segment diseases, such as age-related macular degeneration (AMD) and diabetic retinopathy (DR) are vital factor that seriously threatens human vision health and quality of life, the treatment of which poses a great challenge to ophthalmologists and ophthalmic scientists. In particular, ocular posterior drug delivery in a non-invasive manner is highly desired but still faces many difficulties such as rapid drug clearance, limited permeability and low drug accumulation at the target site. At present, many novel non-invasive topical ocular drug delivery systems are under development aiming to improve drug delivery efficiency and biocompatibility for better therapy of posterior segment oculopathy. The purpose of this review is to present the challenges in the noninvasive treatment of posterior segment diseases, and to propose strategies to tackle these bottlenecks. First of all, barriers to ocular administration were introduced based on ocular physiological structure and behavior, including analysis and discussion on the influence of ocular structures on noninvasive posterior segment delivery. Thereafter, various routes of posterior drug delivery, both invasive and noninvasive, were illustrated, along with the respective anatomical obstacles that need to be overcome. The widespread and risky application of invasive drug delivery, and the need to develop non-invasive local drug delivery with alternative to injectable therapy were described. Absorption routes through topical administration and strategies to enhance ocular posterior drug delivery were then discussed. As a follow-up, an up-to-date research advances in non-invasive delivery systems for the therapy of ocular fundus lesions were presented, including different nanocarriers, contact lenses, and several other carriers. In conclusion, it seems feasible and promising to treat posterior oculopathy via non-invasive local preparations or in combination with appropriate devices.

Drug delivery to the anterior segment of the eye: A review of current and future treatment strategies

Research in the development of ophthalmic drug formulations and innovative technologies over the past few decades has been directed at improving the penetration of medications delivered to the eye. Currently, approximately 90% of all ophthalmic drug formulations (e.g. liposomes, micelles) are applied as eye drops. The major challenge of topical eye drops is low bioavailability, need for frequent instillation due to the short half-life, poor drug solubility, and potential side effects. Recent research has been focused on improving topical drug delivery devices by increasing ocular residence time, overcoming physiological and anatomical barriers, and developing medical devices and drug formulations to increase the duration of action of the active drugs. Researchers have developed innovative technologies and formulations ranging from sub-micron to macroscopic size such as prodrugs, enhancers, mucus-penetrating particles (MPPs), therapeutic contact lenses, and collagen corneal shields. Another approach towards the development of effective topical drug delivery is embedding therapeutic formulations in microdevices designed for sustained release of the active drugs. The goal is to optimize the delivery of ophthalmic medications by achieving high drug concentration with prolonged duration of action that is convenient for patients to administer.

Biomimetic Lipid-Based Nanosystems for Enhanced Dermal Delivery of Drugs and Bioactive Agents

Clinical utility of conventional oral therapies is limited by their inability to deliver therapeutic molecules at the local or targeted site, causing a variety of side effects. Transdermal delivery has made a significant contribution in the management of skin diseases with enhanced therapeutic activities over the past two decades. In the modern era, various biomimetic and biocompatible polymer-lipid hybrid systems have been used to augment the transdermal delivery of therapeutics such as dermal patches, topical gels, iontophoresis, electroporation, sonophoresis, thermal ablation, microneedles, cavitational ultrasound, and nano or microlipid vesicular systems. Nevertheless, the stratum corneum still represents the main barrier to the delivery of vesicles into the skin. Lipid based formulations applied to the skin are at the center of attention and are anticipated to be increasingly functional as the skin offers many advantages for the direction of such systems. Accordingly, this review provides an overview of the development of conventional to advanced biomimetic lipid vesicles for skin delivery of a variety of therapeutics, with special emphasis on recent developments in this field including the development of transferosomes, niosomes, aquasomes, cubosomes, and other new generation lipoidal carriers.

Light activated liposomes: Functionality and prospects in ocular drug delivery

Ocular drug delivery, especially to the retina and choroid, is a major challenge in drug development. Liposome technology may be useful in ophthalmology in enabling new routes of delivery, prolongation of drug action and intracellular drug delivery, but drug release from the liposomes should be controlled. For that purpose, light activation may be an approach to release drug at specified time and site in the eye. Technical advances have been made in the field of light activated drug release, particularly indocyanine green loaded liposomes are a promising approach with safe materials and effective light triggered release of small and large molecules. This review discusses the liposomal drug delivery with light activated systems in the context of ophthalmic drug delivery challenges.

Liposomal flucytosine capped with gold nanoparticle formulations for improved ocular delivery

Nanoliposomes have an organized architecture that provides versatile functions. In this study, liposomes were used as an ocular carrier for nanogold capped with flucytosine antifungal drug. Gold nanoparticles were used as a contrasting agent that provides tracking of the drug to the posterior segment of the eye for treating fungal intraocular endophthalmitis. The nanoliposomes were prepared with varying molar ratios of lecithin, cholesterol, Span 60, a positive charge inducer (stearylamine), and a negative charge inducer (dicetyl phosphate). Formulation F6 (phosphatidylcholine, cholesterol, Span 60, and stearylamine at a molar ratio of 1:1:1:0.15) demonstrated the highest extent of drug released, which reached 7.043 mg/h. It had a zeta potential value of 42.5±2.12 mV and an average particle size approaching 135.1±12.0 nm. The ocular penetration of the selected nanoliposomes was evaluated in vivo using a computed tomography imaging technique. It was found that F6 had both the highest intraocular penetration depth (10.22±0.11 mm) as measured by the computed tomography and the highest antifungal efficacy when evaluated in vivo using 32 infected rabbits' eyes. The results showed a strong correlation between the average intraocular penetration of the nanoparticles capped with flucytosine and the percentage of the eyes healed. After 4 weeks, all the infected eyes (n=8) were significantly healed (P<0.01) when treated with liposomal formulation F6. Overall, the nanoliposomes encapsulating flucytosine have been proven efficient in treating the infected rabbits' eyes, which proves the efficiency of the nanoliposomes in delivering both the drug and the contrasting agent to the posterior segment of the eye.

Enhanced corneal permeation of coumarin-6 using nanoliposomes containing dipotassium glycyrrhizinate: in vitro mechanism and in vivo permeation evaluation

The fluidity of the liposomal membrane differently affected cellular uptake/internalization and in vivo corneal penetration of the nanoliposomes.

Topical delivery of ocular therapeutics: carrier systems and physical methods

OBJECTIVE

The basic concepts, major mechanisms, technological developments and advantages of the topical application of lipid-based systems (microemulsions, nanoemulsions, liposomes and solid lipid nanoparticles), polymeric systems (hydrogels, contact lenses, polymeric nanoparticles and dendrimers) and physical methods (iontophoresis and sonophoresis) will be reviewed.

KEY FINDINGS

Although very convenient for patients, topical administration of conventional drug formulations for the treatment of eye diseases requires high drug doses, frequent administration and rarely provides high drug bioavailability. Thus, strategies to improve the efficacy of topical treatments have been extensively investigated. In general, the majority of the successful delivery systems are present on the ocular surface over an extended period of time, and these systems typically improve drug bioavailability in the anterior chamber whereas the physical methods facilitate drug penetration over a very short period of time through ocular barriers, such as the cornea and sclera.

SUMMARY

Although in the early stages, the combination of these delivery systems with physical methods would appear to be a promising tool to decrease the dose and frequency of administration; thereby, patient compliance and treatment efficacy will be improved.

Design of liposomal colloidal systems for ocular delivery of ciprofloxacin

Ophthalmic drug bioavailability is limited due to protective mechanisms of the eye which require the design of a system to enhance ocular delivery. In this study several liposomal formulations containing ciprofloxacin (CPX) have been formulated using reverse phase evaporation technique with final dispersion of pH 7.4. Different types of phospholipids including Phosphatidylcholine, Dipalmitoylphosphatidylcholine and Dimyristoyl-sn-glycero-3-phosphocholine were utilized. The effect of formulation factors such as type of phospholipid, cholesterol content, incorporation of positively charging inducing agents and ultrasonication on the properties of the liposomal vesicles was studied. Bioavailability of selected liposomal formulations in rabbit eye aqueous humor has been investigated and compared with that of commercially available CPX eye drops (Ciprocin®). Pharmacokinetic parameters including Cmax, Tmax, elimination rate constant, t1/2, MRT and AUC0-∞, were determined. The investigated formulations showed more than three folds of improvement in CPX ocular bioavailability compared with the commercial product.

Sustained release of an anti-glaucoma drug: demonstration of efficacy of a liposomal formulation in the rabbit eye

Topical medication remains the first line treatment of glaucoma; however, sustained ocular drug delivery via topical administration is difficult to achieve. Most drugs have poor penetration due to the multiple physiological barriers of the eye and are rapidly cleared if applied topically. Currently, daily topical administration for lowering the intra-ocular pressure (IOP), has many limitations, such as poor patient compliance and ocular allergy from repeated drug administration. Poor compliance leads to suboptimal control of IOP and disease progression with eventual blindness. The delivery of drugs in a sustained manner could provide the patient with a more attractive alternative by providing optimal therapeutic dosing, with minimal local toxicity and inconvenience. To investigate this, we incorporated latanoprost into LUVs (large unilamellar vesicles) derived from the liposome of DPPC (di-palmitoyl-phosphatidyl-choline) by the film hydration technique. Relatively high amounts of drug could be incorporated into this vesicle, and the drug resides predominantly in the bilayer. Vesicle stability monitored by size measurement and DSC (differential scanning calorimetry) analysis showed that formulations with a drug/lipid mole ratio of about 10% have good physical stability during storage and release. This formulation demonstrated sustained release of latanoprost in vitro, and then tested for efficacy in 23 rabbits. Subconjunctival injection and topical eye drop administration of the latanoprost/liposomal formulation were compared with conventional daily administration of latanoprost eye drops. The IOP lowering effect with a single subconjunctival injection was shown to be sustained for up to 50 days, and the extent of IOP lowering was comparable to daily eye drop administration. Toxicity and localized inflammation were not observed in any treatment groups. We believe that this is the first demonstration, in vivo, of sustained delivery to the anterior segment of the eye that is safe and efficacious for 50 days.

Noninvasive routes of proteins and peptides drug delivery

Recent advances in the field of pharmaceutical biotechnology have led to the formulation of many protein and peptide-based drugs for therapeutic and clinical application. The route of administration has a significant impact on the therapeutic outcome of a drug. The needle and syringe is a well established choice of protein and peptide delivery which has some drawback related to patient and to formulation such as pain, cost, sterility etc. Thus, the noninvasive routes which were of minor importance as parts of drug delivery in the past have assumed added importance in protein and peptide drug delivery and these include nasal, ophthalmic, buccal, vaginal, transdermal and pulmonary routes. The pharmaceutical scientists have some approaches to develop the formulations for protein and peptide delivery by noninvasive routes. But, due to the physiochemical instability and enzymatic barrier of proteins and peptides there are several hurdle to develop suitable formulation. So there is need of penetration enhancers, enzyme inhibitors and suitable vehicles for noninvasive delivery to increase the bioavailability. In this review, the aim is to focus on the approaches to formulation of protein and peptide based drug administration by noninvasive route.

Optimization of gatifloxacin liposomal hydrogel for enhanced transcorneal permeation

The aim of this study was to prepare and characterize a topically effective prolonged-release ophthalmic gatifloxacin liposomal hydrogel formulation. Reverse-phase evaporation was used for the preparation of liposomes consisting of phosphatidylcholine (PC) and cholesterol (CH). The effect of PC:CH molar ratio on the percentage of drug encapsulated was investigated. The effect of additives, such as stearylamine (SA) or dicetyl phosphate (DP), as positive and negative charge inducers, respectively, was studied. Morphology, mean size, encapsulation efficiency, and in vitro release of gatifloxacin from liposomes were evaluated. For hydrogel preparation, carbopol 940 was applied. In vitro transcorneal permeation through excised albino rabbit cornea was also determined. Optimal encapsulation efficiency was found at the 5:3 PC:CH molar ratio; by increasing CH content above this limit, the encapsulation efficiency decreased. Positively charged liposomes showed superior entrapment efficiency over other liposomes. Hydrogel-containing liposomes with lipid content PC, CH, and SA in a molar ratio of 5:3:1, respectively, showed best release and transcorneal permeation. These results suggest that the encapsulation of gatifloxacin into liposomes prolonged the in vitro release, depending on composition of the vesicles. In addition, the polymer hydrogel used in the preparation ensured steady, prolonged transcorneal permeation. In conclusion, gatifloxacin liposomal hydrogel is a suitable delivery system for the improvement of the ocular bioavailability of gatifloxacin.

Characterization of drug release from liposomal formulations in ocular fluid

The successful application of liposomes in topical ophthalmic drug delivery requires knowledge of vesicle stabilization in the presence of tear fluid. The release of procaine hydrochloride (PCH) from large unilamellar liposomes in the presence of simulated tear fluid was studied in vitro as a function of bilayer lipid content and tear protein composition. Reverse-phase evaporation vesicles were prepared from egg phosphatidylcholine, stearylamine or dicetyl phosphate, and cholesterol. The relationship between lipid composition and encapsulation efficiency, vesicle size, drug leakage upon storage at 4 degrees C, and the release of PCH-loaded liposomes was studied. The encapsulation efficiency was found to be dependent upon the lipid composition used in the liposome preparation. In particular, phosphatidylcholine vesicles containing cholesterol and/or charged lipids had a lower entrapment efficiency than liposomes prepared with phosphatidylcholine alone. However, the drug release rate was reduced significantly by inclusion of cholesterol and/or charged lipids in the liposomes. The release kinetics of the entrapped agent seemed to be a biphasic process and the drug-release in both simulated tear fluid (STF) and pH 7.4 phosphate buffered saline (PBS) solutions followed pseudo first-order kinetics in the early stage of the release profile. The drug-release appeared to be diffusion and/or partition controlled. Drug release from liposomes into STF, pH 7.4 PBS, and five different modified tear formulations was also evaluated. While serum-induced leakage is attributed to high-density lipoprotein-mediated destabilization, it was determined that lactoferrin might be the protein component in tear fluid that has the primary influence on the liposome-entrapped drug release rate. Five local anesthetics, benoxinate, proparacaine, procaine, tetracaine, and benzocaine were entrapped in liposomal vesicles by a reverse-phase evaporation (REV) technique. The release of these structurally similar topical anesthetics entrapped in positively charged liposomes (egg phosphatidylcholine, stearylamine, and cholesterol in a 7:2:1 molar ratio) was evaluated in a simulated tear fluid and pH 7.4 phosphate buffered saline solution. The liposomes appeared to be useful carriers for these drugs to retard their in vitro release in tear fluid and perhaps sustain or control their release in the eye for better therapeutic efficacy. An analysis of the release data demonstrated that for this series of drugs, drug partition coefficient has the largest effect on release rate, with molecular weight exhibiting a smaller effect. Release rate was found to decrease with increased lipophilicity or increased molecular weight.

Ciprofloxacin as ocular liposomal hydrogel

The purpose of this study was to prepare and characterize an ocular effective prolonged-release liposomal hydrogel formulation containing ciprofloxacin. Reverse-phase evaporation was used for preparation of liposomes consisting of soybean phosphatidylcholine (PC) and cholesterol (CH). The effect of PC/CH molar ratio on the percentage drug encapsulation was investigated. The effect of additives such as stearylamine (SA) or dicetyl phosphate (DP) as positive and negative charge inducers, respectively, were studied. Morphology, mean size, encapsulation efficiency, and in vitro release of ciprofloxacin from liposomes were evaluated. For hydrogel preparation, Carbopol 940 was applied. In vitro transcorneal permeation through excised albino rabbit cornea was also determined. Optimal encapsulation efficiency of 73.04 +/- 3.06% was obtained from liposomes formulated with PC/CH at molar ratio of 5:3 and by increasing CH content above this limit, the encapsulation decreased. Positively charged liposomes showed superior entrapment efficiency (82.01 +/- 0.52) over the negatively charged and the neutral liposomes. Hydrogel containing liposomes with lipid content PC, CH, and SA in molar ratio 5:3:1, respectively, showed the best release and transcorneal permeation with the percentage permeation of 30.6%. These results suggest that the degree of encapsulation of ciprofloxacin into liposomes and prolonged in vitro release depend on composition of the vesicles. In addition, the polymer hydrogel used in preparation ensure steady and prolonged transcorneal permeation. In conclusion, ciprofloxacin liposomal hydrogel is a suitable delivery system for improving the ocular bioavailability of ciprofloxacin.

Preparation and evaluation of thermosensitive liposomal hydrogel for enhanced transcorneal permeation of ofloxacin

Ofloxacin, available as ophthalmic solution, has two major problems: first, it needs frequent administration every 4 hours or even every 1 hour to treat severe eye infection; second, there is formation of white crystalline deposit on cornea due to its pH-dependent solubility, which is very low at pH of corneal fluid. In order to provide a solution to previous problems, ofloxacin in this study is prepared as topically effective in situ thermosensitive prolonged release liposomal hydrogel. Two preparation procedures were carried out, leading to the formation of multilamellar vesicles (MLVs) and reverse-phase evaporation vesicles (REVs) at pH 7.4. Effects of method of preparation, lipid content, and charge inducers on encapsulation efficiency were studied. For the preparation of in situ thermosensitive hydrogel, chitosan/beta-glycerophosphate system was synthesized and used as carrier for ofloxacin liposomes. The effect of addition of liposomes on gelation temperature, gelation time, and rheological behaviors of the hydrogel were evaluated. In vitro transcorneal permeation was also determined. MLVs entrapped greater amount of ofloxacin than REVs liposomes at pH 7.4; drug loading was increased by including charge-inducing agent and by increasing cholesterol content until a certain limit. The gelation time was decreased by the addition of liposomes into the hydrogel. The prepared liposomal hydrogel enhances the transcorneal permeation sevenfold more than the aqueous solution. These results suggested that the in situ thermosensitive ofloxacin liposomal hydrogel ensures steady and prolonged transcorneal permeation, which improves the ocular bioavailability, minimizes the need for frequent administration, and decreases the ocular side effect of ofloxacin.

Evaluation of the in vivo behaviour of gentamicin sulphate ocular mini-tablets in ponies

The in vivo behaviour of 5% gentamicin sulphate ocular mini-tablets (2-mm diameter, 6.525 mg weight) was compared with gentamicin eye drops in six ponies. Two mini-tablets were inserted on the bulbar conjunctiva of the right eye while a similar dose of gentamicin was administered via eye drops in the left eye. Irritation induced by the mini-tablets and the eye drops was evaluated using a visual analogue scale (0-10). Tears were sampled with ophthalmologic absorption triangles for 1 min for the determination of the concentration of gentamicin sulphate using a microbiological plate diffusion method. Irritation induced by the tablets was minor and clinically acceptable (overall median score of 1.7 +/- 1.4). Eye drops induced a sharp increase in gentamicin sulphate concentration (364.4 microg/mL after 5 min) followed by a fast decline (10.8 microg/mL after 60 min). The increase in concentration induced by the ocular mini-tablets was less pronounced (up to 56.2 microg/mL after 30 min) and followed by a gradual decrease; the concentration remained above 15 microg/mL for 8 h. Ocular 5% gentamicin sulphate mini-tablets are clinically well-tolerated in ponies, assuring a constant concentration in the tears for at least 8 h.

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.

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.

Risk factors for proliferative vitreoretinopathy

Proliferative vitreoretinopathy (PVR) is one of the major causes of failure in retinal detachment surgery. To prevent PVR, it is necessary to determine factors predisposing its development. In primary PVR, large retinal tears, long duration of retinal detachment, vitreous hemorrhages, aphakia and choroidal detachment were demonstrated as clinical risk factors for PVR. In postoperative PVR, it was revealed that large breaks, pre- and postoperative choroidal detachment, minor intra- or postoperative hemorrhages, signs of uveitis, extensive retinal detachment, vitrectomy, cryopexy, air injection and preoperative PVR were risk factors for PVR by multivariate analysis. Almost all risk factors for PVR are associated with intravitreal dispersion of retinal pigment epithelial (RPE) cells or breakdown of the blood-ocular barrier which are prerequisite to development of PVR.

Ophthalmic drug delivery systems--recent advances

Eye-drops are the conventional dosage forms that account for 90% of currently accessible ophthalmic formulations. Despite the excellent acceptance by patients, one of the major problems encountered is rapid precorneal drug loss. To improve ocular drug bioavailability, there is a significant effort directed towards new drug delivery systems for ophthalmic administration. This chapter will focus on three representative areas of ophthalmic drug delivery systems: polymeric gels, colloidal systems, cyclodextrins and collagen shields. Hydrogels generally offer a moderate improvement of ocular drug bioavailability with the disadvantage of blurring of vision. In situ activated gel-forming systems are preferred as they can be delivered in drop form with sustained release properties. Colloidal systems including liposomes and nanoparticles have the convenience of a drop, which is able to maintain drug activity at its site of action and is suitable for poorly water-soluble drugs. Among the new therapeutic approaches in ophthalmology, cyclodextrins represent an alternative approach to increase the solubility of the drug in solution and to increase corneal permeability. Finally, collagen shields have been developed as a new continuous-delivery system for drugs that provide high and sustained levels of drugs to the cornea, despite a problem of tolerance. It seems that new tendency of research in ophthalmic drug delivery systems is directed towards a combination of several drug delivery technologies. There is a tendency to develop systems which not only prolong the contact time of the vehicle at the ocular surface, but which at the same time slow down the elimination of the drug. Combination of drug delivery systems could open a new directive for improving results and the therapeutic response of non-efficacious systems.

Influence of sterilization processes on poly(epsilon-caprolactone) nanospheres

Polymeric vectors and especially poly(epsilon-caprolactone) nanoparticles have already shown promising results in the optimization of the ophthalmic bioavailability of drugs. Any formulation instilled in the eye must be sterile, and preferentially isotonic. Poly(epsilon-caprolactone) nanospheres were thus formulated with Synperonic PE/F68, Synperonic PE/F127, or Cremophor RH40. A tonicity agent, a preservative and, in some cases, a viscosifiant were then added. The pH was finally adjusted to pH 4 or buffered to pH 7. Different sterilization processes were studied to investigate their influence on the physicochemical characteristics of vectors. Autoclaving did not induce any modification on polymer molecular weight or Synperonic nanospheres diameter, but catalysed some reactions with surfactants and tonicity agents. This method could thus be used if the nanosphere excipients are chosen with care. gamma radiation induced preservative degradation and viscosifiant depolymerization. A cross-linking of poly(epsilon-caprolactone) chains was observed, as reflected by a sharp increase of its molecular weight. However, no variation of the mean particle size was detected. Finally, sterile filtration was the only process which ensured the conservation of physicochemical integrity of nanospheres. This process was successfully applied on non-viscosified vectors with a sufficiently small diameter.

In-vivo studies on dexamethasone sodium phosphate liposomes

Dexamethasone Sodium Phosphate (DSP) is a water soluble anti-inflammatory steroid commonly used in the therapy of serious types of ophthalmic inflammation. It has been demonstrated that unless the corneal epithelium is damaged, DSP is poorly absorbed by the cornea (Kupferman et al. 1974). Thus, it is doubtful whether such a drug would cure inflammation of the anterior segments. For this purpose, several liposomal DSP formulations containing phospholipid: charge inducer: cholesterol in molar ratios of 10:1:4 were investigated. Both gel state (PL 90H: SA: Cho1) and liquid state (PL 100: SA: Cho1) liposomes were prepared. For the preparation of liposomes, the film method followed by bath sonication was used. Liposomes were labelled with (99m)-Tc and administered intra-ocularly to New Zealand white rabbits weighing 2.5-3 kg for in vivo experiments. The biodistribution of the labelled liposomes were determined. For this purpose, eye segments (such as cornea, lens, iris, ciliar body, vitreous, aqueous humor, conjuctiva and sclera) and RES organs (such as liver, pancreas, spleen) were removed at fixed time intervals. In the present study, the efficiency of liposomes for the delivery of water-soluble drugs was evaluated in rabbit eyes using DSP as a model drug in different liposomal formulations.

Antiproliferative activity of a liposomal delivery system of mitoxantrone on rabbit subconjunctival fibroblasts in an ex-vivo model

Wound healing is the main cause of the failure of filtering surgery in glaucoma. We developed a liposomal delivery system of mitoxantrone (MITX), an anthracyclin derivative, to allow a single adjuvant administration and to lessen ocular side-effects of the drug. In order to evaluate the antiproliferative activity of liposomal MITX, an ex vivo model consisting in the culture of subconjunctival tissue explants from rabbits pretreated with subconjunctival injections of free or liposomal MITX was used. We found that both forms of MITX decreased the growth rate as well as the explant proliferation surfaces 15 days or 1 month after a single administration of the drug in vivo. A morphometric analysis of the cells showed that the surface of the fibroblasts exposed to both forms of MITX was from 10 to 12 times as important as that of the control cells exposed to the empty liposomes and to the control buffer. A radioautographic study showed that more than 95% of the fibroblasts exposed to both forms of MITX were in the G1 phase of the cell cycle, while the control cell population was equally distributed among the different phases of the cell cycle.

Clinical pharmacokinetics of various topical ophthalmic delivery systems

The eye provides an interesting study in contrasts: it is a delicate structure with a transparent anterior wall as thin as 0.5mm; yet this structure in combination with the ocular adnexa provides a resilient physicochemical barrier. The lids, tears and lacrimal apparatus work in concert to continuously protect the cornea and conjunctiva with a stable tear film, which also serves as the primary refracting surface. This elaborate defence system simultaneously prevents ready intraocular access of pharmaceutical agents. Additionally, the trilaminate structure of the cornea has variable permeability to chemical agents, thereby further limiting the passage of highly hydrophobic and hydrophilic moieties. Presenting topical pharmaceutical agents to the eye via different delivery systems allows clinicians to directly affect the profile of drug bioavailability and, ultimately, bioactivity. While achieving optimum bioavailability is therapeutically important, one must simultaneously limit the occurrence of drug-induced adverse effects, both systemic and local. Utilising the different pharmacokinetic properties of drug delivery systems permits clinicians to maximise their therapeutic plans for addressing specific clinical situations while minimising the potential for adverse drug effects.

Analysis of interactions between the corneal epithelium and liposomes: qualitative and quantitative fluorescence studies of a corneal epithelial cell line

Transcorneal drug transport is normally limited by the intrinsic permeation characteristics of the corneal epithelium. However, liposomes, i.e., phospholipid vesicles composed of phospholipid membranes, have recently attracted attention as carriers of topically applied agents. The present study therefore describes a qualitative and quantitative laboratory investigation of interactions between corneal epithelial cells and liposomes. The lipid bilayers and interior spaces of liposomes were labelled with different fluorophores. Fluorescence microscopy revealed a rapid uptake of rhodamine B-labelled liposome bilayer components by the epithelial cell membrane and the cytoplasm. Simultaneously, intracellular uptake of aqueous liposome content was indicated by uniform fluorescence of the cytoplasm due to carboxyfluorescein (CF). The fluorimetric experiments showed that the uptake of liposomes by SIRC cells depended on liposome concentrations and the time of exposure of the cells to the liposomes, and that saturation effect characteristics were present. Cell fluorescence dropped by approximately 45% when the incubation temperature of the cells was reduced from 37 degrees C to 4 degrees C. Both this phenomenon and a significant reduction in liposome uptake (p < 0.05 and p < 0.01) after incubation with the metabolic inhibitors 2-deoxyglucose and sodium azide indicated active, energy-dependent processes. Phagocytosis in cell-liposome interactions was directly shown by a significant reduction in cell fluorescence (p < 0.05) after application of the actin inhibitor cytochalasin B. The results presented here give concrete data on interactions between liposomes and superficial cells of the eye in vitro.

Prolongation of corneal allograft survival with liposome-encapsulated cyclosporine in the rat eye

PURPOSE

To study the effects of different formulations of topical cyclosporine (Cyclosporin A [CsA]) on corneal allograft rejection in a rat model.

METHODS

Female Lewis rats received penetrating keratoplasties from female Wistar-Furth donors. A total of 78 allogeneic grafts were performed. An additional 15 syngeneic grafts (Lewis) were used as technical controls. Two CsA preparations with equivalent drug concentrations (2.1 mg/ml) were applied as drops: CsA encapsulated in large unilamellar liposomes (CsA-LIP) and CsA dissolved in olive oil (CsA-DR). Allogeneic grafts were randomly assigned to receive CsA-LIP or CsA-DR beginning on the day of surgery five times daily for 10 days. Animals without any treatment or receiving empty liposomes (EM-LIP) were used as treatment controls. Grafts were graded three times weekly and a rejection index was generated based on graft clarity, neovascularization, and vessel size.

RESULTS

All syngeneic grafts remained clear over the observation period of 60 days. Rejected allogeneic grafts without any treatment and those receiving EM-LIP or CsA-DR showed a mean survival time (+/- standard deviation) of 14 +/- 4, 14 +/- 5, and 14 +/- 4 days, respectively. There was no significant difference in mean survival time between the grafts without any treatment and those in CsA-DR or EM-LIP treatment groups. The mean survival time of rejected grafts in animals receiving CsA-LIP was prolonged to 20 +/- 4 days. There was a significant difference in the mean survival time between the CsA-LIP treatment group and groups receiving CsA-DR, EM-LIP, or no treatment (P < or = 0.05). The Kaplan-Meier survival curve of the CsA-LIP treatment group was significantly different from the other experimental groups. The graft survival rate in the CsA-LIP group was 77%, whereas the rate was 37% in the non-treated group, 45% in the CsA-DR group, and 36% in the EM-LIP group.

CONCLUSION

Encapsulation of CsA in liposomes might be a promising formulation for use in the prevention of corneal graft rejection.

Corticosteroids and daunomycin in the prevention of experimental proliferative vitreoretinopathy induced by macrophages

An experimental model of proliferative vitreoretinopathy (PVR) induced by macrophages simulates a special form of wound healing process in the eye and mimics the development of PVR from its initial stage. We used this model for the evaluation of drug efficacy in the prevention of PVR. One mg triamcinolone acetonide (TA), 10 micrograms daunomycin-liposome (DL), 5 micrograms free daunomycin (FD) and 0.1 ml saline or empty liposomes (as controls) were injected into the vitreous in four groups of animals (30 or 40 rabbit eyes each) after macrophage injection. Retinal detachment developed in 77.5% of the control eyes on day 28, compared to 13.3% of the TA-treated eyes (P < 0.01), to 33.3% of the eyes treated with DL (P < 0.01), and 50% of the FD-treated eyes (P < 0.05). TA cleared up from the vitreous within 35-63 days (average 45.5 days). The half-time of FD clearance was 145.5 min. Although DL declined rapidly during the first 2 days, there was an average of 0.64 microgram/ml daunomycin in the vitreous on day 14. Transmission electron microscopy showed that FD at a dosage of over 5 micrograms or DL over 20 micrograms was toxic to the retina and that up to 4 mg TA was nontoxic. These results suggest that steroids such as TA, given at the inflammatory stage, can effectively and safely prevent the development of PVR, and that encapsulation in liposomes of cytotoxic agents such as daunomycin can enhance drug efficacy and reduce toxicity. The time course of initiation and development of PVR is important in the selection of particular drugs.

Liposomes in drug delivery. Clinical, diagnostic and ophthalmic potential

Liposomes (phospholipid-based vesicles) have been investigated since 1970 as a system for the delivery or targeting of drugs to specific sites in the body. Because of their structural versatility in terms of size, composition, surface charge, bilayer fluidity and ability to incorporate almost any drug regardless of solubility, or to carry on their surface cell-specific ligands, liposomes have the potential to be tailored in a variety of ways to ensure the production of formulations that are optimal for clinical use. This includes controlled retention of entrapped drugs in the presence of biological fluids, controlled vesicle residence in the blood circulation or other compartments in the body, and enhanced vesicle uptake by target cells. Accumulated in vivo evidence, particularly in areas such as cancer chemotherapy, antimicrobial therapy, vaccines, diagnostic imaging and the treatment of ophthalmic disorders has indicated clearly that some liposome-entrapped drugs and vaccines exhibit superior pharmacological properties to those observed with conventional formulations. Such work has encouraged the application of liposomes in the treatment of diseases in humans. A large number of trials in patients with cancer or infections suggest that certain liposomal drug formulations are likely to prove clinically useful.

Principles of therapeutics

Topical administration of drugs is the treatment of choice for diseases of the anterior segment. Drug levels attained by this means are usually of short duration, however, necessitating frequent therapy or continuous perfusion if prolonged drug levels are required. A drug-delivery device (collagen shield or contact lens) or subconjunctival injections can be used to augment topical therapy if frequent treatment is not possible. Subconjunctival injections are recommended for drugs that have low solubility and, hence, low corneal penetration. Retrobulbar injections are seldom indicated, except for regional anesthesia. Systemic administration is useful for anti-inflammatory therapy but it may be difficult to establish therapeutic levels of antibiotic agents in the eye because of the blood-ocular barrier. In severe cases, intraocular injection may be required.

Liposomally-entrapped ganciclovir for the treatment of cytomegalovirus retinitis in AIDS patients. Eperimental toxicity and pharmacokinetics, and clinical trial

Treatment of retinitis by cytomegalovirus (CMV) in AIDS patients requires frequent repetitive injections of intravitreal ganciclovir (GCV). This study was undertaken to establish experimentally whether the intravitreal application of liposomally-entrapped GCV could prolong intraocular therapeutic levels when compared with the intravitreal injection of free GCV, and the clinical effectiveness of this approach in AIDS patients. Intraocular concentration of GCV was determined by means of an ELISA test in rabbit vitreous 2, 3, 7, and 14 days after a single intravitreal injection of either different doses of the free drug (0.2-20 mg) or 1 mg of liposomally-entrapped GCV. After 72 h, only the vitreous of rabbits injected with doses of free GCV greater than or equal to 5 mg showed therapeutic levels of the drug; no GCV was detected after 72 h with any of the doses applied. Moreover, the microscopic study revealed GCV-induced damage in retinal structures in the animals injected with a free GCV dose greater than or equal to 15 mg. Intravitreal injection to rabbits of 1 mg of liposomally-encapsulated GCV showed no retinal toxicity at any of the time points studied, and therapeutic levels were detected up to 14 days after injection (4.67 +/- 0.39 microgram/ml). Five AIDS patients suffering CMV retinitis were injected with 0.5 mg of liposomally-entrapped GCV (2 mg of lecithin). Complete remission of the CMV retinitis was observed already at the third injection of 0.5 mg GCV (one per week) and relapse did not occur during the 2-4 month follow-up of the patients. In view of the results presented, it can be concluded that intravitreal injection of liposomally-encapsulated GCV increases the time period required for reinjections in the treatment of CMV retinitis.

Topological distribution of atropine in dipalmitoylphosphatidylcholine liposomes

The topography of atropine entrapped in sn-3-(dipalmitoyl)phosphatidylcholine (DPPC) liposomes was determined by the electron spin resonance (ESR) technique using 5-, 7-, and 16-nitroxy-stearic acid probes. The liposome preparations, with or without entrapped atropine, were passed through a Sepharose-4B column; the entrapment efficiency and lipid recovery were determined using [3H]-atropine and [14C]-DPPC as tracers. It was found that approximately 31 per cent of the added atropine was entrapped in the liposomes. The ESR results established that the temperature range of the gel-to-liquid crystalline transition of DPPC liposomes was not altered significantly by the encapsulation of atropine. The entrapped atropine also caused no significant change in the order of hydrocarbon chains of DPPC vesicles. These results were confirmed by data obtained from differential scanning calorimetry. On the basis of these experiments, it was concluded that atropine does not interact with the hydrophobic region of the lipid bilayers but is exclusively localized within the entrapped aqueous compartment of DPPC liposomes.

Externally triggered release of dye and drugs from liposomes into the eye. An in vitro and in vivo study

Temperature-sensitive liposomes were made with carboxyfluorescein (CF) (100 mM) entrapped in the aqueous compartment. This liposome dye system was used in vitro to evaluate the feasibility of using microwaves as a triggering mechanism for the release of drugs in the anterior chamber of the eye. The in vitro study demonstrated that CF can be released from liposomes in response to a localized temperature rise induced by microwave irradiation. In an in vivo study in rabbits, CF and the antineoplastic agent, cytosine arabinoside, were administered intravenously and selectively released by increasing the temperature of the ciliary body with microwaves. In the eyes receiving liposome-encapsulated dye, the average concentration of CF in the anterior chamber of the heated eyes was 8.0 times higher than in the contralateral unheated control eyes. In the eyes receiving liposome-encapsulated drug, the average concentration of cytosine arabinoside in the aqueous of the heated eyes was 4.1 times higher than the concentration in the contralateral unheated control eyes. The importance of this finding lies in the potential for a new method of targeting the delivery of dyes and drugs to specific sites in the eye.

Wound healing in glaucoma filtering surgery

Successful glaucoma filtering surgery is characterized by the passage of aqueous humor from the anterior chamber to the subconjunctival space, which results in the formation of a filtering bleb. Aqueous in the subconjunctival space may then exit by multiple pathways. Bleb failure most often results from fibroblast proliferation and subconjunctival fibrosis. Factors associated with an increased risk of bleb failure include youth, aphakia, active anterior segment neovascularization, inflammation, previously failed glaucoma filtering surgery, and, possibly, race. Several surgical and pharmacologic techniques have recently been introduced to enhance success in eyes with poor surgical prognoses. To elucidate the scientific rationale of these methods, we summarize the process of wound healing after glaucoma filtering surgery and describe postoperative clinical and histopathologic features, factors which may affect success, and specific methods to improve surgical success.

Liposomes are effective carriers for the ocular delivery of prophylactics

Liposomes containing acetylcholinesterase were prepared by the freeze-drying method. The multilamellar morphology of the vesicles was revealed by freeze-fracture electron microscopy and their size distribution was determined by quasi-elastic light scattering. The vesicle diameters were in the range of about 0.2-4.0 micron. The liposome preparations were tested for their ocular delivery of an entrapped cholinesterase enzyme in counteracting the miotic effect of diisopropylfluorophosphate (DFP), a prototype of a family of organophosphate poisons. The topical application of the enzyme-containing liposomes to the rabbit eye was found to confer a significant level of protection against DFP-induced miosis. In comparing the prophylactic effectiveness of different enzyme-bearing liposomes, positively charged vesicles were found to be more effective than either neutral or negatively charged vesicles. Although the precise protective mechanism is not clear, our in vitro studies indicate that DFP molecules freely associate with liposomes and tear fluid promotes the release of liposome-entrapped enzymes. Thus, it is conceivable that the enzyme-liposome complex may act somewhat like a sponge by sequestering DFP molecules which diffuse into the vesicle, and also by releasing the entrapped enzyme to combine with DFP, thereby neutralizing its in vivo toxic effect.