Intravitreal pharmacokinetics of liposome-encapsulated amikacin in a rabbit model

Formulation Advances in Posterior Segment Ocular Drug Delivery

Posterior segment ocular diseases, such as diabetic retinopathy, age-related macular degeneration, and retinal vein occlusion, are leading causes of vision impairment and blindness worldwide. Effective management of these conditions remains a formidable challenge due to the unique anatomical and physiological barriers of the eye, including the blood-retinal barrier and rapid drug clearance mechanisms. To address these hurdles, nanostructured drug delivery systems are proposed to overcome ocular barriers, target the retina, and enhance permeation while ensuring controlled release. Traditional therapeutic approaches, such as intravitreal injections, pose significant drawbacks, including patient discomfort, poor compliance, and potential complications. Therefore, understanding the physiology and clearance mechanism of eye could aid in the design of novel formulations that could be noninvasive and deliver drugs to reach the target site is pivotal for effective treatment strategies. This review focuses on recent advances in formulation strategies for posterior segment ocular drug delivery, highlighting their potential to overcome these limitations. Furthermore, the potential of nanocarrier systems such as in-situ gel, niosomes, hydrogels, dendrimers, liposomes, nanoparticles, and nanoemulsions for drug delivery more effectively and selectively is explored, and supplemented with illustrative examples, figures, and tables. This review aims to provide insights into the current state of posterior segment drug delivery, emphasizing the need for interdisciplinary approaches to develop patient-centric, minimally invasive, and effective therapeutic solutions.

Emphasis on Nanostructured Lipid Carriers in the Ocular Delivery of Antibiotics

BACKGROUND

Drug distribution to the eye is still tricky because of the eye's intricate structure. Systemic delivery, as opposed to more traditional methods like eye drops and ointments, is more effective but higher doses can be harmful.

OBJECTIVE

The use of solid lipid nanoparticles (SLNPs) as a method of drug delivery has been the subject of research since the 1990s. Since SLNPs are derived from naturally occurring lipids, they pose no health risks to the user. To raise the eye's absorption of hydrophilic and lipophilic drugs, SLNs can promote corneal absorption and improve the ocular bioavailability of SLNPs.

METHODS

To address problems related to ocular drug delivery, many forms of nano formulation were developed. Some of the methods developed are, emulsification and ultra-sonication, high-speed stirring and ultra-sonication, thin layer hydration, adapted melt-emulsification, and ultrasonication techniques, hot o/w micro-emulsion techniques, etc. Results: Nanostructured lipid carriers are described in this review in terms of their ocular penetration mechanism, structural characteristic, manufacturing process, characterization, and advantages over other nanocarriers.

CONCLUSION

Recent developments in ocular formulations with nanostructured bases, such as surfacemodified attempts have been made to increase ocular bioavailability in both the anterior and posterior chambers by incorporating cationic chemicals into a wide variety of polymeric systems.

In vitro Model Systems for Studies Into Retinal Neuroprotection

Therapy development for neurodegenerative diseases of the retina constitutes a major unmet medical need, and this may be particularly relevant for inherited diseases of the retina, which are largely untreatable to this day. Therapy development necessitates appropriate models to improve the understanding of the underlying degenerative mechanisms, as well as for the testing and evaluation of novel treatment approaches. This review provides an overview of various in vitro model systems used to study retinal neuroprotection. The in vitro methods and technologies discussed range from primary retinal cell cultures and cell lines, to retinal organoids and organotypic retinal explants, to the cultivation of whole eyeballs. The advantages and disadvantages of these methods are compared and evaluated, also in view of the 3R principles (i.e., the refinement, reduction, and replacement of live animal testing), to identify suitable in vitro alternatives for in vivo experimentation. The article further expands on the use of in vitro models to test and evaluate neuroprotective treatments and to aid the development of retinal drug delivery systems. Among the pharmacological agents tested and characterized in vitro are such that interfere with aberrant cyclic guanosine monophosphate (cGMP) -signaling or such that inhibit the activities of poly (ADP-ribose) polymerase (PARP), histone deacetylases (HDAC), calpain-type proteases, as well as unfolded protein response-related stress. We then introduce nanoparticle-based drug delivery systems and discuss how different in vitro systems may be used to assess their efficacy in the treatment of retinal diseases. The summary provides a brief comparison of available in vitro models and relates their advantages and limitations to the various experimental requirements, for instance, for studies into disease mechanisms, novel treatments, or retinal toxicity. In many cases, combinations of different in vitro models may be required to obtain a comprehensive view of the efficacy of a given retinal neuroprotection approach.

Investigating Ex Vivo Animal Models to Test the Performance of Intravitreal Liposomal Drug Delivery Systems

There is a strong need for innovative and efficient drug delivery systems for ocular therapy development. However, testing intravitreal drug delivery systems without using live animals is challenging. Ex vivo animal models offer an interesting alternative. We analyzed the potential of using fresh porcine eyes obtained from the local slaughterhouse as a model for testing the intravitreal biodistribution and retention of liposomes with or without polyethylene glycol (PEG) conjugation and with different surface charges. The histology of the eyes was analyzed to localize the liposomes, and it was found that liposomes with PEG absorbed rapidly on the retina (within 1 h), with positively charged and PEG-coated liposomes being retained for at least 24 h. In parallel, fluorophotometry was employed on intact eyes, to determine the pharmacokinetics of the fluorophore calcein, as a substitute for a small hydrophilic therapeutic compound. We found a 4.5-fold increase in the vitreous half-life of calcein loaded in liposomes, compared with the free solution. Retinal toxicity was addressed using murine-derived retinal explant cultures. Liposomes were non-toxic up to 500 µg/mL. Toxicity was observed at 5 mg/mL for anionic and cationic liposomes, with 2-fold and 2.5-fold increased photoreceptor cell death, respectively. Overall, we could show that important ocular drug delivery considerations such as pharmacokinetics and biodistribution can be estimated in ex vivo porcine eyes, and may guide subsequent in vivo experiments.

Safety and Tolerability of Topical Ophthalmic Triamcinolone Acetonide-Loaded Liposomes Formulation and Evaluation of Its Biologic Activity in Patients with Diabetic Macular Edema

Intravitreal injections (IVTs) of corticosteroids as triamcinolone acetonide (TA) are frequently used for the treatment of many vitreous and retinal disorders. However, IVTs are related to severe ocular complications. Lately, a topical ophthalmic TA-loaded liposomes formulation (TALF) was designed to transport TA into the posterior segment of the eye when instilled on the ocular surface. To evaluate the safety, tolerability, and biological activity of TALF, an animal study and a phase I clinical assay were performed. Moreover, four patients with diabetic macular edema (DME) were treated with TALF in order to explore the biological activity of the formulation. No inflammation, lens opacity, swelling, or increase in intraocular pressure were recorded after the instillation of TALF in any of the animal or clinical studies. Mainly, mild and transient adverse events such as dry eye and burning were reported. TALF significantly improves visual acuity and diminishes central foveal thickness in patients with DME. The current data demonstrate the safety, tolerability, and biological activity of TALF. It seems that TALF can be used topically to treat vitreous and retinal diseases that respond to TA such as DME, avoiding the use of corticosteroid IVTs and their associated hazards.

Ocular Drug Delivery to the Retina: Current Innovations and Future Perspectives

Treatment options for retinal diseases, such as neovascular age-related macular degeneration, diabetic retinopathy, and retinal vascular disorders, have markedly expanded following the development of anti-vascular endothelial growth factor intravitreal injection methods. However, because intravitreal treatment requires monthly or bimonthly repeat injections to achieve optimal efficacy, recent investigations have focused on extended drug delivery systems to lengthen the treatment intervals in the long term. Dose escalation and increasing molecular weight of drugs, intravitreal implants and nanoparticles, hydrogels, combined systems, and port delivery systems are presently under preclinical and clinical investigations. In addition, less invasive techniques rather than intravitreal administration routes, such as topical, subconjunctival, suprachoroidal, subretinal, and trans-scleral, have been evaluated to reduce the treatment burden. Despite the latest advancements in the field of ophthalmic pharmacology, enhancing drug efficacy with high ocular bioavailability while avoiding systemic and local adverse effects is quite challenging. Consequently, despite the performance of numerous in vitro studies, only a few techniques have translated to clinical trials. This review discusses the recent developments in ocular drug delivery to the retina, the pharmacokinetics of intravitreal drugs, efforts to extend drug efficacy in the intraocular space, minimally invasive techniques for drug delivery to the retina, and future perspectives in this field.

Nanoparticulate Drug Delivery to the Retina

Retinal diseases, such as age-related macular degeneration and diabetic retinopathy, are the leading causes of blindness worldwide. The mainstay of treatment for these blinding diseases remains to be surgery, and the available pharmaceutical therapies on the market are limited, partially owing to various biological barriers in hindering the delivery of therapeutics to the retina. The nanoparticulate drug delivery system confers the capability for delivering therapeutics to the specific ocular targets and, hence, potentially revolutionizes the current treatment landscape of retinal diseases. While the research to date indicates the enormous therapeutics potentials of the nanoparticulate delivery systems, the successful translation of these systems from the bench to bedside is challenging and requires a combined understanding of retinal pathology, physiology of the eye, and particle and formulation designs of nanoparticles. To this end, the review begins with an overview of the most prevalent retinal diseases and related pharmacotherapy. Highlights of the current challenges encountered in ocular drug delivery for each administration route are provided, followed by critical appraisal of various nanoparticulate drug delivery systems for the retinal diseases, including their formulation designs, therapeutic merits, limitations, and future direction. It is believed that a greater understanding of the nano-biointeraction in eyes will lead to the development of more sophisticated drug delivery systems for retinal diseases.

Topical Triamcinolone Acetonide-Loaded Liposomes as Primary Therapy for Macular Edema Secondary to Branch Retinal Vein Occlusion: A Pilot Study

Purpose: To explore safety and therapeutic efficacy of a topical ophthalmic triamcinolone acetonide-loaded liposome formulation (TA-LF) as primary therapy in patients with macular edema (ME) secondary to branch retinal vein occlusion (BRVO). Methods: Twelve eyes of 12 patients with ME secondary to BRVO were exposed to a topical instillation of 1 drop of TA-LF (TA 0.2%) 6 times a day for 12 weeks to evaluate safety and efficacy. Best corrected visual acuity (BCVA) intraocular pressure (IOP), slit lamp examination, and central foveal thickness (CFT) were analyzed at every visit. In addition, the morphology of TA-LF was analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results: Patients presented a significant improvement of BCVA and CFT without significant IOP modification (P = 0.94). Treated eyes showed BCVA improvement from 40 ± 12.05 to 64.83 ± 15.97 letters and CFT reduction from 682.91 ± 278.60 to 271.58 ± 57.66 μm after 12 weeks of TA-LF therapy (P < 0.001). No adverse events, including IOP rising, were registered. SEM analysis of liposomal formulations showed that liposome (LP) size depends on its concentration. As the concentration of TA increased, the average size of LPs and the number of larger particles increased as well. TEM study displayed that LP formulation efficiently solubilizes TA crystals in nanoparticles and encapsulates them. Conclusion: LPs can function as nanocarriers of TA and they could be used as topical ophthalmic primary therapy instead of intravitreal drugs in patients with ME secondary to BRVO.

Therapies Based on Nanoparticles for Eye Drug Delivery

Eye drug delivery, particularly to the retina, is a technical hurdle that needs to be solved and represents an ongoing current important medical field. Posterior segment eye diseases are a major cause of visual impairment worldwide. Age-related macular degeneration, glaucoma, and diabetic retinopathy are the major causes of blindness. To achieve efficient drug delivery and drug retention time in the posterior segment of the eye, novel delivery systems based on nanoparticles have been developed in the last few years. Nowadays, liposomes represent the most utilized nanoparticles for eye drug delivery and, recently, a broad spectrum of diverse nanoparticles continue to emerge with special characteristics representing ideal candidates for eye drug delivery.

Multifarious Biologic Loaded Liposomes that Stimulate the Mammalian Target of Rapamycin Signaling Pathway Show Retina Neuroprotection after Retina Damage

A common event in optic neuropathies is the loss of axons and death of retinal ganglion cells (RGCs) resulting in irreversible blindness. Mammalian target of rapamycin (mTOR) signaling pathway agonists have been shown to foster axon regeneration and RGC survival in animal models of optic nerve damage. However, many challenges remain in developing therapies that exploit cell growth and tissue remodeling including (i) activating/inhibiting cell pathways synergistically, (ii) avoiding tumorigenesis, and (iii) ensuring appropriate physiological tissue function. These challenges are further exacerbated by the need to overcome ocular physiological barriers and clearance mechanisms. Here we present liposomes loaded with multiple mTOR pathway stimulating biologics designed to enhance neuroprotection after retina damage. Liposomes were loaded with ciliary neurotrophic factor, insulin-like growth factor 1, a lipopeptide N-fragment osteopontin mimic, and lipopeptide phosphatase tension homologue inhibitors for either the ATP domain or the c-terminal tail. In a mouse model of N-methyl-d-aspartic acid induced RGC death, a single intravitreal administration of liposomes reduced both RGC death and loss of retina electrophysiological function. Furthermore, combining liposomes with transplantation of induced pluripotent stem cell derived RGCs led to an improved electrophysiological outcome in mice. The results presented here show that liposomes carrying multiple signaling pathway modulators can facilitate neuroprotection and transplant electrophysiological outcome.

Characterization and Pharmacokinetics of Triamcinolone Acetonide-Loaded Liposomes Topical Formulations for Vitreoretinal Drug Delivery

PURPOSE

To achieve a safer alternative to intravitreal injection of corticosteroids, we developed and characterized triamcinolone acetonide-loaded liposomes formulations (TA-LFs) to be used topically for vitreoretinal drug delivery.

METHODS

Four different 0.2% TA-LFs (TA-LF1 to TA-LF4) were generated and submitted to physicochemical characterization. Posteriorly, an ex vivo diffusion assay was performed using rabbit corneas as membranes. Finally, concentrations of triamcinolone acetonide (TA) were determined by high-performance liquid chromatography in ocular tissues from New Zealand white rabbits after multiple topical doses of TA-LF2 (6 times per day, 14 days). In addition, toxicity and tolerability of TA-LF2 was evaluated by cell viability assay and eye examination of study animals, respectively.

RESULTS

TA-LF2 was the most stable formulation maintaining a stable hidrogenion potential (pH) at 30 and 40°C and even improving encapsulation with higher temperature. TA-LF2 and TA-LF3 presented the best diffusion performance in vitro reaching the highest TA concentrations after 8 h of follow-up. In vivo diffusion and pharmacokinetics analysis showed that concentrations of TA in retina and vitreous reached the highest peak at 12 h after topical administration of TA-LF2 (252.10 ± 90.00 ng/g and 32.6 ± 10.27 ng/g, respectively) and subsequently decline to 24.0 ± 11.72 ng/g and 19.5 ± 13.14 ng/g, respectively, at 14 days of follow-up. Finally, cell viability was unaffected by TA-LF2, and no increase in intraocular pressure nor ocular alterations were observed after topical administration of this formulation in rabbits.

CONCLUSION

TA-loaded liposomes, administered topically, can deliver TA in the vitreous cavity and reach the retina efficiently.

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.

Intravitreal clearance and volume of distribution of compounds in rabbits: In silico prediction and pharmacokinetic simulations for drug development

The aims of this research were to (1) create a curated universal database of intravitreal volumes of distribution (Vss, ivt) and clearances (CL ivt) of small molecular weight compounds and macromolecules and (2) to develop quantitative structure property relationship (QSPR) and pharmacokinetic models for the estimation of vitreal drug concentrations based on the compound structure. Vss, ivt and CL ivt values were determined from the available literature on intravitreal drug administration using compartmental models and curve fitting. A simple QSPR model for CL ivt of small molecular weight compounds was obtained with two descriptors: Log D7.4 and hydrogen bond donor capacity. The model predicted the internal and external test sets reliably with a mean fold error of 1.50 and 1.33, respectively (Q(2)Y=0.62). For 80% of the compounds the Vss, ivt was 1.18-2.28 ml; too narrow range for QSPR model building. Integration of the estimated Vss, ivt and predicted CL ivt parameters into pharmacokinetic simulation models allows prediction of vitreous drug concentrations after intravitreal administration. The present work presents for the first time a database of CL ivt and Vss, ivt values and the dependence of the CL ivt values on the molecular structure. The study provides also useful in silico tools to investigate a priori the intravitreal pharmacokinetic profiles for intravitreally injected candidate compounds and drug delivery systems.

Nanotherapy for posterior eye diseases

It is assumed that more than 50% of the most enfeebling ocular diseases have their origin in the posterior segment. Furthermore, most of these diseases lead to partial or complete blindness, if left untreated. After cancer, blindness is the second most dreaded disease world over. However, treatment of posterior eye diseases is more challenging than the anterior segment ailments due to a series of anatomical barriers and physiological constraints confronted for delivery to this segment. In this regard, nanostructured drug delivery systems are proposed to defy ocular barriers, target retina, and act as permeation enhancers in addition to providing a controlled release. Since an important step towards developing effective treatment strategies is to understand the course or a route a drug molecule needs to follow to reach the target site, the first part of the present review discusses various pathways available for effective delivery to and clearance from the posterior eye. Promise held by nanocarrier systems, viz. liposomes, nanoparticles, and nanoemulsion, for effective delivery and selective targeting is also discussed with illustrative examples, tables, and flowcharts. However, the applicability of these nanocarrier systems as self-administration ocular drops is still an unrealized dream which is in itself a huge technological challenge.

Nanocarriers of nanotechnology in retinal diseases

We are approaching a new era of retinal pharmacotherapy where new drugs are rapidly being worked out for the treatment of posterior-segment disease. Recent development in ocular drug delivery systems research has provided new insights into drug development, and the use of nanoparticles for drug delivery is thus a promising excellent approach for advanced therapy of ocular diseases. The primary goal is to develop a variety of drug delivery systems to complement and further enhance the efficacy of the available new medications. The ideal sustained release technology will provide a high level of safety with continuous release over an extended period of time while maintaining almost total drug bioactivity. The use of nanocarriers, such as cyclodextrin nanoparticle suspension, liposomes, nanospheres and, nanoemulsions for gene therapy of retinal diseases has been highlighted in this review.

Drug delivery implants in the treatment of vitreous inflammation

The eye is a model organ for the local delivery of therapeutics. This proves beneficial when treating vitreous inflammation and other ophthalmic pathologies. The chronicity of certain diseases, however, limits the effectiveness of locally administered drugs. To maintain such treatments often requires frequent office visits and can result in increased risk of infection and toxicity to the patient. This paper focuses on the implantable devices and particulate drug delivery systems that are currently being implemented and investigated to overcome these challenges. Implants currently on the market or undergoing clinical trials include those made of nonbiodegradable polymers, containing ganciclovir, fluocinolone acetonide, triamcinolone acetonide, and ranibizumab, and biodegradable polymers, containing dexamethasone, triamcinolone acetonide, and ranibizumab. Investigational intravitreal implants and particulate drug delivery systems, such as nanoparticles, microparticles, and liposomes, are also explored in this review article.

Liposomes and nanotechnology in drug development: focus on ocular targets

Poor drug delivery to lesions in patients’ eyes is a major obstacle to the treatment of ocular diseases. The accessibility of these areas to drugs is highly restricted by the presence of barriers, including the corneal barrier, aqueous barrier, and the inner and outer blood–retinal barriers. In particular, the posterior segment is difficult to reach for drugs because of its structural peculiarities. This review discusses various barriers to drug delivery and provides comprehensive information for designing nanoparticle-mediated drug delivery systems for the treatment of ocular diseases. Nanoparticles can be designed to improve penetration, controlled release, and drug targeting. As highlighted in this review, the therapeutic efficacy of drugs in ocular diseases has been reported to be enhanced by the use of nanoparticles such as liposomes, micro/nanospheres, microemulsions, and dendrimers. Our recent data show that intravitreal injection of targeted liposomes encapsulating an angiogenesis inhibitor caused significantly greater suppression of choroidal neovascularization than did the injection of free drug. Recent progress in ocular drug delivery systems research has provided new insights into drug development, and the use of nanoparticles for drug delivery is thus a promising approach for advanced therapy of ocular diseases.

Liposomes for intravitreal drug delivery: a state of the art

Intravitreal administration of drugs has raised a large interest during the last two decades improving the treatment of infectious diseases of the posterior segment of the eye or edematous maculopathies. This route of administration allows achieving high drug concentrations in the vitreous and avoiding adverse effects resulting from systemic administration. However, many drugs are rapidly cleared from the vitreous humor; therefore, to reach and to maintain effective therapy, repeated administrations are necessary. Unfortunately, frequent intravitreal injections increase the risk of endophthalmitis, damage to lens, retinal detachment. Moreover, some drugs provoke a local toxicity at their effective dose inducing side-effects and possible retinal lesions. This is the reason why new drug delivery systems, among which liposomes, have been developed to improve the intravitreal administration of drugs. Liposomes can reduce the toxicity and increase the residence time of several active molecules in the eye. In vivo, they can protect poorly-stable drugs such as peptides and nucleic acids from degradation. Successful reports have shown their potential for improving the treatment of retinitis induced by cytomegalovirus in human and more recently for the treatment of uveitis in rats. Moreover, recent preliminary studies about the trafficking of liposomes in ocular tissues and fluids following intravitreal injection attempted to elucidate their fate. All the data discussed in this review support the large interest raised by these colloidal carriers for intravitreal drug delivery.

[Potential of liposomes for the intravitreal injection of therapeutic molecules]

Intravitreal administration has been widely used since 20 years and has been shown to improve the treatment of diseases of the posterior segment of the eye with infectious origin or in edematous maculopathies. This route of administration allows to achieve high concentration of drug in the vitreous and avoids the problems resulting from systemic administration. However, two basic problems limit the use of intravitreal therapy. Many drugs are rapidly cleared from the vitreous humor; therefore, to reach and to maintain effective therapy repeated injections are necessary. Repeated intravitreal injections increase the risk of endophthalmitis, damage to lens, retinal detachment. Moreover, some drugs provoke a local toxicity at their effective dose inducing side-effects and possible retinal lesions. In this context, the development and the use of new drug delivery systems for intravitreal administration are necessary to treat chronic ocular diseases. Among them, particulate systems such as liposomes have been widely studied. Liposomes are easily injectable and permit to reduce the toxicity and to increase the residence time of several drugs in the eye. They are also able to protect in vivo poorly-stable molecules from degradation such as peptides and nucleic acids. Some promising results have been obtained for the treatment of retinitis induced by cytomegalovirus in human and more recently for the treatment of uveitis in animal. Finally, the fate of liposomes in ocular tissues and fluids after their injection into the vitreous and their elimination routes begin to be more known.

Novel approaches to retinal drug delivery

Research into treatment modalities affecting vision is rapidly progressing due to the high incidence of diseases such as diabetic macular edema, proliferative vitreoretinopathy, wet and dry age-related macular degeneration and cytomegalovirus retinitis. The unique anatomy and physiology of eye offers many challenges to developing effective retinal drug delivery systems. Historically, drugs have been administered to the eye as liquid drops instilled in the cul-de-sac. However retinal drug delivery is a challenging area. The transport of molecules between the vitreous/retina and systemic circulation is restricted by the blood-retinal barrier, which is made up of retinal pigment epithelium and endothelial cells of the retinal blood vessels. An increase in the understanding of drug absorption mechanisms into the retina from local and systemic administration has led to the development of various drug delivery systems, such as biodegradable and non-biodegradable implants, microspheres, nanoparticles and liposomes, gels and transporter-targeted prodrugs. Such diversity in approaches is an indication that there is still a need for an optimized noninvasive or minimally invasive drug delivery system to the eye. A number of large molecular weight compounds (i.e., oligonucleotides, RNA aptamers, peptides and monoclonal antibodies) have been and continue to be introduced as new therapeutic entities. However, for high molecular weight polar compounds the mechanism of epithelial transport is primarily through the tight junctions in the retinal pigment epithelium, as these agents undergo limited transcellular diffusion. Delivery and administration of these new drugs in a safe and effective manner is still a major challenge facing pharmaceutical scientists. In this review article, the authors discuss various drug delivery strategies, devices and challenges associated with drug delivery to the retina.

Scleral permeability of a small, single-stranded oligonucleotide

Developing more effective ocular drug delivery systems is essential to improving the treatment of posterior segment eye disease. The large target area provided by the sclera and potentially less vision threatening complications are advantages of transscleral administration compared to more traditional modalities of drug delivery to the posterior segment. We aimed to determine the permeability coefficient for the in vitro diffusion of a small, single-stranded, oligonucleotide across human sclera. Transscleral permeability was measured by placing 100 microL of 2.96 x 10(-4) mol single-stranded, fluorescein-labeled oligonucleotide (MW = 7998.3) on the episcleral surface of sclera mounted in a perfusion chamber. Fractions of choroidal perfusate were collected hourly for 24 hours. The permeability constant or K(trans) for the transscleral diffusion of the naked, single-stranded, fluorescein-labeled oligonucleotide was 7.67 +/- 1.8 x 10(-7) cm/s (mean +/- SEM, N = 7). The permeability constant or K(trans) after intrascleral injection of the same fluorescein-labeled oligonucleotide was 1.32 +/- 0.42 x 10(-7) (mean +/- SEM, N = 4). This analysis demonstrates that diffusion of a naked, 24-base, single-stranded, fluorescein-labeled oligonucleotide can be accomplished by both of the described methods. The ability to deliver single-stranded oligonucleotides across the sclera may prove to be advantageous given the development of several novel therapeutic strategies that use similar molecules.

Versatility of aminoglycosides and prospects for their future

Aminoglycoside antibiotics have had a major impact on our ability to treat bacterial infections for the past half century. Whereas the interest in these versatile antibiotics continues to be high, their clinical utility has been compromised by widespread instances of resistance. The multitude of mechanisms of resistance is disconcerting but also illuminates how nature can manifest resistance when bacteria are confronted by antibiotics. This article reviews the most recent knowledge about the mechanisms of aminoglycoside action and the mechanisms of resistance to these antibiotics.

Intravitreal administration of antisense oligonucleotides: potential of liposomal delivery

Antisense oligonucleotides are short synthetic fragments of genes that are able to inhibit gene expression after being internalized by cells. They can therefore be used as antiviral compounds particularly, for the treatment of ocular viral infections (i.e. Herpes simplex virus or Cytomegalovirus, CMV). Antisense oligonucleotides are however poorly stable in biological fluids and their intracellular penetration is limited. Although oligonucleotides are now currently used in therapeutics for the treatment of CMV by intravitreal injection (Vitravene) their main drawbacks impose to repeat the number of administrations which can be very harmful and damaging. A system that is able to permit a protection of oligonucleotides against degradation and their slow delivery into the vitreous would be more favorable for improving patient compliance. The use of liposomes for intravitreal administration can be very promising since these lipid vesicles are able to protect oligonucleotides against degradation by nucleases and they allow to increase the retention time of many drugs in the vitreous. In this review, the potentialities of liposomes for the intravitreal delivery of oligonucleotides will be discussed.

Targeted delivery of antibiotics using liposomes and nanoparticles: research and applications

This review examines current technologies for increasing the bioavailability of antibiotics by means of liposomes or nanoparticles. The main focus is on liposomes. These carriers were preferentially developed because their composition is compatible with biological constituents. Biodegradable polymers in the form of colloidal particles have also been used and show promise for future applications in antimicrobial chemotherapy. The in vivo behaviour of both types of carriers and consequently their therapeutic potential, are determined by their route of administration. Conventional carrier strategies permit the mononuclear phagocyte system to be targeted by intravenous injection of antibiotics. Stealthy strategies avoid major uptake by these cells and extend the systemic presence of these carriers. The purpose of this review is to provide background information in antibiotic targeting gathered from papers published over the last twenty years. It seems clear that such drug carriers (liposomes, nanoparticles) allow increased drug concentration at infected sites but reduce drug toxicity.

Retinal toxicity of liposome-incorporated and free ofloxacin after intravitreal injection in rabbit eyes

BACKGROUND

Ofloxacin (OFLX) is a fluoroquinolone-antibiotic with a broad antimicrobial spectrum that may have a potential role in the treatment of bacterial endophthalmitis. However, its elimination half life after intravitreal injection is short. To prolong the intravitreal antibacterial level OFLX was incorporated into liposomes. This study was performed to investigate the retinal toxicity of liposome-incorporated and free OFLX.

MATERIALS AND METHODS

OFLX was incorporated into multilamellar large vesicles. 0.1 ml of this suspension (= 180.2 microg OFLX) was injected into the midvitreous of rabbit eyes (n = 6). Free OFLX in doses of 100 microg, 500 microg and 1,000 microg was injected into the midvitreous of a second group of rabbit eyes (n = 18). The other eye served as a control and received empty liposomes or normal saline solution, respectively. Before injection and at the end of follow-up an ERG was obtained. After a follow-up of 1 day, 14 and 28 days the animals were perfused with glutaraldehyde and the eyes were examined by light- and transmission electron microscopy.

RESULTS

The ERG as well as the histologic studies did not reveal any pathological changes after injection of liposome-incorporated OFLX compared to the control eyes. Significant reduction of the ERG was observed after 500 microg free OFLX in 2 out of 6 eyes after 1 and 14 days, respectively, and in 2 eyes 1 day after 1,000 microg free OFLX. Three days after injection of 1,000 microg OFLX the retina showed focal destruction in 1 out of 6 eyes. In another eye with the same dose 14 days after injection the photoreceptor outer segments showed disorganisation.

CONCLUSION

This study shows that liposome-incorporated OFLX did not have any retinal toxicity in this animal model. Free OFLX appears to have no retinal toxicity in rabbit eyes at a dose of 100 microg after intravitreal injection. Injection of higher doses resulted in ERG changes and marked retinal damage.

Preparation of liposome-encapsulating adenosine triphosphate

Liposomes encapsulating adenosine triphosphate (ATP) were prepared by sonication, and the liposomes were evaluated for use in a drug delivery system. The liposomes, which were composed of phosphatidylcholine and cholesterol, were about 1.1 microm in size, as observed under a microscope. From their size, the vesicles were thought to be multilamellar. The maximum concentration of ATP in the liposomes was 1.0 mM, when the initial concentrations of lipid and ATP were 20 mM and 300 mM, respectively. The maximum entrapment ratio of ATP in the liposomes was 88%, when the initial concentrations of lipid and ATP were 20 mM and 500 mM, respectively. About 4% of ATP was encapsulated in these experiments. When liposomes contained 4-7% of cholesterol, about 35% of encapsulated ATP was released from the liposomes for 90 hours at 37 degrees C in vitro. These findings indicated that liposomes encapsulating ATP could be used for the treatment of ischemic retina.

Liposomes as delivery systems in the prevention and treatment of infectious diseases

Research on the potential application of liposomes in the prevention and treatment of infectious diseases has focussed on improvement of the therapeutic index of antimicrobial drugs and immunomodulators and on stimulation of the immune response to otherwise weak antigens in vaccines composed of purified micro-organism subunits. In this review current approaches in this field are outlined. The improved therapeutic index of antimicrobial drugs after encapsulation in liposomes is a result of enhanced drug delivery to infected tissue or infected cells and/or a reduction of drug toxicity of potentially toxic antibiotics. Liposomal encapsulation of immunomodulators that activate macrophages aims at reducing the toxicity of these agents and targeting them to the cells of the mononuclear phagocyte system in order to increase the nonspecific resistance of the host against infections. Studies on the immunogenicity of liposomal antigens have demonstrated that liposomes can potentiate the humoral and cell mediated immunity to a variety of antigens.