The role of blood–ocular barrier transporters in retinal drug disposition: an overview

Application of iontophoresis in ophthalmic practice: an innovative strategy to deliver drugs into the eye

Delivery of drugs to special locations of ocular lesions, while minimizing systemic and local toxic effects, is recognized as a critical challenge in the ophthalmic practice. The special anatomy and physiology barriers within the eyeball entail effective drug delivery systems. Emerging attempts in drug delivery has led to developments in ocular iontophoresis, which acts as a noninvasive technology to enhance drug penetration using a small electric current. This technique offers greater flexibility to deliver desired drug dose in a controlled and tolerable manner. In previous studies, this technique has been testified to deliver antibiotics, corticoid, proteins and other gene drugs into the eye with the potency of treating or alleviating diverse ophthalmological diseases including uveitis, cataract, retinoblastoma, herpes simplex and cytomegalovirus retinitis (CMVR), etc. In this review, we will introduce the recent developments in iontophoresis device. We also summarize the latest progress in coulomb controlled iontophoresis (CCI), hydrogel ionic circuit (HIC) and EyeGate II delivery system (EGDS), as well as overview the potential toxicity of iontophoresis. We will discuss these factors that affect the efficacy of iontophoresis experiments, and focus on the latest progress in its clinical application in the treatment of eye diseases.

Polymorphic nanobody crystals as long-acting intravitreal therapy for wet age-related macular degeneration

Wet age-related macular degeneration (wet AMD) is the most common cause of blindness, and chronic intravitreal injection of anti-vascular endothelial growth factor (VEGF) proteins has been the dominant therapeutic approach. Less intravitreal injection and a prolonged inter-injection interval are the main drivers behind new wet AMD drug innovations. By rationally engineering the surface residues of a model anti-VEGF nanobody, we obtained a series of anti-VEGF nanobodies with identical protein structures and VEGF binding affinities, while drastically different crystallization propensities and crystal lattice structures. Among these nanobody crystals, the P212121 lattice appeared to be denser and released protein slower than the P1 lattice, while nanobody crystals embedding zinc coordination further slowed the protein release rate. The polymorphic protein crystals could be a potentially breakthrough strategy for chronic intravitreal administration of anti-VEGF proteins.

Ocular barriers as a double-edged sword: preventing and facilitating drug delivery to the retina

In recent decades, the growing of the aging population in the world brings increasingly heavy burden of vision-threatening retinal diseases. One of the biggest challenges in the treatment of retinal diseases is the effective drug delivery to the diseased area. Due to the existence of multiple anatomical and physiological barriers of the eye, commonly used oral drugs or topical eye drops cannot effectively reach the retinal lesions. Innovations in new drug formulations and delivery routes have been continuously applied to improve current drug delivery to the back of the eye. Unique ocular anatomical structures or physiological activities on these ocular barriers, in turn, can facilitate drug delivery to the retina if compatible formulations or delivery routes are properly designed or selected. This paper focuses on key barrier structures of the eye and summarizes advances of corresponding drug delivery means to the retina, including various local drug delivery routes by invasive approaches, as well as systemic eye drug delivery by non-invasive approaches.

Targeting Cell Membranes, Depleting ROS by Dithiane and Thioketal-Containing Polymers with Pendant Cholesterols Delivering Necrostatin-1 for Glaucoma Treatment

Glaucoma is the leading cause of irreversible blindness worldwide, characterized by progressive vision loss due to the selective damage to retinal ganglion cells (RGCs) and their axons. Oxidative stress is generally believed as one key factor of RGCs death. Recently, necroptosis was identified to play a key role in glaucomatous injury. Therefore, depletion of reactive oxygen species (ROS) and inhibition of necroptosis in RGCs has become one of treatment strategies for glaucoma. However, existing drugs without efficient drug enter into the retina and have controlled release due to a short drug retention. Herein, we designed a glaucomatous microenvironment-responsive drug carrier polymer, which is characterized by the presence of thioketal bonds and 1,4-dithiane unit in the main chain for depleting ROS as well as the pendant cholesterols for targeting cell membranes. This polymer was adopted to encapsulate an inhibitor of necroptosis, necrostatin-1, into nanoparticles (designated as NP1). NP1 with superior biosafety could scavenge ROS in RGCs both in vitro and in vivo of an acute pathological glaucomatous injury model. Further, NP1 was found to effectively inhibit the upregulation of the necroptosis pathway, reducing the death of RGCs. The findings in this study exemplified the use of nanomaterials as potential strategies to treat glaucoma.

Advances in biomaterials for the treatment of retinoblastoma

Retinoblastoma is the most common primary intraocular malignancy in children. Although traditional chemotherapy has shown some success in retinoblastoma management, there are several shortcomings to this approach, including inadequate pharmacokinetic parameters, multidrug resistance, low therapeutic efficiency, nonspecific targeting, and the need for adjuvant therapy, among others. The revolutionary developments in biomaterials for drug delivery have enabled breakthroughs in cancer management. Today, biomaterials are playing a crucial role in developing more efficacious retinoblastoma treatments. The key goal in the evolution of drug delivery biomaterials for retinoblastoma therapy is to resolve delivery-associated obstacles and lower nonlocal exposure while ameliorating certain adverse effects. In this review, we will first delve into the historical perspective of retinoblastoma with a focus on the classical treatments currently used in clinics to enhance patients' quality of life and survival rate. As we move along, we will discuss biomaterials for drug delivery applications. Various aspects of biomaterials for drug delivery will be dissected, including their features and recent advances. In accordance with the current advances in biomaterials, we will deliver a synopsis on the novel chemotherapeutic drug delivery strategies and evaluate these approaches to gain new insights into retinoblastoma treatment.

Bibliometric and visualized analysis of ocular drug delivery from 2001 to 2020

OBJECTIVE

To perform a bibliometric analysis in the field of ocular drug delivery research to characterize the current international trends and to present visual representations of the past and emerging trends on ocular drug delivery research over the past decade.

METHOD

In this cross-sectional study, a bibliometric analysis of data retrieved and extracted from the Web of Science Core Collection (WoSCC) database was performed to analyze evolution and theme trends on ocular drug delivery research from January 1, 2001, to December 31, 2020. A total of 4334 articles on ocular drug delivery were evaluated for specific characteristics, such as publication year, journals, authors, institutions, countries/regions, references, and keywords. Co-authorship analysis, co-occurrence analysis, co-citation analysis, and network visualization were constructed by VOSviewer. Some important subtopics identified by bibliometric characterization were further discussed and reviewed.

RESULTS

From 2001 to 2020, the annual global publications increased by 746.15%, from 52 to 440. International Journal of Pharmaceutics published the most manuscripts (250 publications) and produced the highest citations (9509 citations), followed by Investigative Ophthalmology & Visual Science (202 publications) and Journal of Ocular Pharmacology and Therapeutics (136 publications). The United States (1289 publications, 31,512 citations), the University of Florida (82 publications, 2986 citations), and Chauhan, Anuj (52 publications, 2354 citations) were the most productive and impactful institution, country, and author respectively. The co-occurrence cluster analysis of the top 100 keywords form five clusters: (1) micro/nano ocular drug delivery systems; (2) the treatment of inflammation and posterior diseases; (3) macroscopic ocular drug delivery systems/devices; (4) the characteristics of drug delivery systems; (5) and the ocular drug delivery for glaucoma treatment. Diabetic macular edema, anti-VEGF, ranibizumab, bevacizumab, micelles and latanoprost, were the latest high-frequency keywords, indicating the emerging frontiers of ocular drug delivery. Further discussions into the subtopics were provided to assist researchers to determine the range of research topics and plan research direction.

CONCLUSIONS

Over the last two decades there has been a progressive increase in the number of publications and citations on research related to ocular drug delivery across many countries, institutions, and authors. The present study sheds light on current trends, global collaboration patterns, basic knowledge, research hotspots, and emerging frontiers of ocular drug delivery. Novel solutions for ocular drug delivery and the treatment of inflammation and posterior diseases were the major themes over the last 20 years.

Stimuli-Responsive Polymers for Transdermal, Transmucosal and Ocular Drug Delivery

Despite their conventional and widespread use, oral and intravenous routes of drug administration face several limitations. In particular, orally administered drugs undergo enzymatic degradation in the gastrointestinal tract and first-pass metabolism in the liver, which tend to decrease their bioavailability. Intravenous infusions of medications are invasive, painful and stressful for patients and carry the risk of infections, tissue damage and other adverse reactions. In order to account for these disadvantages, alternative routes of drug delivery, such as transdermal, nasal, oromucosal, ocular and others, have been considered. Moreover, drug formulations have been modified in order to improve their storage stability, solubility, absorption and safety. Recently, stimuli-responsive polymers have been shown to achieve controlled release and enhance the bioavailability of multiple drugs. In this review, we discuss the most up-to-date use of stimuli-responsive materials in order to optimize the delivery of medications that are unstable to pH or undergo primary metabolism via transdermal, nasal, oromucosal and ocular routes. Release kinetics, diffusion parameters and permeation rate of the drug via the mucosa or skin are discussed as well.

Exploring the systemic delivery of a poorly water-soluble model drug to the retina using PLGA nanoparticles

During the drug development process, many pharmacologically active compounds are discarded because of poor water solubility, but nanoparticle-based formulations are increasingly proposed as a solution for this problem. We therefore studied the distribution of nanoparticulate carriers and the delivery of their poorly water-soluble cargo to a structure of the central nervous system, the retina, under naive and pathological conditions. The lipophilic fluorescent dye coumarin 6 (Cou6) was encapsulated into poly(lactic-co-glycolic acid) PLGA nanoparticles (NPs). After intravenous administration in rats, we analyzed the distribution of cargo Cou6 and of the NP carrier covalently labeled with Cy5.5 in healthy animals and animals with optic nerve crush (ONC). In vivo real-time retina imaging revealed that Cou6 was rapidly released from PLGA NPs and penetrated the inner blood-retina barrier (BRB) within 15 min and PLGA NPs were gradually eliminated from the retinal blood circulation. Ex vivo microscopy of retinal flat mounts indicated that the Cou6 accumulated predominantly in the extracellular space and to a lesser extent in neurons. While the distribution of Cou6 in healthy animals and post ONC was comparable at early time point post-operation, the elimination of the NPs from the vessels was faster on day 7 post ONC. These results demonstrate the importance of considering different kinetics of nano-carrier and poorly water-soluble cargo, emphasizing the critical role of their parenchymal distribution, i.e. cellular/extracellular, and function of different physiological and pathological conditions.

Extranodal NK/T-Cell Lymphoma Occurring Primarily in the Eyes and with Central Nervous System Relapse

A 56-year-old man complaining of blurred vision was referred to the ophthalmologist. He was initially diagnosed with uveitis and showed no response to steroid treatment. Diagnostic vitrectomy and skin lesion biopsy performed later confirmed the diagnosis of extranodal NK/T cell lymphoma (ENKTL), nasal type. Despite sufficient systemic chemotherapy, the patient developed central nervous system (CNS) relapse 3 months after achieving systemic remission. The diagnosis and treatment of this patient required multidisciplinary teamwork between ophthalmologists, pathologists, and hematologists. This report focused on the diagnostic planning and optimal treatment strategy for this patient.

Therapeutic nanoemulsions in ophthalmic drug administration: Concept in formulations and characterization techniques for ocular drug delivery

The eye is the specialized part of the body and is comprised of numerous physiological ocular barriers that limit the drug absorption at the action site. Regardless of various efforts, efficient topical ophthalmic drug delivery remains unsolved, and thus, it is extremely necessary to advance the contemporary treatments of ocular disorders affecting the anterior and posterior cavities. Nowadays, the advent of nanotechnology-based multicomponent nanoemulsions for ophthalmic drug delivery has gained popularity due to the enhancement of ocular penetrability, improve bioavailability, increase solubility, and stability of lipophilic drugs. Nanoemulsions offer the sustained/controlled drug release and increase residence time which depend on viscosity, compositions, and stabilization process, etc.; hence, decrease the instillation frequency and improve patient compliance. Further, due to the nanosized of nanoemulsions, the sterilization process is easy as conventional solutions and cause no blur vision. The review aims to summarizes the various ocular barriers, manufacturing techniques, possible mechanisms to the retention and deep penetration into the eye, and appropriate excipients with their under-lying selection principles to prevent destabilization of nanoemulsions. This review also discusses the characterization parameters of ocular drug delivery to spike the interest of those contemplating a foray in this field. Here, in short, nanoemulsions are abridged with concepts to design clinically advantageous ocular drug delivery.

Efficacy of systemic administration of riboflavin on a rabbit model of corneal alkali burn

Changes in the barrier mechanisms in the eye should determine the rational route for the administration and dosage of each drug in the treatment of traumatic injuries and other pathologies. The aim of this study was to examine the efficacy of intra-arterial delivery of 14C-riboflavin (as an "indicator") and compare it with intravenous and intramuscular administration in an animal model of chemical eye burn. 14C-riboflavin (14C-I) was administered by intra-arterial (carotid artery), intravenous (femoral vein) and intramuscular (femoral muscle) routes. The total radioactivity was determined over 2 h in the plasma and structures of the rabbit's eyes using a scintillation counter. The results of the study show that intravascular administration of 14C-I gives significantly higher concentrations of total radioactivity in the blood and is accompanied by a significant increase in the permeability of the blood-barrier and barrier in eyes suffering from burns. The highest concentration in the plasma and aqueous humour of the anterior chamber of the eye was observed during the first hour with the intra-arterial route of administration of 14C-I in either burnt and unburnt eyes. The distribution of total radioactivity in the structures of the eye over the 2 h of the experiment showed a higher level of the drug under intra-arterial administered in the uveal regions, namely: the iris, ciliary body, choroid, retina and also the sclera and cornea. This experimental model shows that intra-arterial administration can increase the bioavailability of a drug to the structures of the eye within a short period of time.

Intractable Ocular Diseases and Treatment Progress

In recent years, with the aging of the population and the frequent use of electronic devices, many eye diseases have shown a linear upward trend, such as dry eye disease, glaucoma, cataract, age-related macular degeneration, and diabetic retinopathy. These diseases are often chronic and difficult to cure. Based on the structure and barrier of the human eye, this review describes the pathogenesis and treatments of several intractable eye diseases and summarizes the advanced ocular drug delivery systems to provide new treatment ideas for these diseases. Finally, we also look forward to the prospect of RNAi therapy in the treatment of eye diseases.

Membrane transporter data to support kinetically-informed chemical risk assessment using non-animal methods: Scientific and regulatory perspectives

Humans are continuously exposed to low levels of thousands of industrial chemicals, most of which are poorly characterised in terms of their potential toxicity. The new paradigm in chemical risk assessment (CRA) aims to rely on animal-free testing, with kinetics being a key determinant of toxicity when moving from traditional animal studies to integrated in vitro-in silico approaches. In a kinetically informed CRA, membrane transporters, which have been intensively studied during drug development, are an essential piece of information. However, how existing knowledge on transporters gained in the drug field can be applied to CRA is not yet fully understood. This review outlines the opportunities, challenges and existing tools for investigating chemical-transporter interactions in kinetically informed CRA without animal studies. Various environmental chemicals acting as substrates, inhibitors or modulators of transporter activity or expression have been shown to impact TK, just as drugs do. However, because pollutant concentrations are often lower in humans than drugs and because exposure levels and internal chemical doses are not usually known in contrast to drugs, new approaches are required to translate transporter data and reasoning from the drug sector to CRA. Here, the generation of in vitro chemical-transporter interaction data and the development of transporter databases and classification systems trained on chemical datasets (and not only drugs) are proposed. Furtheremore, improving the use of human biomonitoring data to evaluate the in vitro-in silico transporter-related predicted values and developing means to assess uncertainties could also lead to increase confidence of scientists and regulators in animal-free CRA. Finally, a systematic characterisation of the transportome (quantitative monitoring of transporter abundance, activity and maintenance over time) would reinforce confidence in the use of experimental transporter/barrier systems as well as in established cell-based toxicological assays currently used for CRA.

A multidimensional concept for mercury neuronal and sensory toxicity in fish - From toxicokinetics and biochemistry to morphometry and behavior

BACKGROUND

Neuronal and sensory toxicity of mercury (Hg) compounds has been largely investigated in humans/mammals with a focus on public health, while research in fish is less prolific and dispersed by different species. Well-established premises for mammals have been governing fish research, but some contradictory findings suggest that knowledge translation between these animal groups needs prudence [e.g. the relative higher neurotoxicity of methylmercury (MeHg) vs. inorganic Hg (iHg)]. Biochemical/physiological differences between the groups (e.g. higher brain regeneration in fish) may determine distinct patterns. This review undertakes the challenge of identifying sensitive cellular targets, Hg-driven biochemical/physiological vulnerabilities in fish, while discriminating specificities for Hg forms.

SCOPE OF REVIEW

A functional neuroanatomical perspective was conceived, comprising: (i) Hg occurrence in the aquatic environment; (ii) toxicokinetics on central nervous system (CNS)/sensory organs; (iii) effects on neurotransmission; (iv) biochemical/physiological effects on CNS/sensory organs; (v) morpho-structural changes on CNS/sensory organs; (vi) behavioral effects. The literature was also analyzed to generate a multidimensional conceptualization translated into a Rubik's Cube where key factors/processes were proposed.

MAJOR CONCLUSIONS

Hg neurosensory toxicity was unequivocally demonstrated. Some correspondence with toxicity mechanisms described for mammals (mainly at biochemical level) was identified. Although the research has been dispersed by numerous fish species, 29 key factors/processes were pinpointed.

GENERAL SIGNIFICANCE

Future trends were identified and translated into 25 factors/processes to be addressed. Unveiling the neurosensory toxicity of Hg in fish has a major motivation of protecting ichtyopopulations and ecosystems, but can also provide fundamental knowledge to the field of human neurodevelopment.

Ocular Drug Delivery: Role of Degradable Polymeric Nanocarriers for Ophthalmic Application

Ocular drug delivery has been a major challenge for clinical pharmacologists and biomaterial scientists due to intricate and unique anatomical and physiological barriers in the eye. The critical requirement varies from anterior and posterior ocular segments from a drug delivery perspective. Recently, many new drugs with special formulations have been introduced for targeted delivery with modified methods and routes of drug administration to improve drug delivery efficacy. Current developments in nanoformulations of drug carrier systems have become a promising attribute to enhance drug retention/permeation and prolong drug release in ocular tissue. Biodegradable polymers have been explored as the base polymers to prepare nanocarriers for encasing existing drugs to enhance the therapeutic effect with better tissue adherence, prolonged drug action, improved bioavailability, decreased toxicity, and targeted delivery in eye. In this review, we summarized recent studies on sustained ocular drug/gene delivery and emphasized on the nanocarriers made by biodegradable polymers such as liposome, poly lactic-co-glycolic acid (PLGA), chitosan, and gelatin. Moreover, we discussed the bio-distribution of these nanocarriers in the ocular tissue and their therapeutic applications in various ocular diseases.

Recent advances in drug and nutrient transport across the blood-retinal barrier

INTRODUCTION

The blood-retinal barrier (BRB) is the barrier separating the blood and neural retina, and transport systems for low-weight molecules at the BRB are expected to be useful for developing drugs for the treatment of ocular neural disorders and maintaining a healthy retina. Areas covered: This review discusses blood-to-retina and retina-to-blood transport of drugs and nutrients at the BRB. In particular, P-gp (ABCB1/MDR1) has low impact on the transport of cationic drugs at the BRB, suggesting a significant role of novel organic cation transporters in influx and efflux transport of lipophilic cationic drugs between blood and the retina. The transport of pravastatin at the BRB involves transporters including organic anion transporting polypeptide 1a4 (Oatp1a4). Recent studies have shown the involvement of solute carrier transporters in the blood-to-retina transport of nutrients including riboflavin, L-ornithine, β-alanine, and L-histidine, implying that dipeptide transport at the BRB is minimal. Expert opinion: Novel organic cation transport systems and the elimination-dominant transport of pravastatin at the BRB are expected to be useful in systemic drug delivery to the neural retina without CNS side effects. The mechanism of nutrient transport at the BRB is expected to provide a new strategy for delivery of nutrient-mimetic drugs.

Zebrafish as a visual and dynamic model to study the transport of nanosized drug delivery systems across the biological barriers

With the wide application of nanotechnology to drug delivery systems, a simple, dynamic and visual in vivo model for high-throughput screening of novel formulations with fluorescence markers across biological barriers is desperately needed. In vitro cell culture models have been widely used, although they are far from a complimentary in vivo system. Mammalian animal models are common predictive models to study transport, but they are costly and time consuming. Zebrafish (Danio rerio), a small vertebrate model, have the potential to be developed as an "intermediate" model for quick evaluations. Based on our previously established coumarin 6 nanocrystals (C6-NCs), which have two different sizes, the present study investigates the transportation of C6-NCs across four biological barriers, including the chorion, blood brain barrier (BBB), blood retinal barrier (BRB) and gastrointestinal (GI) barrier, using zebrafish embryos and larvae as in vivo models. The biodistribution and elimination of C6 from different organs were quantified in adult zebrafish. The results showed that compared to 200nm C6-NCs, 70nm C6-NCs showed better permeability across these biological barriers. A FRET study suggested that intact C6-NCs together with the free dissolved form of C6 were absorbed into the larval zebrafish. More C6 was accumulated in different organs after incubation with small sized NCs via lipid raft-mediated endocytosis in adult zebrafish, which is consistent with the findings from in vitro cell monolayers and the zebrafish larvae model. C6-NCs could be gradually eliminated in each organ over time. This study demonstrated the successful application of zebrafish as a simple and dynamic model to simultaneously assess the transport of nanosized drug delivery systems across several biological barriers and biodistribution in different organs, especially in the brain, which could be used for central nervous system (CNS) drug and delivery system screening.

Loss of amplitude of accommodation in pre-presbyopic HIV and AIDS patients under treatment with antiretrovirals

Background: The prevalence of HIV and AIDS is causing an enormous public health burden. Its manifestations spare no organ. Ocular complications are mainly attributed to various opportunistic infections which are directly or indirectly caused by immune deficiency.Purpose and aim: The purpose of this study was to determine the effect of HIV and AIDS on subjective amplitude of accommodation of patients under treatment with antiretrovirals and then to compare their results to those of control subjects.Methods: The study took place over a period of 10 months. A quantitative study was carried out on 58 subjects (29 ± 5.5 years) with HIV and AIDS and 35 (28.67 ± 4.6 years) controls of similar age. Amplitude of accommodation was measured using the subjective Royal Air Force push-up method. The influence of CD4+ cell count was also recorded.Results: People with HIV and AIDS had lower mean amplitude of accommodation (5.69 ± 0.88 D) compared to controls (8.53 ± 1.2 D). The decrease in amplitude of accommodation did not show any correlation with the CD4+ cell count. Lower amplitude of accommodation exists in people living with HIV and AIDS when compared with age-related healthy people.Conclusion: The results suggest that patients with HIV and AIDS on antiretroviral drugs (ARVs) have reduced amplitude of accommodation and might experience presbyopia earlier in life than participants without HIV and AIDS. The reduced amplitude of accommodation could be the initial presentation of HIV infection before the systemic manifestation. The possible causes could be the direct neuronal infection by HIV-1, ARVs use, pathological changes of the lens and ciliary muscle or the sensory component of the visual system. It is unknown whether the reduced amplitude of accommodation occurred prior to antiretroviral therapy or represents an ongoing injury to the eye and visual system by the HIV.

Prediction of Ocular Drug Distribution from Systemic Blood Circulation

Systemically circulating drugs may distribute to ocular tissues across the blood-ocular barriers. Ocular distribution is utilized in the treatment of ocular diseases with systemic medications, but ocular delivery of systemic drugs and xenobiotics may also lead to adverse ocular effects. Ocular distribution after systemic drug administration has not been predicted or modeled. In this study, distribution clearance between vitreous and plasma was obtained from a previous QSPR model for clearance of intravitreal drugs. These values were used in a pharmacokinetic simulation model to describe entry of unbound drug from plasma to vitreous. The simulation models predicted ocular distribution of 10 systemic drugs in rabbit eyes within 1.96 mean fold error and the distribution of cefepime from plasma to vitreous in humans. This is the first attempt to predict ocular distribution of systemic drugs. Reliable predictions were obtained using systemic concentrations of unbound drug, computational value of ocular distribution clearance, and a simple pharmacokinetic model. This approach can be used in drug discovery to estimate ocular drug exposure at an early stage.

Delivery strategies for treatment of age-related ocular diseases: From a biological understanding to biomaterial solutions

Age-related ocular diseases, such as age-related macular degeneration (AMD), diabetic retinopathy, and glaucoma, result in life-long functional deficits and enormous global health care costs. As the worldwide population ages, vision loss has become a major concern for both economic and human health reasons. Due to recent research into biomaterials and nanotechnology major advances have been gained in the field of ocular delivery. This review provides a summary and discussion of the most recent strategies employed for the delivery of both drugs and cells to the eye to treat a variety of age-related diseases. It emphasizes the current challenges and limitations to ocular delivery and how the use of innovative materials can overcome these issues and ultimately provide treatment for age-related degeneration and regeneration of lost tissues. This review also provides critical considerations and an outlook for future studies in the field of ophthalmic delivery.

In vitro and in vivo release characteristics of Tacrolimus (FK506) from an episcleral drug-delivery implant

PURPOSE

To investigate the in vitro and in vivo release characteristics of Tacrolimus (FK506) from an episcleral drug-delivery implant.

METHODS

For in vitro experiments, Tacrolimus-loaded implants (0.5 mL; at concentrations of 0.25, 0.5, and 1.0 mg/mL) were immersed in a balanced salt solution. Samples of the surrounding liquid were aspirated at different times over a 96-h period. For in vivo experiments, the experimental group received an implant loaded with Tacrolimus (0.5 mg/mL; 0.5 mL); the control group was given a subconjunctival injection of 0.5 mL Tacrolimus (0.5 mg/mL). On postoperative days 3, 7, 14, 28, and 56, 3 animals were sacrificed, and their eyes were enucleated. Tacrolimus concentrations were determined by liquid chromatographic-tandem mass spectrometry. Ocular toxicity was evaluated by slit-lamp photography, fundus photography, intraocular pressure (IOP), and histology.

RESULTS

The implants released Tacrolimus in a biphasic pattern for 96 h in the in vitro study. The release kinetics were not dependent on the drug concentrations. The in vivo study showed statistically significant differences between the 2 treatment groups. Tacrolimus levels were particularly high in the conjunctiva, iris, ciliary body, cornea, sclera, choroid, and retina in the experimental group, while concentrations were low and only lasted for 1 week in the controls. Slit-lamp photography, fundus photography, IOP, and histology showed no evidence of toxic effects.

CONCLUSIONS

The episcleral drug-delivery implant mechanically released Tacrolimus through the apertures of capsules and, consequently, may be a promising drug vehicle for the treatment of immune-mediated ocular disorders.

Reversal of multidrug resistance by the inhibition of ATP-binding cassette pumps employing "Generally Recognized As Safe" (GRAS) nanopharmaceuticals: A review

Pumps of the ATP-binding cassette superfamily (ABCs) regulate the access of drugs to the intracellular space. In this context, the overexpression of ABCs is a well-known mechanism of multidrug resistance (MDR) in cancer and infectious diseases (e.g., viral hepatitis and the human immunodeficiency virus) and is associated with therapeutic failure. Since their discovery, ABCs have emerged as attractive therapeutic targets and the search of compounds that inhibit their genetic expression and/or their functional activity has gained growing interest. Different generations of pharmacological ABC inhibitors have been explored over the last four decades to address resistance in cancer, though clinical results have been somehow disappointing. "Generally Recognized As Safe" (GRAS) is a U.S. Food and Drug Administration designation for substances that are accepted as safe for addition in food. Far from being "inert", some amphiphilic excipients used in the production of pharmaceutical products have been shown to inhibit the activity of ABCs in MDR tumors, emerging as a clinically translatable approach to overcome resistance. The present article initially overviews the classification, structure and function of the different ABCs, with emphasis on those pumps related to drug resistance. Then, the different attempts to capitalize on the activity of GRAS nanopharmaceuticals as ABC inhibitors are discussed.

Methylmercury targets photoreceptor outer segments

Human populations experience widespread low level exposure to organometallic methylmercury compounds through consumption of fish and other seafood. At higher levels, methylmercury compounds specifically target nervous systems, and among the many effects of their exposure are visual disturbances, including blindness, which previously were thought to be due to methylmercury-induced damage to the visual cortex. Here, we employ high-resolution X-ray fluorescence imaging using beam sizes of 500 × 500 and 250 × 250 nm(2) to investigate the localization of mercury at unprecedented resolution in sections of zebrafish larvae ( Danio rerio ), a model developing vertebrate. We demonstrate that methylmercury specifically targets the outer segments of photoreceptor cells in both the retina and pineal gland. Methylmercury distribution in both tissues was correlated with that of sulfur, which, together with methylmercury's affinity for thiolate donors, suggests involvement of protein cysteine residues in methylmercury binding. In contrast, in the lens, the mercury distribution was different from that of sulfur, with methylmercury specifically accumulating in the secondary fiber cells immediately underlying the lens epithelial cells rather than in the lens epithelial cells themselves. Since methylmercury targets two main eye tissues (lens and photoreceptors) that are directly involved in visual perception, it now seems likely that the visual disruption associated with methylmercury exposure in higher animals including humans may arise from direct damage to photoreceptors, in addition to injury of the visual cortex.

Novel engineered systems for oral, mucosal and transdermal drug delivery

Technological advances in drug discovery have resulted in increasing number of molecules including proteins and peptides as drug candidates. However, how to deliver drugs with satisfactory therapeutic effect, minimal side effects and increased patient compliance is a question posted before researchers, especially for those drugs with poor solubility, large molecular weight or instability. Microfabrication technology, polymer science and bioconjugate chemistry combine to address these problems and generate a number of novel engineered drug delivery systems. Injection routes usually have poor patient compliance due to their invasive nature and potential safety concerns over needle reuse. The alternative non-invasive routes, such as oral, mucosal (pulmonary, nasal, ocular, buccal, rectal, vaginal), and transdermal drug delivery have thus attracted many attentions. Here, we review the applications of the novel engineered systems for oral, mucosal and transdermal drug delivery.

Hypoxia alters ocular drug transporter expression and activity in rat and calf models: implications for drug delivery

Chronic hypoxia, a key stimulus for neovascularization, has been implicated in the pathology of proliferative diabetic retinopathy, retinopathy of prematurity, and wet age related macular degeneration. The aim of the present study was to determine the effect of chronic hypoxia on drug transporter mRNA expression and activity in ocular barriers. Sprague-Dawley rats were exposed to hypobaric hypoxia (PB = 380 mmHg) for 6 weeks, and neonatal calves were maintained under hypobaric hypoxia (PB = 445 mmHg) for 2 weeks. Age matched controls for rats, and calves were maintained at ambient altitude and normoxia. The effect of hypoxia on transporter expression was analyzed by qRT-PCR analysis of transporter mRNA expression in hypoxic and control rat choroid-retina. The effect of hypoxia on the activity of PEPT, OCT, ATB(0+), and MCT transporters was evaluated using in vitro transport studies of model transporter substrates across calf cornea and sclera-choroid-RPE (SCRPE). Quantitative gene expression analysis of 84 transporters in rat choroid-retina showed that 29 transporter genes were up regulated or down regulated by ≥1.5-fold in hypoxia. Nine ATP binding cassette (ABC) families of efflux transporters including MRP3, MRP4, MRP5, MRP6, MRP7, Abca17, Abc2, Abc3, and RGD1562128 were up-regulated. For solute carrier family transporters, 11 transporters including SLC10a1, SLC16a3, SLC22a7, SLC22a8, SLC29a1, SLC29a2, SLC2a1, SLC3a2, SLC5a4, SLC7a11, and SLC7a4 were up regulated, while 4 transporters including SLC22a2, SLC22a9, SLC28a1, and SLC7a9 were down-regulated in hypoxia. Of the three aquaporin (Aqp) water channels, Aqp-9 was down-regulated, and Aqp-1 was up-regulated during hypoxia. Gene expression analysis showed down regulation of OCT-1, OCT-2, and ATB(0+) and up regulation of MCT-3 in hypoxic rat choroid-retina, without any effect on the expression of PEPT-1 and PEPT-2. Functional activity assays of PEPT, OCT, ATB(0+), and MCT transporters in calf ocular tissues showed that PEPT, OCT, and ATB(0+) functional activity was down-regulated, whereas MCT functional activity was up-regulated in hypoxic cornea and SCRPE. Gene expression analysis of these transporters in rat tissues was consistent with the functional transport assays except for PEPT transporters. Chronic hypoxia results in significant alterations in the mRNA expression and functional activity of solute transporters in ocular tissues.

Gene expression profiling of transporters in the solute carrier and ATP-binding cassette superfamilies in human eye substructures

The barrier epithelia of the cornea and retina control drug and nutrient access to various compartments of the human eye. While ocular transporters are likely to play a critical role in homeostasis and drug delivery, little is known about their expression, localization and function. In this study, the mRNA expression levels of 445 transporters, metabolic enzymes, transcription factors and nuclear receptors were profiled in five regions of the human eye: cornea, iris, ciliary body, choroid and retina. Through RNA expression profiling and immunohistochemistry, several transporters were identified as putative targets for drug transport in ocular tissues. Our analysis identified SLC22A7 (OAT2), a carrier for the antiviral drug acyclovir, in the corneal epithelium, in addition to ABCG2 (BCRP), an important xenobiotic efflux pump, in retinal nerve fibers and the retinal pigment epithelium. Collectively, our results provide an understanding of the transporters that serve to maintain ocular homeostasis and which may be potential targets for drug delivery to deep compartments of the eye.

Closed gateways--can neuroprotectants shield the retina in glaucoma?

Neuroprotection for glaucoma is a therapeutic approach that aims to prevent optic nerve damage or cell death. An appropriate drug that reaches an adequate concentration across the blood retinal barrier is expected to shield the retina in glaucoma. Several in vitro and in vivo attempts in experimental models indicate the possibility of successful neuroprotection. However, clinical trials might not show the same level of neuroprotection as a result of subtherapeutic concentrations of the drug in the eye. The study by Zhong et al. in this issue of Drugs in R&D could not attribute the observed improvement in visual field indices to any one of the individual active constituents of Erigeron breviscapus (vant.) Hand. Mazz. (EBHM). One of the major constituents of EBHM is scutellarin, which is known to have poor oral bioavailability and an unclear ability to penetrate inside the eye. Therefore, before recognizing EBHM as a neuroprotectant in glaucoma for further clinical studies and practice, its active constituents and their pharmacokinetics (systemic as well as ocular) need to be explored.

Strategies for therapy of retinal diseases using systemic drug delivery: relevance of transporters at the blood-retinal barrier

INTRODUCTION

There is an increasing need for managing rapidly progressing retinal diseases because of the potential loss of vision. Although systemic drug administration is one possible route for treating retinal diseases, retinal transfer of therapeutic drugs from the circulating blood is strictly regulated by the blood-retinal barrier (BRB).

AREAS COVERED

This review discusses the constraints and challenges of drug delivery to the retina. In addition, this article discusses the properties of drugs and the conditions of the BRB that affect drug permeability. The reader will gain insights into the strategies for developing therapeutic drugs that are able to cross the BRB for treating retinal diseases. Further, the reader will gain insights into the role of BRB physiology including barrier functions, and the effect of influx and efflux transporters on retinal drug delivery.

EXPERT OPINION

When designing and selecting optimal drug candidates, it's important to consider the fact that they should be recognized by influx transporters and that efflux transporters at the BRB should be avoided. Although lipophilic cationic drugs are known to be transported to the brain across the blood-brain barrier, verapamil transport to the retina is substantially higher than to the brain. Therefore, lipophilic cationic drugs do have a great ability to increase influx transport across the BRB.

Adult peripheral blood mononuclear cells transdifferentiate in vitro and integrate into the retina in vivo

Adult peripheral blood-derived cells are able to differentiate into a variety of cell types, including nerve cells, liver-like cells and epithelial cells. However, their differentiation into retina-like cells is controversial. In the present study, transdifferentiation potential of human adult peripheral blood mononuclear cells into retina-like cells and integration into the retina of mice were investigated. Freshly isolated adult peripheral blood mononuclear cells were divided into two groups: cells in group I were cultured in neural stem cell medium, and cells in group II were exposed to conditioned medium from rat retinal tissue culture. After 5 days, several distinct cell morphologies were observed, including standard mononuclear, neurons with one or two axons and elongated glial-like cells. Immunohistochemical analysis of neural stem cell, neuron and retina cell markers demonstrated that cells in both groups were nestin-, MAP2 (microtubule-associated protein)- and GFAP (glial fibrillary acidic protein)-positive. Flow cytometry results suggested a significant increase in nestin-, MAP2- and CD16-positive cells in group I and nestin-, GFAP-, MAP2-, vimentin- and rhodopsin-positive cells in group II. To determine survival, migration and integration in vivo, cell suspensions (containing group I or group II cells) were injected into the vitreous or the peritoneum. Tissue specimens were obtained and immunostained 4 weeks after transplantation. We found that cells delivered by intravitreal injection integrated into the retina. Labelled cells were not detected in the retina of mice receiving differentiated cells by intraperitoneal injection, but cells (groups I and II) were detected in the liver and spleen. Our findings revealed that human adult peripheral blood mononuclear cells could be induced to transdifferentiate into neural precursor cells and retinal progenitor cells in vitro, and the differentiated peripheral blood mononuclear cells can migrate and integrate into the retina in vivo.

Drug transporters, the blood-testis barrier, and spermatogenesis

The blood-testis barrier (BTB), which is created by adjacent Sertoli cells near the basement membrane, serves as a 'gatekeeper' to prohibit harmful substances from reaching developing germ cells, most notably postmeiotic spermatids. The BTB also divides the seminiferous epithelium into the basal and adluminal (apical) compartment so that postmeiotic spermatid development, namely spermiogenesis, can take place in a specialized microenvironment in the apical compartment behind the BTB. The BTB also contributes, at least in part, to the immune privilege status of the testis, so that anti-sperm antibodies are not developed against antigens that are expressed transiently during spermatogenesis. Recent studies have shown that numerous drug transporters are expressed by Sertoli cells. However, many of these same drug transporters are also expressed by spermatogonia, spermatocytes, round spermatids, elongating spermatids, and elongated spermatids, suggesting that the developing germ cells are also able to selectively pump drugs 'in' and/or 'out' via influx or efflux pumps. We review herein the latest developments regarding the role of drug transporters in spermatogenesis. We also propose a model utilized by the testis to protect germ cell development from 'harmful' environmental toxicants and xenobiotics and/or from 'therapeutic' substances (e.g. anticancer drugs). We also discuss how drug transporters that are supposed to protect spermatogenesis can work against the testis in some instances. For example, when drugs (e.g. male contraceptives) that can perturb germ cell adhesion and/or maturation are actively pumped out of the testis or are prevented from entering the apical compartment, such as by efflux pumps.