Hydrogel Containing Silica Shell Cross-Linked Micelles for Ocular Drug Delivery

Mitigation of pesticide-mediated ocular toxicity via nanotechnology-based contact lenses: a review

The xenobiotic stress exerted by pesticides leads to the deterioration of human and animal health including ocular health. Acute or prolonged exposure to these agricultural toxicants has been implicated in a number of pathological conditions of the eye such as irritation, epiphora or hyper-lacrimation, abrasions on the ocular surface, and decreased visual acuity. The issue is compounded by the fact that tissues of the eye absorb pesticides faster than other organs of the body and are more susceptible to damage as well. However, there is a lacuna in our knowledge regarding the ways by which pesticide exposure-mediated ocular insult might be counteracted. Topical instillation of drugs known to combat the pesticide induced toxicity has been explored to mitigate the detrimental impact of pesticide exposure. However, topical eye drop solutions exhibit very low bioavailability and limited drug residence duration in the tear film decreasing their efficacy. Contact lenses have been explored in this respect to increase bioavailability of ocular drugs, while nanoparticles have lately been utilized to increase drug bioavailability and increase drug residence duration in different tissues. The current review focuses on drug delivery and futuristic aspects of corneal protection from ocular toxicity using contact lenses.

Evolution of Hybrid Hydrogels: Next-Generation Biomaterials for Drug Delivery and Tissue Engineering

Hydrogels, being hydrophilic polymer networks capable of absorbing and retaining aqueous fluids, hold significant promise in biomedical applications owing to their high water content, permeability, and structural similarity to the extracellular matrix. Recent chemical advancements have bolstered their versatility, facilitating the integration of the molecules guiding cellular activities and enabling their controlled activation under time constraints. However, conventional synthetic hydrogels suffer from inherent weaknesses such as heterogeneity and network imperfections, which adversely affect their mechanical properties, diffusion rates, and biological activity. In response to these challenges, hybrid hydrogels have emerged, aiming to enhance their strength, drug release efficiency, and therapeutic effectiveness. These hybrid hydrogels, featuring improved formulations, are tailored for controlled drug release and tissue regeneration across both soft and hard tissues. The scientific community has increasingly recognized the versatile characteristics of hybrid hydrogels, particularly in the biomedical sector. This comprehensive review delves into recent advancements in hybrid hydrogel systems, covering the diverse types, modification strategies, and the integration of nano/microstructures. The discussion includes innovative fabrication techniques such as click reactions, 3D printing, and photopatterning alongside the elucidation of the release mechanisms of bioactive molecules. By addressing challenges, the review underscores diverse biomedical applications and envisages a promising future for hybrid hydrogels across various domains in the biomedical field.

Visible light degradable micelles for intraocular corticosteroid delivery

Visible light responsive micellar drug delivery formulations are of notable interest for the treatment of ocular diseases, as their successful development would enable controlled drug release at the back of the eye, improving efficacy and reducing side-effects when compared to existing approaches. In this work, an aliphatic polycarbonate-based visible light responsive micelle formulation based on mPEG-b-poly(5-hydroxy-trimethylene carbonate) (PHTMC) was prepared wherein the pendant hydroxyl groups of the PHTMC repeating units were protected by blue light-labile [7-(diethylamino)coumarin-4-yl]methyl (DEACM). The photo-labile DEACM provided a photo-triggered release profile, as, upon the removal of these protecting groups by photo-irradiation, the micelles underwent structural disruption, leading to the release of the payload. The removal of DEACM also deprotected the pendant hydroxyl groups of PHTMC, leading to PHTMC backbone degradation via intramolecular cyclization.

Developments in Emerging Topical Drug Delivery Systems for Ocular Disorders

According to the current information, using nano gels in the eyes have therapeutic benefits. Industry growth in the pharmaceutical and healthcare sectors has been filled by nanotechnology. Traditional ocular preparations have a short retention duration and restricted drug bioavailability because of the eye's architectural and physiological barriers, a big issue for physicians, patients, and chemists. In contrast, nano gels can encapsulate drugs within threedimensional cross-linked polymeric networks. Because of their distinctive structural designs and preparation methods, they can deliver loaded medications in a controlled and sustained manner, enhancing patient compliance and therapeutic efficacy. Due to their excellent drugloading capacity and biocompatibility, nano-gels outperform other nano-carriers. This study focuses on using nano gels to treat eye diseases and provides a brief overview of their creation and response to stimuli. Our understanding of topical drug administration will be advanced using nano gel developments to treat common ocular diseases such as glaucoma, cataracts, dry eye syndrome, bacterial keratitis, and linked medication-loaded contact lenses and natural active ingredients.

Role of Polymeric Micelles in Ocular Drug Delivery: An Overview of Decades of Research

Local drug delivery to the eye through conventional means has faced many challenges due to three essential barriers: (a) the complex structure of the cornea limiting drug absorption, (b) the capacity of ocular absorptive cells in drug metabolism, and (c) the washing effect of eye tears. Polymeric micelles (PMs) have been the focus of much interest for ocular drug delivery due to several advantages they provide for this application, including the capacity for the solubilization of hydrophobic drugs, nonirritability, nanoscopic diameter, and the clarity of their aqueous solution not interfering with vision. The potential to increase the release and residence time of incorporated medication at the site of absorption is also a bonus advantage for these delivery systems. This Review covers research conducted on single or mixed micelles prepared from small amphiphilic molecules, copolymers (diblock, triblock, and graft), and gel systems containing micelles. The purpose of this review is to provide an update on the status of micellar ocular delivery systems for different indications, with a focus on preclinical and clinical drug development. In this context, we are discussing the anatomy of the eye, various ocular barriers, different micellar formulations, and their benefits in ocular drug delivery, as well as the role of PMs in the management of ocular diseases both in preclinical models and in clinic. The encouraging preclinical effectiveness findings from experiments conducted in both laboratory settings and live animals have paved the way for the advancement of micellar systems in clinical trials for ocular administration and the first nanomicallar formulation approved for clinical use by the United States Food and Drug Administration (marketed as Cequa by Sun Pharmaceuticals).

Matrix metalloproteinase responsive hydrogel microplates for programmed killing of invasive tumour cells

Interactive materials are an emerging class of systems that can offer control over response and adaptivity in polymer structures towards the meso- and macroscale. Here, we use enzyme regulated cleavage of peptide crosslinkers in polymer hydrogels to release a cytotoxic therapeutic nanoparticle with an adaptable mechanism. Hydrogel microplates were formed through polyethylene glycol/peptide photoinitiated thiol-ene chemistry in a soft-lithography process to give square plates of 20 by 20 μm with a height of 10 μm. The peptide was chosen to be degradable in the presence of matrix metalloproteinase 2/9 (MMP-2/9). The hydrogel material's mechanical properties, swelling, and protease degradation were characterised. The microfabricated hydrogels were loaded with docetaxel (DTXL) containing poly(dl-lactide-co-glycolide) (PLGA) nanoparticles, and characterised for enzyme responsivity, and toxicity to MMP-2/9 overexpressing brain cancer cell line U87-MG. A 5-fold decrease in EC50 was seen compared to free DTXL, and a 20-fold decrease was seen for the MMP responsive microplates versus a non-degradable control microplate. Potential applications of this system in post-resection glioblastoma treatment are envisioned.

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.

Recent development of polymer nanomicelles in the treatment of eye diseases

The eye, being one of the most intricate organs in the human body, hosts numerous anatomical barriers and clearance mechanisms. This highlights the importance of devising a secure and efficacious ocular medication delivery system. Over the past several decades, advancements have been made in the development of a nano-delivery platform based on polymeric micelles. These advancements encompass diverse innovations such as poloxamer, chitosan, hydrogel-encapsulated micelles, and contact lenses embedded with micelles. Such technological evolutions allow for sustained medication retention and facilitate enhanced permeation within the eye, thereby standing as the avant-garde in ocular medication technology. This review provides a comprehensive consolidation of ocular medications predicated on polymer nanomicelles from 2014 to 2023. Additionally, it explores the challenges they pose in clinical applications, a discussion intended to aid the design of future clinical research concerning ocular medication delivery formulations.

Hot-melt extruded in situ gelling systems (MeltDrops Technology): Formulation development, in silico modelling and in vivo studies

In situ gelling systems (ISGS) can prolong retention time and bioavailability of ophthalmic solutions. The complexity and cost of ISGS avert their industrial scale-up and clinical implementation. In this study, we demonstrate novel application of hot-melt extrusion (HME) technology for continuous manufacturing of ISGS (MeltDrops Technology). Timolol maleate (TIM) and dorzolamide hydrochloride (DRZ) loaded MeltDrops were successfully developed using HME for glaucoma management, thereby resolving issues with batch manufacturing of ISGS, prolonging retention time thus improving bioavailability. The MeltDrops technology involves one-step, i.e., passing all the ingredients through an extruder at a screw speed between 20-50 rpm and barrel temperature of 80 °C. The comparative evaluation of MeltDrops and batch-processed ISGS demonstrated that MeltDrops exhibited better physical and chemical content uniformity. The extrusion temperature and screw speed were critical factors influencing content uniformity and properties of the MeltDrops. MeltDrops showed sustained drug release for >12 hours in vitro (TIM= 83.07%; DRZ = 60.43%, 12hours) versus marketed eyedrops. The developed MeltDrops followed Peppas-Sahlin model, combining Fickian diffusion and swelling processes. The in vivo study in New Zealand rabbits revealed superior effectiveness and safety of the MeltDrops as compared to the marketed eyedrops. Herein we conclude, MeltDrops would serve as a cutting-edge platform technology that can be used to manufacture various ISGS with one-step processability, cost-effectiveness, and improved product quality, which are otherwise processed by batch manufacturing that involves numerous complex processing steps.

Review of Potential Drug-Eluting Contact Lens Technologies

The field of ophthalmology is expanding exponentially, both in terms of diagnostic and therapeutic capabilities, as well as the worldwide increasing incidence of eye-related diseases. Due to an ageing population and climate change, the number of ophthalmic patients will continue to increase, overwhelming healthcare systems and likely leading to under-treatment of chronic eye diseases. Since drops are the mainstay of therapy, clinicians have long emphasised the unmet need for ocular drug delivery. Alternative methods, i.e., with better compliance, stability and longevity of drug delivery, would be preferred. Several approaches and materials are being studied and used to overcome these drawbacks. We believe that drug-loaded contact lenses are among the most promising and are a real step toward dropless ocular therapy, potentially leading to a transformation in clinical ophthalmic practice. In this review, we outline the current role of contact lenses in ocular drug delivery, focusing on materials, drug binding and preparation, concluding with a look at future developments.

Advances in Nanogels for Topical Drug Delivery in Ocular Diseases

Nanotechnology has accelerated the development of the pharmaceutical and medical technology fields, and nanogels for ocular applications have proven to be a promising therapeutic strategy. Traditional ocular preparations are restricted by the anatomical and physiological barriers of the eye, resulting in a short retention time and low drug bioavailability, which is a significant challenge for physicians, patients, and pharmacists. Nanogels, however, have the ability to encapsulate drugs within three-dimensional crosslinked polymeric networks and, through specific structural designs and distinct methods of preparation, achieve the controlled and sustained delivery of loaded drugs, increasing patient compliance and therapeutic efficiency. In addition, nanogels have higher drug-loading capacity and biocompatibility than other nanocarriers. In this review, the main focus is on the applications of nanogels for ocular diseases, whose preparations and stimuli-responsive behaviors are briefly described. The current comprehension of topical drug delivery will be improved by focusing on the advances of nanogels in typical ocular diseases, including glaucoma, cataracts, dry eye syndrome, and bacterial keratitis, as well as related drug-loaded contact lenses and natural active substances.

Application of Convergent Science and Technology toward Ocular Disease Treatment

Eyes are one of the main critical organs of the body that provide our brain with the most information about the surrounding environment. Disturbance in the activity of this informational organ, resulting from different ocular diseases, could affect the quality of life, so finding appropriate methods for treating ocular disease has attracted lots of attention. This is especially due to the ineffectiveness of the conventional therapeutic method to deliver drugs into the interior parts of the eye, and the also presence of barriers such as tear film, blood-ocular, and blood-retina barriers. Recently, some novel techniques, such as different types of contact lenses, micro and nanoneedles and in situ gels, have been introduced which can overcome the previously mentioned barriers. These novel techniques could enhance the bioavailability of therapeutic components inside the eyes, deliver them to the posterior side of the eyes, release them in a controlled manner, and reduce the side effects of previous methods (such as eye drops). Accordingly, this review paper aims to summarize some of the evidence on the effectiveness of these new techniques for treating ocular disease, their preclinical and clinical progression, current limitations, and future perspectives.

Therapeutic contact lenses for the treatment of corneal and ocular surface diseases: advances in extended and targeted drug delivery

The eye is one of the most important organs in the human body providing critical information on the environment. Many corneal diseases can lead to vision loss affecting the lives of people around the world. Ophthalmic drug delivery has always been a major challenge in the medical sciences. Since traditional methods are less efficient (∼ 5%) at delivering drugs to ocular tissues, contact lenses have generated growing interest in ocular drug delivery due to their potential to enhance drug bioavailability in ocular tissues. The main techniques used to achieve sustained release are discussed in this review, including soaking in drug solutions, incorporating drug into multilayered contact lenses, use of vitamin E barriers, molecular imprinting, nanoparticles, micelles and liposomes. The most clinically relevant results on different eye pathologies are presented. In addition, this review summarizes the benefits of contact lenses over eye drops, strategies for incorporating drugs into lenses to achieve sustained release, results of in vitro and in vivo studies, and the recent advances in the commercialization of therapeutic contact lenses for allergic conjunctivitis.

Enhanced topical corticosteroids delivery to the eye: A trade-off in strategy choice

Topical corticosteroids are the primary treatment of ocular inflammation caused by surgery, injury, or other conditions. Drug pre-corneal residence time, drug water solubility, and drug corneal permeability coefficient are the major factors that determine the ocular drug bioavailability after topical administration. Although growing research successfully enhanced local delivery of corticosteroids utilizing various strategies, rational and dynamic approaches to strategy selection are still lacking. Within this review, an overview of the various strategies as well as their performance in retention, solubility, and permeability coefficient of corticosteroids are provided. On this basis, the tradeoff of strategy selection is discussed, which may shed light on the rational choice and application of ophthalmic delivery enhancement strategies.

Pharmaceutical Perspective in Wearable Drug Delivery Systems

Humans have been dealing with health problems for millions of years. Normal health services need well-trained personnel and high-cost diagnostic tests, which forces patients to go to hospitals if medical treatment is required. To address this, prototype testing has been carried out into the wearable drug delivery health care perspectives. Researchers have devised a wide variety of formulations for the treatment of various diseases at home by performing real-time monitoring of different routes of drug administration such as ocular, transdermal, intraoral, intracochlear, and several more. A comprehensive review of the different types of wearable drug delivery systems with respect to their manufacturing, mechanism of action and specifications has been done. In the pharmaceutical context, these devices are technologically well-equipped interfaces for diverse physicochemical signals. Above mentioned information with a broader perspective has also been discussed in this article. Several wearable drug delivery systems have been introduced in the market in recent years. But a lot of testing needs to be conducted to address the numerous obstacles before the wearable devices are successfully launched in the market.

Contact Lenses as Ophthalmic Drug Delivery Systems: A Review

Ophthalmic drugs used for the treatment of various ocular diseases are commonly administered by eye drops. However, due to anatomical and physiological factors, there is a low bioavailability of the active principle. In order to increase the drug residence time on the cornea to adequate levels, therapeutic contact lenses have recently been proposed. The polymeric support that constitutes the contact lens is loaded with the drug; in this way, there is a direct and effective pharmacological action on the target organ, promoting a prolonged release of the active principle. The incorporation of ophthalmic drugs into contact lenses can be performed by different techniques; nowadays, the soaking method is mainly employed. To improve the therapeutic performance of drug-loaded contact lenses, innovative methods have recently been proposed, including the impregnation with supercritical carbon dioxide. This updated review of therapeutic contact lenses production and application provides useful information on the most effective preparation methodologies, recent achievements and future perspectives.

Fouling in ocular devices: implications for drug delivery, bioactive surface immobilization, and biomaterial design

The last 30 years has seen a proliferation of research on protein-resistant biomaterials targeted at designing bio-inert surfaces, which are prerequisite for optimal performance of implantable devices that contact biological fluids and tissues. These efforts have only been able to yield minimal results, and hence, the ideal anti-fouling biomaterial has remained elusive. Some studies have yielded biomaterials with a reduced fouling index among which high molecular weight polyethylene glycols have remained dominant. Interestingly, the field of implantable ocular devices has not experienced an outflow of research in this area, possibly due to the assumption that biomaterials tested in other body fluids can be translated for application in the ocular space. Unfortunately, progression in the molecular understanding of many ocular conditions has brought to the fore the need for treatment options that necessitates the use of anti-fouling biomaterials. From the earliest implanted horsehair and silk seton for glaucoma drainage to the recent mini telescopes for sight recovery, this review provides a concise incursion into the gradual evolution of biomaterials for the design of implantable ocular devices as well as approaches used to overcome the challenges with fouling. The implication of fouling for drug delivery, the design of immune-responsive biomaterials, as well as advanced surface immobilization approaches to support the overall performance of implantable ocular devices are also reviewed.

Rebaudioside A/TPGS mixed nanomicelles as promising nanocarriers for nimodipine ocular delivery

Nimodipine (NMD), a calcium channel blocker, has demonstrated benefits in treating glaucoma. However, its ocular therapeutic application remains limited due to its poor aqueous solubility, which restrains the development of an ophthalmic formulation. Thus, the present study aimed to formulate an NMD micelle ophthalmic solution to enhance the potential of NMD in an ocular topical formulation to treat glaucoma. The NMD micelle ophthalmic solution was formulated with nanocarriers composed of rebaudioside A and D-α-tocopheryl polyethylene glycol 1000 succinate. Spherical mixed micelles were optimized and obtained at a small micelle size 13.429 ± 0.181 nm with a narrow size distribution (polydispersity index 0.166 ± 0.023) and high encapsulation efficiency rate (99.59 ± 0.09%). Compared with free NMD, NMD in micelles had much greater in vitro membrane permeability and antioxidant activity. The NMD micelle ophthalmic solution was well tolerated in rabbit eyes. It profoundly improved the in vivo intraocular permeation of NMD, and in vivo intraocular pressure reduction and improved miosis were also observed. Accordingly, this NMD micelle ophthalmic solution might be a promising ocular formulation to treat glaucoma. Graphical abstract.

Hybrid hydrogels for biomedical applications

The use of hydrogels in biomedical applications dates back multiple decades, and the engineering potential of these materials continues to grow with discoveries in chemistry and biology. The approaches have led to increasing complex hydrogels that incorporate both synthetic and natural polymers and functional domains for tunable release kinetics, mediated cell response, and ultimately use in clinical and research applications in biomedical practice. This review focuses on recent advances in hybrid hydrogels that incorporate nano/microstructures, their synthesis, and applications in biomedical research. Examples discussed include the implementation of click reactions, photopatterning, and 3D printing for the facile production of these hybrid hydrogels, the use of biological molecules and motifs to promote a desired cellular outcome, and the tailoring of kinetic and transport behavior through hybrid-hydrogel engineering to achieve desired biomedical outcomes. Recent progress in the field has established promising approaches for the development of biologically relevant hybrid hydrogel materials with potential applications in drug discovery, drug/gene delivery, and regenerative medicine.

Ocular Drug Delivery

Although the eye is an accessible organ for direct drug application, ocular drug delivery remains a major challenge due to multiple barriers within the eye. Key barriers include static barriers imposed by the cornea, conjunctiva, and retinal pigment epithelium and dynamic barriers including tear turnover and blood and lymphatic clearance mechanisms. Systemic administration by oral and parenteral routes is limited by static blood-tissue barriers that include epithelial and endothelial layers, in addition to rapid vascular clearance mechanisms. Together, the static and dynamic barriers limit the rate and extent of drug delivery to the eye. Thus, there is an ongoing need to identify novel delivery systems and approaches to enhance and sustain ocular drug delivery. This chapter summarizes current and recent experimental approaches for drug delivery to the anterior and posterior segments of the eye.

Ophthalmic gels: Past, present and future

Aqueous gels formulated using hydrophilic polymers (hydrogels) along with those based on stimuli responsive polymers (in situ gelling or gel forming systems) continue to attract increasing interest for various eye health-related applications. They allow the incorporation of a variety of ophthalmic pharmaceuticals to achieve therapeutic levels of drugs and bioactives at target ocular sites. The integration of sophisticated drug delivery technologies such as nanotechnology-based ones with intelligent and environment responsive systems can extend current treatment duration to provide more clinically relevant time courses (weeks and months instead of hours and days) which will inevitably reduce dose frequency, increase patient compliance and improve clinical outcomes. Novel applications and design of contact lenses and intracanalicular delivery devices along with the move towards integrating gels into various drug delivery devices like intraocular pumps, injections and implants has the potential to reduce comorbidities caused by glaucoma, corneal keratopathy, cataract, diabetic retinopathies and age-related macular degeneration. This review describes ophthalmic gelling systems with emphasis on mechanism of gel formation and application in ophthalmology. It provides a critical appraisal of the techniques and methods used in the characterization of ophthalmic preformed gels and in situ gelling systems along with a thorough insight into the safety and biocompatibility of these systems. Newly developed ophthalmic gels, hydrogels, preformed gels and in situ gelling systems including the latest in the area of stimuli responsive gels, molecularly imprinted gels, nanogels, 3D printed hydrogels; 3D printed devices comprising ophthalmic gels are covered. Finally, new applications of gels in the production of artificial corneas, corneal wound healing and hydrogel contact lenses are described.

Nanoparticles for drug delivery to the anterior segment of the eye

Commercially available ocular drug delivery systems are effective but less efficacious to manage diseases/disorders of the anterior segment of the eye. Recent advances in nanotechnology and molecular biology offer a great opportunity for efficacious ocular drug delivery for the treatments of anterior segment diseases/disorders. Nanoparticles have been designed for preparing eye drops or injectable solutions to surmount ocular obstacles faced after administration. Better drug pharmacokinetics, pharmacodynamics, non-specific toxicity, immunogenicity, and biorecognition can be achieved to improve drug efficacy when drugs are loaded in the nanoparticles. Despite the fact that a number of review articles have been published at various points in the past regarding nanoparticles for drug delivery, there is not a review yet focusing on the development of nanoparticles for ocular drug delivery to the anterior segment of the eye. This review fills in the gap and summarizes the development of nanoparticles as drug carriers for improving the penetration and bioavailability of drugs to the anterior segment of the eye.

Approaches in topical ocular drug delivery and developments in the use of contact lenses as drug-delivery devices

Drug-delivery approaches have diversified over the last two decades with the emergence of nanotechnologies, smart polymeric systems and multimodal functionalities. The intended target for specific treatment of disease is the key defining developing parameter. One such area which has undergone significant advancements relates to ocular delivery. This has been expedited by the development of material advancement, mechanistic concepts and through the deployment of advanced process technologies. This review will focus on the developments within lens-based drug delivery while touching on conventional and current methods of topical ocular drug delivery. A summary table will provide quick reference to note the key findings in this area. In addition, the review also elucidates current theranostic and diagnostic approaches based on ocular lenses.

Dry Eye Treatment Based on Contact Lens Drug Delivery: A Review

Dry eye disease affects a substantial segment of the word population with increasing frequency. It is a multifactorial disease of the ocular surface and tear film, which causes ocular discomfort, visual disturbances, and tear instability with potential damage to the cornea and conjunctiva. Because of its multifactorial etiology, the use of different pharmacological treatment for dry eye treatment has been proposed, which include anti-inflammatory molecules, lubricants or comfort agents, and secretagogues. However, in some cases these pharmacological approaches only relieve symptoms temporarily, and consequently, eye care professionals continue to have difficulties managing dry eye. To improve pharmacological therapy that allows a more efficient and long-term action, effective ocular drug delivery of the currently available drugs for dry eye treatment is required. Contact lenses are emerging as alternative ophthalmic drugs delivery systems that provide an increased residence time of the drug at the eye, thus leading to enhanced bioavailability and more convenient and efficacious therapy. In this article, we reviewed the different techniques used to prepare contact lens-based drug delivery systems and focused on articles that describe the delivery of compounds for dry eye treatment through contact lenses.

Effect of surfactant chain length on drug release kinetics from microemulsion-laden contact lenses

The effect of surfactant chain lengths [sodium caprylate (C₈), Tween 20 (C₁₂), Tween 80 (C₁₈)] and the molecular weight of block copolymers [Pluronic F68 and Pluronic F 127] were studied to determine the stability of the microemulsion and its effect on release kinetics from cyclosporine-loaded microemulsion-laden hydrogel contact lenses in this work. Globule size and dilution tests (transmittance) suggested that the stability of the microemulsion increases with increase in the carbon chain lengths of surfactants and the molecular weight of pluronics. The optical transmittance of direct drug-laden contact lenses [DL-100] was low due to the precipitation of hydrophobic drugs in the lenses, while in microemulsion-laden lenses, the transmittance was improved when stability of the microemulsion was achieved. The results of in vitro release kinetics revealed that drug release was sustained to a greater extent as the stability of microemulsion was improved as well. This was evident in batch PF127-T80, which showed sustained release for 15days in comparison to batch DL-100, which showed release up to 7days. An in vivo drug release study in rabbit tear fluid showed significant increase in mean residence time (MRT) and area under curve (AUC) with PF-127-T80 lenses (stable microemulsion) in comparison to PF-68-SC lenses (unstable microemulsion) and DL-100 lenses. This study revealed the correlation between the stability of microemulsion and the release kinetics of drugs from contact lenses. Thus, it was inferred that the stable microemulsion batches sustained the release of hydrophobic drugs, such as cyclosporine from contact lenses for an extended period of time without altering critical lens properties.

Polymeric micelles for ocular drug delivery: From structural frameworks to recent preclinical studies

Effective intraocular drug delivery poses a major challenge due to the presence of various elimination mechanisms and physiological barriers that result in low ocular bioavailability after topical application. Over the past decades, polymeric micelles have emerged as one of the most promising drug delivery platforms for the management of ocular diseases affecting the anterior (dry eye syndrome) and posterior (age-related macular degeneration, diabetic retinopathy and glaucoma) segments of the eye. Promising preclinical efficacy results from both in-vitro and in-vivo animal studies have led to their steady progression through clinical trials. The mucoadhesive nature of these polymeric micelles results in enhanced contact with the ocular surface while their small size allows better tissue penetration. Most importantly, being highly water soluble, these polymeric micelles generate clear aqueous solutions which allows easy application in the form of eye drops without any vision interference. Enhanced stability, larger cargo capacity, non-toxicity, ease of surface modification and controlled drug release are additional advantages with polymeric micelles. Finally, simple and cost effective fabrication techniques render their industrial acceptance relatively high. This review summarizes structural frameworks, methods of preparation, physicochemical properties, patented inventions and recent advances of these micelles as effective carriers for ocular drug delivery highlighting their performance in preclinical studies.

A review on therapeutic contact lenses for ocular drug delivery

Contact lenses for ophthalmic drug delivery have become very popular, due to their unique advantages like extended wear and more than 50% bioavailability. To achieve controlled and sustained drug delivery from contact lenses, researchers are working on various systems like polymeric nanoparticles, microemulsion, micelle, liposomes, use of vitamin E, etc. Numerous scientists are working on different areas of therapeutic contact lenses to treat ocular diseases by implementing techniques like soaking method, molecular imprinting, entrapment of drug-laden colloidal nanoparticles, drug plate/film, ion ligand polymeric systems, supercritical fluid technology, etc. Though sustained drug delivery was achieved using contact lens, the critical properties such as water content, tensile strength (mechanical properties), ion permeability, transparency and oxygen permeability were altered, which limit the commercialization of therapeutic contact lenses. Also issues like drug stability during processing/fabrication (drug integrity test), zero order release kinetics (prevent burst release), drug release during monomer extraction step after fabrication (to remove un-reacted monomers), protein adherence, drug release during storage in packaging solution, shelf life study, cost-benefit analysis, etc. are still to be addressed. This review provides an expert opinion on different methodology to develop therapeutic contact lenses with special remark of their advantages and limitations.

Chitosan grafted methoxy poly(ethylene glycol)-poly(ε-caprolactone) nanosuspension for ocular delivery of hydrophobic diclofenac

This study aimed to develop a cationic nanosuspension of chitosan (CS) and methoxy poly(ethylene glycol)-poly(ε-caprolactone) (MPEG-PCL) for ocular delivery of diclofenac (DIC). MPEG-PCL-CS block polymer was synthesized by covalent coupling of MPEG-PCL with CS. The critical micelle concentration of the MPEG-PCL-CS block polymer was 0.000692 g/L. DIC/MPEG-PCL-CS nanosuspension (mean particle size = 105 nm, zeta potential = 8 mV) was prepared and characterized by Fourier transform infrared spectroscopy, X-ray diffraction, and differential scanning calorimetry. The nanosuspension was very stable without apparent physical property changes after storage at 4 °C or 25 °C for 20 days, but it was unstable in the aqueous humor solution after 24 h incubation. Sustained release of the encapsulated DIC from the nanosuspension occurred over 8 h. Neither a blank MPEG-PCL-CS nanosuspension nor a 0.1% (mass fraction) DIC/MPEG-PCL-CS nanosuspension caused ocular irritation after 24 h of instillation. Enhanced penetration and retention in corneal tissue was achieved with a Nile red/MPEG-PCL-CS nanosuspension compared with a Nile red aqueous solution. In vivo pharmacokinetics studies showed enhanced pre-corneal retention and penetration of the DIC/MPEG-PCL-CS nanosuspension, which resulted in a higher concentration of DIC (C_max) in the aqueous humor and better bioavailability compared with commercial DIC eye drops (P < 0.01).

Development of a Layer-by-Layer Assembled Film on Hydrogel for Ocular Drug Delivery

Hydrogel is a kind of attractive drug carriers because of its good biocompatibility and transparency. But traditional hydrogel showed some restrictions in its application in ocular drug delivery. A simple surface modification technique based on layer-by-layer (LbL) self-assembled multilayer for ocular drug delivery was developed in this work. Polycarboxymethyl-β-cyclodextrin (poly(CM-β-CD))/poly-l-lysine (PLL) multilayer film was designed and constructed for ocular drug delivery, sinceβ-CD showed good drug delivery property. The properties such as the contact angle and transparency varied a little with the deposition of poly(CM-β-CD)/PLL multilayer. Orfloxacin and puerarin were loaded into multilayer during the self-assembly procedure by two methods, which were tracked by the largest drug absorbance of UV spectrum. The loaded drug amount by incorporating drugs into poly(CM-β-CD) solution was larger than that by incorporating drugs into PLL solution. The loaded drug in the multilayer could gradually be released from multilayer in some period either for orfloxacin or for puerarin. The drug release behavior was influenced by drug loading method and pH value of released medium. Moreover, the balanced released drug amount by incorporating drugs into poly(CM-β-CD) solution is much smaller than that by incorporating drugs into PLL solution.

Multi-arm PEG/silica hydrogel for sustained ocular drug delivery

In the present study, a series of sustained drug delivery multiarm poly(ethylene glycol) (PEG)/silica hydrogels were prepared and characterized. The hydrogels were formed by hydrolysis and condensation of poly(4-arm PEG silicate) using the sol-gel method. The relationships between water content in the PEG/silica hydrogel and stability as well as rheological properties were evaluated. Scanning electron microscopy analysis of the PEG/silica hydrogels revealed water content-dependent changes in microstructure. An increase in water content resulted in larger pores within the hydrogel, longer gelation time and higher viscosity. The PEG/silica hydrogels were loaded with dexamethasone (DMS) or dexamethasone sodium phosphate (DMSP), drugs that are hydrophobic and hydrophilic in nature, respectively. Evaluation of in vitro release revealed a zero-order release profile for DMS over the first 6 days, suggesting that degradation of the silica hydrogel matrix was the primary mechanism of drug release. It was also found that the drug-release profile could be tailored by varying the water content used during hydrogel preparation. In contrast, more than 90% of DMSP was released within 1 h, suggesting that DMSP release was only controlled by diffusion. Overall, results from this study indicate that PEG/silica hydrogels may be promising drug-eluting depot materials for the sustained delivery of hydrophobic, ophthalmic drugs.

Soft contact lenses for controlled ocular delivery: 50 years in the making

The use of contact lenses as ocular bandages for drug delivery was envisioned nearly 50 years ago by Wichterle and co-workers. Despite the therapeutic advantages that can be obtained, this application has to face up to the poor affinity shown by commercially available contact lenses for most ophthalmic drugs, resulting in small amounts of drug being loaded and short time of therapeutic levels in the eye structures. Novel strategies that appeared in the beginning of 21st century, for example coating lenses with vitamin E, incorporation of drug nanocarriers or application of molecular imprinting technology, are becoming relevant tools for development of true drug/contact lens combination products that may be available for ocular therapy in the foreseeable future.