Polymer based sustained drug delivery to the ocular posterior segment: barriers and future opportunities for the treatment of neovascular pathologies

Extended siponimod release via low-porosity PLGA fibres: a comprehensive three-month in vitro evaluation for neovascular ocular diseases

Neovascular ocular diseases are among the most common causes of preventable or treatable vision loss. Their management involves lifelong, intravitreal injections of anti-vascular endothelial growth factor (VEGF) therapeutics to inhibit neovascularization, the key pathological step in these diseases. Anti-VEGF products approved for ocular administration are expensive biological agents with limited stability and short half-life. Additionally, their therapeutic advantages are hindered by high treatment resistance, poor patient compliance and the need for frequent, invasive administration. Herein, we used electrospinning to develop a unique, non-porous, PLGA implant for the ocular delivery of siponimod to improve ocular neovascular disease management. Siponimod is an FDA-approved drug for multiple sclerosis with a novel indication as a potential ocular angiogenesis inhibitor. The electrospinning conditions were optimised to produce a microfibrous, PLGA matte that was cut and rolled into the desired implant size. Physical characterisation techniques (Raman, PXRD, DSC and FTIR) indicated siponimod was distributed uniformly within the electrospun fibres as a stabilised, amorphous, solid dispersion with a character modifying drug-polymer interaction. Siponimod dispersion and drug-polymer interactions contributed to the formation of smooth fibres, with reduced porous structures. The apparent reduced porosity, coupled with the drug's hydrophobic dispersion, afforded resistance to water penetration. This led to a slow, controlled, Higuchi-type drug diffusion, with ∼30% of the siponimod load released over 90 days. The released drug inhibited human retinal microvascular endothelial cell migration and did not affect the cells' metabolic activity at different time points. The electrospun implant was physically stable after incubation under stress conditions for three months. This novel siponimod intravitreal implant broadens the therapeutic possibilities for neovascular ocular diseases, representing a potential alternative to biological, anti-VEGF treatments due to lower financial and stability burdens. Additionally, siponimod interaction with PLGA provides a unique opportunity to sustain the drug release from the electrospun fibres, thereby reducing the frequency of intravitreal injection and improving patient adherence.

Fabrication of acetazolamide loaded leciplex for intraocular delivery: Optimization by 32 full factorial design, in vitro, ex vivo and in vivo pharmacodynamics

The complex structure of the eye poses challenges in delivering drugs effectively, which can be circumvented by employing nanotechnologies. The present study aimed to prepareacetazolamide-loadedleciplex (ACZ - LP) using a simple one-step fabrication approach followed byoptimization employing a 32 Full Factorial Design. The ACZ - LP demonstrated high entrapment efficiency (93.25 ± 2.32 %), average diameter was recorded around 171.03 ± 3.32 with monodisperse size distribution and zeta potential of 41.33 ± 2.10 mV. Invitro release and ex vivo permeation studies of prepared formulation demonstrated an initial burst release in 1 h followed by sustained release pattern as compared to plain acetazolamide solution. Moreover, an ex vivo corneal drug retention (27.05 ± 1.20 %) and in vitro mucoadhesive studies with different concentration of mucin indicated strong electrostatic bonding confirming the mucoadhesive characteristics of the formulation. Additionally, the histopathological studies ensured that the formulation was non-irritant and nontoxic while and HET-CAM ensured substantial tolerability of the formulation. The in vivo pharmacodynamic investigation carried out on a rabbit model demonstrated that treatment with ACZ - LP resulted in a significant and prolonged reduction in intraocular pressure as compared to plain acetazolamide solution, acetazolamide oral tablet, and Brinzox®. In summary, the ACZ - LP is anefficient and versatile drug delivery approach which demonstrates significant potential in controlling glaucoma.

Long-acting delivery and therapies for neovascular age-related macular degeneration

INTRODUCTION

Neovascular age-related macular degeneration (nAMD) represents a leading cause of severe visual impairment in individuals over 50 years of age in developed nations. Intravitreal anti-vascular endothelial growth factor (VEGF) injections have become the standard of care for treating nAMD; however, monthly or bimonthly dosing represents significant time and cost burden due to the disease's chronic nature and limited medication half-life.

AREAS COVERED

This review summarizes innovative therapeutics and delivery methods for nAMD. Emerging methods for extended drug delivery include high molar concentration anti-VEGF drugs, intravitreal sustained-release polymers and devices, reservoirs for intravitreal delivery, suprachoroidal delivery of small molecular suspensions and gene therapy biofactories. In addition to VEGF-A, therapies targeting inhibition of VEGF-C and D, the angiopoetin-2 (Ang-2)/Tie-2 pathway, tyrosine kinases, and integrins are reviewed.

EXPERT OPINION

The evolving therapeutic landscape of nAMD is rapidly expanding our toolkit for effective and durable treatment. Recent FDA approvals of faricimab (Vabysmo) and high-dose aflibercept (Eylea HD) for nAMD with potential extension of injection intervals up to four months have been promising developments for patients and providers alike. Further research and innovation, including novel delivery techniques and pharmacologic targets, is necessary to validate the efficacy of developing therapeutics and characterize real-world outcomes, demonstrating promise in expanding treatment durability.

Multifunctional Nanotherapeutics with Long-Acting Release against Macular Degeneration by Minimally Invasive Administration

Neovascular age-related macular degeneration (AMD), a leading cause of blindness, requires frequent intravitreal injection of antivascular endothelial growth factor (anti-VEGF), which could generate a succession of complications with poor patient compliance. The current VEGF-targeting therapies often fail in half of patients due to the complex pathologic microenvironment of excessive reactive oxygen species (ROS) production, and increased levels of inflammation are accompanied by choroidal neovascularization (CNV). We herein reported multifunctional nanotherapeutics featuring superior antioxidant and anti-inflammation properties that aim to reverse the pathological condition, alongside its strong targeted antiangiogenesis to CNV and its ability to provide long-term sustained bioactive delivery via the minimally invasive subconjunctival injection, so as to achieve satisfactory wet AMD treatment effects. Concretely, the nanomedicine was designed by coencapsulation of astaxanthin (AST), a red pigmented carotenoid known for its antioxidative, anti-inflammatory and antiapoptotic properties, and axitinib (AXI), a small molecule tyrosine kinase inhibitor that selectively targets the vascular epidermal growth factor receptor for antiangiogenesis, into the Food and Drug Administration (FDA) approved poly(lactic-co-glycolic acid) (PLGA), which forms the nanodrug of PLGA@AST/AXI. Our results demonstrated that a single-dose subconjunctival administration of PLGA@AST/AXI showed a rational synergistic effect by targeting various prevailing risk factors associated with wet AMD, ensuring persistent drug release profiles, maintaining good ocular biocompatibility, and causing no obvious mechanical damage. Such attributes are vital and hold significant potential in treating ocular posterior segment diseases. Moreover, this nanotherapeutic strategy represents a versatile and broad-spectrum nanoplatform, offering a promising alternative for the complex pathological progression of other neovascular diseases.

ROS-responsive charge reversal mesoporous silica nanoparticles as promising drug delivery system for neovascular retinal diseases

Intravitreal injection of biodegradable implant drug carriers shows promise in reducing the injection frequency for neovascular retinal diseases. However, current intravitreal ocular devices have limitations in adjusting drug release rates for individual patients, thereby affecting treatment effectiveness. Accordingly, we developed mesoporous silica nanoparticles (MSNs) featuring a surface that reverse its charge in response to reactive oxygen species (ROS) for efficient delivery of humanin peptide (HN) to retinal epithelial cells (ARPE-19). The MSN core, designed with a pore size of 2.8 nm, ensures a high HN loading capacity 64.4% (w/w). We fine-tuned the external surface of the MSNs by incorporating 20% Acetyl-L-arginine (Ar) to create a partial positive charge, while 80% conjugated thioketal (TK) methoxy polyethylene glycol (mPEG) act as ROS gatekeeper. Ex vivo experiments using bovine eyes revealed the immobilization of Ar-MSNs-TK-PEG (mean zeta potential: 2 mV) in the negatively charged vitreous. However, oxidative stress reversed the surface charge to -25 mV by mPEG loss, facilitating the diffusion of the nanoparticles impeded with HN. In vitro studies showed that ARPE-19 cells effectively internalize HN-loaded Ar-MSNs-TK, subsequently releasing the peptide, which offered protection against oxidative stress-induced apoptosis, as evidenced by reduced TUNEL and caspase3 activation. The inhibition of retinal neovascularization was further validated in an in vivo oxygen-induced retinopathy (OIR) mouse model.

Polymers and their engineered analogues for ocular drug delivery: Enhancing therapeutic precision

Ocular drug delivery is constrained by anatomical and physiological barriers, necessitating innovative solutions for effective therapy. Natural polymers like hyaluronic acid, chitosan, and gelatin, alongside synthetic counterparts such as PLGA and PEG, have gained prominence for their biocompatibility and controlled release profiles. Recent strides in polymer conjugation strategies have enabled targeted delivery through ligand integration, facilitating tissue specificity and cellular uptake. This versatility accommodates combined drug delivery, addressing diverse anterior (e.g., glaucoma, dry eye) and posterior segment (e.g., macular degeneration, diabetic retinopathy) afflictions. The review encompasses an in-depth exploration of each natural and synthetic polymer, detailing their individual advantages and disadvantages for ocular drug delivery. By transcending ocular barriers and refining therapeutic precision, these innovations promise to reshape the management of anterior and posterior segment eye diseases.

In Vivo Ocular Pharmacokinetics and Toxicity of Siponimod in Albino Rabbits

Siponimod is a promising agent for the inhibition of ocular neovascularization in diabetic retinopathy and age-related macular degeneration. Siponimod's development for ophthalmological application is hindered by the limited information available on the drug's solubility, stability, ocular pharmacokinetics (PK), and toxicity in vivo. In this study, we investigated the aqueous stability of siponimod under stress conditions (up to 60 °C) and its degradation behavior in solution. Additionally, siponimod's ocular PK and toxicity were investigated using intravitreal injection of two different doses (either 1300 or 6500 ng) in an albino rabbit model. Siponimod concentration was quantified in the extracted vitreous, and the PK parameters were calculated. The drug half-life after administration of the low and high doses was 2.8 and 3.9 h, respectively. The data obtained in vivo was used to test the ability of published in silico models to predict siponimod's PK accurately. Two models that correlated siponimod's molecular descriptors with its elimination from the vitreous closely predicted the half-life. Furthermore, 24 h and 7 days after intravitreal injections, the retinas showed no signs of toxicity. This study provides important information necessary for the formulation and development of siponimod for ophthalmologic applications. The short half-life of siponimod necessitates the development of a sustained drug delivery system to maintain therapeutic concentrations over an extended period, while the lack of short-term ocular toxicity observed in the retinas of siponimod-treated rabbits supports possible clinical use.

Drug-eluting contact lenses: Progress, challenges, and prospects

Topical ophthalmic solutions (eye drops) are becoming increasingly popular in treating and preventing ocular diseases for their safety, noninvasiveness, and ease of handling. However, the static and dynamic barriers of eyes cause the extremely low bioavailability (<5%) of eye drops, making ocular therapy challenging. Thus, drug-eluting corneal contact lenses (DECLs) have been intensively investigated as a drug delivery device for their attractive properties, such as sustained drug release and improved bioavailability. In order to promote the clinical application of DECLs, multiple aspects, i.e., drug release and penetration, safety, and biocompatibility, of these drug delivery systems were thoroughly examined. In this review, we systematically discussed advances in DECLs, including types of preparation materials, drug-loading strategies, drug release mechanisms, strategies for penetrating ocular barriers, in vitro and in vivo drug delivery and penetration detection, safety, and biocompatibility validation methods, as well as challenges and future perspectives.

Multifunctional nano-in-micro delivery systems for targeted therapy in fundus neovascularization diseases

Fundus neovascularization diseases are a series of blinding eye diseases that seriously impair vision worldwide. Currently, the means of treating these diseases in clinical practice are continuously evolving and have rapidly revolutionized treatment opinions. However, key issues such as inadequate treatment effectiveness, high rates of recurrence, and poor patient compliance still need to be urgently addressed. Multifunctional nanomedicine can specifically respond to both endogenous and exogenous microenvironments, effectively deliver drugs to specific targets and participate in activities such as biological imaging and the detection of small molecules. Nano-in-micro (NIM) delivery systems such as metal, metal oxide and up-conversion nanoparticles (NPs), quantum dots, and carbon materials, have shown certain advantages in overcoming the presence of physiological barriers within the eyeball and are widely used in the treatment of ophthalmic diseases. Few studies, however, have evaluated the efficacy of NIM delivery systems in treating fundus neovascular diseases (FNDs). The present study describes the main clinical treatment strategies and the adverse events associated with the treatment of FNDs with NIM delivery systems and summarizes the anatomical obstacles that must be overcome. In this review, we wish to highlight the principle of intraocular microenvironment normalization, aiming to provide a more rational approach for designing new NIM delivery systems to treat specific FNDs.

Ocular RNA nanomedicine: engineered delivery nanoplatforms in treating eye diseases

Ocular disorders remain a major global health challenge with unmet medical needs. RNA nanomedicine has shown significant therapeutic benefits and safety profiles in patients with complex eye disorders, already benefiting numerous patients with gene-related eye disorders. The effective delivery of RNA to the unique structure of the eye is challenging owing to RNA instability, off-target effects, and ocular physiological barriers. Specifically tailored RNA medication, coupled with sophisticated engineered delivery platforms, is crucial to guide and advance developments in treatments for oculopathy. Herein we review recent advances in RNA-based nanomedicine, innovative delivery strategies, and current clinical progress and present challenges in ocular disease therapy.

Combination of PEGylation and Cationization on Phospholipid-Coated Cyclosporine Nanosuspensions for Enhanced Ocular Drug Delivery

Strong precorneal clearance mechanisms including reflex blink, constant tear drainage, and rapid mucus turnover constitute great challenges for eye drops for effective drug delivery to the ocular epithelium. In this study, cyclosporine A (CsA) for the treatment of dry eye disease (DED) was selected as the model drug. Two strategies, PEGylation for mucus penetration and cationization for potent cellular uptake, were combined to construct a novel CsA nanosuspension (NS@lipid-PEG/CKC) by coating nanoscale drug particles with a mixture of lipids, DSPE-PEG2000, and a cationic surfactant, cetalkonium chloride (CKC). NS@lipid-PEG/CKC with the mean size ∼173 nm and positive zeta potential ∼+40 mV showed promoted mucus penetration, good cytocompatibility, more cellular uptake, and prolonged precorneal retention without obvious ocular irritation. More importantly, NS@lipid-PEG/CKC recovered tear production and goblet cell density more efficiently than the commercial cationic nanoemulsion on a dry eye disease rat model. All results indicated that a combination of PEGylation and cationization might provide a promising strategy to coordinate mucus penetration and cellular uptake for enhanced drug delivery to the ocular epithelium for nanomedicine-based eye drops.

In-situ forming biodegradable implants for sustained Fluocinolone acetonide release to the posterior eye: In-vitro and in-vivo investigations in rabbits

Delivering medication to the posterior segment of the eye presents a significant challenge. Intravitreal injection has emerged as the preferred method for drug delivery to this area. However, current injectable non-biodegradable implants for fluocinolone acetonide (FA) require surgical removal after prolonged drug release, potentially affecting patient compliance. This study aimed to develop an in-situ forming biodegradable implant (ISFBI) optimal formulation containing PLGA504H and PLGA756S (50:50 w/w%) with the additive NMP solvent. The goal was to achieve slow and controlled release of FA over a two-month period with lower burst release, following a single intravitreal injection. Through morphology, rheology, stability and in-vitro release evaluations, the optimal formulation demonstrated low viscosity (0.12-1.25 Pa. s) and sustained release of FA at a rate of 0.36 µg/day from the third day up to two months. Furthermore, histopathology and in-vivo studies were conducted after intravitreal injection of the optimal formulation in rabbits' eye. Pharmacokinetic analysis demonstrated mean residence time (MRT) of 20.02 ± 0.6 days, half-life (t1/2) of 18.80 ± 0.4 days, and clearance (Cl) of 0.29 ± 0.03 ml/h for FA in the vitreous humor, indicating sustained and slow absorption of FA by the targeted retinal tissue from vitrea over the two-month period and eliminating through the anterior section of the eye, as revealed by its presence in the aqueous humor. Additionally, FA exhibited no detection in the blood and no evidence of systemic side effects or damage on the retinal layer and other organs. Based on these findings, it can be concluded that in-situ forming injectable biodegradable PLGA implants can show promise as a long-acting and controlled-release system for intraocular drug delivery.

Light-responsive polymeric nanoparticles for retinal drug delivery: design cues, challenges and future perspectives

A multitude of sight-threatening retinal diseases, affecting hundreds of millions around the globe, lack effective pharmacological treatments due to ocular barriers and common drug delivery limitations. Polymeric nanoparticles (PNPs) are versatile drug carriers with sustained drug release profiles and tunable physicochemical properties which have been explored for ocular drug delivery to both anterior and posterior ocular tissues. PNPs can incorporate a wide range of drugs and overcome the challenges of conventional retinal drug delivery. Moreover, PNPs can be engineered to respond to specific stimuli such as ultraviolet, visible, or near-infrared light, and allow precise spatiotemporal control of the drug release, enabling tailored treatment regimens and reducing the number of required administrations. The objective of this study is to emphasize the therapeutic potential of light-triggered drug-loaded polymeric nanoparticles to treat retinal diseases through an exploration of ocular pathologies, challenges in drug delivery, current production methodologies and recent applications. Despite challenges, light-responsive PNPs hold the promise of substantially enhancing the treatment landscape for ocular diseases, aiming for an improved quality of life for patients.

Investing in vision: Innovation in retinal therapeutics and the influence on venture capital investment

Since the groundbreaking approval of the first anti-VEGF therapy in 2004, the retinal therapeutics field has undergone a remarkable transformation, witnessing a surge in novel, disease-modifying therapeutics for a broad spectrum of retinal diseases, extending beyond exudative VEGF-driven conditions. The surge in scientific advancement and the pressing, unmet, medical need have captured the attention of venture capital investors, who have collectively invested close to $10 billion in research and development of new retinal therapeutics between 2004 and 2023. Notably, the field of exudative diseases has gradually shifted away from trying to outcompete anti-VEGF therapeutics towards lowering the overall treatment burden by reducing injection frequency. Simultaneously, a new era has emerged in the non-exudative field, targeting prevalent conditions like dry AMD and rare indications such as Retinitis pigmentosa. This has led to promising drug candidates in development, culminating in the landmark approval of Luxturna for a rare form of Retinitis pigmentosa. The validation of new mechanisms, such as the complement pathway in dry AMD has paved the way for the approvals of Syvovre (Apellis) and Izervay (Iveric/Astellas), marking the first two therapies for this condition. In this comprehensive review, we share our view on the cumulative lessons from the past two decades in developing retinal therapeutics, covering both positive achievements and challenges. We also contextualize the investments, strategic partnering deals, and acquisitions of biotech companies, pharmaceutical companies venture capital investors in retinal therapeutics, respectively. Finally, we provide an outlook and potentially a forward-looking roadmap on novel retinal therapeutics, highlighting the emergence of potential new intervention strategies, such as cell-based therapies, gene editing, and combination therapies. We conclude that upcoming developments have the potential to further stimulate venture capital investments, which ultimately could facilitate the development and delivery of new therapies to patients in need.

Fabrication of architectonic nanosponges for intraocular delivery of Brinzolamide: An insight into QbD driven optimization, in vitro characterization, and pharmacodynamics

The intricate structure of the eye poses difficulties in drug targeting, which can be surmounted with the help of nanoformulation strategies. With this view, brinzolamide nanosponges (BNS) were prepared using the emulsion solvent evaporation technique and optimized via Box-Behnken statistical design. The optimized BNS were further incorporated into a poloxamer 407 in situ gel (BNS-ISG) and evaluated. The optimized BNS showed spherical morphology, entrapment efficiency of 83.12 ± 1.2 % with particle size of 114 ± 2.32 nm and PDI of 0.11 ± 0.01. The optimized BNS-ISG exhibited a pseudoplastic behavior and depicted a gelling temperature and gelation time of 35 ± 0.5 °C and 10 ± 2 s respectively. In-vitro release and ex- vivo permeation studies of BNS-ISG demonstrated a sustained release pattern as compared to Brinzox®. Additionally, the HET-CAM and in vitro cytotoxicity studies (using SIRC cell line) ensured that the formulation was non-irritant and nontoxic for ophthalmic delivery. The in vivo pharmacodynamic study using rabbit model depicted that BNS-ISG treatment significantly lowers the intra ocular pressure for prolonged period of time when compared with Brinzox®. In conclusion, the BNS-ISG is an efficient and scalable drug delivery system with significant potential as the targeted therapy of posterior segment eye diseases.

Nanomaterial surface modification toolkit: Principles, components, recipes, and applications

Surface-functionalized nanostructures are at the forefront of biotechnology, providing new opportunities for biosensors, drug delivery, therapy, and bioimaging applications. The modification of nanostructures significantly impacts the performance and success of various applications by enabling selective and precise targeting. This review elucidates widely practiced surface modification strategies, including click chemistry, cross-coupling, silanization, aldehyde linkers, active ester chemistry, maleimide chemistry, epoxy linkers, and other protein and DNA-based methodologies. We also delve into the application-focused landscape of the nano-bio interface, emphasizing four key domains: therapeutics, biosensing, environmental monitoring, and point-of-care technologies, by highlighting prominent studies. The insights presented herein pave the way for further innovations at the intersection of nanotechnology and biotechnology, providing a useful handbook for beginners and professionals. The review draws on various sources, including the latest research articles (2018-2023), to provide a comprehensive overview of the field.

Design, optimization and pharmacokinetic evaluation of PLGA phosphatidylcholine hybrid nanoparticles of triamcinolone acetonide loaded in situ gel for topical ocular delivery

Posterior uveitis (PU), which often has an autoimmune origin, can be treated effectively with synthetic glucocorticoid triamcinolone acetonide (TAA). Due to the limitations of topical TAA administration reaching the posterior segment of the eye, the drug is injected directly into the eye through an intravitreal injection. In this study, we prepared TAA loaded poly(lactic-co-glycolic acid) phosphatidylcholine hybrid nanoparticles (TAA-PLHNPs) using the principles of design of experiments (DoE) for topical ocular administration. The mean particle size (nm) and drug loading efficiency (LE%) for the optimized formulations were 163 ± 2.8 nm and 39 ± 1.9%, respectively. The TAA-PLHNPs were then loaded into the dual responsive in situ gel that we reported in our previous work. In vitro assessments were done to show that the formulations are safe for ocular administration. Finally, in vivo ocular pharmacokinetic studies were performed to compare pharmacokinetic parameters of TAA-PLHNPs and TAA-PLHNPs loaded in situ gel with each other and with the previously reported conventional formulation of TAA (aqueous suspension of TAA with 20% hydroxypropyl β-cyclodextrin (TAA-HP-β-CD-Susp)). TAA-PLHNPs loaded dual responsive in situ gel (TAA-PLHNP-ISG) achieved higher concentrations of TAA in the vitreous humor (Cmax of 946.53 ng/mL) and sustained (MRT0-∞ of 16.26 h) the drug concentrations for longer period of time compared to aqueous suspension of TAA-PLHNPs (TAA-PLHNP-Susp) and TAA-HP-β-CD-Susp.

Trends in Formulation Approaches for Sustained Drug Delivery to the Posterior Segment of the Eye

The eye, an intricate organ comprising physical and physiological barriers, poses a significant challenge for ophthalmic physicians seeking to treat serious ocular diseases affecting the posterior segment, such as age-related macular degeneration (AMD) and diabetic retinopathy (DR). Despite extensive efforts, the delivery of therapeutic drugs to the rear part of the eye remains an unresolved issue. This comprehensive review delves into conventional and innovative formulation strategies for drug delivery to the posterior segment of the eye. By utilizing alternative nanoformulation approaches such as liposomes, nanoparticles, and microneedle patches, researchers and clinicians can overcome the limitations of conventional eye drops and achieve more effective drug delivery to the posterior segment of the eye. These innovative strategies offer improved drug penetration, prolonged residence time, and controlled release, enhancing therapeutic outcomes for ocular diseases. Moreover, this article explores recently approved delivery systems that leverage diverse polymer technologies, such as chitosan and hyaluronic acid, to regulate drug-controlled release over an extended period. By offering a comprehensive understanding of the available formulation strategies, this review aims to empower researchers and clinicians in their pursuit of developing highly effective treatments for posterior-segment ocular diseases.

An insight on ophthalmic drug delivery systems: Focus on polymeric biomaterials-based carriers

Presently, different types of eye diseases, such as glaucoma, myopia, infection, and dry eyes are treated with topical eye drops. However, due to ocular surface barriers, eye drops require multiple administrations, which may cause several risks, thereby necessitating additional strategies. Some of the key characteristics of an ideal ocular drug delivery system are as follows: (a) good penetration into cornea, (b) high drug retention in the ocular tissues, (c) targetability to the desired regions of the eye, and (d) good bioavailability. It is worthy to note that the corneal epithelial tight junctions hinder the permeation of therapeutics through the cornea. Therefore, it is necessary to design nanocarriers that can overcome these barriers and enhance drug penetration into the inner parts of the eye. Moreover, intelligent multifunctional nanocarriers can be designed to include cavities, which may help encapsulate sufficient amount of the drug. In addition, nanocarriers can be modified with the targeting moieties. Different types of nanocarriers have been developed for ocular drug delivery applications, including emulsions, liposomes, micelles, and nanoparticles. However, these formulations may be rapidly cleared from the eye. The therapeutic use of the nanoparticles (NPs) is also hindered by the non-specific adsorption of proteins on NPs, which may limit their interaction with the cellular moieties or other targeted biological factors. Functional drug delivery systems (DDS), which can offer targeted ocular drug delivery while avoiding the non-specific protein adsorption could exhibit great potential. This could be further realized by the on-demand DDS, which can respond to the stimuli in a spatio-temporal fashion. The cell-mediated DDS offer another valuable platform for ophthalmological drug delivery.

Development and Bioactivity of Zinc Sulfate Cross-Linked Polysaccharide Delivery System of Dexamethasone Phosphate

Improving the biopharmaceutical properties of glucocorticoids (increasing local bioavailability and reducing systemic toxicity) is an important challenge. The aim of this study was to develop a dexamethasone phosphate (DexP) delivery system based on hyaluronic acid (HA) and a water-soluble cationic chitosan derivative, diethylaminoethyl chitosan (DEAECS). The DexP delivery system was a polyelectrolyte complex (PEC) resulting from interpolymer interactions between the HA polyanion and the DEAECS polycation with simultaneous incorporation of zinc ions as a cross-linking agent into the complex. The developed PECs had a hydrodynamic diameter of 244 nm and a ζ-potential of +24.4 mV; the encapsulation efficiency and DexP content were 75.6% and 45.4 μg/mg, respectively. The designed DexP delivery systems were characterized by both excellent mucoadhesion and prolonged drug release (approximately 70% of DexP was released within 10 h). In vitro experiments showed that encapsulation of DexP in polysaccharide nanocarriers did not reduce its anti-inflammatory activity compared to free DexP.

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.

Microbubble-Assisted Ultrasound for Drug Delivery to the Retina in an Ex Vivo Eye Model

Drug delivery to the retina is one of the major challenges in ophthalmology due to the biological barriers that protect it from harmful substances in the body. Despite the advancement in ocular therapeutics, there are many unmet needs for the treatment of retinal diseases. Ultrasound combined with microbubbles (USMB) was proposed as a minimally invasive method for improving delivery of drugs in the retina from the blood circulation. This study aimed to investigate the applicability of USMB for the delivery of model drugs (molecular weight varying from 600 Da to 20 kDa) in the retina of ex vivo porcine eyes. A clinical ultrasound system, in combination with microbubbles approved for clinical ultrasound imaging, was used for the treatment. Intracellular accumulation of model drugs was observed in the cells lining blood vessels in the retina and choroid of eyes treated with USMB but not in eyes that received ultrasound only. Specifically, 25.6 ± 2.9% of cells had intracellular uptake at mechanical index (MI) 0.2 and 34.5 ± 6.0% at MI 0.4. Histological examination of retinal and choroid tissues revealed that at these USMB conditions, no irreversible alterations were induced at the USMB conditions used. These results indicate that USMB can be used as a minimally invasive targeted means to induce intracellular accumulation of drugs for the treatment of retinal diseases.

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.

Overcoming Treatment Challenges in Posterior Segment Diseases with Biodegradable Nano-Based Drug Delivery Systems

Posterior segment eye diseases present a challenge in treatment due to the complex structures in the eye that serve as robust static and dynamic barriers, limiting the penetration, residence time, and bioavailability of topical and intraocular medications. This hinders effective treatment and requires frequent dosing, such as the regular use of eye drops or visits to the ophthalmologist for intravitreal injections, to manage the disease. Moreover, the drugs must be biodegradable to minimize toxicity and adverse reactions, as well as small enough to not affect the visual axis. The development of biodegradable nano-based drug delivery systems (DDSs) can be the solution to these challenges. First, they can stay in ocular tissues for longer periods of time, reducing the frequency of drug administration. Second, they can pass through ocular barriers, offering higher bioavailability to targeted tissues that are otherwise inaccessible. Third, they can be made up of polymers that are biodegradable and nanosized. Hence, therapeutic innovations in biodegradable nanosized DDS have been widely explored for ophthalmic drug delivery applications. In this review, we will present a concise overview of DDSs utilized in the treatment of ocular diseases. We will then examine the current therapeutic challenges faced in the management of posterior segment diseases and explore how various types of biodegradable nanocarriers can enhance our therapeutic arsenal. A literature review of the pre-clinical and clinical studies published between 2017 and 2023 was conducted. Through the advances in biodegradable materials, combined with a better understanding of ocular pharmacology, the nano-based DDSs have rapidly evolved, showing great promise to overcome challenges currently encountered by clinicians.

NIR photothermal-activable drug-conjugated microcapsules for in vitro targeted delivery and release: An alternative treatment of diabetic retinopathy

Diabetic retinopathy (DR) is one of the most serious complications of diabetes, which leads to blindness. By addressing the traditional treatment limitations, we developed a novel light-responsive targeted polymeric microcapsule able to encapsulate a near infrared (NIR) photoactive fluorophore - Indocyanine Green, owing to its photothermal properties. Moreover, for an efficient in vitro targeted drug delivery, the fluorescent microsystem was conjugated with a therapeutic agent, i.e., Avastin drug - a Food and Drug Administration approved therapeutic antibody. The microcapsules were fabricated and evaluated in terms of morphology, encapsulation and drug conjugation efficiency and its release capacity. Avastin-conjugated microcapsules with an average dimension of 4.5 ± 0.35 μm were obtained, according to Scanning Electron Microscopy and Re-Scanning Confocal Microscopy (RCM) investigations. The capacity of the microcapsules to operate as effective phototherapeutic agents by generating heat under NIR laser irradiation was evaluated, followed by the investigation of the microcapsule's shell rupture and NIR laser-induced release of Avastin. The biocompatibility of the Avastin-conjugated microcapsules was proven by WST-1 assay. In vitro cellular internalization and localization of the Avastin microcarriers were determined through Conventional fluorescence microscopy, RCM and Transmission Electron Microscopy imaging techniques. Finally, the Avastin-conjugated microcapsules were validated for in vitro targeted drug delivery and release directly under simulated DR conditions, which could certainly become a successful strategy in DR fighting.

Recent Progress of Solid Lipid Nanoparticles and Nanostructured Lipid Carriers as Ocular Drug Delivery Platforms

Sufficient ocular bioavailability is often considered a challenge by the researchers, due to the complex structure of the eye and its protective physiological mechanisms. In addition, the low viscosity of the eye drops and the resulting short ocular residence time further contribute to the observed low drug concentration at the target site. Therefore, various drug delivery platforms are being developed to enhance ocular bioavailability, provide controlled and sustained drug release, reduce the number of applications, and maximize therapy outcomes. Solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) exhibit all these benefits, in addition to being biocompatible, biodegradable, and susceptible to sterilization and scale-up. Furthermore, their successive surface modification contributes to prolonged ocular residence time (by adding cationic compounds), enhanced penetration, and improved performance. The review highlights the salient characteristics of SLNs and NLCs concerning ocular drug delivery, and updates the research progress in this area.

Intraocular nano-microscale drug delivery systems for glaucoma treatment: design strategies and recent progress

Glaucoma is a leading cause of irreversible visual impairment and blindness, affecting over 76.0 million people worldwide in 2020, with a predicted increase to 111.8 million by 2040. Hypotensive eye drops remain the gold standard for glaucoma treatment, while inadequate patient adherence to medication regimens and poor bioavailability of drugs to target tissues are major obstacles to effective treatment outcomes. Nano/micro-pharmaceuticals, with diverse spectra and abilities, may represent a hope of removing these obstacles. This review describes a set of intraocular nano/micro drug delivery systems involved in glaucoma treatment. Particularly, it investigates the structures, properties, and preclinical evidence supporting the use of these systems in glaucoma, followed by discussing the route of administration, the design of systems, and factors affecting in vivo performance. Finally, it concludes by highlighting the emerging notion as an attractive approach to address the unmet needs for managing glaucoma.

How can machine learning and multiscale modeling benefit ocular drug development?

The eyes possess sophisticated physiological structures, diverse disease targets, limited drug delivery space, distinctive barriers, and complicated biomechanical processes, requiring a more in-depth understanding of the interactions between drug delivery systems and biological systems for ocular formulation development. However, the tiny size of the eyes makes sampling difficult and invasive studies costly and ethically constrained. Developing ocular formulations following conventional trial-and-error formulation and manufacturing process screening procedures is inefficient. Along with the popularity of computational pharmaceutics, non-invasive in silico modeling & simulation offer new opportunities for the paradigm shift of ocular formulation development. The current work first systematically reviews the theoretical underpinnings, advanced applications, and unique advantages of data-driven machine learning and multiscale simulation approaches represented by molecular simulation, mathematical modeling, and pharmacokinetic (PK)/pharmacodynamic (PD) modeling for ocular drug development. Following this, a new computer-driven framework for rational pharmaceutical formulation design is proposed, inspired by the potential of in silico explorations in understanding drug delivery details and facilitating drug formulation design. Lastly, to promote the paradigm shift, integrated in silico methodologies were highlighted, and discussions on data challenges, model practicality, personalized modeling, regulatory science, interdisciplinary collaboration, and talent training were conducted in detail with a view to achieving more efficient objective-oriented pharmaceutical formulation design.

Polymer- and lipid-based nanocarriers for ocular drug delivery: Current status and future perspectives

Ocular diseases seriously affect patients' vision and life quality, with a global morbidity of over 43 million blindness. However, efficient drug delivery to treat ocular diseases, particularly intraocular disorders, remains a huge challenge due to multiple ocular barriers that significantly affect the ultimate therapeutic efficacy of drugs. Recent advances in nanocarrier technology offer a promising opportunity to overcome these barriers by providing enhanced penetration, increased retention, improved solubility, reduced toxicity, prolonged release, and targeted delivery of the loaded drug to the eyes. This review primarily provides an overview of the progress and contemporary applications of nanocarriers, mainly polymer- and lipid-based nanocarriers, in treating various eye diseases, highlighting their value in achieving efficient ocular drug delivery. Additionally, the review covers the ocular barriers and administration routes, as well as the prospective future developments and challenges in the field of nanocarriers for treating ocular diseases.

Framework Nucleic Acids as Blood-Retinal-Barrier-Penetrable Nanocarrier for Periocular Administration

Designing an ocular drugs delivery system that can permeate the outer blood-retinal barrier (oBRB) is crucial for the microinvasive or noninvasive treatment of ocular fundus diseases. However, due to the lack of a nanocarrier that can maintain structure and composition at the oBRB, only intravitreal injection at the eyeball can deliver therapeutics directly to the ocular fundus via paracellular and intercellular routes, despite the intraocular operations risks. Here, we demonstrated tetrahedral framework nucleic acids (tFNAs) can penetrate the oBRB and deliver therapeutic nucleic acids to the retina of the rat eye in vivo following subconjunctival injection. We also discovered that tFNAs were transported via a paracellular route across the intercellular tight junctions at the oBRB. The histology analysis for ocular layers indicated that individual and aptamer/doxorubicin-loaded tFNAs penetrated all layers of the posterior segment of the eyeball to reach the innermost retina and persisted for over 3 days with minimal systemic biodistribution. We expect that the programmability and penetrability of tFNAs will provide a promising method for drug delivery across oBRB and long-term sustenance at the target site via periocular administration to various tissues.

Ocular Delivery of Therapeutic Proteins: A Review

Therapeutic proteins, including monoclonal antibodies, single chain variable fragment (ScFv), crystallizable fragment (Fc), and fragment antigen binding (Fab), have accounted for one-third of all drugs on the world market. In particular, these medicines have been widely used in ocular therapies in the treatment of various diseases, such as age-related macular degeneration, corneal neovascularization, diabetic retinopathy, and retinal vein occlusion. However, the formulation of these biomacromolecules is challenging due to their high molecular weight, complex structure, instability, short half-life, enzymatic degradation, and immunogenicity, which leads to the failure of therapies. Various efforts have been made to overcome the ocular barriers, providing effective delivery of therapeutic proteins, such as altering the protein structure or including it in new delivery systems. These strategies are not only cost-effective and beneficial to patients but have also been shown to allow for fewer drug side effects. In this review, we discuss several factors that affect the design of formulations and the delivery of therapeutic proteins to ocular tissues, such as the use of injectable micro/nanocarriers, hydrogels, implants, iontophoresis, cell-based therapy, and combination techniques. In addition, other approaches are briefly discussed, related to the structural modification of these proteins, improving their bioavailability in the posterior segments of the eye without affecting their stability. Future research should be conducted toward the development of more effective, stable, noninvasive, and cost-effective formulations for the ocular delivery of therapeutic proteins. In addition, more insights into preclinical to clinical translation are needed.

A Comprehensive Review of Cross-Linked Gels as Vehicles for Drug Delivery to Treat Central Nervous System Disorders

Gels are attractive candidates for drug delivery because they are easily producible while offering sustained and/or controlled drug release through various mechanisms by releasing the therapeutic agent at the site of action or absorption. Gels can be classified based on various characteristics including the nature of solvents used during preparation and the method of cross-linking. The development of novel gel systems for local or systemic drug delivery in a sustained, controlled, and targetable manner has been at the epitome of recent advances in drug delivery systems. Cross-linked gels can be modified by altering their polymer composition and content for pharmaceutical and biomedical applications. These modifications have resulted in the development of stimuli-responsive and functionalized dosage forms that offer many advantages for effective dosing of drugs for Central Nervous System (CNS) conditions. In this review, the literature concerning recent advances in cross-linked gels for drug delivery to the CNS are explored. Injectable and non-injectable formulations intended for the treatment of diseases of the CNS together with the impact of recent advances in cross-linked gels on studies involving CNS drug delivery are discussed.