Diabetic eye: associated diseases, drugs in clinic, and role of self-assembled carriers in topical treatment

Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=iedd20 Expert Opinion on Drug Delivery ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/iedd20 Diabetic eye: associated diseases, drugs in clinic, and role of self-assembled carriers in topical treatment Axel Kattar, Angel Concheiro & Carmen Alvarez-Lorenzo To cite this article: Axel Kattar, Angel Concheiro & Carmen Alvarez-Lorenzo (2021) Diabetic eye: associated diseases, drugs in clinic, and role of self-assembled carriers in topical treatment, Expert Opinion on Drug Delivery, 18:11, 1589-1607, DOI: 10.1080/17425247.2021.1953466 To link to this article: https://doi.org/10.1080/17425247.2021.1953466 © 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. Published online: 01 Sep 2021. Submit your article to this journal Article views: 453 View related articles View Crossmark data REVIEW Diabetic eye: associated diseases, drugs in clinic, and role of self-assembled carriers in topical treatment Axel Kattar , Angel Concheiro and Carmen Alvarez-Lorenzo Departamento de Farmacología, Farmacia y Tecnología Farmacéutica, I+D Farma Group (GI-1645), Facultad de Farmacia and Health Research Institute of Santiago de Compostela (IDIS), Universidade de Santiago de Compostela, Santiago de Compostela, Spain ABSTRACT Introduction: Diabetes is a pandemic disease that causes relevant ocular pathologies. Diabetic retinopathy, macular edema, cataracts, glaucoma, or keratopathy strongly impact the quality of life of the patients. In addition to glycemic control, intense research is devoted to finding more efficient ocular drugs and improved delivery systems that can overcome eye barriers. Areas covered: The aim of this review is to revisit first the role of diabetes in the development of chronic eye diseases. Then, commercially available drugs and new candidates in clinical trials are tackled together with the pros and cons of their administration routes. Subsequent sections deal with self-assembled drug carriers suitable for eye instillation combining patient-friendly administration with high ocular bioavailability. Performance of topically administered polymeric micelles, liposomes, and niosomes for the management of diabetic eye diseases is analyzed in the light of ex vivo and in vivo results and outcomes of clinical trials. Expert opinion: Self-assembled carriers are being shown useful for efficient delivery of not only a variety of small drugs but also macromolecules (e.g. antibodies) and genes. Successful design of drug carriers may offer alternatives to intraocular injections and improve the treatment of both anterior and posterior segments diabetic eye diseases. ARTICLE HISTORY Received 27 April 2021 Accepted 6 July 2021 KEYWORDS Diabetic eye; topical ocular delivery; liposomes; polymeric micelles; niosomes; clinical trials 1. Introduction Diabetes mellitus currently affects 8.5% people worldwide, and it is expected to impact on the lives of 570 million people in 2025 [1]. There are five different forms of diabetes, with diabetes type 1 (failure in the production of insulin) and type 2 (deficient insulin sensitivity) being the most common. The other three forms are monogenic diabetes, which is hereditary due to a single gene mutation; gestational diabetes, related to pregnancy; and cystic fibrosis-related diabetes, which is linked to scarring of the pancreas that leads to insulin abnormalities. For people with type 1 diabetes, the immune system attacks pancreatic cells responsible for the production of insulin, disrupting their normal function. Both genetic and environmental factors have been identified as causal agents. Type 2 diabetes is indicative of insulin resistance, which may be caused by excess body weight. Diabetes is considered a pandemic disease with an increasing morbidity and the highest rate in years of life lost due to disability in both high-income and lower-middle-income countries [2]. Such a high incidence results in an increase in diseases secondary to diabetes. Diabetes-associated diseases range from cardiovascular problems to diabetic neuropathy including kidney failures and ocular diseases. Indeed, the term diabetic eye disease has a broad meaning as it may encompass multiple illnesses in different parts of the eye, mainly diabetic retinopathy, macular edema, cataracts, glaucoma, and keratopathy [3–6]. Control of hyperglycemia is a critical main measure to avoid a fast progression of damage in ocular structures. Depending on the time lag between the first symptoms of diabetes and effective regulation of glycemia levels, the eyes may already be affected when therapeutic measurements are taken. Therefore, early diagnosis may prevent the apparition of diabetes-associated diseases. Nevertheless, continuous high basal glucose levels over time inevitably cause damage to a wide variety of tissues, especially those where glucose is freely accessible [7]. Since diabetes is a chronic disease, ocular treatments may have to be applied for years. Therefore, finding drug delivery systems that combine the patient-friendly administration of topical formulations with the high ocular bioavailability of intraocular injections is an unmet clinical need. In the last decades, eye drops in which the drug molecules are encapsulated in nanocarriers have demonstrated notable enhancements in drug levels in both anterior and posterior eye segments [8,9]. Nanomicelles, liposomes, lipid nanoparticles, and polymer nanospheres provide protection against premature degradation, enhanced retention on eye surface, and novel pathways of penetration through corneal and transscleral routes [10,11]. Carriers that are spontaneously formed by self-assembly of their components in water are CONTACT Carmen Alvarez-Lorenzo [email protected] Departamento de Farmacología, Farmacia y Tecnología Farmacéutica, I+D Universidade de Santiago de Compostela, 15782- Santiago de Compostela, Spain EXPERT OPINION ON DRUG DELIVERY 2021, VOL. 18, NO. 11, 1589–1607 https://doi.org/10.1080/17425247.2021.1953466 © 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way. advantageous in terms of preparation and scale up because only few steps and energy are required. Polymeric micelles have outstanding capability to encapsulate hydrophobic drugs increasing apparent drug solubility, although concerns about premature disassembly may arise [12]. Lipid-based vesicles, such as liposomes, may host both hydrophobic and hydrophilic active substances [13], but the constituent lipids may be prone to chemical degradation [14,15]. In this context, niosomes as vesicles made of self-assembled nonionic surfactants may gather the advantages of both micelles and liposomes while providing improved physical and chemical stability. The development of niosomes as eye drops components is still incipient, but preliminary results have evidenced their potential [16]. Other supramolecular structures such as self-assembling polypeptides have shown to be excellent drug carriers for other administration routes and may offer new avenues in the treatment of eye diseases not only as carriers but also as therapeutic agents [17–19]. The aim of this review is to revisit first the role of diabetes in the development of eye diseases. Then, the pros and cons of different drug administration routes for diabetic eye treatments are considered. Commercially available drugs as well as those in clinical trials are analyzed in detail. Subsequent sections deal with the main self-assembled nano- and microcarriers suitable for topical eye administration, namely, polymeric micelles, liposomes, and niosomes. Nanocarriers for treatment of diabetes-associated diseases using other administration routes have been tackled elsewhere [20]. Finally, recent advances in the design of self-assembled carriers for topical diabetic eye drug administration are presented. Publications containing ex vivo or in vivo results or reporting on clinical trials have been prioritized. 2. Diabetic eye diseases Diabetes may affect the anterior segment triggering the development of cataracts, dry eye syndrome, corneal ulcers, warts, tortuous conjunctival vessels, and keratopathy [4,21]. In the posterior segment, diabetes may contribute to glaucoma, retinopathy, and macular edema [22]. Diabetic patients can also suffer from ocular neuropathy, difficulties in healing ocular wounds, and increased infection probabilities [4]. Cataracts are described as the clouding of the lens. Three pathogenesis routes have been described [23]: the polyol pathway where an accumulation of sorbitol induces hydropic lens fiber degeneration, the oxidative and osmotic stress leading to apoptosis of epithelial cells of the lens, and the autoimmunity. The polyol pathway is the one referred to most often and the most researched [24]. Sugars (e.g. glucose and galactose) at elevated concentrations are favorable substrates for aldose reductase and generate intense osmotic stress (as explained in Section 3.1). Also, reducing sugars promote glycation (i.e. non-enzymatic glycosylation) of lens proteins, triggering cataract formation [25]. Dry eye syndrome, which is characterized by an abnormal tear film, can be classified as either aqueous tear-deficient or evaporative (associated to a deficient tear film lipid layer) [26]. Direct correlations have been found between prevalence of the syndrome and glycated hemoglobin and duration of diabetes. In vivo studies suggest that lachrymal glands undergo histological changes in diabetic patients and that oxidative stress derived from hyperglycemia might be involved in dry eye syndrome [27]. Corneal ulcers are sores on the cornea that can alter the vision [28]. The primary cause of corneal ulcers may be unrelated to diabetes (eye surgery, accidents), but the lengthier wound healing process makes ulcers a heavier burden for diabetic patients [29,30]. Therapeutic approaches to deal with diabetic keratopathy have been reviewed elsewhere [31]. Glaucoma is the leading cause of blindness worldwide and is defined by damage of the retinal ganglion cells, leading to irreversible damage of the optic nerve [32]. This is often accompanied by a rise of the intraocular pressure (IOP) triggered through different mechanisms. Open-angle glaucoma is caused by blocking of the trabecular meshwork, which in turn hinders fluid drainage and increases pressure. This is the most common form of glaucoma and happens at slow pace. Angle closure glaucoma is provoked by the iris coming forward and blocking the drainage angle between the iris and the cornea. It can happen over time or suddenly. Secondary angle closure glaucoma, in which the angle can be opened or closed, is caused by a secondary factor that leads to drainage hindrance, for example excessive pigment release blocking the trabecular meshwork (pigmentary glaucoma). Although the role of diabetes is unclear, direct correlations were found between Article highlights ●Diabetes mellitus currently affects 8.5% people worldwide and causes the highest lost in years of quality of life due to disability. ●The term ‘diabetic eye disease’ has a broad meaning as it may encompass multiple illnesses in different eye tissues. ●Pharmacological treatments for diabetic eye rely on reducing the intraocular pressure, blocking the abnormal growth of blood vessels, or inhibiting negative chemical pathways. ●More than half a thousand clinical studies in phases 1–4 are currently in progress with a variety of drugs, biologics and gene therapy for diabetic eye diseases. ●Chronic ocular treatments demand drug delivery systems that can become an alternative to intraocular injections combining patientfriendly administration and high ocular bioavailability. ●Drug carriers for topical instillation prevent from premature degradation, enhance the retention on eye surface, and use alternative pathways for cornea and sclera penetration. ●Polymeric micelles with hydrophobic core and hydrophilic shell show a polarity gradient from inside to outside facilitating the encapsulation of apolar drugs. ●Bilayered vesicles formed by amphiphilic components assembled as concentric cell membrane-like bilayers show stepped apolar-polar regions, which allow for encapsulation of both hydrophilic and hydrophobic compounds. ●Niosomes as vesicles made of self-assembled nonionic surfactants gather the advantages of micelles and liposomes while providing improved physical and chemical stability. ●The development of drug-loaded niosomes as eye drops is still incipient, but preliminary results have evidenced their potential. ●Despite the success of subretinal and intravitreal administration of nioplexes, their suitability for topical ocular administration is still to be explored. ●This box summarizes key points contained in the article. 1590 A. KATTAR ET AL. diabetes duration and fasting glucose levels and the increase in IOP [33]. Diabetic retinopathy involves damage to the retinal blood vessels, which has disastrous effects on the retina [34]. In nonproliferative diabetic retinopathy, the retinal blood vessels are damaged and start leaking, which increases the pressure in the tissue. This condition can progress from microaneurysms to severe macular edema. In the proliferative diabetic retinopathy, the damaged blood vessels close and new blood vessels start to grow abnormally. The consequences may include increased IOP, accumulation of scar tissue, and ultimately nerve damage. Diabetic retinopathy affects one out of two persons with type 1 diabetes [35]. The duration of diabetes is a risk factor, and the prevalence of diabetic retinopathy increases from 8% after 3 years of diabetes to 80% after 15 years [36]. Diabetic macular edema may be a consequence of diabetic retinopathy, where leaking blood vessels increase the fluid volume in the macula, resulting in vision loss [37]. Two types of diabetic macular edema can be clinically differentiated: focal macular edema, which is characterized by microaneurysms on the retinal capillaries, and diffuse macular edema, which shows leakage from the blood-retinal barrier due to generalized damage [38]. 3. Delivery of drugs to diabetic eye 3.1. Main drug classes and current administration routes As described earlier, diabetes-related eye diseases are varied and evolve along time to become chronic. This means that their treatment should be designed according to the prolonged time the drug should be administered. In this regard, three main strategies are so far the most investigated and used ones: (i) topical administration, mostly for the treatment of anterior segment diseases, with the limitation of poor ocular bioavailability; (ii) intraocular administration, which is more efficient in terms of ocular bioavailability but entails relevant risks for the patient; and (iii) oral administration, which is the most patient friendly route, but the ocular bioavailability is quite low even when large doses are administered. Pros and cons of the different administration routes are explained in Table 1. Efficient ocular drug delivery is a difficult goal to reach in any case. Depending on the administration procedure and the dosage form, ocular barriers can be either physiological or anatomical, and either static or dynamic [39]. Different barriers located in the anterior and the posterior segments protect the eye against foreign substances coming from outside or inside the body (e.g. the bloodstream). Also, the goal of drug delivery can be different depending on the target cells. If the drug must reach a tissue protected by many barriers, drug permeation through these barriers is critical; prodrugs, penetration enhancers, and encapsulation in nanocarriers may be helpful tools [40,41]. If the aim is to continuously supply the drug to an area with important dynamic turnover of fluids, sustained drug delivery systems may be required [42]. Regarding topical administration, the eye is dynamically protected by the blinking reflex, the tear clearance rate, and the nasolacrimal drainage. Adhesion to the corneal surface and promotion of the penetration might overcome these barriers. Drugs can follow three routes: corneal, scleral, and conjunctival [43–47]. Major static corneal barriers are the corneal epithelium, which limits the absorption of macromolecules and hydrophilic drugs through tight junctions, and the corneal stroma, which limits the penetration of lipophilic molecules due to its high aqueous content [44]. Thus, midlipophilic drugs are the most suitable candidates for corneal penetration and subsequent diffusion through aqueous humor for intraocular distribution. The iris and the nonpigmented ciliary epithelium further block drugs from passing to the aqueous humor, making make up the blood–aqueous barrier [47]. Furthermore, the aqueous humor flow from the ciliary body to the cornea counteracts the diffusion of hydrophilic molecules trying to enter further in the eye. On this blood–aqueous barrier and also on the corneal epithelium, there are efflux pumps that expel the drugs back to the front of the eye [48]. The conjunctiva opposes to drug entry in the eye tissues mainly due to the presence of conjunctival blood capillaries and lymph vessels, which reroute a major fraction of the drug dose to the blood stream. Drug access through the conjunctiva to the posterior segment may occur via passive or active transport [45]. Although there are tight junctions among conjunctival epithelium cells, polar solutes up to 20 kDa can enter through paracellular diffusion across 5-nm pores [49]. Peptides and proteins may find the additional barrier of enzymatic degradation and require co-administration with a protease inhibitor [50]. Lipophilic drugs can still penetrate better via the transcellular route; the surface area is larger than for paracellular pathway, although efflux pumps pose a relevant challenge. Intense active carrier-mediated transport occurs at Table 1. Administration routes for delivery of drugs to the eye. Administration route Advantages Disadvantages Topical Noninvasive approach, well accepted by the patient All barriers except the blood–eye barriers must be overcome Self-administration may be not feasible in all cases Oral High patient compliance, no corneal or anterior segment barriers Systemic untoward effects, blood-retinal barrier, low on-site concentration Intracameral High drug concentration in the anterior chamber Clinical injection Subconjunctival Scleral route to the retina, suitable for depot formulations Clinical injection Intravitreal High drug concentration in the posterior segment, hydraulic pressure gradient Clinical injection (more invasive than subconjunctival), dependent on vitreous diffusion, the visual axis can be obscured if the formulation is opaque Retrobulbar Low risk for intraorbital injury, low influence on IOP Clinical injection, risk of optic nerve damage Peribulbar Low risk for intraorbital injury Clinical injection Posterior juxta scleral Available for inserts Retinal pigment epithelium is still a barrier EXPERT OPINION ON DRUG DELIVERY 1591 conjunctiva for some ions and nutrients, which may be exploited for drug and prodrug absorption [46]. Drugs encapsulated in nanocarriers take benefit of the additional pathway of endocytosis, which is feasible both in cornea and conjunctiva [51–53]. Once conjunctiva is crossed, the drug can move through sclera into the uvea and the retinal pigmented epithelium, and then move forward to neural retina and vitreous humor. The sclera performs as a size exclusion barrier, with the permeability decreasing exponentially with molecular radius and lipophilicity [54]. The sclera is negatively charged at physiological pH and therefore electrostatic interactions must be considered too. Bruch’s–choroid complex traps positively charged lipophilic drugs [55], and since Bruch’s membrane becomes less elastic with age (due to calcification of elastin and crosslinking of collagen) drug diffusion is hindered in elderly patients. Systemic administration of ocular drugs is compromised by the blood-retinal barrier. The inner limiting membrane of the retinal pigment epithelium prevents passage of highmolecular-weight molecules from blood to vitreous and vice versa [56]. Müller cells and astrocytes form tight junctions to regulate the passage of molecules between the outer choroid and the inner retina. Although the information on ocular bioavailability after drug systemic administration in humans is limited, some reports evidenced that for small drugs, such as ciprofloxacin, similar drug levels can be obtained in aqueous humor after topical instillation of the free drug or oral administration. The levels in vitreous humor are commonly higher after oral administration, but at expenses of exposing the whole organism to high drug dose [57]. Also, interestingly, the drug can be found in tear fluid after oral administration but not because of distribution through the eye, as reported for cyclosporine A [58]. The blood-retinal barrier efficiently prevents cyclosporine A diffusion from blood to the anterior segment, except during concomitance of inflammatory processes [59]. Pharmacological treatments intended to stop ocular damage caused by hyperglycemia or at least delay the process rely on (i) reducing IOP (Table 2), (ii) blocking the abnormal growth of blood vessels, or (iii) inhibiting negative chemical pathways. Drugs like prostaglandins [60], rho kinase inhibitors [61], nitric oxides [62], or miotic/cholinergic agents [63] drain ocular fluids. Alpha-adrenergic agonists, β-blockers, and carbonic anhydrase inhibitors lower the amount of fluid produced in the eye [64]. Both strategies result in a lowering of the IOP and are usually addressed using eye drops (Table 2). Additionally, new drug candidates are intended to act on the heme oxygenase 1 (HO-1)/carbon monoxide (CO) physiological pathway that regulates the IOP. The HO-1 produces protection against ischemic insult by producing CO, which has anti-inflammatory properties. Incidentally, CO protects retinal ganglion cells from ischemic/reperfusion injury. Decreased CO levels have been related to increased IOP and, therefore, drugs that release CO may be useful in glaucoma treatment [65]. The growth of abnormal ocular blood vessels can be handled with anti-vascular endothelial growth factor (anti-VEGF) drugs [66]. Bevacizumab and ranibizumab, which are respectively full antibody and antibody fragment that bind VEGF-A, and aflibercept, a recombinant protein that traps VEGF-A and VEGF-B, are the cornerstones for the therapy of diabetes-related macular edema and retinopathy [67]. They require intravitreal injection, which is not absent of complications [68]. Intraocular injections should be used as infrequently as possible, according to pro re nata or treat-and-extend protocols [69]. Biodegradable delivery systems that sustain intraocular release avoiding multiple treatment and maintaining drug stability are under investigation [70,71]. Since each available anti-VEGF agent interacts quite differently with VEGF, characterization of the molecular interactions can improve the design of novel biological drugs potentially useful in clinical practice [72]. Hyperglycemia is also responsible for triggering the polyol pathway. Under normoglycemic conditions, the Embden– Meyerhof–Parnas catabolism route that transforms glucose into pyruvate, NADH, and ATP becomes saturated. Consequently, the polyol pathway, which commonly transforms 3% glucose, enters into action with the participation of two enzymes: (i) aldose reductase that transforms glucose into sorbitol with the consumption of NADPH and (ii) sorbitol dehydrogenase that slowly converts sorbitol into fructose while consuming NAD+. The polyol pathway, which is very active in retina and lens, metabolizes more than 30% glucose under diabetic conditions [73]. Accumulation of sorbitol causes osmotic stress, triggers leukocyte accumulation, disrupts blood-retinal barrier, favors cells apoptosis, and starts a cascade of oxidative stress-mediated reactions [74]. The excess of fructose acts as precursor of advanced glycation-end products (AGEs). In this context, aldose reductase inhibitors are gaining increased attention, and epalrestat is approved in some countries for oral administration. As an alternative, drugs that accelerate the metabolic rate of sorbitol dehydrogenase and, thus, decrease the levels of sorbitol are being tested [73]. In the later stages of the disease, laser treatment (mainly for photocoagulation) or surgery (when blood vessel leakage becomes excessive or there is scar tissue) can be proposed Table 2. Some active substances of medicines used to reduce the IOP. Data from the European Medicines Agency, https://www.ema.europa.eu/en/ medicines. Drug class Drug Dosage form Prostaglandin or analog Travoprost Eye drop Bimatoprost Eye drop Latanoprost Eye drop Unoprostone Eye drop Prostaglandin analog nitric oxide Latanoprostene bunod Eye drop Rho kinase inhibitor Netarsudil Eye drop Ripasudil Eye drop Miotic agent Pilocarpine Eye drop Cholinergic agonist Carbachol Eye drop or intraocular injection Alpha-adrenergic agonist Brimonidine Eye drop Apraclonidine Eye drop Beta blocker Betaxolol Eye drop or oral tablet Timolol Eye drop Carteolol Eye drop Carbonic anhydrase inhibitor Methazolamide Oral tablet Acetazolamide Eye drop Brinzolamide Eye drop Dorzolamide Eye drop 1592 A. KATTAR ET AL. [75,76]. Vitreoretinal surgery, for example, involves the removal of part of the vitreous and scar tissue in order to ameliorate the patient’s vision [77]. 3.2. Drugs, biologics, and gene therapy in clinical trials Relevance of the morbidity caused by diabetes on eye structures is exemplified by the 868 clinical studies in phases 1 to 4 in February 2021 when searching for ‘diabetic eye’ in the ClinicalTrials.gov database. Refinement of the information to select recruiting, enrolling, active, terminated, or completed trails rendered an outcome of 657 studies, with an ample distribution worldwide (Figure 1). Most clinical trials are focused on the efficacy and safety of new molecules or novel administration routes, drug combinations, or delivery systems such as implants, microparticle depot formulations, or biopolymer–antibody conjugates. Microneedle patches that can be applied onto cornea or sclera for direct drug delivery in the aqueous or vitreous humor, respectively, are gaining increasing interest, although still in the preclinical phase [78–80]. Most clinical trials related to diabetic eye refer to the conditions macular edema (Table 3) and retinopathy (Table 4), and most interventions deal with drugs or biologics, particularly intravitreal injection of antibodies. However, the interest for oral administration as well as topical formulations does not decrease but is gaining attention, spearheaded by the search for novel active substances with improved ocular bioavailability and new therapeutic targets. Intense research on small molecules that perform as anti-inflammatory (e.g. nepafemac, loteprednol etabonate) or as anti-angiogenic/angiolytic (e.g. EXN407, OC-10X) is being carried out. For corneal epithelial defects, clinical trials deal with combinations of anti-inflammatory and antimicrobial drugs or autologous serum [81]. Fonadelpar (SJP 0035), a peroxisome proliferator-activated receptor delta agonist, is in Phase III for dry eyes and Phase II for corneal disorders [82]. Topical insulin and naltrexone eye drops have been shown to accelerate corneal epithelial healing and ameliorate dry eye symptoms in a variety of animal models [83,84]. Results of topical insulin and naltrexone clinical trials have not been posted yet [85,86]. Gene therapy of diabetic eye diseases is also an active field of research and clinical translation. The approval of Luxturna®, a virus-based gene delivery system for inherited retinal dystrophy, paved the road for other developments [87–89]. There are currently 492 recruiting or active clinical trials on ocular gene therapy, most of which use adeno-associated viruses as carriers, according to ClinicalTrials.gov, clinicaltrialsregister.eu and rctportal.niph.go.jp. Differently to the repeated administration of drugs and biologics, gene therapy approaches pursue potential one-time treatment, namely the cells are instructed once to produce the needed therapeutic substance or to not produce the harmful substance. In the case of diabetes-related macular edema, three clinical trials with intravitreal formulations and one clinical trial with a suprachoroidal formulation for gene therapy are ongoing (Table 3). ADVM-022 (AAV.7m8-aflibercept) and RGX-314 (AAV8 vector containing a transgene for anti-VEGF fab) are intended to provide durable expression of an anti-VEFG antibody [90,91]. RGX-314 is also being tested for diabetic retinopathy [92]. RNA interference therapy is in clinical trials too, although it may require repeated injections [93]. For example, iCo-007 is a single-stranded antisense that degrades messenger RNA intended to target c-Raf kinase for diabetic macular edema treatment. Phase II results using intravitreal injections were not conclusive about safety and efficacy [94]. PF-04523655 (RTP801I-14), a small-interfering RNA (siRNA) that may inhibit RTP801 gene transcription, is under evaluation as direct intravitreal injection. RTP801 is strongly upregulated in diabetic eyes and is associated with hypoxia and stress-related damage to retina cells [95]. Gene therapy also offers excellent opportunities to address ocular inflammation triggered by sorbitol accumulation and AGEs [74,96,97]. Gene therapy may allow for regulation of pro- and anti-inflammatory cytokines and neovascularization in keratitis, as reviewed elsewhere [98]. Figure 1. Regional distribution of clinical trials related to diabetic eye. Data source: ClinicalTrials.gov. There were 657 outcomes for ‘diabetic eye’ on February 2021. Applied filters were Recruiting, Active not recruiting, Completed, Enrolling by invitation, and Terminated. EXPERT OPINION ON DRUG DELIVERY 1593 Table 3. Pharmacological treatments in clinical trials for diabetes-related macular edema classified as a function of the administration route, drug/biologic active substance, and number of clinical studies. Administration route Drug/Biologic class Active substance Number of clinical trials Intravitreal Antibodies or blockers Aflibercept and biosimilars 42 Anti-angiopoietin-2 antibody REGN910 1 REGN910-3 (co-formulation of REGN910 and aflibercept) 1 Anti-erythropoietin LKA651 1 Anti-PlGF recombinant monoclonal antibody 1 Anti-ROBO 4 antibody DS-7080a 1 Bevacizumab 16 Bevasiranib 1 Tofacitinib (BI 764,524) 1 Conbercept (KH902) 1 Faricimab 1 Infliximab, anti-TNFα 1 OPT-302, anti-VEGF-C and anti-VEGF-D 1 Pegaptanib 7 Ranibizumab 40 Teprotumumab 1 Small molecules Dexamethasone 31 Fluocinolone acetonide 9 Triamcinolone acetonide 11 Anti-VEGF drugs 7 KVD001 plasma kallikrein inhibitor 2 AR-13,503, small-molecule inhibitor of both Rho kinase and protein kinase C 1 UBX1325, inhibitor of Bcl-xL (anti-apoptotic regulatory protein) 1 Peptides AXT107, tyrosine kinase blocking collagen IV–derived peptide 1 Luminate (Alg-1001) integrin inhibitor 1 Proteases Ocriplasmin 1 Gene therapy ADVM-022 gene therapy (AAV.7m8-aflibercept) 1 iCo-007, a single-stranded antisense that degrades messenger RNA (mRNA) 1 PF-04523655, small-interfering RNA (siRNA) 1 Oral Small molecules GSK2798745, transient receptor potential vanilloid 4 (TRPV4) channel blocker 1 Aliskiren 1 Danazol 1 Fenofibrate/pemafibrate 2 Imatinib mesylate (YD312) 1 Levosulpiride 1 Minocycline 1 MS-533 protein kinase inhibitor 1 Ruboxistaurin 1 Semaglutide 1 Dietary supplements Alzer®, Diamel®, others 2 Topical eyedrops Small molecules Bromfenac 1 Dexamethasone 2 Diclofenac 1 EXN407, specific serine/threonine-protein kinase 1 (SRPK1) inhibitor 1 FOV2304, inhibitor of bradykinin B1 receptor 1 Fluocinolone acetonide 1 Ketorolac 3 Nepafenac 5 OC-10X tubulin inhibitor 1 Loteprednol etabonate 1 Mecamylamine nonspecific nACh receptor blocker 1 SF0166 small-molecule αvβ3 antagonist 1 Vitamin E 2 Peptides Elamipretide (MTP-131), mitochondria-targeting peptide 1 Intravenous Small molecules Methotrexate 1 Intramuscular Peptides Octreotide acetate in microspheres 1 Episcleral Small molecules Dexamethasone implant 1 Subconjunctival Antibody Bevacizumab 1 Small molecule Rapamycin 2 Subcutaneous Small molecule Razuprotafib (AKB-9778), inhibitor of VE-PTP (vascular endothelial protein tyrosine phosphatase) 2 Sub-macular Antibodies Ranibizumab 1 Suprachoroidal Gene therapy RGX-314 (AAV8 vector containing a transgene for anti-VEGF fab) 1 Data source: ClinicalTrials.gov. Outcomes for ‘diabetic eye AND macular edema’ on February 2021. Applied filters were Recruiting, Active not recruiting, Completed, Enrolling by invitation, and Terminated. 1594 A. KATTAR ET AL. 4. Self-assembled nanocarriers for topical diabetic eye drugs Most of drugs and new drug candidates in clinical trials for diabetic eye therapy, as referred in Tables 3 and Table 4, are quite hydrophobic and must be formulated as suspensions or ointments, showing limited ocular bioavailability after instillation. Thus, noninvasive topical therapeutic approaches may be notably improved if the drugs are formulated into delivery systems that could enhance their solubility and ocular permanence. In preclinical tests, micelles and cyclodextrin aggregates have been shown to enhance cornea and sclera accumulation and permeation of various hydrophobic diabetic eye drugs [99–101]. Even in the search for novel delivery strategies, contact lenses have been designed specifically to deliver epalrestat [102] and naltrexone [103] and viral-based gene vectors [104]. Regarding intravitreal gene therapy, siRNAs are extremely labile and rapidly cleared, and thus demand adequate nanocarriers. However, most non-viral vectors are strongly cationic polymers or lipids that may interact with negatively charged glycosaminoglycans in the vitreous humor. Such an interaction may prematurely break the poly/ lipoplexes or alter the cell transfection and, thus, a very fine equilibrium in surface charge or a shell able to minimize retention in vitreous is required [105,106]. Table 4. Pharmacological treatments in clinical trials for diabetes-related retinopathy classified as a function of the administration route, drug/biologic active substance, and number of clinical studies. Administration route Drug/Biologic class Active substance Number of clinical trials Intravitreal Antibodies or blockers Aflibercept and biosimilars 14 Anti-PlGF recombinant monoclonal antibody 1 Bevacizumab 14 Tofacitinib (BI 764,524) 1 Conbercept (KH902) 2 Pegaptanib 2 Ranibizumab 20 Small molecules Dexamethasone 3 Triamcinolone acetonide 5 Anti-VEGF Drugs 1 Gene therapy PF-04523655, small-interfering RNA (siRNA) 1 Oral Small molecules Acetazolamide 1 Alpha-lipoic acid 1 Aminoguanidine 1 Brimonidine 1 Darapladib 1 Doxycycline 1 Emixustat hydrochloride 1 Empaglifozin 1 Fenofibrate/pemafibrate 3 Finerenone 1 Melatonin 1 RG7774 1 Ruboxistaurin 2 Semaglutide 1 Sinemet 1 Sulodexide 1 Tientine 1 Ubiquinone 1 Dietary supplements Alpha-lipoic acid, carotenoid vitamins, omega 3, multi-component nutritional supplement (vitamin C, mixed tocopherols/tocotrienols, vitamin D, fish oil, lutein, zeaxanthin, pine bark extract, benfotiamine, green tea extract, curcumin), Ocufolin® 5 Topical eyedrops Small molecules Anecortave acetate 1 Citicoline 1 Curcumin, homotaurine, and vitamin D3 1 Dexamethasone 1 Diclofenac 1 Dorzolamide 1 Ketorolac 3 Latanoprost 2 Napafenac 3 OC-10X tubulin inhibitor 2 Prednisolone acetate 2 Squalamine lactate 1 TG100801 multikinases inhibitor 1 Peptides Somatostatin (with brimonidine) 1 Intravenous Protein Pulsatile insulin 2 Intramuscular Peptides Octreotide acetate in microspheres 4 Subconjunctival Small molecule Rapamycin 1 Subcutaneous Small molecule Razuprotafib (AKB-9778), inhibitor of VE-PTP (vascular endothelial protein tyrosine phosphatase) 1 Suprachoroidal Gene therapy RGX-314 (AAV8 vector containing a transgene for anti-VEGF fab) 1 Data source: ClinicalTrials.gov. Outcomes for ‘diabetic eye AND retinopathy’ on February 2021. Applied filters were Recruiting, Active not recruiting, Completed, Enrolling by invitation, and Terminated. EXPERT OPINION ON DRUG DELIVERY 1595 Self-assembled drug carriers that can be prepared in few steps and can encapsulate both small and large active substances, while providing a highly biocompatible, stealth interface are gaining increasing attention for ocular delivery. Although a plethora of novel self-assembled carriers are being tested, polymeric micelles, liposomes, and niosomes have already demonstrated in vivo promising performances [13,16,99,107]. Below, first the variables that drive the formation of these carriers and the main preparation protocols are revisited. Then, specific applications for management of diabetic eye diseases are analyzed. 4.1. The self-assembly process Self-assembled carriers rely on amphiphilic components that bear regions of different affinity for water [108]. The simplest self-assembled structure is that of common surfactant micelles. In contact with water, small surfactants move to the air–water interface with the polar head immersed in water Figure 2. Dependence of the architecture of the self-assembled nanocarrier on the critical packing parameter (CPP). Figure 3. Progesterone (PG) apparent solubility in Soluplus and Pluronic F68 micelles, and permeability coefficients of cornea and sclera recorded for PG encapsulated in Soluplus 20% micelles or Pluronic F68 20% micelles. Reproduced from Alambiaga-Caravaca et al. [126] (Creative Commons Attribution License). 1596 A. KATTAR ET AL. employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties. Reviewer disclosures Peer reviewers on this manuscript have no relevant financial or other relationships to disclose. ORCID Axel Kattar http://orcid.org/0000-0002-5892-5933 Angel Concheiro http://orcid.org/0000-0003-0507-049X Carmen Alvarez-Lorenzo http://orcid.org/0000-0002-8546-7085 References Papers of special note have been highlighted as either of interest (•) or of considerable interest (••) to readers. 1. Lin X, Xu Y, Pan X, et al. Global, regional, and national burden and trend of diabetes in 195 countries and territories: an analysis from 1990 to 2025. Sci Rep. 2020;10:14790. 2. Greiner W, Patel K, Crossman-Barnes CJ, et al. High-expenditure disease in the EU-28: does drug spend correspond to clinical and economic burden in oncology, autoimmune disease and diabetes? Pharmacoecon Open. 2021. DOI:10.1007/s41669-020-00253-4 3. National Institute of Diabetes and Digestive and Kidney Diseases. Diabetic Eye Disease. [cited Feb 2021]. https://www.niddk.nih.gov/ health-information/diabetes/overview/preventing-problems/dia betic-eye-disease 4. Zhu L, Titone R, Robertson DM. 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