Safety and effect on rod function of ACU-4429, a novel small-molecule visual cycle modulator

Advances and therapeutic opportunities in visual cycle modulation

The visual cycle is a metabolic pathway that enables continuous vision by regenerating the 11-cis-retinal chromophore for photoreceptors opsins. Although integral to normal visual function, the flux of retinoids through this cycle can contribute to a range of retinal pathologies, including Stargardt disease, age-related macular degeneration, and diabetic retinopathy. In such conditions, intermediates and byproducts of the visual cycle, such as bisretinoid components of lipofuscin, can accumulate, concomitant with cellular damage and eventual photoreceptor loss. This has inspired efforts to modulate the visual cycle, aiming to slow or prevent the formation of these toxic intermediates and thus preserve retinal structure and function. Over the past two decades, multiple strategies to modulate the visual cycle have emerged. These include both intrinsic approaches, targeting key enzymes, retinoid-binding proteins, or receptors within the pigment epithelium or photoreceptors (e.g., RPE65, CRBP1, and rhodopsin inhibitors/antagonists) and extrinsic strategies that indirectly alter retinoid availability within the retina (e.g., RBP4 antagonists). Many of these agents have shown promise in animal models of visual cycle-associated retinal diseases, reducing pathological changes, and improving retinal survival. Several have advanced into clinical studies, although none are currently FDA-approved. Challenges remain in optimizing drug specificity and duration of action while minimizing side effects such as nyctalopia. In this review, we comprehensively examine current and emerging visual cycle modulators, discuss their medicinal chemistry, mechanisms of action, efficacy in preclinical and clinical studies, and highlight future opportunities for drug discovery aimed at safely and effectively preserving vision through modulation of this biochemical pathway.

The Retina-Based Visual Cycle

The continuous function of vertebrate photoreceptors requires regeneration of their visual pigment following its destruction upon activation by light (photobleaching). For rods, the chromophore required for the regeneration of rhodopsin is derived from the adjacent retinal pigmented epithelium (RPE) cells through a series of reactions collectively known as the RPE visual cycle. Mounting biochemical and functional evidence demonstrates that, for cones, pigment regeneration is supported by the parallel supply with chromophore by two pathways-the canonical RPE visual cycle and a second, cone-specific retina visual cycle that involves the Müller glial cells in the neural retina. In this article, we review historical information that led to the discovery of the retina visual cycle and discuss what is currently known about the reactions and molecular components of this pathway and its functional role in supporting cone-mediated vision.

Emerging Oral Pharmaceuticals for Dry Age-Related Macular Degeneration: Mechanism of Action, Current Clinical Status, and Future Directions

Dry age-related macular degeneration (AMD) has been historically managed with lifestyle modifications, monitoring for conversion to wet AMD, and vitamins. Recently there has been a flurry of research focused on discovering new targets to prevent worsening of dry AMD. In 2023, the US Food and Drug Administration approved the first two intravitreal complement inhibitors to slow the rate of geographic atrophy progression. However, serial intravitreal injections for a chronic progressive disease are burdensome for patients and have procedural risks. Therefore, there is significant research to discover novel oral medications to manage dry AMD. Several oral medications are currently in phase 2 and 3 clinical trials for dry AMD, whereas others have had recent readouts on their clinical trials and efficacy. The purpose of this review is to describe the therapeutic pathways currently being investigated and to provide an update on the clinical status of novel oral medications for the management of dry AMD. [Ophthalmic Surg Lasers Imaging Retina 2024;55:528-534.].

Stargardt macular dystrophy and therapeutic approaches

Stargardt macular dystrophy (Stargardt disease; STGD1; OMIM 248200) is the most prevalent inherited macular dystrophy. STGD1 is an autosomal recessive disorder caused by multiple pathogenic sequence variants in the large ABCA4 gene (OMIM 601691). Major advances in understanding both the clinical and molecular features, as well as the underlying pathophysiology, have culminated in many completed, ongoing and planned human clinical trials of novel therapies.The aims of this concise review are to describe (1) the detailed phenotypic and genotypic characteristics of the disease, multimodal imaging findings, natural history of the disease, and pathogenesis, (2) the multiple avenues of research and therapeutic intervention, including pharmacological, cellular therapies and diverse types of genetic therapies that have either been investigated or are under investigation and (3) the exciting novel therapeutic approaches on the translational horizon that aim to treat STGD1 by replacing the entire 6.8 kb ABCA4 open reading frame.

Updates on Emerging Interventions for Autosomal Recessive ABCA4-Associated Stargardt Disease

Autosomal recessive Stargardt disease (STGD1) is an inherited retinal degenerative disease associated with a mutated ATP-binding cassette, subfamily A, member 4 (ABCA4) gene. STGD1 is the most common form of juvenile macular degeneration with onset in late childhood to early or middle adulthood and causes progressive, irreversible visual impairment and blindness. No effective treatment is currently available. In the present article, we review the most recent updates in clinical trials targeting the management of STGD1, including gene therapy, small molecule therapy, and stem cell therapy. In gene therapy, dual adeno-associated virus and non-viral vectors have been successful in delivering the human ABCA4 gene in preclinical studies. For pharmaceutical therapies ALK-001, deuterated vitamin A shows promise with preliminary data for phase 2 trial, demonstrating a decreased atrophy growth rate after two years. Stem cell therapy using human pluripotent stem cell-derived retinal pigment epithelium cells demonstrated long-term safety three years after implantation and visual acuity improvements in the first two years after initiation of therapy. Many other treatment options have ongoing investigations and clinical trials. While multiple potential interventions have shown promise in attenuating disease progression, further exploration is necessary to demonstrate treatment safety and efficacy.

Tuning the Metabolic Stability of Visual Cycle Modulators through Modification of an RPE65 Recognition Motif

In the eye, the isomerization of all-trans-retinal to 11-cis-retinal is accomplished by a metabolic pathway termed the visual cycle that is critical for vision. RPE65 is the essential trans-cis isomerase of this pathway. Emixustat, a retinoid-mimetic RPE65 inhibitor, was developed as a therapeutic visual cycle modulator and used for the treatment of retinopathies. However, pharmacokinetic liabilities limit its further development including: (1) metabolic deamination of the γ-amino-α-aryl alcohol, which mediates targeted RPE65 inhibition, and (2) unwanted long-lasting RPE65 inhibition. We sought to address these issues by more broadly defining the structure-activity relationships of the RPE65 recognition motif via the synthesis of a family of novel derivatives, which were tested in vitro and in vivo for RPE65 inhibition. We identified a potent secondary amine derivative with resistance to deamination and preserved RPE65 inhibitory activity. Our data provide insights into activity-preserving modifications of the emixustat molecule that can be employed to tune its pharmacological properties.

Identification of Small Molecular Chaperones Binding P23H Mutant Opsin through an In Silico Structure-Based Approach

N-terminal P23H opsin mutation accounts for most of retinitis pigmentosa (RP) cases. P23H functions and folding can be rescued by small chaperone ligands, which contributes to validate mutant opsin as a suitable target for pharmacological treatment of RP. However, the lack of structural details on P23H mutant opsin strongly impairs drug design, and new chemotypes of effective chaperones of P23H opsin are in high demand. Here, a computational-boosted workflow combining homology modeling with molecular dynamics (MD) simulations and virtual screening was used to select putative P23H opsin chaperones among different libraries through a structure-based approach. In vitro studies corroborated the reliability of the structural model generated in this work and identified a number of novel chemotypes of safe and effective chaperones able to promote P23H opsin trafficking to the outer cell membrane.

Human retinal dark adaptation tracked in vivo with the electroretinogram: insights into processes underlying recovery of cone- and rod-mediated vision

The substantial time taken for regaining visual sensitivity (dark adaptation) following bleaching exposures has been investigated for over a century. Psychophysical studies yielded the classic biphasic curve representing recovery of cone-driven and rod-driven vision. The electroretinogram (ERG) permits direct assessment of recovery at the level of the retina (photoreceptors, bipolar cells), with the first report over 70 years ago. Over the last two decades, ERG studies of dark adaptation have generated insights into underlying physiological processes. After large bleaches, rod photoreceptor circulating current, estimated from the rod-isolated bright-flash ERG a-wave, takes 30 min to recover, indicating that products of bleaching, thought to be free opsin (unbound to 11-cis-retinal), continue to activate phototransduction, shutting off rod circulating current. In contrast, cone current, assessed with cone-driven bright-flash ERG a-waves, recovers within 100 ms following similar exposures, suggesting that free opsin is less able to shut off cone current. The cone-driven dim-flash a-wave can be used to track recovery of cone photopigment, showing regeneration is 'rate-limited' rather than first order. Recoveries of the dim-flash ERG b-wave are consistent also with rate-limited rod photopigment regeneration (where free opsin, desensitising the visual system as an 'equivalent background', is removed by rate-limited delivery of 11-cis-retinal). These findings agree with psychophysical and retinal densitometry studies, although there are unexplained points of divergence. Post-bleach ERG recovery has been explored in age-related macular degeneration and in trials of visual cycle inhibitors for retinal diseases. ERG tracking of dark adaptation may prove useful in future clinical contexts.

The novel visual cycle inhibitor (±)-RPE65-61 protects retinal photoreceptors from light-induced degeneration

The visual cycle refers to a series of biochemical reactions of retinoids in ocular tissues and supports the vision in vertebrates. The visual cycle regenerates visual pigments chromophore, 11-cis-retinal, and eliminates its toxic byproducts from the retina, supporting visual function and retinal neuron survival. Unfortunately, during the visual cycle, when 11-cis-retinal is being regenerated in the retina, toxic byproducts, such as all-trans-retinal and bis-retinoid is N-retinylidene-N-retinylethanolamine (A2E), are produced, which are proposed to contribute to the pathogenesis of the dry form of age-related macular degeneration (AMD). The primary biochemical defect in Stargardt disease (STGD1) is the accelerated synthesis of cytotoxic lipofuscin bisretinoids, such as A2E, in the retinal pigment epithelium (RPE) due to mutations in the ABCA4 gene. To prevent all-trans-retinal-and bisretinoid-mediated retinal degeneration, slowing down the retinoid flow by modulating the visual cycle with a small molecule has been proposed as a therapeutic strategy. The present study describes RPE65-61, a novel, non-retinoid compound, as an inhibitor of RPE65 (a key enzyme in the visual cycle), intended to modulate the excessive activity of the visual cycle to protect the retina from harm degenerative diseases. Our data demonstrated that (±)-RPE65-61 selectively inhibited retinoid isomerase activity of RPE65, with an IC50 of 80 nM. Furthermore, (±)-RPE65-61 inhibited RPE65 via an uncompetitive mechanism. Systemic administration of (±)-RPE65-61 in mice resulted in slower chromophore regeneration after light bleach, confirming in vivo target engagement and visual cycle modulation. Concomitant protection of the mouse retina from high-intensity light damage was also observed. Furthermore, RPE65-61 down-regulated the cyclic GMP-AMP synthase stimulator of interferon genes (cGAS-STING) pathway, decreased the inflammatory factor, and attenuated retinal apoptosis caused by light-induced retinal damage (LIRD), which led to the preservation of the retinal function. Taken together, (±)-RPE65-61 is a potent visual cycle modulator that may provide a neuroprotective therapeutic benefit for patients with STGD and AMD.

Drug Discovery Strategies for Inherited Retinal Degenerations

Inherited retinal degeneration is a group of blinding disorders afflicting more than 1 in 4000 worldwide. These disorders frequently cause the death of photoreceptor cells or retinal ganglion cells. In a subset of these disorders, photoreceptor cell death is a secondary consequence of retinal pigment epithelial cell dysfunction or degeneration. This manuscript reviews current efforts in identifying targets and developing small molecule-based therapies for these devastating neuronal degenerations, for which no cures exist. Photoreceptors and retinal ganglion cells are metabolically demanding owing to their unique structures and functional properties. Modulations of metabolic pathways, which are disrupted in most inherited retinal degenerations, serve as promising therapeutic strategies. In monogenic disorders, great insights were previously obtained regarding targets associated with the defective pathways, including phototransduction, visual cycle, and mitophagy. In addition to these target-based drug discoveries, we will discuss how phenotypic screening can be harnessed to discover beneficial molecules without prior knowledge of their mechanisms of action. Because of major anatomical and biological differences, it has frequently been challenging to model human inherited retinal degeneration conditions using small animals such as rodents. Recent advances in stem cell-based techniques are opening new avenues to obtain pure populations of human retinal ganglion cells and retinal organoids with photoreceptor cells. We will discuss concurrent ideas of utilizing stem-cell-based disease models for drug discovery and preclinical development.

Recent Advances in Age-Related Macular Degeneration Therapies

Age-related macular degeneration (AMD) was described for the first time in the 1840s and is currently the leading cause of blindness for patients over 65 years in Western Countries. This disease impacts the eye’s posterior segment and damages the macula, a retina section with high levels of photoreceptor cells and responsible for the central vision. Advanced AMD stages are divided into the atrophic (dry) form and the exudative (wet) form. Atrophic AMD consists in the progressive atrophy of the retinal pigment epithelium (RPE) and the outer retinal layers, while the exudative form results in the anarchic invasion by choroidal neo-vessels of RPE and the retina. This invasion is responsible for fluid accumulation in the intra/sub-retinal spaces and for a progressive dysfunction of the photoreceptor cells. To date, the few existing anti-AMD therapies may only delay or suspend its progression, without providing cure to patients. However, in the last decade, an outstanding number of research programs targeting its different aspects have been initiated by academics and industrials. This review aims to bring together the most recent advances and insights into the mechanisms underlying AMD pathogenicity and disease evolution, and to highlight the current hypotheses towards the development of new treatments, i.e., symptomatic vs. curative. The therapeutic options and drugs proposed to tackle these mechanisms are analyzed and critically compared. A particular emphasis has been given to the therapeutic agents currently tested in clinical trials, whose results have been carefully collected and discussed whenever possible.

Retinal pigment epithelium 65 kDa protein (RPE65): An update

Vertebrate vision critically depends on an 11-cis-retinoid renewal system known as the visual cycle. At the heart of this metabolic pathway is an enzyme known as retinal pigment epithelium 65 kDa protein (RPE65), which catalyzes an unusual, possibly biochemically unique, reaction consisting of a coupled all-trans-retinyl ester hydrolysis and alkene geometric isomerization to produce 11-cis-retinol. Early work on this isomerohydrolase demonstrated its membership to the carotenoid cleavage dioxygenase superfamily and its essentiality for 11-cis-retinal production in the vertebrate retina. Three independent studies published in 2005 established RPE65 as the actual isomerohydrolase instead of a retinoid-binding protein as previously believed. Since the last devoted review of RPE65 enzymology appeared in this journal, major advances have been made in a number of areas including our understanding of the mechanistic details of RPE65 isomerohydrolase activity, its phylogenetic origins, the relationship of its membrane binding affinity to its catalytic activity, its role in visual chromophore production for rods and cones, its modulation by macromolecules and small molecules, and the involvement of RPE65 mutations in the development of retinal diseases. In this article, I will review these areas of progress with the goal of integrating results from the varied experimental approaches to provide a comprehensive picture of RPE65 biochemistry. Key outstanding questions that may prove to be fruitful future research pursuits will also be highlighted.

Stargardt disease and progress in therapeutic strategies

Background: Stargardt disease (STGD1) is an autosomal recessive retinal dystrophy due to mutations in ABCA4, characterized by subretinal deposition of lipofuscin-like substances and bilateral centrifugal vision loss. Despite the tremendous progress made in the understanding of STGD1, there are no approved treatments to date. This review examines the challenges in the development of an effective STGD1 therapy.Materials and Methods: A literature review was performed through to June 2021 summarizing the spectrum of retinal phenotypes in STGD1, the molecular biology of ABCA4 protein, the in vivo and in vitro models used to investigate the mechanisms of ABCA4 mutations and current clinical trials.Results: STGD1 phenotypic variability remains an challenge for clinical trial design and patient selection. Pre-clinical development of therapeutic options has been limited by the lack of animal models reflecting the diverse phenotypic spectrum of STDG1. Patient-derived cell lines have facilitated the characterization of splice mutations but the clinical presentation is not always predicted by the effect of specific mutations on retinoid metabolism in cellular models. Current therapies primarily aim to delay vision loss whilst strategies to restore vision are less well developed.Conclusions: STGD1 therapy development can be accelerated by a deeper understanding of genotype-phenotype correlations.

Rational Alteration of Pharmacokinetics of Chiral Fluorinated and Deuterated Derivatives of Emixustat for Retinal Therapy

Recycling of all-trans-retinal to 11-cis-retinal through the visual cycle is a fundamental metabolic pathway in the eye. A potent retinoid isomerase (RPE65) inhibitor, (R)-emixustat, has been developed and tested in several clinical trials; however, it has not received regulatory approval for use in any specific retinopathy. Rapid clearance of this drug presents challenges to maintaining concentrations in eyes within a therapeutic window. To address this pharmacokinetic inadequacy, we rationally designed and synthesized a series of emixustat derivatives with strategically placed fluorine and deuterium atoms to slow down the key metabolic transformations known for emixustat. Crystal structures and quantum chemical analysis of RPE65 in complex with the most potent emixustat derivatives revealed the structural and electronic bases for how fluoro substituents can be favorably accommodated within the active site pocket of RPE65. We found a close (∼3.0 Å) F-π interaction that is predicted to contribute ∼2.4 kcal/mol to the overall binding energy.

Peptide Derivatives of Retinylamine Prevent Retinal Degeneration with Minimal Side Effects on Vision in Mice

Safe and effective molecular therapeutics for prophylactic treatment of retinal degenerative diseases are greatly needed. Disruptions in the clearance of all-trans-retinal (atRAL) by the visual (retinoid) cycle of the retina can lead to the accumulation of atRAL and its condensation products known to initiate progressive retinal dystrophy. Retinylamine (Ret-NH₂) and its analogues are known to be effective in lowering the concentration of atRAL within the eye and thus preventing retinal degeneration in mouse models of human retinopathies. Here, we chemically modified Ret-NH₂ with amino acids and peptides to improve the stability and ocular bioavailability of the resulting derivatives and to minimize their side effects. Fourteen Ret-NH₂ derivatives were synthesized and tested in vitro and in vivo. These derivatives exhibited structure-dependent therapeutic efficacy in preventing light-induced retinal degeneration in Abca4^-/-Rdh8^-/- double-knockout mice, with the compounds containing glycine and/or L-valine generally exhibiting greater protective effects than Ret-NH₂ or other tested amino acid derivatives of Ret-NH₂. Ret-NH₂-L-valylglycine amide (RVG) exhibited good stability in storage; and effective uptake and prolonged retention in mouse eyes. RVG readily formed a Schiff base with atRAL and did not inhibit RPE65 enzymatic activity. Administered by oral gavage, this retinoid also provided effective protection against light-induced retinal degeneration in Abca4^-/-Rdh8^-/- mice. Notably, the treatment with RVG had minimal effects on the regeneration of 11-cis-retinal and recovery of retinal function. RVG holds promise as a lead therapy for effective and safe treatment of human retinal degenerative diseases.

Randomised study evaluating the pharmacodynamics of emixustat hydrochloride in subjects with macular atrophy secondary to Stargardt disease

Background/Aims

Stargardt disease is a rare, inherited, degenerative disease of the retina that is the most common type of hereditary macular dystrophy. Currently, no approved treatments for the disease exist. The purpose of this study was to characterise the pharmacodynamics of emixustat, an orally available small molecule that targets the retinal pigment epithelium–specific 65 kDa protein (RPE65), in subjects with macular atrophy secondary to Stargardt disease.

Methods

In this multicentre study conducted at six study sites in the USA, 23 subjects with macular atrophy secondary to Stargardt disease were randomised to one of three doses of daily emixustat (2.5 mg, 5 mg or 10 mg) and treated for 1 month. The primary outcome was the suppression of the rod b-wave recovery rate on electroretinography after photobleaching, which is an indirect measure of RPE65 inhibition.

Results

Subjects who received 10 mg emixustat showed near-complete suppression of the rod b-wave amplitude recovery rate postphotobleaching (mean=91.86%, median=96.69%), whereas those who received 5 mg showed moderate suppression (mean=52.2%, median=68.0%). No effect was observed for subjects who received 2.5 mg emixustat (mean=−3.31%, median=−12.23%). The adverse event profile was consistent with prior studies in other patient populations and consisted primarily of ocular adverse events likely related to RPE65 inhibition.

Conclusion

This study demonstrated dose-dependent suppression of rod b-wave amplitude recovery postphotobleaching, confirming emixustat’s biological activity in patients with Stargardt disease. These findings informed dose selection for a 24-month phase 3 trial (SeaSTAR Study) that is now comparing emixustat to placebo in the treatment of Stargardt disease-associated macular atrophy.

[New possibilities in the treatment of Stargardt disease]

Stargardt disease is a hereditary retinal dystrophy associated with mutations in the ABCA4 gene. Currently, no etiopathogenetic drugs nor treatment methods for Stargardt disease have completely passed clinical trials. The review summarizes experimental and clinical studies of drugs aimed at reducing the accumulation of vitamin A dimers, lipofuscin, complement inhibition and RPE regeneration by stem cell transplantation, as well as gene therapy studies with intravitreal vector injection of the ABCA4 functional gene.

Clinical and genetic characteristics of Stargardt disease in a large Western China cohort: Report 1

Stargardt disease 1 (STGD1) is the most prevalent retinal dystrophy caused by pathogenic biallelic ABCA4 variants. Forty-two unrelated patients mostly originating from Western China were recruited. Comprehensive ophthalmological examinations, including visual acuity measurements (subjective function), fundus autofluorescence (retinal imaging), and full-field electroretinography (objective function), were performed. Next-generation sequencing (target/whole exome) and direct sequencing were conducted. Genotype grouping was performed based on the presence of deleterious variants. The median age of onset/age was 10.0 (5-52)/29.5 (12-72) years, and the median visual acuity in the right/left eye was 1.30 (0.15-2.28)/1.30 (0.15-2.28) in the logarithm of the minimum angle of resolution unit. Ten patients (10/38, 27.0%) showed confined macular dysfunction, and 27 (27/37, 73.7%) had generalized retinal dysfunction. Fifty-eight pathogenic/likely pathogenic ABCA4 variants, including 14 novel variants, were identified. Eight patients (8/35, 22.8%) harbored multiple deleterious variants, and 17 (17/35, 48.6%) had a single deleterious variant. Significant associations were revealed between subjective functional, retinal imaging, and objective functional groups, identifying a significant genotype-phenotype association. This study illustrates a large phenotypic/genotypic spectrum in a large well-characterized STGD1 cohort. A distinct genetic background of the Chinese population from the Caucasian population was identified; meanwhile, a genotype-phenotype association was similarly represented.

All-trans-Retinaldehyde Contributes to Retinal Vascular Permeability in Ischemia Reperfusion

Purpose

Extracellular accumulation of all-trans-retinaldehyde (atRAL), a highly reactive visual cycle intermediate, is toxic to cells of the outer retina and contributes to retinal and macular degenerations. However, the contribution of atRAL to retinal capillary function has not been studied. We hypothesized that atRAL released from the outer retina can contribute to retinal vascular permeability. We, therefore, tested the contribution of atRAL to retinal ischemia-reperfusion (IR)-induced vascular permeability.

Methods

IR was induced in mice by transient increase in intraocular pressure followed by natural reperfusion. The visual cycle was ablated in the Lrat-/- mice, reduced by dark adaptation or the use of the RPE65 inhibitor and atRAL scavenger emixustat. Accumulation of FITC-BSA was used to assess vascular permeability and DNA fragmentation quantified cell death after IR. Primary bovine retinal endothelial cell (BREC) culture was used to measure the direct effects of atRAL on endothelial permeability and cell death.

Results

Inhibition of the visual cycle by Lrat-/-, dark adaptation, or with emixustat, all reduced approximately half of IR induced vascular permeability at 48 hours. An increase in BREC permeability with atRAL coincided with lactate dehydrogenase (LDH) release, a measure of cell death. Both permeability and toxicity were blocked by emixustat.

Conclusions

Outer retinal pathology may contribute to vascular permeability by release of atRAL, which can act directly on vascular endothelial cells to alter barrier properties and induce cell death. These studies may have implications for a variety of blinding eye diseases that include outer retinal damage and retinal vascular permeability.

Ocular Disease Therapeutics: Design and Delivery of Drugs for Diseases of the Eye

The ocular drug discovery field has evidenced significant advancement in the past decade. The FDA approvals of Rhopressa, Vyzulta, and Roclatan for glaucoma, Brolucizumab for wet age-related macular degeneration (wet AMD), Luxturna for retinitis pigmentosa, Dextenza (0.4 mg dexamethasone intracanalicular insert) for ocular inflammation, ReSure sealant to seal corneal incisions, and Lifitegrast for dry eye represent some of the major developments in the field of ocular therapeutics. A literature survey also indicates that gene therapy, stem cell therapy, and target discovery through genomic research represent significant promise as potential strategies to achieve tissue repair or regeneration and to attain therapeutic benefits in ocular diseases. Overall, the emergence of new technologies coupled with first-in-class entries in ophthalmology are highly anticipated to restructure and boost the future trends in the field of ophthalmic drug discovery. This perspective focuses on various aspects of ocular drug discovery and the recent advances therein. Recent medicinal chemistry campaigns along with a brief overview of the structure-activity relationships of the diverse chemical classes and developments in ocular drug delivery (ODD) are presented.

Emixustat Reduces Metabolic Demand of Dark Activity in the Retina

Purpose

In the dark, photoreceptor outer segments contain high levels of cyclic guanosine 3'-5' monophosphate (cGMP), which binds to ion channels, holding them open and allowing an influx of cations. Ion pumping activity, which balances cation influx, uses considerable amounts of adenosine triphosphate (ATP) and oxygen. Light reduces cation influx and thereby lowers metabolic demand. Blood vessels are compromised in the diabetic retina and may not be able to meet the higher metabolic demand in darkness. Emixustat is a visual cycle modulator (VCM) that reduces chromophore levels and, therefore, may mimic light conditions. We evaluated the effect of emixustat on oxygen consumption and cation influx in dark conditions.

Methods

Cation influx was measured in rats using Mn2+-magnetic resonance imaging (MEMRI). Retinal oxygen profiles were recorded to evaluate oxygen consumption. In the MEMRI protocol, animals were treated with either emixustat or vehicle. In the oxygen protocol, animals were untreated or treated with emixustat.

Results

In vehicle-treated animals, cation channel activity increased in the dark. Emixustat treatment reduced cation channel activity; activity was comparable to vehicle-treated controls in light conditions. In vehicle-treated animals, minimum retinal oxygen tension decreased as the retina recovered from a photobleach, indicating that more oxygen was being consumed. Emixustat treatment prevented the decrease in oxygen pressure after photobleach.

Conclusions

Emixustat reduced the cation influx and retinal oxygen consumption associated with dark conditions. VCMs are a promising potential treatment for ischemic retinal neovascularization, such as that in diabetic retinopathy.

Pharmacotherapy for metabolic and cellular stress in degenerative retinal diseases

Retinal photoreceptors continually endure stresses associated with prolonged light exposure and the metabolic demands of dark adaptation. Although healthy photoreceptors are able to withstand these stresses for several decades, the disease-affected retina functions at a reduced capacity and is at an increased risk for dysfunction. To alleviate cellular and metabolic stressors in degenerative retinal diseases, a new class of drugs that modulate the metabolic activity of the retina have been developed. A clinical candidate in this class (emixustat) has been shown to reduce retinal pathology in various animal models of human retinal disease and is currently under clinical study. Here, we describe the pharmacological properties of emixustat, its mechanisms of action, and potential for use in the treatment of specific retinal diseases.

Impacts of deletion and ichthyosis prematurity syndrome-associated mutations in fatty acid transport protein 4 on the function of RPE65

The retinal pigment epithelium-specific 65 kDa (RPE65) isomerase plays a pivotal role in photoreceptor survival and function. RPE65-catalyzed synthesis of 11-cis-retinol from all-trans-retinyl esters in the visual cycle is negatively regulated, through a heretofore unknown mechanism, by the fatty acid transport protein FATP4, mutations in which are associated with ichthyosis prematurity syndrome (IPS). Here, we analyzed the interaction between deletion mutants of FATP4 and RPE65 and the impacts of IPS-associated FATP4 mutations on RPE65 expression, 11-cis-retinol synthesis, and all-trans-retinyl ester synthesis. Our results suggest that the interaction between FATP4 and RPE65 contributes to the inhibition of RPE65 function and that IPS-associated nonsense and missense mutations in FATP4 have different effects on the visual cycle.

Oxidative Deamination of Emixustat by Human Vascular Adhesion Protein-1/Semicarbazide-Sensitive Amine Oxidase

Emixustat potently inhibits the visual cycle isomerase retinal pigment epithelium protein 65 (RPE65) to reduce the accumulation of toxic bisretinoid by-products that lead to various retinopathies. Orally administered emixustat is cleared rapidly from the plasma, with little excreted unchanged. The hydroxypropylamine moiety that is critical in emixustat's inhibition of RPE65 is oxidatively deaminated to three major carboxylic acid metabolites that appear rapidly in plasma. These metabolites greatly exceed the plasma concentrations of emixustat and demonstrate formation-rate-limited metabolite kinetics. This study investigated in vitro deamination of emixustat in human vascular membrane fractions, plasma, and recombinant human vascular adhesion protein-1 (VAP-1), demonstrating single-enzyme kinetics for the formation of a stable aldehyde intermediate (ACU-5201) in all in vitro systems. The in vitro systems used herein established sequential formation of the major metabolites with addition of assay components for aldehyde dehydrogenase and cytochrome P450. Reaction phenotyping experiments using selective chemical inhibitors and recombinant enzymes of monoamine oxidase, VAP-1, and lysyl oxidase showed that only VAP-1 deaminated emixustat. In individually derived human vascular membranes from umbilical cord and aorta, rates of emixustat deamination were highly correlated to VAP-1 marker substrate activity (benzylamine) and VAP-1 levels measured by enzyme-linked immunosorbent assay. In donor-matched plasma samples, soluble VAP-1 activity and levels were lower than in aorta membranes. A variety of potential comedications did not strongly inhibit emixustat deamination in vitro.

Abnormal Cannabidiol Modulates Vitamin A Metabolism by Acting as a Competitive Inhibitor of CRBP1

Cellular retinol-binding proteins (CRBPs) facilitate the uptake and intracellular transport of vitamin A. They integrate retinoid metabolism, playing an important role in regulating the synthesis of bioactive vitamin A metabolites. Thus, CRBPs constitute potential pharmacological targets to modulate cellular retinoid status that in turn may have applications in the treatment of certain immunological, metabolic, and ocular disorders. Here we identify abnormal cannabidiol (abn-CBD) as a nonretinoid inhibitor of cellular retinol-binding protein 1 (CRBP1). X-ray crystal structures of CRBP1 in complex with abn-CBD and its derivatives revealed a distinctive mode of protein-ligand interaction and provided a molecular basis for the high affinity and selectivity of this compound. We demonstrated that abn-CBD modulates the flux of retinoids via the retinoid cycle in vivo. Furthermore, the biological activity of abn-CBD was evidenced by its ability to protect against light-induced retinal damage in Balb/cJ mice. Altogether, our findings indicate that targeting selected CRBPs with a small-molecule inhibitor can potentially lead to the development of new therapeutic agents to counteract diseases with etiologies involving imbalance in retinoid metabolism or signaling.

Validation and reproducibility of an LC-MS/MS method for emixustat and its three deaminated metabolites in human plasma

Aim: A sensitive method to quantify emixustat and its rapidly formed three major deaminated metabolites in human plasma was necessary to determine exposure in clinical trials. Methods: An LC-MS/MS method was validated for accuracy and precision, linearity, carry over, selectivity, recovery, matrix effects, hematocrit effects and stability. Results: A quantitative procedure for the determination of emixustat, ACU-5116, ACU-5124 and ACU-5149 in human plasma over the concentration range of 0.0500/1.00/1.00/1.00-10.0/1000/1000/1000 ng/ml, was successfully validated and has been used to successfully analyze samples in three clinical trials. Incurred sample reanalysis was performed for all four analytes in each study with >92% of the repeat results and original results within 20% of the mean of the two values.

Stargardt macular dystrophy and evolving therapies

INTRODUCTION

Stargardt macular dystrophy (STGD1) is a hereditary retinal degeneration that lacks effective treatment options. Gene therapy, stem cell therapy, and pharmacotherapy with visual cycle modulators (VCMs) and complement inhibitors are discussed as potential treatments.

AREAS COVERED

Investigational therapies for STGD1 aim to reduce toxic bisretinoids and lipofuscin in the retina and retinal pigment epithelium (RPE). These agents include C20-D3-vitamin A (ALK-001), isotretinoin, VM200, emixustat, and A1120. Avacincaptad pegol is a C5 complement inhibitor that may reduce inflammation-related RPE damage. Animal models of STGD1 show promising data for these treatments, though proof of efficacy in humans is lacking. Fenretinide and emixustat are VCMs for dry AMD and STGD1 that failed to halt geographic atrophy progression or improve vision in trials for AMD. A1120 prevents retinol transport into RPE and may spare side effects typically seen with VCMs (nyctalopia and chromatopsia). Stem cell transplantation suggests potential biologic plausibility in a phase I/II trial. Gene therapy aims to augment the mutated ABCA4 gene, though results of a phase I/II trial are pending.

EXPERT OPINION

Stem cell transplantation, ABCA4 gene therapy, VCMs, and complement inhibitors offer biologically plausible treatment mechanisms for treatment of STGD1. Further trials are warranted to assess efficacy and safety in humans.

A novel small molecule chaperone of rod opsin and its potential therapy for retinal degeneration

Rhodopsin homeostasis is tightly coupled to rod photoreceptor cell survival and vision. Mutations resulting in the misfolding of rhodopsin can lead to autosomal dominant retinitis pigmentosa (adRP), a progressive retinal degeneration that currently is untreatable. Using a cell-based high-throughput screen (HTS) to identify small molecules that can stabilize the P23H-opsin mutant, which causes most cases of adRP, we identified a novel pharmacological chaperone of rod photoreceptor opsin, YC-001. As a non-retinoid molecule, YC-001 demonstrates micromolar potency and efficacy greater than 9-cis-retinal with lower cytotoxicity. YC-001 binds to bovine rod opsin with an EC50 similar to 9-cis-retinal. The chaperone activity of YC-001 is evidenced by its ability to rescue the transport of multiple rod opsin mutants in mammalian cells. YC-001 is also an inverse agonist that non-competitively antagonizes rod opsin signaling. Significantly, a single dose of YC-001 protects Abca4 -/- Rdh8 -/- mice from bright light-induced retinal degeneration, suggesting its broad therapeutic potential.

Emixustat Hydrochloride for Geographic Atrophy Secondary to Age-Related Macular Degeneration: A Randomized Clinical Trial

PURPOSE

To determine whether emixustat hydrochloride (emixustat) reduces the rate of enlargement of geographic atrophy (GA) compared with placebo in subjects with age-related macular degeneration (AMD) and to evaluate the safety and tolerability of emixustat over 24 months of treatment.

DESIGN

Multicenter, randomized, double-masked, placebo-controlled, phase 2b/3 clinical trial.

PARTICIPANTS

Patients with GA secondary to AMD, a visual acuity score of at least 35 letters, and GA with a total area of 1.25 to 18 mm² were enrolled.

METHODS

Subjects were randomized (1:1:1:1) to emixustat 2.5 mg, 5 mg, 10 mg, or placebo, administered orally once daily for 24 months. Visits included screening, baseline, and months 1, 2, 3, 6, 9, 12, 15, 18, 21, 24, and 25.

MAIN OUTCOME MEASURES

The primary efficacy end point was the mean annual growth rate of total GA area in the study eye, as measured by a central reading center using fundus autofluorescence (FAF) images. The change from baseline in normal luminance best-corrected visual acuity (NL-BCVA) was a secondary efficacy end point.

RESULTS

Of 508 randomized subjects, 320 completed the study. Demographics and baseline characteristics were comparable between treatment groups. On average, GA lesions in the study eye grew at a similar rate in each group (emixustat: 1.69 to 1.84 mm²/year; placebo: 1.69 mm²/year; P ≥ 0.81). Changes in NL-BCVA were also comparable between groups. Subjects with a larger low luminance deficit (LLD) at baseline (≥20 letters) demonstrated a more rapid growth of GA over 24 months. No relationship was observed between the risk-allele status of the AMD-associated single-nucleotide polymorphisms tested and the growth rate of GA. The most common adverse events in emixustat-treated subjects were delayed dark adaptation (55%), chromatopsia (18%), visual impairment (15%), and erythropsia (15%).

CONCLUSIONS

Emixustat did not reduce the growth rate of GA in AMD. The most common adverse events were ocular in nature and likely related to the drug's mechanism of action. Data gained from this study over a 2-year period add to the understanding of the natural history of GA and the baseline characteristics affecting the growth rate of GA.

A novel RPE65 inhibitor CU239 suppresses visual cycle and prevents retinal degeneration

The retinoid visual cycle is an ocular retinoid metabolism specifically dedicated to support vertebrate vision. The visual cycle serves not only to generate light-sensitive visual chromophore 11-cis-retinal, but also to clear toxic byproducts of normal visual cycle (i.e. all-trans-retinal and its condensation products) from the retina, ensuring both the visual function and the retinal health. Unfortunately, various conditions including genetic predisposition, environment and aging may attribute to a functional decline of the all-trans-retinal clearance. To combat all-trans-retinal mediated retinal degeneration, we sought to slow down the retinoid influx from the RPE by inhibiting the visual cycle with a small molecule. The present study describes identification of CU239, a novel non-retinoid inhibitor of RPE65, a key enzyme in the visual cycle. Our data demonstrated that CU239 selectively inhibited isomerase activity of RPE65, with IC₅₀ of 6 μM. Further, our results indicated that CU239 inhibited RPE65 via competition with its substrate all-trans-retinyl ester. Mice with systemic injection of CU239 exhibited delayed chromophore regeneration after light bleach, and conferred a partial protection of the retina against injury from high intensity light. Taken together, CU239 is a potent visual cycle modulator and may have a therapeutic potential for retinal degeneration.

Direct Treatment Costs of Neovascular Age-related Macular Degeneration and Comparison of Gained and/or Preserved Vision with Expenditure

Objectives:

The aim was to quantify the direct medical cost of neovascular age-related macular degeneration (AMD) versus gained or preserved vision.

Materials and Methods:

Data of patients treated for neovascular AMD between January 2009 to January 2014 were reviewed. Patients with complete follow-up for two years, treated with only intravitreal ranibizumab injections and with no intraocular surgery were included. Demographics, diagnostic investigations, the number of visits and injections, changes in visual acuity (VA) at one year and two years from baseline were noted. Total cost was calculated for the first and second years, and the cost of improving or preserving initial vision level was determined with subgroup analysis.

Results:

Two-hundred eyes of 175 patients (86 male and 89 female) with a mean age of 72.3±7.8 years were included. Mean VA was 0.67 logMAR at baseline, 0.60 logMAR at the end of the first year, and 0.67 logMAR at the end of the second year. At the end of the 2 years, VA increased in 82 eyes (41%), remained the same in 42 eyes (21%), and decreased in 76 eyes (38%). The mean number of visits in the first and second years were 6.56 (3-12) and 5.74 (3-10), respectively. An average of 4.42 (1-8) injections were performed in the first year and 2.25 (0-7) in the second. The total direct medical cost for AMD was 9,628 TL (Turkish Lira) per patient for 2 years, which consisted of 529 TL in visit costs, 115 TL in fluorescein and indocyanine angiography costs, 611 TL in injection procedure costs, and 8,371 TL in drug costs. The cost of one line of VA gain was 11,911 TL in the first year.

Conclusion:

This study showed that treatment increased or stabilized vision in a reasonable proportion of patients, that cost of management decreases in the second year, and that drug expenses are the leading item in reimbursement.

Retinoid isomerase inhibitors impair but do not block mammalian cone photoreceptor function

RPE65 is a retinoid isomerase essential for rod function, but its contribution to cone vision is enigmatic. Using selective RPE65 inhibitors, Kiser et al. demonstrate that cone function depends only partially on continuous RPE65 activity, providing support for cone-specific regeneration mechanisms.

Visual function in vertebrates critically depends on the continuous regeneration of visual pigments in rod and cone photoreceptors. RPE65 is a well-established retinoid isomerase in the pigment epithelium that regenerates rhodopsin during the rod visual cycle; however, its contribution to the regeneration of cone pigments remains obscure. In this study, we use potent and selective RPE65 inhibitors in rod- and cone-dominant animal models to discern the role of this enzyme in cone-mediated vision. We confirm that retinylamine and emixustat-family compounds selectively inhibit RPE65 over DES1, the putative retinoid isomerase of the intraretinal visual cycle. In vivo and ex vivo electroretinography experiments in Gnat1^−/− mice demonstrate that acute administration of RPE65 inhibitors after a bleach suppresses the late, slow phase of cone dark adaptation without affecting the initial rapid portion, which reflects intraretinal visual cycle function. Acute administration of these compounds does not affect the light sensitivity of cone photoreceptors in mice during extended exposure to background light, but does slow all phases of subsequent dark recovery. We also show that cone function is only partially suppressed in cone-dominant ground squirrels and wild-type mice by multiday administration of an RPE65 inhibitor despite profound blockade of RPE65 activity. Complementary experiments in these animal models using the DES1 inhibitor fenretinide show more modest effects on cone recovery. Collectively, these studies demonstrate a role for continuous RPE65 activity in mammalian cone pigment regeneration and provide further evidence for RPE65-independent regeneration mechanisms.

New GABA modulators protect photoreceptor cells from light-induced degeneration in mouse models

No clinically approved therapies are currently available that prevent the onset of photoreceptor death in retinal degeneration. Signaling between retinal neurons is regulated by the release and uptake of neurotransmitters, wherein GABA is the main inhibitory neurotransmitter. In this work, novel 3-chloropropiophenone derivatives and the clinical anticonvulsants tiagabine and vigabatrin were tested to modulate GABA signaling and protect against light-induced retinal degeneration. Abca4^-/-Rdh8^-/- mice, an accelerated model of retinal degeneration, were exposed to intense light after prophylactic injections of one of these compounds. Imaging and functional assessments of the retina indicated that these compounds successfully protected photoreceptor cells from degeneration to maintain a full-visual-field response. Furthermore, these compounds demonstrated a strong safety profile in wild-type mice and did not compromise visual function or damage the retina, despite repeated administration. These results indicate that modulating inhibitory GABA signaling can offer prophylactic protection against light-induced retinal degeneration.-Schur, R. M., Gao, S., Yu, G., Chen, Y., Maeda, A., Palczewski, K., Lu, Z.-R. New GABA modulators protect photoreceptor cells from light-induced degeneration in mouse models.

Current drug and molecular therapies for the treatment of atrophic age-related macular degeneration: phase I to phase III clinical development

INTRODUCTION

Age-related macular degeneration (AMD) is the leading cause of vision loss among the elderly. Atrophic AMD, including early, intermediate and geographic atrophy (GA), accounts for ~90% of all cases. It is a multifactorial degeneration characterized by chronic inflammation, oxidative stress and aging components. Although no FDA-approved treatment yet exists for the late stage of atrophic AMD, multiple pathological mechanisms are partially known and several promising therapies are in various stages of development. Areas covered: Underlying mechanisms that define atrophic AMD will help provide novel therapeutic targets that will address this largely unmet clinical need. The purpose of this paper is to review current promising drugs that are being evaluated in clinical trials. Because no pharmacological treatments are currently available for late stage of atrophic AMD, any new therapy would have extensive market potential. Expert opinion: The number of AMD patients is predicted to increase to ~30 million worldwide by 2020. In response to this enormous unmet clinical need, new promising therapies are being developed and evaluated in clinical trials. We propose that the assessment of novel interventions will also need to consider the genotypes of participants, as the benefit may be determined by polymorphisms in an individual's genetic background.

Recent developments in age-related macular degeneration: a review

Background

Visual impairment in elderly people is a considerable health problem that significantly affects quality of life of millions worldwide. The magnitude of this issue is becoming more evident with an aging population and an increasing number of older individuals.

Objective

The objective of this article was to review the clinical and pathological aspects of age-related macular degeneration (AMD), diagnostic tools, and therapeutic modalities presently available or underway for both atrophic and wet forms of the disease.

Methods

An online review of the PubMed database was performed, searching for the key words. The search was limited to articles published since 1980 to date.

Results

Several risk factors have been linked to AMD, such as age (>60 years), lifestyle (smoking and diet), and family history. Although the pathogenesis of AMD remains unclear, genetic factors have been implicated in the condition. Treatment for atrophic AMD is mainly close observation, coupled with nutritional supplements such as zinc and antioxidants, whereas treatment of wet AMD is based on targeting choroidal neovascular membranes.

Conclusion

Identification of modifiable risk factors would improve the possibilities of preventing the progression of AMD. The role of anti-vascular endothelial growth factor (anti-VEGF) agents has transformed the therapeutic approach of the potentially blinding disease “wet AMD” into a more favorable outcome.

Disposition, profiling and identification of emixustat and its metabolites in humans

1. Emixustat is a small molecule that potently inhibits retinal pigment epithelium 65 isomerohydrolase. Emixustat is in clinical development for the treatment of various retinopathies (i.e. Stargardt disease and diabetic retinopathy). 2. A human absorption, distribution, metabolism, and excretion (ADME) study was conducted with a single dose of [¹⁴C]-emixustat in healthy male subjects. Total ¹⁴C content in plasma, urine, and faeces was determined using accelerator mass spectrometry (AMS), and metabolic profiles in pooled plasma and urine were investigated by both HPLC-AMS and 2D LC-MS/MS. 3. After a single, oral 40-mg dose of [¹⁴C]-emixustat, recovery of total ¹⁴C was nearly complete within 24 h. Urine was the major route of ¹⁴C elimination; accounting for > 90% of the administered dose. 4. Biotransformation of emixustat occurred primarily at two structural moieties; oxidation of the cyclohexyl moiety and oxidative deamination of the 3R-hydroxypropylamine, both independently and in combination to produce secondary metabolites. Metabolite profiling in pooled plasma samples identified 3 major metabolites: ACU-5124, ACU-5116 and ACU-5149, accounting for 29.0%, 11.5%, and 10.6% of total ¹⁴C, respectively. Emixustat was metabolized in human hepatocytes with unchanged emixustat accounting for 33.7% of sample radioactivity and predominantly cyclohexanol metabolites observed.

Rational Tuning of Visual Cycle Modulator Pharmacodynamics

Modulators of the visual cycle have been developed for treatment of various retinal disorders. These agents were designed to inhibit retinoid isomerase [retinal pigment epithelium-specific 65 kDa protein (RPE65)], the rate-limiting enzyme of the visual cycle, based on the idea that attenuation of visual pigment regeneration could reduce formation of toxic retinal conjugates. Of these agents, certain ones that contain primary amine groups can also reversibly form retinaldehyde Schiff base adducts, which contributes to their retinal protective activity. Direct inhibition of RPE65 as a therapeutic strategy is complicated by adverse effects resulting from slowed chromophore regeneration, whereas effective retinal sequestration can require high drug doses with potential off-target effects. We hypothesized that the RPE65-emixustat crystal structure could help guide the design of retinaldehyde-sequestering agents with varying degrees of RPE65 inhibitory activity. We found that addition of an isopropyl group to the central phenyl ring of emixustat and related compounds resulted in agents effectively lacking in vitro retinoid isomerase inhibitory activity, whereas substitution of the terminal 6-membered ring with branched moieties capable of stronger RPE65 interaction potentiated inhibition. The isopropyl derivative series produced discernible visual cycle suppression in vivo, albeit much less potently than compounds with a high affinity for the RPE65 active site. These agents were distributed into the retina and formed Schiff base adducts with retinaldehyde. Except for one compound [3-amino-1-(3-isopropyl-5-((2,6,6-trimethylcyclohex-1-en-1-yl)methoxy)phenyl)propan-1-ol (MB-007)], these agents conferred protection against retinal phototoxicity, suggesting that both direct RPE65 inhibition and retinal sequestration are mechanisms of potential therapeutic relevance.

Retinol Dehydrogenases Regulate Vitamin A Metabolism for Visual Function

The visual system produces visual chromophore, 11-cis-retinal from dietary vitamin A, all-trans-retinol making this vitamin essential for retinal health and function. These metabolic events are mediated by a sequential biochemical process called the visual cycle. Retinol dehydrogenases (RDHs) are responsible for two reactions in the visual cycle performed in retinal pigmented epithelial (RPE) cells, photoreceptor cells and Müller cells in the retina. RDHs in the RPE function as 11-cis-RDHs, which oxidize 11-cis-retinol to 11-cis-retinal in vivo. RDHs in rod photoreceptor cells in the retina work as all-trans-RDHs, which reduce all-trans-retinal to all-trans-retinol. Dysfunction of RDHs can cause inherited retinal diseases in humans. To facilitate further understanding of human diseases, mouse models of RDHs-related diseases have been carefully examined and have revealed the physiological contribution of specific RDHs to visual cycle function and overall retinal health. Herein we describe the function of RDHs in the RPE and the retina, particularly in rod photoreceptor cells, their regulatory properties for retinoid homeostasis and future therapeutic strategy for treatment of retinal diseases.

Retinoids and Retinal Diseases

Recent progress in molecular understanding of the retinoid cycle in mammalian retina stems from painstaking biochemical reconstitution studies supported by natural or engineered animal models with known genetic lesions and studies of humans with specific genetic blinding diseases. Structural and membrane biology have been used to detect critical retinal enzymes and proteins and their substrates and ligands, placing them in a cellular context. These studies have been supplemented by analytical chemistry methods that have identified small molecules by their spectral characteristics, often in conjunction with the evaluation of models of animal retinal disease. It is from this background that rational therapeutic interventions to correct genetic defects or environmental insults are identified. Thus, most presently accepted modulators of the retinoid cycle already have demonstrated promising results in animal models of retinal degeneration. These encouraging signs indicate that some human blinding diseases can be alleviated by pharmacological interventions.

Vitamin A and Vision

Visual systems detect light by monitoring the effect of photoisomerization of a chromophore on the release of a neurotransmitter from sensory neurons, known as rod and cone photoreceptor cells in vertebrate retina. In all known visual systems, the chromophore is 11-cis-retinal complexed with a protein, called opsin, and photoisomerization produces all-trans-retinal. In mammals, regeneration of 11-cis-retinal following photoisomerization occurs by a thermally driven isomerization reaction. Additional reactions are required during regeneration to protect cells from the toxicity of aldehyde forms of vitamin A that are essential to the visual process. Photochemical and phototransduction reactions in rods and cones are identical; however, reactions of the rod and cone visual pigment regeneration cycles differ, and perplexingly, rod and cone regeneration cycles appear to use different mechanisms to overcome the energy barrier involved in converting all-trans- to 11-cis-retinoid. Abnormal processing of all-trans-retinal in the rod regeneration cycle leads to retinal degeneration, suggesting that excessive amounts of the retinoid itself or its derivatives are toxic. This line of reasoning led to the development of various approaches to modifying the activity of the rod visual cycle as a possible therapeutic approach to delay or prevent retinal degeneration in inherited retinal diseases and perhaps in the dry form of macular degeneration (geographic atrophy). In spite of great progress in understanding the functioning of rod and cone regeneration cycles at a molecular level, resolution of a number of remaining puzzling issues will offer insight into the amelioration of several blinding retinal diseases.

Predicting Progression of ABCA4-Associated Retinal Degenerations Based on Longitudinal Measurements of the Leading Disease Front

PURPOSE

To evaluate the progression of the earliest stage of disease in ABCA4-associated retinal degenerations (RDs).

METHODS

Near-infrared excited reduced-illuminance autofluorescence imaging was acquired across the retina up to 80 degrees eccentricity in 44 patients with two ABCA4 alleles. The eccentricity of the leading disease front (LDF) corresponding to the earliest stage of disease was measured along the four meridians. A mathematical model describing the expansion of the LDF was developed based on 6 years of longitudinal follow-up.

RESULTS

The extent of LDF along the superior, inferior, and temporal meridians showed a wide spectrum from 3.5 to 70 degrees. In patients with longitudinal data, the average centrifugal expansion rate was 2 degrees per year. The nasal extent of LDF between the fovea and ONH ranged from 4.3 to 16.5 degrees and expanded at 0.35 degrees per year. The extent of LDF beyond ONH ranged from 19 to 75 degrees and expanded on average at 2 degrees per year. A mathematical model fit well to the longitudinal data describing the expansion of the LDF.

CONCLUSIONS

The eccentricity of the LDF in ABCA4-RD shows a continuum from parafovea to far periphery along all four meridians consistent with a wide spectrum of severity observed clinically. The model of progression may provide a quantitative prediction of the LDF expansion based on the age and eccentricity of the LDF at a baseline visit, and thus contribute significantly to the enrollment of candidates appropriate for clinical trials planning specific interventions, efficacy outcomes, and durations.

Geographic atrophy in patients with advanced dry age-related macular degeneration: current challenges and future prospects

Geographic atrophy (GA) of the retinal pigment epithelium (RPE) is a devastating complication of age-related macular degeneration (AMD). GA may be classified as drusen-related (drusen-associated GA) or neovascularization-related (neovascular-associated GA). Drusen-related GA remains a large public health concern due to the burden of blindness it produces, but pathophysiology of the condition is obscure and there are no proven treatment options. Genotyping, cell biology, and clinical imaging point to upregulation of parainflammatory pathways, oxidative stress, and choroidal sclerosis as contributors, among other factors. Onset and monitoring of progression is accomplished through clinical imaging instrumentation such as optical coherence tomography, photography, and autofluorescence, which are the tools most helpful in determining end points for clinical trials at present. A number of treatment approaches with diverse targets are in development at this time, some of which are in human clinical trials. Neovascular-associated GA is a consequence of RPE loss after development of neovascular AMD. The neovascular process leads to a plethora of cellular stresses such as ischemia, inflammation, and dramatic changes in cell environment that further taxes RPE cells already dysfunctional from drusen-associated changes. GA may therefore develop secondary to the neovascular process de novo or preexisting drusen-associated GA may continue to worsen with the development of neovascular AMD. Neovascular-associated GA is a prominent cause of continued vision loss in patients with otherwise successfully treated neovascular AMD. Clearly, treatment with vascular endothelial growth factor (VEGF) inhibitors early in the course of the neovascular disease is of great clinical benefit. However, there is a rationale and some suggestive evidence that anti-VEGF agents themselves could be toxic to RPE and enhance neovascular-associated GA. The increasing prevalence of legal blindness from this condition due to the aging of the general population lends urgency to the search for a therapy to ameliorate GA.

Current therapeutic developments in atrophic age-related macular degeneration

Age-related macular degeneration (AMD), a degenerative disorder of the central retina, is the leading cause of irreversible blindness in the elderly. The underlying mechanism of the advanced form of dry AMD, also named geographic atrophy (GA) or atrophic AMD, remains unclear. Consequently, no cure is available for dry AMD or GA. The only prevention option currently available is the Age-Related Eye Disease Study (AREDS) formulation, which has been demonstrated to slow down the progression of dry AMD. This review summarises recent advances in therapy for dry AMD and GA. Building on the new understanding of the disease and recent technological breakthroughs, numerous ongoing clinical trials have the goal of meeting the need to cure AMD. Therapeutic agents are being developed to target the key features of the disease, including inhibiting the complement pathway and other inflammatory pathways, reducing oxidative stress and protecting retinal pigment epithelial (RPE) cells, inhibiting lipofuscin and visual cycle, regenerating RPE cells from stem cells and restoring choroidal blood flow. Some of these therapeutic options, especially the stem cell-based therapy, hold great promise, which brings great hope for this devastating blinding disease.

Phase ii, randomized, placebo-controlled, 90-day study of emixustat hydrochloride in geographic atrophy associated with dry age-related macular degeneration

PURPOSE

This study assessed the safety, tolerability, and pharmacodynamics of emixustat hydrochloride (ACU-4429), a novel visual cycle modulator, in subjects with geographic atrophy associated with dry age-related macular degeneration.

METHODS

Subjects were randomly assigned to oral emixustat (2, 5, 7, or 10 mg once daily) or placebo (3:1 ratio) for 90 days. Recovery of rod photoreceptor sensitivity after a photobleach was measured by electroretinography. Safety evaluations included analysis of adverse events and ophthalmic examinations.

RESULTS

Seventy-two subjects (54 emixustat and 18 placebo) were evaluated. Emixustat suppressed rod photoreceptor sensitivity in a dose-dependent manner. Suppression plateaued by Day 14 and was reversible within 7 days to 14 days after drug cessation. Most systemic adverse events were not considered treatment related. Dose-related ocular adverse events (chromatopsia, 57% emixustat vs. 17% placebo and delayed dark adaptation, 48% emixustat vs. 6% placebo) were mild to moderate in severity, and the majority resolved on study or within 7 days to 14 days after study drug cessation. Reversibility of these adverse events with long-term administration, however, is undetermined.

CONCLUSION

In this Phase II study, emixustat produced a dose-dependent reversible effect on rod function that is consistent with the proposed mechanism of action. These results support further testing of emixustat for the treatment of geographic atrophy associated with dry age-related macular degeneration.

Complement pathway biomarkers and age-related macular degeneration

In the age-related macular degeneration (AMD) 'inflammation model', local inflammation plus complement activation contributes to the pathogenesis and progression of the disease. Multiple genetic associations have now been established correlating the risk of development or progression of AMD. Stratifying patients by their AMD genetic profile may facilitate future AMD therapeutic trials resulting in meaningful clinical trial end points with smaller sample sizes and study duration.

Bioanalysis of emixustat (ACU-4429) in whole blood collected with volumetric absorptive microsampling by LC–MS/MS

Background: A method to quantify emixustat (an investigational drug agent) in human blood collected using volumetric absorptive microsampling (VAMS) could be more practical for sample collection at sites with limited facilities for processing and storage of plasma. Methods: A LC–MS/MS method was developed and evaluated for accuracy and precision, linearity, carryover, selectivity, recovery, matrix effects, hematocrit effects and stability. Results: Core validation parameters met acceptance criteria within the normal ranges of hematocrit levels for adults (30–55%). Stability of emixustat in blood collected with and without anticoagulant (NaF/KOx) on the VAMS device at ambient, refrigerated and frozen conditions was established. Conclusion: The method has been validated and is suitable for the bioanalysis of emixustat in human blood collected by VAMS.

Rescue of the Stargardt phenotype in Abca4 knockout mice through inhibition of vitamin A dimerization

Stargardt disease, an ATP-binding cassette, subfamily A, member 4 (ABCA4)-related retinopathy, is a genetic condition characterized by the accelerated accumulation of lipofuscin in the retinal pigment epithelium, degeneration of the neuroretina, and loss of vision. No approved treatment exists. Here, using a murine model of Stargardt disease, we show that the propensity of vitamin A to dimerize is responsible for triggering the formation of the majority of lipofuscin and transcriptional dysregulation of genes associated with inflammation. Data further demonstrate that replacing vitamin A with vitamin A deuterated at the carbon 20 position (C20-D3-vitamin A) impedes the dimerization rate of vitamin A--by approximately fivefold for the vitamin A dimer A2E--and subsequent lipofuscinogenesis and normalizes the aberrant transcription of complement genes without impairing retinal function. Phenotypic rescue by C20-D3-vitamin A was also observed noninvasively by quantitative autofluorescence, an imaging technique used clinically, in as little as 3 months after the initiation of treatment, whereas upon interruption of treatment, the age-related increase in autofluorescence resumed. Data suggest that C20-D3-vitamin A is a clinically amiable tool to inhibit vitamin A dimerization, which can be used to determine whether slowing the dimerization of vitamin A can prevent vision loss caused by Stargardt disease and other retinopathies associated with the accumulation of lipofuscin in the retina.