Effects of emixustat hydrochloride in patients with proliferative diabetic retinopathy: a randomized, placebo-controlled phase 2 study

Clinical Evidence of a Photoreceptor Origin in Diabetic Retinal Disease

Clinical Relevance

Although diabetes is associated with a classic microvascular disease of the retina, it is also increasingly being recognized as a cause of retinal neuropathy. Preclinical evidence suggests that retinal neuropathy in diabetes manifests in part as photoreceptor dysfunction, preceding the development of vascular features in experimental models. It remains unknown whether such findings are relevant to patients with diabetes.

Methods

Here, we review 4 lines of clinical evidence suggesting that diabetes-associated photoreceptor pathology is linked to the development of retinal microvascular disease.

Results

First, a major population-based investigation of susceptibility loci for diabetic retinopathy (DR) implicated a photoreceptor protein product as a protective factor. Next, electroretinography and other studies of visual function collectively show that rod and/or cone-derived abnormalities occur decades before the development of vascular features of DR. Third, protection from DR seemingly develops in patients with coincident retinitis pigmentosa, as suggested by several case series. Finally, based on anatomic features, we propose that the beneficial effect of macular laser in DR occurs via ablation of diseased photoreceptors.

Conclusions

The evidence we present is limited due to the small patient populations used in the studies we cite and due to the lack of methodologies that allow causative relationships to be inferred. Collectively, however, these clinical observations suggest that photoreceptors are involved in early diabetic retinal disease and may in fact give rise to the classic features of DR.

Financial Disclosures

Proprietary or commercial disclosures may be found in the Footnotes and Disclosures at the end of this article.

Full-field stimulus threshold testing: a scoping review of current practice

The full-field stimulus threshold (FST) is a psychophysical measure of whole-field retinal light sensitivity. It can assess residual visual function in patients with severe retinal disease and is increasingly being adopted as an endpoint in clinical trials. FST applications in routine ophthalmology clinics are also growing, but as yet there is no formalised standard guidance for measuring FST. This scoping review explored current variability in FST conduct and reporting, with an aim to inform further evidence synthesis and consensus guidance. A comprehensive electronic search and review of the literature was carried out according to the Preferred Reporting Items for Systematic Reviews and Meta-analysis Extension for Scoping Reviews (PRISMA-ScR) checklist. Key source, participant, methodology and outcomes data from 85 included sources were qualitatively and quantitatively compared and summarised. Data from 85 sources highlight how the variability and insufficient reporting of FST methodology, including parameters such as units of flash luminance, colour, duration, test strategy and dark adaptation, can hinder comparison and interpretation of clinical significance across centres. The review also highlights an unmet need for paediatric-specific considerations for test optimisation. Further evidence synthesis, empirical research or structured panel consultation may be required to establish coherent standardised guidance on FST methodology and context or condition dependent modifications. Consistent reporting of core elements, most crucially the flash luminance equivalence to 0 dB reference level is a first step. The development of criteria for quality assurance, calibration and age-appropriate reference data generation may further strengthen rigour of measurement.

Increasing Energetic Demands on Photoreceptors in Diabetes Corrects Retinal Lipid Dysmetabolism and Reduces Subsequent Microvascular Damage

Mechanisms responsible for the pathogenesis of diabetic retinal disease remain incompletely understood, but they likely involve multiple cellular targets, including photoreceptors. Evidence suggests that dysregulated de novo lipogenesis in photoreceptors is a critical early target of diabetes. Following on this observation, the present study aimed to determine whether two interventions shown to improve diabetic retinopathy in mice-pharmacologic visual cycle inhibition and prolonged dark adaptation-reduce photoreceptor anabolic lipid metabolism. Elevated retinal lipid biosynthetic signaling was observed in two mouse models of diabetes, with both models showing reduced retinal AMP-activated kinase (AMPK) signaling, elevated acetyl CoA carboxylase (ACC) signaling, and increased activity of fatty acid synthase, which promotes lipotoxicity in photoreceptors. Although retinal AMPK-ACC axis signaling was dependent on systemic glucose fluctuations in healthy animals, mice with diabetes lacked such regulation. Visual cycle inhibition and prolonged dark adaptation reversed abnormal retinal AMPK-ACC signaling in mice with diabetes. Although visual cycle inhibition reduced the severity of diabetic retinopathy in control mice, as assessed by retinal capillary atrophy, this intervention was ineffective in fatty acid synthase gain-of-function mice. These results suggest that early diabetic retinopathy is characterized by glucose-driven elevations in retinal lipid biosynthetic activity, and that two interventions known to increase photoreceptor glucose demands alleviate disease by reversing these signals.

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.

Novel oral medications for retinal disease: an update on clinical development

PURPOSE OF REVIEW

Intravitreal and periocular injections for retinal disease provide a targeted delivery of medication to the eye. However, given risks of injections, including endophthalmitis, pain and treatment burden for both patients and retina specialists, there has been significant interest and effort in developing oral medications for the management of retinal disease. This article provides clinical and preclinical details of new oral medications in the pipeline for management of retinal disease.

RECENT FINDINGS

Several new oral medications show clinical and preclinical promise for the management of retinal disease, including macular degeneration, diabetic retinopathy and Stargardt disease.

SUMMARY

Oral medications provide promise for treating retinal disease, possibly increasing compliance, and reducing side effects of intravitreal or periocular injections. However, difficulties in this approach include systemic side effects and efficacy targeting the eye. There are multiple medications that are currently under investigation with the potential to act as stand-alone treatment or as an adjunct treatment for management of retinal diseases such as diabetic retinopathy, macular degeneration and Stargardt disease.

The role of inflammation in immune system of diabetic retinopathy: Molecular mechanisms, pathogenetic role and therapeutic implications

Diabetic retinopathy is one of the most common complications of diabetes mellitus and the leading cause of low vision and blindness worldwide. Mounting evidence demonstrates that inflammation is a key mechanism driving diabetes-associated retinal disturbance, yet the pathophysiological process and molecular mechanisms of inflammation underlying diabetic retinopathy are not fully understood. Cytokines, chemokines, and adhesion molecules interact with each other to form a complex molecular network that propagates the inflammatory and pathological cascade of diabetic retinopathy. Therefore, it is important to understand and elucidate inflammation-related mechanisms behind diabetic retinopathy progression. Here, we review the current understanding of the pathology and pathogenesis of inflammation in diabetic retinopathy. In addition, we also summarize the relevant clinical trials to further suggest inflammation-targeted therapeutics for prevention and management of diabetic retinopathy.

Changes in aqueous and vitreous inflammatory cytokine levels in proliferative diabetic retinopathy: a systematic review and meta-analysis

BACKGROUND

Diabetic retinopathy is a major complication of diabetes mellitus, where in its most advanced form ischemic changes lead to the development of retinal neovascularization, termed proliferative diabetic retinopathy (PDR). While the development of PDR is often associated with angiogenic and inflammatory cytokines, studies differ on which cytokines are implicated in disease pathogenesis and on the strength of these associations. We therefore conducted a systematic review and meta-analysis to quantitatively assess the existing body of data on intraocular cytokines as biomarkers in PDR.

METHODS

A comprehensive search of the literature without year limitation was conducted to January 18, 2021, which identified 341 studies assessing vitreous or aqueous cytokine levels in PDR, accounting for 10379 eyes with PDR and 6269 eyes from healthy controls. Effect sizes were calculated as standardized mean differences (SMD) of cytokine concentrations between PDR and control patients.

RESULTS

Concentrations (SMD, 95% confidence interval, and p-value) of aqueous IL-1β, IL-6, IL-8, MCP-1, TNF-α, and VEGF, and vitreous IL-2, IL-4, IL-6, IL-8, angiopoietin-2, eotaxin, erythropoietin, GM-CSF, GRO, HMGB-1, IFN-γ, IGF, IP-10, MCP-1, MIP-1, MMP-9, PDGF-AA, PlGF, sCD40L, SDF-1, sICAM-1, sVEGFR, TIMP, TNF-α, and VEGF were significantly higher in patients with PDR when compared to healthy nondiabetic controls. For all other cytokines no differences, failed sensitivity analyses or insufficient data were found.

CONCLUSIONS

This extensive list of cytokines speaks to the complexity of PDR pathogenesis, and informs future investigations into disease pathogenesis, prognosis, and management.

Novel Treatments for Diabetic Macular Edema and Proliferative Diabetic Retinopathy

PURPOSE OF REVIEW

Diabetic retinopathy (DR), a common cause of vision loss, is projected to increase worldwide, and is associated with significant morbidity. The current standard-of-care treatments can preserve and significantly improve vision in many patients affected by DR. However, challenges such as heavy treatment burden and refractory disease remain. The purpose of this review is to highlight and discuss investigative agents in development for the treatment of DR.

RECENT FINDINGS

There are several novel agents with unique mechanisms that may offer greater durability and efficacy compared to existing drugs. Some target new pathways, others leverage a slow-release delivery system, and some modify gene expression through a single-dose treatment. While unfavorable adverse events, such as intraocular inflammation, have been observed with longer-durability agents, many investigational products show excellent efficacy and safety profiles. The outcomes of ongoing and future trials may revolutionize the current treatment paradigm for DR.

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.

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.