Bicyclic [3.3.0]-Octahydrocyclopenta[c]pyrrolo Antagonists of Retinol Binding Protein 4: Potential Treatment of Atrophic Age-Related Macular Degeneration and Stargardt Disease

Oxidative Stress, Persistent Inflammation and Blood Coagulation Alterations in Serum Proteome of Patients with Neovascular Age-Related Macular Degeneration

Neovascular age-related macular degeneration (AMD) is a major cause of irreversible blindness in elderly populations in developed countries. AMD's etiopathology is multifactorial, with strong environmental and genetic components, but the exact molecular pathomechanisms underlying the disease are still unknown. In this study, we analyzed blood serum collected from 74 neovascular AMD patients and 58 healthy controls to identify proteins that may serve as potential biomarkers and expand our knowledge about the etiopathogenesis of the disease. The study revealed 17 differentially expressed proteins-11 up-regulated and 6 down-regulated-in neovascular AMD, which are involved in the biological processes previously linked with the disease-oxidative stress and persistent inflammation, impaired cellular transport, lipid metabolism and blood coagulation. In conclusion, the differences in the expressions of the proteins identified in this study may contribute to our understanding of the mechanisms underlying AMD and possibly serve in future as promising biomarkers.

Recent Advances on Small-Molecule Inhibitors of Lipocalin-like Proteins

Lipid transfer proteins (LTPs) are crucial players in nonvesicular lipid trafficking. LTPs sharing a lipocalin lipid transfer domain (lipocalin-like proteins) have a wide range of biological functions, such as regulating immune responses and cell proliferation, differentiation, and death as well as participating in the pathogenesis of inflammatory, metabolic, and neurological disorders and cancer. Therefore, the development of small-molecule inhibitors targeting these LTPs is important and has potential clinical applications. Herein, we summarize the structure and function of lipocalin-like proteins, mainly including retinol-binding proteins, lipocalins, and fatty acid-binding proteins and discuss the recent advances on small-molecule inhibitors for these protein families and their applications in disease treatment. The findings of our Perspective can provide guidance for the development of inhibitors of these LTPs and highlight the challenges that might be faced during the procedures.

Discovery of Nonretinoid Inhibitors of CRBP1: Structural and Dynamic Insights for Ligand-Binding Mechanisms

The dysregulation of retinoid metabolism has been linked to prevalent ocular diseases including age-related macular degeneration and Stargardt disease. Modulating retinoid metabolism through pharmacological approaches holds promise for the treatment of these eye diseases. Cellular retinol-binding protein 1 (CRBP1) is the primary transporter of all-trans-retinol (atROL) in the eye, and its inhibition has recently been shown to protect mouse retinas from light-induced retinal damage. In this report, we employed high-throughput screening to identify new chemical scaffolds for competitive, nonretinoid inhibitors of CRBP1. To understand the mechanisms of interaction between CRBP1 and these inhibitors, we solved high-resolution X-ray crystal structures of the protein in complex with six selected compounds. By combining protein crystallography with hydrogen/deuterium exchange mass spectrometry, we quantified the conformational changes in CRBP1 caused by different inhibitors and correlated their magnitude with apparent binding affinities. Furthermore, using molecular dynamic simulations, we provided evidence for the functional significance of the "closed" conformation of CRBP1 in retaining ligands within the binding pocket. Collectively, our study outlines the molecular foundations for understanding the mechanism of high-affinity interactions between small molecules and CRBPs, offering a framework for the rational design of improved inhibitors for this class of lipid-binding proteins.

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.

Administration of Melatonin in Diabetic Retinopathy Is Effective and Improves the Efficacy of Mesenchymal Stem Cell Treatment

Stem cell transplantation is a promising therapeutic technique for the treatment of a variety of diseases; nevertheless, stem cell therapy may not always work as well as it could. The goal of this study was to test the hypothesis that employing a powerful antioxidant like melatonin improves stem cell transplantation success and potentiates stem cell function in the therapy of diabetic retinopathy. For this purpose, 50 adult male rats were divided into the following: control group: this group received 0.5 ml of 0.1 M of sodium citrate buffer (pH = 4.5) (intraperitoneal (I.P.)). The confirmed diabetic rats were divided into 4 groups: diabetic group: confirmed diabetic rats received no treatments with a regular follow of the blood glucose profile for 8 weeks; melatonin group: confirmed diabetic rats received melatonin (5 mg/kg/day); stem cell group: the confirmed diabetic rats were given intravitreal injection of stem cells (2 μl cell suspension of stem cells (3 × 10⁴ cells/μl)); and melatonin+stem cell group: confirmed diabetic rats received melatonin (5 mg/kg/day), orally once daily for 8 weeks, and 2 μl cell suspension of stem cells (3 × 10⁴ cells/μl) was carefully injected into the vitreous cavity. Our results showed that administration of melatonin and/or stem cell restored the retinal oxidative/antioxidant redox and reduced retinal inflammatory mediators. Coadministration of melatonin and stem cells enhanced the number of transplanted stem cells in the retinal tissue and significantly reduced retinal BDEF, VEGF, APOA1, and RBP4 levels as compared to melatonin and/or stem alone. We may conclude that rats treated with melatonin and stem cells had their retinal oxidative/antioxidant redox values restored to normal and their histological abnormalities reduced. These findings support the hypothesis that interactions with the BDEF, VEGF, APOA1, and RBP4 signaling pathways are responsible for these effects.

Novel Possible Protein Targets in Neovascular Age-Related Macular Degeneration: A Pilot Study Experiment

Age-related macular degeneration (AMD) is among the world's leading causes of blindness. In its neovascular form (nAMD), around 25% of patients present further anatomical and visual deterioration due to persistence of neovascular activity, despite gold-standard treatment protocols using intravitreal anti-VEGF medications. Thus, to comprehend, the molecular pathways that drive choroidal neoangiogenesis, associated with the vascular endothelial growth factor (VEGF), are important steps to elucidate the mechanistic events underneath the disease development. This is a pilot study, a prospective, translational experiment, in a real-life context aiming to evaluate the protein profiles of the aqueous humor of 15 patients divided into three groups: group 1, composed of patients with nAMD, who demonstrated a good response to anti-VEGF intravitreal injections during follow-up (good responsive); group 2, composed of patients with anti-VEGF-resistant nAMD, who demonstrated choroidal neovascularization activity during follow-up (poor/non-responsive); and group 3, composed of control patients without systemic diseases or signs of retinopathy. For proteomic characterization of the groups, mass spectrometry (label-free LC-MS/MS) was used. A total of 2,336 proteins were identified, of which 185 were distinctly regulated and allowed the differentiation of the clinical conditions analyzed. Among those, 39 proteins, including some novel ones, were analyzed as potential disease effectors through their pathophysiological implications in lipid metabolism, oxidative stress, complement system, inflammatory pathways, and angiogenesis. So, this study suggests the participation of other promising biomarkers in neovascular AMD, in addition to the known VEGF.

Retinol binding protein 4 antagonists and protein synthesis inhibitors: Potential for therapeutic development

Retinol-binding protein 4 (RBP4) is a serum protein that transports Vitamin A. RBP4 is correlated with numerous diseases and metabolic syndromes, including insulin resistance in type 2 diabetes, cardiovascular diseases, obesity, and macular degeneration. Recently, RBP4 antagonists and protein synthesis inhibitors are under development to regulate the effect of RBP4. Several RBP4 antagonists, especially BPN-14136, have demonstrated promising safety profiles and potential therapeutic benefits in animal studies. Two RBP4 antagonists, specifically tinlarebant (Belite Bio) and STG-001 (Stargazer) are currently undergoing clinical trials. Some antidiabetic drugs and nutraceuticals have been reported to reduce RBP4 expression, but more clinical data is needed to evaluate their therapeutical benefits. As regulating RBP4 levels or its activities would benefit a wide range of patients, further research is highly recommended to develop clinically useful RBP4 antagonists or protein synthesis inhibitors.

Discovery of phenylpyrrolidine derivatives as a novel class of retinol binding protein 4 (RBP4) reducers

Retinol-binding protein 4 (RBP4) is a potential drug target for metabolic and ophthalmologic diseases. A high-throughput screening of our compound library has identified a small-molecule RBP4 reducer 7a, as a hit compound. Aiming to provide a suitable tool for investigating the pharmacological effects of RBP4 reducers, we conducted a structure-activity relationship study of 7a. Exploration of the aryl head, oxazole core, and propanoic acid tail of 7a resulted in the discovery of novel, potent, and orally available phenylpyrrolidine derivatives 43b and 43c. Compound 43b had a potent and long-lasting blood RBP4-level-reducing effect when orally administered to mice at a dose as low as 0.3 mg/kg.

Retinol-binding protein 4 in obesity and metabolic dysfunctions

Excessive increased adipose tissue mass in obesity is associated with numerous co-morbid disorders including increased risk of type 2 diabetes, fatty liver disease, hypertension, dyslipidemia, cardiovascular diseases, dementia, airway disease and some cancers. The causal mechanisms explaining these associations are not fully understood. Adipose tissue is an active endocrine organ that secretes many adipokines, cytokines and releases metabolites. These biomolecules referred to as adipocytokines play a significant role in the regulation of whole-body energy homeostasis and metabolism by influencing and altering target tissues function. Understanding the mechanisms of adipocytokine actions represents a hot topic in obesity research. Among several secreted bioactive signalling molecules from adipose tissue and liver, retinol-binding protein 4 (RBP4) has been associated with systemic insulin resistance, dyslipidemia, type 2 diabetes and other metabolic diseases. Here, we aim to review and discuss the current knowledge on RBP4 with a focus on its role in the pathogenesis of obesity comorbid diseases.

Identification of Transthyretin Tetramer Kinetic Stabilizers That Are Capable of Inhibiting the Retinol-Dependent Retinol Binding Protein 4-Transthyretin Interaction: Potential Novel Therapeutics for Macular Degeneration, Transthyretin Amyloidosis, and Their Common Age-Related Comorbidities

Dissociation of transthyretin (TTR) tetramers may lead to misfolding and aggregation of proamyloidogenic monomers, which underlies TTR amyloidosis (ATTR) pathophysiology. ATTR is a progressive disease resulting from the deposition of toxic fibrils in tissues that predominantly presents clinically as amyloid cardiomyopathy and peripheral polyneuropathy. Ligands that bind to and kinetically stabilize TTR tetramers prohibit their dissociation and may prevent ATTR onset. Drawing from clinically investigated AG10, we designed a constrained congener (14) that exhibits excellent TTR tetramer binding potency, prevents TTR aggregation in a gel-based assay, and possesses desirable pharmacokinetics in mice. Additionally, 14 significantly lowers murine serum retinol binding protein 4 (RBP4) levels despite a lack of binding at that protein's all-trans-retinol site. We hypothesize that kinetic stabilization of TTR tetramers via 14 is allosterically hindering all-trans-retinol-dependent RBP4-TTR tertiary complex formation and that the compound could present ancillary therapeutic utility for indications treated with RBP4 antagonists, such as macular degeneration.

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.

Comparative pharmacokinetics and pharmacodynamics of the advanced Retinol-Binding Protein 4 antagonist in dog and cynomolgus monkey

Accumulation of lipofuscin bisretinoids in the retina contributes to pathogenesis of macular degeneration. Retinol-Binding Protein 4 (RBP4) antagonists reduce serum retinol concentrations thus partially reducing retinol delivery to the retina which decreases bisretinoid synthesis. BPN-14136 is a novel RBP4 antagonist with good in vitro potency and selectivity and optimal rodent pharmacokinetic (PK) and pharmacodynamic (PD) characteristics. To select a non-rodent species for regulatory toxicology studies, we conducted PK and PD evaluation of BPN-14136 in dogs and non-human primates (NHP). PK properties were determined following oral and intravenous administration of BPN-14136 in beagle dogs and cynomolgus monkeys. Dynamics of plasma RBP4 reduction in response to compound administration was used as a PD marker. BPN-14136 exhibited favorable PK profile in both species. Dose-normalized exposure was significantly higher in NHP than in dog. Baseline concentrations of RBP4 were considerably lower in dog than in NHP, reflecting the atypical reliance of canids on non-RBP4 mechanisms of retinoid trafficking. Oral administration of BPN-14136 to NHP induced a strong 99% serum RBP4 reduction. Dynamics of RBP4 lowering in both species correlated with compound exposure. Despite adequate PK and PD characteristics of BPN-14136 in dog, reliance of canids on non-RBP4 mechanisms of retinoid trafficking precludes evaluation of on-target toxicities for RBP4 antagonists in this species. Strong RBP4 lowering combined with good PK attributes and high BPN-14136 exposure achieved in NHP, along with the biology of retinoid trafficking that is similar to that of humans, support the choice of NHP as a non-rodent safety species.

Design, Synthesis, and Preclinical Efficacy of Novel Nonretinoid Antagonists of Retinol-Binding Protein 4 in the Mouse Model of Hepatic Steatosis

Retinol-binding protein 4 (RBP4) serves as a transporter for all- trans-retinol (1) in the blood, and it has been proposed to act as an adipokine. Elevated plasma levels of the protein have been linked to diabetes, obesity, cardiovascular diseases, and nonalcoholic fatty liver disease (NAFLD). Recently, adipocyte-specific overexpression of RBP4 was reported to cause hepatic steatosis in mice. We previously identified an orally bioavailable RBP4 antagonist that significantly lowered RBP4 serum levels in Abca4^-/- knockout mice with concomitant normalization of complement system protein expression and reduction of bisretinoid formation within the retinal pigment epithelium. We describe herein the discovery of novel RBP4 antagonists 48 and 59, which reduce serum RBP4 levels by >80% in mice upon acute oral dosing. Furthermore, 59 demonstrated efficacy in the transgenic adi-hRBP4 murine model of hepatic steatosis, suggesting that RBP4 antagonists may also have therapeutic utility for the treatment of NAFLD.

A non-retinoid antagonist of retinol-binding protein 4 rescues phenotype in a model of Stargardt disease without inhibiting the visual cycle

A primary pathological defect in the heritable eye disorder Stargardt disease is excessive accumulation of cytotoxic lipofuscin bisretinoids in the retina. Age-dependent accumulation of lipofuscin in the retinal pigment epithelium (RPE) matches the age-dependent increase in the incidence of the atrophic (dry) form of age-related macular degeneration (AMD) and therefore may be one of several pathogenic factors contributing to AMD progression. Lipofuscin bisretinoid synthesis in the retina depends on the influx of serum retinol from the circulation into the RPE. Formation of the tertiary retinol-binding protein 4 (RBP4)-transthyretin-retinol complex in the serum is required for this influx. Herein, we report the pharmacological effects of the non-retinoid RBP4 antagonist, BPN-14136. BPN-14136 dosing in the Abca4^-/- mouse model of increased lipofuscinogenesis significantly reduced serum RBP4 levels and inhibited bisretinoid synthesis, and this inhibition correlated with a partial reduction in visual cycle retinoids such as retinaldehydes serving as bisretinoid precursors. BPN-14136 administration at doses inducing maximal serum RBP4 reduction did not produce changes in the rate of the visual cycle, consistent with minimal changes in dark adaptation. Abca4^-/- mice exhibited dysregulation of the complement system in the retina, and BPN-14136 administration normalized the retinal levels of proinflammatory complement cascade components such as complement factors D and H, C-reactive protein, and C3. We conclude that BPN-14136 has several beneficial characteristics, combining inhibition of bisretinoid synthesis and reduction in retinaldehydes with normalization of the retinal complement system. BPN-14136, or a similar compound, may be a promising drug candidate to manage Stargardt disease and dry AMD.

Vitreous levels of apolipoprotein A1 and retinol binding protein 4 in human rhegmatogenous retinal detachment associated with choroidal detachment

Purpose

This study aims to quantify the concentration of apolipoprotein A1 (APOA1) and retinol binding protein (RBP4) expressed in the vitreous humors of patients with rhegmatogenous retinal detachment associated with choroidal detachment (RRDCD), rhegmatogenous retinal detachment (RRD), and idiopathic epimacular membrane (IEM). This study also aims to investigate the potential role of APOA1 and RBP4 as biomarkers of RRDCD.

Methods

Enzyme-linked immunosorbent assay (ELISA) kits were used to obtain levels of APOA1 and RBP4 from the vitreous humor samples of 76 primary patients. These patients included 23 patients with RRDCD, 28 patients with RRD, and 24 patients with IEM. All patients were undergoing planned pars plana vitrectomy. The differences between the concentrations of the molecular biomarkers among different patient groups were analyzed using the Mann-Whitney U-test for nonparametric values and independent samples t-test or one-way ANOVA analysis for parametric data. The relationship between the molecular biomarkers, grades of proliferative vitreoretinopathy (PVR), and quadrants of retinal detachment were analyzed using nonparametric Spearman's rank correlation analysis.

Results

The vitreous concentrations of APOA1 and RBP4 were statistically significantly higher in the RRDCD group compared to the RRD and IEM groups. Patients with severe PVR demonstrated a higher concentration of APOA1 and RBP4 compared to those with mild PVR, but this finding was not statistically significant. There was a statistically significant positive correlation between APOA1 and RBP4 in the RRDCD and RRD groups. Nonparametric Spearman's rank correlation analysis revealed that levels of APOA1 and RBP4 increased statistically significantly with an increasing number of detached retinal quadrants in the RRDCD and RRD groups.

Conclusions

The findings of this study allude to the potential of APOA1 and RBP4 as specific biomarkers of RRDCD. The findings of this study may contribute to increased understanding regarding the role of APOA1 and RBP4 in RRDCD.

Drugs for rare disorders

Estimates of the frequencies of rare disorders vary from country to country; the global average defined prevalence is 40 per 100 000 (0.04%). Some occur in only one or a few patients. However, collectively rare disorders are fairly common, affecting 6-8% of the US population, or about 30 million people, and a similar number in the European Union. Most of them affect children and most are genetically determined. Diagnosis can be difficult, partly because of variable presentations and partly because few clinicians have experience of individual rare disorders, although they may be assisted by searching databases. Relatively few rare disorders have specific pharmacological treatments (so-called orphan drugs), partly because of difficulties in designing trials large enough to determine benefits and harms alike. Incentives have been introduced to encourage the development of orphan drugs, including tax credits and research aids, simplification of marketing authorization procedures and exemption from fees, and extended market exclusivity. Consequently, the number of applications for orphan drugs has grown, as have the costs of using them, so much so that treatments may not be cost-effective. It has therefore been suggested that not-for-profit organizations that are socially motivated to reduce those costs should be tasked with producing them. A growing role for patient organizations, improved clinical and translational infrastructures, and developments in genetics have also contributed to successful drug development. The translational discipline of clinical pharmacology is an essential component in drug development, including orphan drugs. Clinical pharmacologists, skilled in basic pharmacology and its links to clinical medicine, can be involved at all stages. They can contribute to the delineation of genetic factors that determine clinical outcomes of pharmacological interventions, develop biomarkers, design and perform clinical trials, assist regulatory decision making, and conduct postmarketing surveillance and pharmacoepidemiological and pharmacoeconomic assessments.

Vitamin A-aldehyde adducts: AMD risk and targeted therapeutics

Although currently available treatment options for age-related macular degeneration (AMD) are limited, particularly for atrophic AMD, the identification of predisposing genetic variations has informed clinical studies addressing therapeutic options such as complement inhibitors and anti-inflammatory agents. To lower risk of early AMD, recommended lifestyle interventions such as the avoidance of smoking and the intake of low glycemic antioxidant-rich diets have largely followed from the identification of nongenetic modifiable factors. On the other hand, the challenge of understanding the complex relationship between aging and cumulative damage leading to AMD has fueled investigations of the visual cycle adducts that accumulate in retinal pigment epithelial (RPE) cells and are a hallmark of aging retina. These studies have revealed properties of these compounds that provide insights into processes that may compromise RPE and could contribute to disease mechanisms in AMD. This work has also led to the design of targeted therapeutics that are currently under investigation.

Dry Age-Related Macular Degeneration Pharmacology

Age-related macular degeneration (AMD), the most common form of irreversible blindness in the industrially developed world, can present years before a patient begins to lose vision. For most of these patients, AMD never progresses past its early stages to the advanced forms that are principally responsible for the vast majority of vision loss. Advanced AMD can manifest as either an advanced avascular form known as geographic atrophy (GA) marked by regional retinal pigment epithelium (RPE) cell death or as an advanced form known as neovascular AMD marked by the intrusion of fragile new blood vessels into the normally avascular retina. Physicians have several therapeutic interventions available to combat neovascular AMD, but GA has no approved effective therapies as of yet. In this chapter, we will discuss the current strategies for limiting dry AMD in patients. We will also discuss previous attempts at pharmacological intervention that were tested in a clinical setting and consider reasons why these putative therapeutics did not perform successfully in large-scale trials. Despite the number of unsuccessful past trials, new pharmacological interventions may succeed. These future therapies may aid millions of AMD patients worldwide.