Targeting Schlemm's Canal in the Medical Therapy of Glaucoma: Current and Future Considerations

Five-Year Outcomes of Single Trabecular Microbypass Stent (iStent®) Implantation with Phacoemulsification in Korean Patients

INTRODUCTION

This study reports the long-term intraocular pressure (IOP)-lowering efficacy and safety of a single trabecular microbypass stent (iStent®; Glaukos Corp., San Clemente, CA, USA) for medically controlled open-angle glaucoma in Korean patients.

METHODS

This retrospective observational study included 42 eyes of 35 patients with primary open-angle glaucoma (POAG). All subjects underwent single first-generation iStent® implantation with phacoemulsification by a single surgeon with 5 years follow-up. The primary outcomes were changes in IOP and the number of antiglaucoma medications compared to the preoperative values. The secondary outcome was the proportion of eyes with IOP ≤ 18 mmHg without medication, ≤ 15 mmHg without medication, and ≤ 18 mmHg with or without medication. Adverse events and need for secondary glaucoma surgery were also recorded.

RESULTS

The mean IOP decreased from 15.8 ± 2.8 to 13.8 ± 1.7 mmHg and the mean number of medications was reduced from 2.24 ± 1.18 to 0.83 ± 1.12, respectively, at year 5. At 3 and 5 years, 80.6% and 78.6% of eyes, respectively, were receiving fewer medications than preoperative numbers. In contrast, only 50% of eyes on four preoperative medications showed medication reductions at 5 years. At years 3 and 5, 61.3% and 53.5% of eyes achieved IOP ≤ 18 mmHg without medication, whereas 90.3% and 89.3% of eyes achieved ≤ 18 mmHg regardless of medication use, respectively. Four eyes required additional glaucoma surgery (two Ahmed glaucoma valve implantations, one trabeculectomy, and one XEN 45 Gel Stent implantation), and all were receiving four preoperative antiglaucoma medications. Transient IOP elevation (14.3%) was the most common complication, followed by five hyphema, one stent obstruction, one stent malposition, and one severe anterior chamber reaction.

CONCLUSION

This study demonstrated a good safety profile and sustained IOP reduction after the implantation of a single trabecular microbypass stent (iStent®) with phacoemulsification in Korean patients. The majority of subjects with POAG showed a relatively good response; however, eyes receiving a higher number of medications preoperatively (especially four medications) had difficulty achieving a low target IOP.

Evaluation of Schlemm Canal Parameters Using Swept-Source Optical Coherence Tomography in Eyes That Underwent Keratoplasty

OBJECTIVES

In this study, we aimed to evaluate Schlemm canal parameters using anterior segment swept-source optical coherence tomography in eyes that underwent keratoplasty and compare them with keratoconus and healthy control groups.

METHODS

The study included 32 patients who underwent penetrating keratoplasty or deep anterior lamellar keratoplasty once due to keratoconus and age-matched and sex-matched 20 keratoconus patients and 30 healthy controls. In all the patients, a single horizontal image centered on the central cornea was obtained from the nasal and temporal quadrants with low-intensity scanning to visualize Schlemm canal.

RESULTS

There was no statistically significant difference between the groups for age and gender ( P ˃0.05). In the keratoplasty group, the area and diameter of Schlemm canal were 2.266±1.141μm 2 and 160.77±65.08 μm, respectively, in the nasal quadrant and 2.623±1.277 μm 2 and 158.81±68.05 μm, respectively in the temporal quadrant, which were statistically significantly lower compared with other groups ( P <0.001 for all). There was no significant difference between the penetrating and deep anterior lamellar keratoplasty subgroups for Schlemm canal parameters.

CONCLUSION

This is the first study to report anterior segment optical coherence tomography after surgery shows SC parameters that are, on average, less than age-matched and keratoconus controls.

Novel discovery of a lymphatic bridge connecting Schlemm's canal to limbal and conjunctival lymphatic pathway

PURPOSE

Schlemm's canal (SC) is a critical structure regulating aqueous humor (AH) drainage and intraocular pressure (IOP). It is known that in the conventional outflow pathway, AH flows from SC to episcleral veins. We recently reported a high-resolution three-dimensional (3D) imaging technology for intact eyeballs, SC and ocular surface. Using this advanced technology, we herein report the discovery of a new structure, termed lymphatic bridge, that directly connects SC to the limbal and conjunctival lymphatic pathway. Further investigation on this novel outflow pathway may provide new mechanisms and therapeutic approaches for glaucoma.

METHODS

As reported previously, intact eyeballs were harvested from Prox-1-GFP (green fluorescent protein) mice and processed by a tissue clearing technique with CLARITY. Samples were immunolabeled with specific antibodies for CD31 (pan-endothelial marker) and LYVE-1 (lymphatic vessel endothelial hyaluronan receptor-1) and imaged by light-sheet fluorescent microscopy. The limbal areas were examined to locate connecting channels between SC and limbal and conjunctival lymphatic vessels. Moreover, in vivo anterior chamber dye injection was performed with Texas Red dextran for functional analysis on AH outflow.

RESULTS

A novel lymphatic bridge structure that expressed both Prox-1 and LYVE-1 was discovered between the SC and limbal lymphatic vessels connected with conjunctival lymphatic pathway. Results from the anterior chamber dye injection also confirmed AH drainage into the conjunctival lymphatic outflow pathway.

CONCLUSIONS

This study provides the first evidence on the direct connection between SC and conjunctival lymphatic pathway. This new pathway is distinctive from the traditional episcleral vein pathway and merits further investigation.

Isolation and Culture of Vascular Distal Outflow Pathway (VDOP) Cells From Human Donor Eyes

Glaucoma, a progressive neurodegenerative ocular disease, is the leading cause of irreversible blindness worldwide. Elevated intraocular pressure (IOP) is the most common-and the only treatable-risk factor for glaucoma. IOP is generated by the balance between production and removal of aqueous humor in the anterior part of the eye, and the latter happens either through the uveoscleral or the conventional pathway. Although both pathways are important for aqueous humor removal, dysfunction within the conventional pathway is more commonly associated with increased resistance leading to elevated IOP and glaucoma. The conventional pathway can be separated into proximal (trabecular meshwork and inner wall of Schlemm's canal) and distal (outer wall of Schlemm's canal, collector channels, and episcleral vasculature) regions. Both regions contribute resistance to aqueous humor removal, but the proximal region has been studied more extensively due to the availability of model systems. In contrast, little is known about the role of the distal region in outflow resistance, largely due to the lack of suitable in vitro models. To address this, we have developed a novel method of isolating and culturing vascular distal outflow pathway (VDOP) cells from the distal outflow region of human eyes. VDOP cells can be used to study the physiological and molecular functions of cells in the distal outflow region and can help in the development of ocular hypotensive drugs that specifically target this area. We also provide a protocol describing immunohistochemical methods to validate the molecular profile of these cells, utilizing cell surface markers that distinguish them from adjacent cells. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Isolation and culture of VDOP cells Basic Protocol 2: Analysis of cell surface markers.

Schlemm's canal: the outflow 'vessel'

In a healthy eye, the aqueous humour (AH) flows via the ciliary body and trabecular meshwork into the collector channels, which carry it to the episcleral veins. In glaucoma, a heterogeneous group of eye disorders affecting approximately 60 million individuals worldwide, the juxtacanalicular meshwork offers greater resistance to the outflow of the AH, leading to an increase in outflow resistance that gradually results in elevated intraocular pressure (IOP). The present review comprehensively covers the morphology of Schlemm’s canal (SC) and AH pathways. The path of the AH from the anterior chamber through the trabeculum into suprascleral and conjunctival veins via collector channels is described, and the role of SC in the development of glaucoma and outflow resistance is discussed. Finally, channelography is presented as a precise method of assessing the conventional drainage pathway and facilitating localization of an uncollapsed collector and aqueous veins. Attention is also given to the relationship between aqueous and episcleral veins and heartbeat. Possible directions of future research are proposed.

Evaluation of different OCT systems in quantitative imaging of human Schlemm's canal

We examined the performance of human Schlemm's canal (SC) imaging using different OCT devices: CIRRUS 5000 (840 nm, spectral-domain (SD)-OCT), PLEX Elite 9000 (1060 nm, swept-source (SS)-OCT) and CASIA SS-1000 (1310 nm, SS-OCT), and analyzed potential impact factors on visualization and the quantitative assessment of SC morphology in a pilot study. Ten healthy subjects were imaged using three OCT devices by a single experienced operator on the same day. Each eye underwent two cubic scans by each device, one on nasal and the other on temporal quadrant. The B-scan showing the largest SC was manually selected for processing. Four quantitative metrics, including one morphological metric as cross-sectional area (CSA), and three performance metrics as contrast, continuity, and coverage, were derived from the datasets. Repeated-measures ANOVA was used to investigate the difference between these parameters from the three devices (P < 0.05). We found the CSA measured from CIRRUS was significantly larger than PLEX, followed by CASIA. The contrast was highest in CIRRUS, followed by PLEX and CASIA. The coverage was also higher in CIRRUS as compared to PLEX and CASIA. No significant difference was seen in the continuity from the three devices. In summary, we showed the measurements from the three devices were not interchangeable.

[Xalacom: possibilities and advantages of the drug in the treatment of patients with glaucoma]

The strategy of glaucoma therapy is aimed at preserving visual functions and ensuring an acceptable quality of life for patients. To achieve this strategic goal, clinicians in their practice use drugs that affect the main factor in the progression of the disease - intraocular pressure (IOP), aiming to reduce it to an individual target level. It is not always possible to achieve optimal IOP values with monotherapy. Many patients require a combination of drugs from different pharmacological groups. Xalacom is a fixed drug with good tolerability and t hypotensive effect. This review focuses on the benefits of this drug for the treatment of glaucoma.

The Application of Nitric Oxide for Ocular Hypertension Treatment

Despite of various therapeutic methods for treating ocular hypertension and glaucoma, it still remains the leading cause of irreversible blindness. Intraocular pressure (IOP) lowering is the most effective way to slow disease progression and prevent blindness. Among the ocular hypotensive drugs currently in use, only a couple act on the conventional outflow system, which is the main pathway for aqueous humor outflow and the major lesion site resulting in ocular hypertension. Nitric oxide (NO) is a commendable new class of glaucoma drugs that acts on the conventional outflow pathway. An increasing number of nitric oxide donors have been developed for glaucoma and ocular hypertension treatment. Here, we will review how NO lowers IOP and the types of nitric oxide donors that have been developed. And a brief analysis of the advantages and challenges associated with the application will be made. The literature used in this review is based on Pubmed database search using ‘nitric oxide’ and ‘glaucoma’ as key words.

Endogenous dual stimuli-activated NO generation in the conventional outflow pathway for precision glaucoma therapy

High intraocular pressure (IOP) has been regarded as a predominant risk factor for glaucoma. Nitric oxide (NO) is shown to lower IOP, but the magnitude and duration of IOP reduction are not satisfying due to the poor cornea penetration of NO drugs and limited NO generation in the trabecular meshwork (TM)/Schlemm's canal (SC) area. Herein, we introduce deep cornea penetrating biodegradable hollow mesoporous organosilica (HOS) nanocapsules for the efficient co-delivery of hydrophobic JS-K (J_R) and hydrophilic l-Arginine (L_O). The resulting HOS-J_RL_O can be reduced and oxidized by the ascorbic acid (AA) and catalysis of endothelial nitric oxide synthase (eNOS) in the TM/SC microenvironment to release NO for inducing appreciable IOP reduction in various glaucoma mouse models. In addition to developing an endogenous stimuli-responsive NO nanotherapeutic, this study is also expected to establish a versatile, non-invasive, and efficacious treatment paradigm for precision glaucoma therapy.

Isolation and characterization of novel primary cells from the human distal outflow pathway

Ocular hypertension occurs due to increased resistance to aqueous humor removal through the conventional outflow pathway. Unlike the proximal region of the conventional outflow pathway, the distal region has not been well studied, mostly due to lack of model systems. Here we describe isolation and characterization of human primary vascular distal outflow pathway (VDOP) cells from the distal region of the conventional outflow pathway. Tissue from the distal region was isolated from human corneo-scleral rims, digested with collagenase type I (100 U/ml) and placed on gelatin coated plates to allow cellular growth in Dulbecco's Modified Eagle's Medium (low glucose) containing fetal bovine serum and antibiotic/antimycotic. VDOP cells showed consistent proliferation for up to 7 passages, retained endothelial-like nature of the parent tissues and showed a unique marker phenotype of Lectin+VEGFR2-CD34-NG2- that was distinct from neighboring trabecular meshwork (Lectin+VEGFR2-CD34-NG2+) and Schlemm's canal (Lectin+VEGFR2+CD34+NG2+) cells. Dexamethasone treated VDOP cells did not express myocilin and did not form cross-linked actin networks, in contrast to trabecular meshwork cells. These data show that VDOP cells are unique to the distal outflow region and can be used as a viable in vitro model system to understand the biology of the distal outflow pathway and intraocular pressure regulation.

Targeted Delivery of Cell Softening Micelles to Schlemm's Canal Endothelial Cells for Treatment of Glaucoma

Increased stiffness of the Schlemm's canal (SC) endothelium in the aqueous humor outflow pathways has been associated with elevated intraocular pressure (IOP) in glaucoma. Novel treatments that relax this endothelium, such as actin depolymerizers and rho kinase inhibitors, are in development. Unfortunately, these treatments have undesirable off-target effects and a lower than desired potency. To address these issues, a targeted PEG-b-PPS micelle loaded with actin depolymerizer latrunculin A (tLatA-MC) is developed. Targeting of SC cells is achieved by modifying the micelle surface with a high affinity peptide that binds the VEGFR3/FLT4 receptor, a lymphatic lineage marker found to be highly expressed by SC cells relative to other ocular cells. During in vitro optimization, increasing the peptide surface density increased micellar uptake in SC cells while unexpectedly decreasing uptake by human umbilical vein endothelial cells (HUVEC). The functional efficacy of tLatA-MC, as measured by decreased SC cell stiffness compared to non-targeted micelles (ntLatA-MC) or targeted blank micelles (tBL-MC), is verified using atomic force microscopy. tLatA-MC reduced IOP in an in vivo mouse model by 30-50%. The results validate the use of a cell-softening nanotherapy to selectively modulate stiffness of SC cells for therapeutic reduction of IOP and treatment of glaucoma.

Clinical outcomes of trabecular microbypass stent (iStent) implantation in medically controlled open-angle glaucoma in the Korean population

To evaluate the safety and efficacy [intraocular pressure (IOP)-lowering effect and medication use] of a single trabecular microbypass stent (iStent; Glaukos Corp, San Clemente, CA) for medically controlled open-angle glaucoma.This retrospective case series included 42 eyes of 34 patients with medically controlled open-angle glaucoma with IOP less than 21 mm Hg. Clinical outcomes analyzed were IOP, medication use, corrected distance visual acuity (CDVA), and surgical complications. Surgical success was defined according to 4 criteria: IOP < 21 mm Hg without medication; IOP < 18 mm Hg without medication; IOP < 15 mm Hg without medications; and IOP < 18 mm Hg with or without medication. Patients were followed for a minimum of 6 months postoperatively.Mean IOP was reduced from 15.8 ± 2.8 mm Hg to 14.5 ± 2.8 mm Hg (P < .001), while mean number of medications decreased from 2.2 ± 1.2 to 0.8 ± 1.1 at final visit (P < .001). Surgical success rates were 78.6%, 61.9%, 57.1%, and 97.6% at 6 months and 78.6%, 59.5%, 52.4%, and 95.2% at final visits according to criteria A, B, C, and D. Meanwhile, 59.5% of patients were medication-free at their final visit. The relative risk of surgical failure by Criteria B and C was 4.337 (95% confidence interval: 1.799-10.454) and 3.717 (95% confidence interval: 1.516-9.116) times greater in the higher-medication group (3 or more preoperative medications), respectively. CDVA was significantly improved from 0.41 ± 0.10 to 0.09 ± 0.07 LogMAR in the combined phacoemulsification and iStent implantation group (P < .001). There was no case whose vision was threatened (vision loss of 2 or more lines) or who showed severe complications after surgery.Single trabecular microbypass stent implantation was effective in reducing IOP and medication usage in patients with open-angle glaucoma with a low preoperative IOP. Our results imply that it is more difficult to achieve low target IOP control in eyes with higher numbers of preoperative medications.

Local ocular renin-angiotensin-aldosterone system: any connection with intraocular pressure? A comprehensive review

The renin-angiotensin system (RAS) is one of the oldest and most extensively studied human peptide cascades, well-known for its role in regulating blood pressure. When aldosterone is included, RAAS is involved also in fluid and electrolyte homeostasis. There are two main axes of RAAS: (1) Angiotensin (1-7), angiotensin converting enzyme 2 and Mas receptor (ACE2-Ang(1-7)-MasR), (2) Angiotensin II, angiotensin converting enzyme 1 and angiotensin II type 1 receptor (ACE1-AngII-AT1R). In its entirety, RAAS comprises dozens of angiotensin peptides, peptidases and seven receptors. The first mentioned axis is known to counterbalance the deleterious effects of the latter axis. In addition to the systemic RAAS, tissue-specific regulatory systems have been described in various organs, evidence that RAAS is both an endocrine and an autocrine system. These local regulatory systems, such as the one present in the vascular endothelium, are responsible for long-term regional changes. A local RAAS and its components have been detected in many structures of the human eye. This review focuses on the local ocular RAAS in the anterior part of the eye, its possible role in aqueous humour dynamics and intraocular pressure as well as RAAS as a potential target for anti-glaucomatous drugs.KEY MESSAGESComponents of renin-angiotensin-aldosterone system have been detected in different structures of the human eye, introducing the concept of a local intraocular renin-angiotensin-aldosterone system (RAAS).Evidence is accumulating that the local ocular RAAS is involved in aqueous humour dynamics, regulation of intraocular pressure, neuroprotection and ocular pathology making components of RAAS attractive candidates when developing new effective ways to treat glaucoma.

Minimally Invasive Glaucoma Surgery: What Do We Know? Where Should We Go?

With the arrival of a plethora of new and revolving minimally invasive glaucoma surgery techniques, glaucoma specialists currently are fortunate to have various surgical options that aim to recovery of the function of the aqueous outflow system in different ways. Meanwhile, the aqueous outflow system has become the hot point of researching. In ARVO 2019, a special interest group session was held on new perspectives on minimally invasive glaucoma surgery. Ten surgeons, clinical professors, and experimental scientists were invited to report their latest studies and discussed on five hot topics in this special interest group. This review summarizes the special interest group session and posts the issues of greatest concern, providing insight to the aqueous outflow system and areas that require further study.

Cataract surgery and intraocular pressure in glaucoma

PURPOSE OF REVIEW

To review the current literature on the relationship between cataract extraction and intraocular pressure (IOP).

RECENT FINDINGS

Cataract extraction can be an effective IOP lowering treatment for open and closed angle glaucoma as well as ocular hypertension. In comparative trials studying novel micro-invasive glaucoma surgeries in open angle glaucoma, the control group undergoing cataract extraction alone routinely achieved significant reductions in IOP and medication use postoperatively. Data from the Effectiveness in Angle Closure Glaucoma of Lens Extraction (EAGLE) trials have demonstrated that lens extraction is more effective at lowering IOP than peripheral iridotomy in patients with angle closure and should be considered as first line therapy. Additionally, patients in the ocular hypertension treatment study who underwent cataract extraction over the course of follow-up demonstrated significant IOP lowering sustained over 3 years.

SUMMARY

Cataract extraction is an effective method to lower IOP in patients with glaucoma. Pressure lowering is more significant in eyes with narrow angles and those with higher baseline IOP levels. In eyes with angle closure, phacoemulsification alone can lower IOP, but when combined with GSL it may be even more effective. Recent large multicenter randomized trials have further elucidated the benefit of standalone cataract extraction to treat mild to moderate primary open angle glaucoma. Prospective and longitudinal studies that systematically investigate the variables that may influence degree and duration of IOP lowering post cataract extraction are lacking.

Modulation of Schlemm's canal endothelial cell stiffness via latrunculin loaded block copolymer micelles

Increased stiffness of Schlemm's canal endothelial cells (SC cells) is a major contributing factor to the increased pressure characteristic of primary open-angle glaucoma. New treatments for glaucoma are being developed using actin depolymerizers and rho kinase inhibitors to address this increased stiffness. However, these agents have off-target effects and are not as potent as had been hoped. We have developed a micellar nanocarrier assembled from poly(ethylene glycol)-bl-poly(propylene sulfide) copolymers capable of encapsulating latrunculin A (Lat A) with the goal of modulating SC cell stiffness. Lat A-loaded nanocarriers were similar in size and morphology to unloaded poly (ethylene glycol)-bl-poly(propylene sulfide) (PEG-bl-PPS) micelles, loaded Lat A at 62% encapsulation efficiency, and retained loaded Lat A for at least 22 days. The continued functional activity of Lat A following encapsulation within micelles was verified in murine macrophages, which are known to display decreased endocytosis in response to Lat A-dependent cytoskeletal disruption. Endocytic inhibition remained unchanged when comparing equal concentrations of micelle-loaded versus free form Lat A. Uptake of Lat A-loaded micelles by human SC cells was verified in vitro with no sign of cytotoxicity, and modulation of SC cell stiffness was measured by atomic force microscopy. Lat A-loaded micelles significantly decreased SC cell stiffness, which resulted in visible changes in cell morphology as observed by confocal microscopy. Our results demonstrate that PEG-bl-PPS micelles represent a tunable platform for the controlled intracellular delivery of latrunculin. These self-assembled polymeric nanobiomaterials may support the rational design and engineering of delivery systems for the treatment of glaucoma. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1771-1779, 2018.

Nitric oxide-donating compounds for IOP lowering in glaucoma

INTRODUCTION

An elevated intraocular pressure (IOP) remains the main risk factor for progression of glaucoma upon which we can efficiently act. Pharmacological strategies to reduce IOP are directed towards the reduction of aqueous humour (AH) production and/or the increase in AH drainage through the uveoscleral pathway. However, there are no drugs currently available as first-line treatment to increase AH outflow primarily via the conventional route. Ocular nitric oxide (NO) production takes place in AH outflow pathways and in the ciliary muscle, modulating the cellular response to elevated IOP.

METHODS

This review describes the mechanism of action of endogenous NO and NO-donating compounds that are under research. It includes information regarding pre-clinical and clinical studies previously conducted with these compounds, discussing their role and therapeutic potential in the pharmacological treatment of ocular hypertension in glaucoma.

RESULTS

The topical ocular administration of NO-donating compounds significantly lowered IOP and maintained it in animal models of glaucoma and subjects with ocular hypertension.

CONCLUSIONS

The mechanism of action of these compounds is novel and scientific evidence that shows promising results. However, there is a need for more comprehensive studies to assess long-term safety and tolerability in order to properly evaluate their use in chronic therapies.