Unconventional aqueous humor outflow: A review

Specific Biomarkers in the Aqueous Humour of Glaucoma Patients

Glaucoma, a multifactorial neurodegenerative disease, is the second most common cause of blindness. Since early diagnosis facilitates timely treatment, it is therefore essential to identify appropriate markers. In the future, so-called biomarkers could be helpful in early detection and follow-up. In glaucoma, these parameters could be obtained in the aqueous humour. Altered antibodies, proteins, microRNA (miRNA) and trace element levels have already been identified. This review provides insight into possible changes in the aqueous humour of patients with primary open-angle glaucoma (POAG), normal tension glaucoma (NTG) or pseudoexfoliation glaucoma (PEXG). Studies on antibody changes in POAG patients identified an upregulation of immune system associated antibodies such as heat shock protein (HSP) 27. HSP27 was also upregulated in PEXG patients but decreased in NTG. In POAG and PEXG samples, the levels of certain proteins, including interleukins and endothelin-1, were elevated. The vasoconstrictor endothelin-1 may play a role in regulating intraocular pressure. By contrast, proteins playing a role in the response to oxidative stress were downregulated. In NTG patients, proteins responsible for the elimination of toxic by-products from the respiratory chain were downregulated. In addition, the aqueous humour of POAG and PEXG patients contained several miRNAs that have been linked to tissue development, neurological disease and cellular organisation. Other miRNAs regulated in glaucoma play a role in extracellular matrix remodelling and thus may affect drainage resistance in the trabecular meshwork. It is also interesting to note that the aqueous humour of glaucoma patients showed changes in the levels of trace elements such as zinc and selenium. The elevated zinc levels could be responsible for the imbalance of intraocular matrix metalloproteinases and thus for increased intraocular pressure. All these studies demonstrate the complex changes in aqueous humour in glaucoma. Some of these biomarkers may be useful in the future for early diagnosis of the disease.

Consensus Recommendation for Mouse Models of Ocular Hypertension to Study Aqueous Humor Outflow and Its Mechanisms

Due to their similarities in anatomy, physiology, and pharmacology to humans, mice are a valuable model system to study the generation and mechanisms modulating conventional outflow resistance and thus intraocular pressure. In addition, mouse models are critical for understanding the complex nature of conventional outflow homeostasis and dysfunction that results in ocular hypertension. In this review, we describe a set of minimum acceptable standards for developing, characterizing, and utilizing mouse models of open-angle ocular hypertension. We expect that this set of standard practices will increase scientific rigor when using mouse models and will better enable researchers to replicate and build upon previous findings.

Pathogenesis of glaucoma: Extracellular matrix dysfunction in the trabecular meshwork-A review

The trabecular meshwork regulates aqueous humour outflow from the anterior chamber of the eye. It does this by establishing a tunable outflow resistance, defined by the interplay between cells and their extracellular matrix (ECM) milieu, and the molecular interactions between ECM proteins. During normal tissue homeostasis, the ECM is remodelled and trabecular cell behaviour is modified, permitting increased aqueous fluid outflow to maintain intraocular pressure (IOP) within a relatively narrow physiological pressure. Dysfunction in the normal homeostatic process leads to increased outflow resistance and elevated IOP, which is a primary risk factor for glaucoma. This review delineates some of the changes in the ECM that lead to gross as well as some more subtle changes in the structure and function of the ECM, and their impact on trabecular cell behaviour. These changes are discussed in the context of outflow resistance and glaucoma.

Ocular Autonomic Nervous System: An Update from Anatomy to Physiological Functions

The autonomic nervous system (ANS) confers neural control of the entire body, mainly through the sympathetic and parasympathetic nerves. Several studies have observed that the physiological functions of the eye (pupil size, lens accommodation, ocular circulation, and intraocular pressure regulation) are precisely regulated by the ANS. Almost all parts of the eye have autonomic innervation for the regulation of local homeostasis through synergy and antagonism. With the advent of new research methods, novel anatomical characteristics and numerous physiological processes have been elucidated. Herein, we summarize the anatomical and physiological functions of the ANS in the eye within the context of its intrinsic connections. This review provides novel insights into ocular studies.

Bleb-Independent Glaucoma Surgery to Activate the Uveolymphatic Route of Non-Trabecular Aqueous Humor Outflow: Short-Term Clinical and OCT Results

The deep sclerectomy technique was modified to enhance aqueous humor (AH) outflow via the non-trabecular pathway. A pilot study was carried out to assess its safety and effectiveness. Thirty-eight patients were under observation. After superficial scleral flap (4 × 4 mm), deep scleral layers were divided into three parts by three parallel-to-limbus incisions. Deep sclerectomy without creating a window in the Descemetes' membrane was carried out in the distal part. A collagen implant was placed under the sclera of the remaining two parts with one end in the intrascleral pool. The third proximal part was excised to expose the uvea and implant. A Nd:YAG laser trabeculotomy at the surgery site was made on postoperative days 7-10. Outcome measures were IOP change, use of hypotensive medication(s), complications, and the need for a second surgery. At six months, the mean IOP decreased from 29.1 ± 9.2 mm Hg to 14.0 ± 4.3 mm Hg (p = 1.4 × 10-9); hypotensive medication use reduced from 2.9 ± 0.9 to 0.6 ± 1.0 (p = 1.3 × 10-10); complete success was achieved in 68.4% of cases and partial success was achieved in 31.6% of cases. Intraoperative and postoperative complications were rare and manageable. The OCT of the surgery site revealed the absence of bleb in all cases. Lymphatic vessels with characteristic bicuspid valves in their lumen were detected in conjunctiva near the operation site and over it in 32 patients. IOP decrease in the proposed technique was achieved by activation of the uveolymphatic route of AH outflow.

Tie2 Activation via VE-PTP Inhibition With Razuprotafib as an Adjunct to Latanoprost in Patients With Open Angle Glaucoma or Ocular Hypertension

Purpose

To evaluate the ocular hypotensive efficacy and safety of razuprotafib, a novel Tie2 activator, when used as an adjunct to latanoprost in patients with open-angle glaucoma (OAG) or ocular hypertension (OHT).

Methods

Subjects with OAG or OHT and an unmedicated IOP from ≥22 mm Hg to <36 mm Hg were randomized to one of three treatment arms: razuprotafib every day (QD) + latanoprost; razuprotafib twice daily (BID) + latanoprost; or latanoprost monotherapy. The primary endpoint was change in mean diurnal IOP from baseline at day 28.

Results

A total of 194 subjects were randomized, and 193 (99.5%) completed the study. Razuprotafib BID + latanoprost resulted in a significantly larger reduction in diurnal IOP than latanoprost alone (7.95 ± 0.26 mmHg vs. 7.04 ± 0.26 mm Hg, P < 0.05). A smaller improvement was observed after 14 days of treatment (7.62 ± 0.26 mm Hg vs. 7.03 ± 0.26 mm Hg, P = 0.11). Razuprotafib QD dosing did not demonstrate additional IOP lowering compared to latanoprost alone. Conjunctival hyperemia on Day 28 increased by 1.1 units on the four-point Efron scale two hours post dose from a baseline value of 0.6 units, and decreased thereafter.

Conclusions

Topical ocular razuprotafib as an adjunct to latanoprost therapy was well tolerated and significantly reduced IOP in patients with OAG/OHT.

Translational Relevance

These data support the IOP lowering efficacy of targeting Tie2 activation in Schlemm's canal in the relevant patient population.

Application of an organotypic ocular perfusion model to assess intravitreal drug distribution in human and animal eyes

Intravitreal (ITV) drug delivery is a new cornerstone for retinal therapeutics. Yet, predicting the disposition of formulations in the human eye remains a major translational hurdle. A prominent, but poorly understood, issue in pre-clinical ITV toxicity studies is unintended particle movements to the anterior chamber (AC). These particles can accumulate in the AC to dangerously raise intraocular pressure. Yet, anatomical differences, and the inability to obtain equivalent human data, make investigating this issue extremely challenging. We have developed an organotypic perfusion strategy to re-establish intraocular fluid flow, while maintaining homeostatic pressure and pH. Here, we used this approach with suitably sized microbeads to profile anterior and posterior ITV particle movements in live versus perfused porcine eyes, and in human donor eyes. Small-molecule suspensions were then tested with the system after exhibiting differing behaviours in vivo. Aggregate particle size is supported as an important determinant of particle movements in the human eye, and we note these data are consistent with a poroelastic model of bidirectional vitreous transport. Together, this approach uses ocular fluid dynamics to permit, to our knowledge, the first direct comparisons between particle behaviours from human ITV injections and animal models, with potential to speed pre-clinical development of retinal therapeutics.

Screening and Functional Analysis of TEK Mutations in Chinese Children With Primary Congenital Glaucoma

Purposes: Recent studies have suggested that loss-of-function mutations of the tunica intima endothelial receptor tyrosine kinase (TEK) are responsible for approximately 5% of primary congenital glaucoma (PCG) cases in diverse populations. However, the causative role of TEK mutations has not been studied in Chinese PCG patients. Here, we report the mutation spectrum of TEK after screening a large cohort of PCG patients of Chinese Han origin and analyze the identified variants in functional assays.

Methods: TEK-targeted next-generation sequencing (NGS) was performed in 200 PCG patients. Candidate variants were prioritized by mutation type and allele frequency in public datasets. Plasmids containing wild type and identified variants of TEK were constructed and used to assess protein expression, solubility, receptor auto-phosphorylation, and response to ligand stimulation in cell-based assays.

Results: Ten missense and one nonsense heterozygous variants were detected by NGS in 11 families. The clinical features of TEK variants carriers were comparable to that of TEK-mutated patients identified in other populations and CYP1B1-mutated individuals from in-house database. Functional analysis confirmed four variants involving evolutionarily conserved residues to be loss-of-function, while one variant (p.R1003H) located in tyrosine kinase domain seemed to be an activating mutation. However, our results did not support the pathogenicity of the other five variants (p.H52R, p.M131I, p.M228V, p.H494Y, and p.L888P).

Conclusion: We provide evidence for TEK variants to be causative in Chinese PCG patients for the first time. Attention needs to be paid to TEK mutations in future genetic testing.

Conventional glaucoma implants and the new MIGS devices: a comprehensive review of current options and future directions

Glaucoma is a progressive optic neuropathy that is the second leading cause of preventable blindness worldwide, after cataract formation. A rise in the intraocular pressure (IOP) is considered to be a major risk factor for glaucoma and is associated with an abnormal increase of resistance to aqueous humour outflow from the anterior chamber. Glaucoma drainage devices have been developed to provide an alternative pathway through which aqueous humour can effectively exit the anterior chamber, thereby reducing IOP. These devices include the traditional aqueous shunts with tube-plate design, as well as more recent implants, such as the trabeculectomy-modifying EX-PRESS^® implant and the new minimally invasive glaucoma surgery (MIGS) devices. In this review, we will describe each implant in detail, focusing on their efficacy in reducing IOP and safety profile. Additionally, a critical and evidence-based comparison between these implants will be provided. Finally, we will propose potential developments that may help to improve the performance of current devices.

Deletion of transcription factor AP-2β from the developing murine trabecular meshwork region leads to progressive glaucomatous changes

Glaucoma is one of the leading causes of irreversible blindness and can result from abnormalities in anterior segment structures required for aqueous humor outflow, including the trabecular meshwork (TM) and Schlemm's canal (SC). Transcription factors such as AP-2β play critical roles in anterior segment development. Here, we show that the Mgp-Cre knock-in (Mgp-Cre.KI) mouse can be used to target the embryonic periocular mesenchyme giving rise to the TM and SC. Fate mapping of male and female mice indicates that AP-2β loss causes a decrease in iridocorneal angle cells derived from Mgp-Cre.KI-expressing populations compared to controls. Moreover, histological analyses revealed peripheral iridocorneal adhesions in AP-2β mutants that were accompanied by a decrease in expression of TM and SC markers, as observed using immunohistochemistry. In addition, rebound tonometry showed significantly higher intraocular pressure (IOP) that was correlated with a progressive significant loss of retinal ganglion cells, reduced retinal thickness, and reduced retinal function, as measured using an electroretinogram, in AP-2β mutants compared with controls, reflecting pathology described in late-stage glaucoma patients. Importantly, elevated IOP in AP-2β mutants was significantly reduced by treatment with latanoprost, a prostaglandin analog that increases unconventional outflow. These findings demonstrate that AP-2β is critical for TM and SC development, and that these mutant mice can serve as a model for understanding and treating progressive human primary angle-closure glaucoma.

Aqueous outflow channels and its lymphatic association: A review

The human eye has a unique immune architecture and behavior. While the conjunctiva is known to have a well-defined lymphatic drainage system, the cornea, sclera, and uveal tissues were historically considered "alymphatic" and thought to be immune privileged. The very fact that the aqueous outflow channels carry a clear fluid (aqueous humor) along the outflow pathway makes it hard to ignore its lymphatic-like characteristics. The development of novel lymphatic lineage markers and expression of these markers in aqueous outflow channels and improved imaging capabilities has sparked a renewed interest in the study of ocular lymphatics. Ophthalmic lymphatic research has had a directional shift over the last decade, offering an exciting new physiological platform that needs further in-depth understanding. The evidence of a presence of distinct lymphatic channels in the human ciliary body is gaining significant traction. The uveolymphatic pathway is an alternative new route for aqueous outflow and adds a new dimension to pathophysiology and management of glaucoma. Developing novel animal models, markers, and non-invasive imaging tools to delineate the core anatomical structure and physiological functions may help pave some crucial pathways to understand disease pathophysiology and help develop novel targeted therapeutic approaches for glaucoma.

Beta-adrenergic glaucoma drugs reduce lymphatic clearance from the eye: A sequential photoacoustic imaging study

Our study aims to determine whether the beta-adrenergic system is involved in the regulation of lymphatic drainage from the eye. For this purpose, we assessed the effect of 2 topical beta-adrenergic blockers, timolol and betaxolol, commonly used as glaucoma drugs, on lymphatic clearance of albumin from the aqueous humor to neck lymph nodes. Adult mice were treated with either topical timolol, a non-selective β-blocker, 0.5% (n = 8), or topical betaxolol, a selective β₁-adrenergic blocker, 0.5% (n = 6) twice daily for 14 days and compared to respective control groups (n = 5 and n = 7). Changes in lymphatic clearance from the eye were assessed using a quantitative in vivo photoacoustic imaging approach. In all subjects, right eye and neck lymph nodes were longitudinally assessed by sequential photoacoustic imaging just prior to near-infrared dye injection into the anterior chamber of the eye, and 20 min, 2 and 4 h after injection. Repeat measurements of mean pixel intensities (MPIs) of right eyes and nodes were performed at all timepoints. The areas under the curves (AUC) were calculated and the AUC of the treated-group was compared to that of controls using the Mann-Whitney U test. The slopes of MPI of each region of interest over time were compared using the linear mixed model after adjusting for IOP decrease after treatment and other parameters such as sex and body weight. In the timolol-treated group, right neck nodes showed significant decrease in AUC signal intensity compared with controls (P = 0.003), and significant decrease in slope of MPI compared with controls (P = 0.0025). In the betaxolol-treated group, right neck nodes showed significant decrease in AUC signal intensity compared with controls (P = 0.02), and significant decrease in slope of MPI compared with controls (P = 0.0069). Topical treatment with timolol and betaxolol reduced lymphatic clearance of albumin from the aqueous humor to the neck lymph nodes. This finding may be relevant for the management of secondary glaucomas and inflammatory eye disease in which the clearance of accumulated proteins and antigen from the eye is important to disease recovery and sight protection. This study suggests that the beta-adrenergic system plays a role in the regulation of lymphatic clearance from the eye.

Effect of high-intensity focused ultrasound (HiFU) treatment on intraocular pressure and aqueous humour dynamics: 12 -months results

PURPOSE

High intensity focused ultrasound (HiFU) is a cyclodestructive therapy for controlling intraocular pressure (IOP) in glaucoma. The mechanism of action is thought to be through destruction of the ciliary epithelium as well as increased uveoscleral outflow. We reviewed the change in aqueous humour dynamics parameters including aqueous humour flow rate, tonographic outflow facility (TOF) and uveoscleral outflow at 12 months.

PATIENTS AND METHODS

This is a prospective observational study. Consecutive patients with open angle glaucoma (OAG) or ocular hypertension (OHT) requiring further IOP lowering were enroled in the study between August 2016 and January 2017. Patients were commenced on medication washout period prior to baseline and twelve months' visit.

RESULTS

Sixteen patients (OAG) in the treatment group underwent assessment at twelve months follow up. Mean age was 63.1 ± 11 years. Eleven patients were African/Caribbean and 5 were Caucasian. Nine patients were female and 7 were male. Mean post-washout IOP was reduced by 21% (28.3 ± 5.7 at baseline vs 22.4 ± 8.4 mmHg at 12 months, p = 0.04). Aqueous humour flow rate was reduced by 16% at twelve months (2.40 ± 0.6 at baseline vs 2.02 ± 0.6 µl/min at 12 months, p = 0.0493). There was no statistically significant change in the TOF (0.12 ± 0.09 at baseline vs 0.08 ± 0.05 µl/min/mmHg at 12 months, p = 0.08) or uveoscleral outflow (0.6 ± 1.3 at baseline vs 1.3 ± 0.85 µl/min at 12 months, p = 0.15).

CONCLUSION

In this study, we demonstrated that the observed IOP reduction was likely due to aqueous humour flow rate reduction. The TOF and uveoscleral outflow were not detectibly changed.

Changes in peripapillary and subfoveal choroidal thickness in patients with central retinal vein occlusion

PURPOSE

We sought to evaluate changes of mean peripapillary choroidal thickness (PCT) and subfoveal choroidal thickness (SFCT) over 12 months in patients with unilateral central retinal vein occlusion (CRVO).

METHODS

Our retrospective, observational study included 19 patients with treatment-naïve, unilateral CRVO who completed at least 12 months of follow-up period. Mean PCT and mean SFCT in CRVO-affected eyes and unaffected contralateral eyes were measured at each follow-up visit, and then compared. Differences between baseline and 12 months (ΔSFCT and ΔPCT) and percentage changes (ΔSFCT or ΔPCT/baseline×100%) were determined. We also investigated the predictive factors for visual outcome in the CRVO-affected eyes.

RESULTS

In the CRVO-affected eyes, mean PCT was 146.7±41.9 μm at baseline, and 106.5±24.2 μm at 12 months (P < 0.001). Mean PCT of the contralateral eyes was 129.8±42.6 μm at baseline and 124.6±39.7 μm at 12 months (P = 0.089). Mean SFCT of CRVO-affected eyes was 225.8±77.9 μm at baseline, and 199.4±66.6 μm at 12 months (P = 0.009). Mean SFCT of the contralateral eyes was 218.4±83.0 μm at baseline, and 208.4±78.1 μm at 12 months (P = 0.089). Δ PCT was -41.6±25.3 μm in the CRVO-affected eyes, and -5.2±5.8 μm in the contralateral eyes (P<0.001). % PCT was -24.9±14.0% in the CRVO-affected eyes, and -4.0±0.4% in the contralateral eyes (P = 0.001). Δ SFCT was -26.4±24.6 μm in the CRVO-affected eyes, and -9.5±16.7μm in the contralateral eyes (P = 0.016). % SFCT was -10.4±9.8% in the CRVO-affected eyes, and -3.4±6.4% in the contralateral eyes (P = 0.015). Among the various factors, BCVA at baseline (β = 0.797, P = 0.001) and % SFCT (β = 0.712, P = 0.001) were significantly associated with visual outcome at 12 months in the CRVO-affected eyes.

CONCLUSION

Both peripapillary and subfoveal choroidal thickness reduced significantly over 12 months in the CRVO-affected eyes, but not in the contralateral eyes. In addition, the absolute reduction amount and reduction ratio of PCT and SFCT were significantly greater in the CRVO-affected eyes than the contralateral eyes.

Intraocular Pressure Reduction by Femtosecond Laser Created Trabecular Channels in Perfused Human Anterior Segments

Purpose

This study investigated the initial feasibility of using femtosecond laser trabeculotomy (FLT) to create open channels through the trabecular meshwork into Schlemm's canal to lower intraocular pressure (IOP) in a perfused anterior segment model.

Methods

Human anterior segments (12 eyes) were assigned to either treatment (n = 6) or sham treatment (n = 6) groups. Both groups were perfused until a baseline IOP was recorded upon which a direct FLT treatment or a sham treatment was administered. IOP was recorded before and after the treatment. Spectral domain optical coherence tomography and second harmonic generation imaging we used to investigate the FLT channels.

Results

In the FLT group, there was a significant mean decrease in the IOP of 22% compared with the pre-FLT IOP (7.13 ± 2.95 mm Hg to 5.34 ± 1.62 mm Hg; P < 0.05). In the control group, the post-sham IOP remained relatively unchanged compared with the pre-sham IOP (6.39 ± 3.69 mm Hg to 6.67 ± 4.12 mm Hg).

Conclusions

The results of this study indicate that FLT treatment can significantly decrease the IOP in a perfusion model and may provide a potential noninvasive treatment option for primary open angle glaucoma.

Translational Relevance

Investigating the use of femtosecond lasers for photodisrupting the trabecular meshwork can lead to a clinically relevant alternative to current glaucoma procedures.

Lymphatics in Eye Fluid Homeostasis: Minor Contributors or Significant Actors?

Lymphatic vessels exert major effects on the maintenance of interstitial fluid homeostasis, immune cell trafficking, lipid absorption, tumor progression and metastasis. Recently, novel functional roles for the lymphatic vasculature have emerged, which can be associated with pathological situations. Among them, lymphatics have been proposed to participate in eye aqueous humor drainage, with potential consequences on intraocular pressure, a main risk factor for progression of glaucoma disease. In this review, after the description of eye fluid dynamics, we provide an update on the data concerning the distribution of ocular lymphatics. Particular attention is given to the results of investigations allowing the three dimensional visualization of the ocular surface vasculature, and to the molecular mechanisms that have been characterized to regulate ocular lymphatic vessel development. The studies concerning the potential role of lymphatics in aqueous humor outflow are reported and discussed. We also considered the novel studies mentioning the existence of an ocular glymphatic system which may have, in connection with lymphatics, important repercussions in retinal clearance and in diseases affecting the eye posterior segment. Some remaining unsolved questions and new directions to explore are proposed to improve the knowledge about both lymphatic and glymphatic system interactions with eye fluid homeostasis.

Intraocular pressure and choroidal thickness respond differently to lower body negative pressure during spaceflight

Spaceflight-associated neuro-ocular syndrome (SANS) develops during long-duration (>1 mo) spaceflight presumably because of chronic exposure to a headward fluid shift that occurs in weightlessness. We aimed to determine whether reversing this headward fluid shift with acute application of lower body negative pressure (LBNP) can influence outcome measures at the eye. Intraocular pressure (IOP) and subfoveal choroidal thickness were therefore evaluated by tonometry and optical coherence tomography (OCT), respectively, in 14 International Space Station crewmembers before flight in the seated, supine, and 15° head-down tilt (HDT) postures and during spaceflight, without and with application of 25 mmHg LBNP. IOP in the preflight seated posture was 14.4 mmHg (95% CI, 13.5-15.2 mmHg), and spaceflight elevated this value by 1.3 mmHg (95% CI, 0.7-1.8 mmHg, P < 0.001). Acute exposure to LBNP during spaceflight reduced IOP to 14.2 mmHg (95% CI, 13.4-15.0 mmHg), which was equivalent to that of the seated posture (P > 0.99), indicating that venous fluid redistribution by LBNP can influence ocular outcome variables during spaceflight. Choroidal thickness during spaceflight (374 µm, 95% CI, 325-423 µm) increased by 35 µm (95% CI, 25-45 µm, P < 0.001), compared with the preflight seated posture (339 µm, 95% CI, 289-388 µm). Acute use of LBNP during spaceflight did not affect choroidal thickness (381 µm, 95% CI, 331-430 µm, P = 0.99). The finding that transmission of reduced venous pressure by LBNP did not decrease choroidal thickness suggests that engorgement of this tissue during spaceflight may reflect changes that are secondary to the chronic cerebral venous congestion associated with spaceflight.NEW & NOTEWORTHY Spaceflight induces a chronic headward fluid shift that is believed to underlie ocular changes observed in astronauts. The present study demonstrates, for the first time, that reversing this headward fluid shift via application of lower body negative pressure (LBNP) during spaceflight may alter the ocular venous system, as evidenced by a decrease in intraocular pressure. This finding indicates that LBNP has the potential to be an effective countermeasure against the headward fluid shift during spaceflight, which may then be beneficial in preventing or reversing associated ocular changes.

siRNA Therapeutics in Ocular Diseases

The field of RNAi therapeutics has quickly adapted to the treatment of ocular diseases. Although the eye provides a unique system for the delivery of siRNAs, its complex structure and composition fostered the development of novel strategies for efficient gene silencing in the target compartment. Moreover, anterior and posterior segments differ in their multiple drug barriers and clearance mechanisms. This chapter summarizes the recent achievements in terms of routes of administration, chemical modifications, and delivery systems for siRNAs that specifically apply to eye disorders. Methods employed for siRNA detection/quantitation in ocular tissues are also described, together with safety concerns that need to be addressed to fulfill regulatory requirements of new drug approval. Even though RNAi therapies for ocular diseases have not yet translated into patient care, we document herein the rising number of candidate drugs currently under preclinical or clinical development.

An ocular glymphatic clearance system removes β-amyloid from the rodent eye

Despite high metabolic activity, the retina and optic nerve head lack traditional lymphatic drainage. We here identified an ocular glymphatic clearance route for fluid and wastes via the proximal optic nerve in rodents. β-amyloid (Aβ) was cleared from the retina and vitreous via a pathway dependent on glial water channel aquaporin-4 (AQP4) and driven by the ocular-cranial pressure difference. After traversing the lamina barrier, intra-axonal Aβ was cleared via the perivenous space and subsequently drained to lymphatic vessels. Light-induced pupil constriction enhanced efflux, whereas atropine or raising intracranial pressure blocked efflux. In two distinct murine models of glaucoma, Aβ leaked from the eye via defects in the lamina barrier instead of directional axonal efflux. The results suggest that, in rodents, the removal of fluid and metabolites from the intraocular space occurs through a glymphatic pathway that might be impaired in glaucoma.

Modeling the effects of glaucoma surgery on intraocular pressure

Minimally invasive glaucoma surgeries (MIGS) offer an effective way to lower intraocular pressure without inducing extensive trauma to the anterior segment. In order to predict their efficacy, an analytical model of the conventional aqueous humor outflow pathway is developed using a resistor network. The model describes outflow through the normal eye and allows for the effects of geometric changes in the outflow pathway as IOP changes. By selectively removing these resistors, the model can be used to examine and predict the outcomes of several surgical procedures currently used to treat glaucoma. Treatments examined include traditional trabeculectomy, several ab interno methods for trabeculotomy and trabeculectomy, as well as recently developed trabecular stents that bypass the trabecular meshwork and dilate Schlemm canal. The model's predictions for the efficacy of these procedures generally matched well with the efficacy determined in experimental studies, although it tended to somewhat overestimate the efficacy of these procedures. Matching the model to experimental data indicated that a partial trabeculotomy substantially increases flow to collector channels within that region and approximately 1.5 clock hours past the ends of the trabeculotomized region. Similarly, trabecular bypass stents substantially increase flow to collector channels up to 1.5 clock hours past the open ends of the stent. The resistor model we have developed can be used to predict the efficacy of a variety of MIGS procedures. Circumferential flow in Schlemm canal extends the efficacy of MIGS, but this effect is limited to a few clock hours.

Piezo2 downregulation via the Cre-lox system affects aqueous humor dynamics in mice

Purpose

Proper aqueous humor (AH) dynamics is crucial for maintaining the intraocular pressure (IOP) in the eye. This study aims to investigate the function of Piezo2, a newly discovered mechanosensitive ion channel, in regulating AH dynamics.

Methods

Immunohistochemistry (IHC) analysis and western blotting were performed to detect Piezo2 expression. The Cre-lox system was applied to create a conditional knockout model of Piezo2. IOP and aqueous humor outflow facility in live animals were recorded with a Tonometer and a syringe-pump system for up to 2 weeks.

Results

We first detected Piezo2 with robust expression in the human trabecular meshwork (TM), Schlemm's canal (SC), the ciliary body's epithelium, and ciliary muscle. In addition, we found Piezo2 in human retinal ganglion cells (RGCs) and astrocytes in the optic nerve head (ONH). Through the Cre-lox system, Piezo2 can be successfully downregulated in mouse iridocorneal angle tissues. However, Piezo2 downregulation cannot significantly influence the IOP and outflow facility through the conventional pathway. Instead, we observed an effect of downregulated Piezo2 on decreasing the intercept in the flow rate versus pressure plot. According to the Goldmann equation, Piezo2 may function in regulating unconventional outflow, AH production, and episcleral venous pressure.

Conclusions

These findings, for the first time, demonstrate that Piezo2 acts as an essential mechanosensor in maintaining the proper aqueous humor dynamics in the eye.

Immunology and Pathology in Ocular Drug Development

Clear vision is dependent on features that protect the anatomical integrity of the eye (cornea and sclera) and those that contribute to internal ocular homeostasis by conferring hemangiogenic (avascular tissues and antiangiogenic factors), lymphangiogenic (lack of draining lymphatics), and immunologic (tight junctions that form blood-ocular barriers, immunosuppressive cells, and modulators) privileges. The later examples are necessary components that enable the eye to maintain an immunosuppressive environment that responds to foreign invaders in a deviated manner, minimizing destructive inflammation that would impair vision. These conditions allowed for the observations made by Medawar, in 1948, of delayed rejection of allogenic tissue grafts in the anterior chamber of mouse eye and permit the sequestration of foreign invaders (eg, Toxoplasma gondii) within the retina of healthy individuals. Yet successful development of intraocular drugs (biologics and delivery devices) has been stymied by adverse ocular pathology, much of which is driven by immune pathways. The eye can be intolerant of foreign protein irrespective of delivery route, and endogenous ocular cells have remarkable plasticity when recruited to preserve visual function. This article provides a review of current understanding of ocular immunology and the potential role of immune mechanisms in pathology observed with intraocular drug delivery.

Topically instilled caffeine selectively alters emmetropizing responses in infant rhesus monkeys

Oral administration of the adenosine receptor (ADOR) antagonist, 7-methylxanthine (7-MX), reduces both form-deprivation and lens-induced myopia in mammalian animal models. We investigated whether topically instilled caffeine, another non-selective ADOR antagonist, retards vision-induced axial elongation in monkeys. Beginning at 24 days of age, a 1.4% caffeine solution was instilled in both eyes of 14 rhesus monkeys twice each day until the age of 135 days. Concurrent with the caffeine regimen, the monkeys were fitted with helmets that held either -3 D (-3D/pl caffeine, n = 8) or +3 D spectacle lenses (+3D/pl caffeine, n = 6) in front of their lens-treated eyes and zero-powered lenses in front of their fellow-control eyes. Refractive errors and ocular dimensions were measured at baseline and periodically throughout the lens-rearing period. Control data were obtained from 8 vehicle-treated animals also reared with monocular -3 D spectacles (-3D/pl vehicle). In addition, historical comparison data were available for otherwise untreated lens-reared controls (-3D/pl controls, n = 20; +3D/pl controls, n = 9) and 41 normal monkeys. The vehicle controls and the untreated lens-reared controls consistently developed compensating axial anisometropias (-3D/pl vehicle = -1.44 ± 1.04 D; -3D/pl controls = -1.85 ± 1.20 D; +3D/pl controls = +1.92 ± 0.56 D). The caffeine regime did not interfere with hyperopic compensation in response to +3 D of anisometropia (+1.93 ± 0.82 D), however, it reduced the likelihood that animals would compensate for -3 D of anisometropia (+0.58 ± 1.82 D). The caffeine regimen also promoted hyperopic shifts in both the lens-treated and fellow-control eyes; 26 of the 28 caffeine-treated eyes became more hyperopic than the median normal monkey (mean (±SD) relative hyperopia = +2.27 ± 1.65 D; range = +0.31 to +6.37 D). The effects of topical caffeine on refractive development, which were qualitatively similar to those produced by oral administration of 7-MX, indicate that ADOR antagonists have potential in treatment strategies for preventing and/or reducing myopia progression.

Hyperbranched Cationic Glycogen Derivative-Mediated IκBα Gene Silencing Regulates the Uveoscleral Outflow Pathway in Rats

The role of the IκB/NF-κB signaling pathway in the uveoscleral outflow pathway was investigated with IκBα gene silencing mediated by the 3-(dimethylamino)-1-propylamine-conjugated glycogen (DMAPA-Glyp) derivative. The IκBα-siRNA-loaded DMAPA-Glyp complex was transfected into the ciliary muscles of rats by intracameral injection (labeled as the DMAPA-Glyp+siRNA group). The Lipofectamine™ 2000 (Lipo)/siRNA complex and the naked siRNA were set as the controls. The mRNA and protein expression of IκBα, NF-κBp65, and MMP-2 were analyzed by real-time PCR, western blotting, and in situ gelatin zymography. Nuclear translocation of NF-κBp65 was analyzed by immunofluorescence. Rat intraocular pressure (IOP) was monitored pre- and postinjection. Gene transfection efficiency and toxicity of the DMAPA-Glyp derivative were also evaluated. After RNA interference (RNAi), IκBα mRNA and protein expression were significantly inhibited. NF-κBp65 mRNA and protein expression showed no significant differences. Nevertheless, nuclear translocation of NF-κBp65 occurred in the DMAPA-Glyp+siRNA group. Both mRNA expression and activity of MMP-2 increased, with the largest increase in the DMAPA-Glyp+siRNA group. IOP in the DMAPA-Glyp+siRNA group fell to the lowest level on day 3 after RNAi. The levels of Cy3-siRNA in the ciliary muscle of the DMAPA-Glyp+siRNA group did not significantly decrease over time. At 7 and 14 d after RNAi, no significant pathological damage was detectable in the eyes injected with the DMAPA-Glyp derivative or the DMAPA-Glyp/siRNA complex. Taken together, our results suggest that downregulation of IκBα expression in the ciliary muscle plays a crucial role in reducing the IOP values of rats. IκBα may become a new molecular target for lowering IOP in glaucoma. The DMAPA-Glyp derivative is safe and feasible as an effective siRNA vector in rat eyes.

Drug-Free, Nonsurgical Reduction of Intraocular Pressure for Four Months after Suprachoroidal Injection of Hyaluronic Acid Hydrogel

Glaucoma is the leading cause of irreversible blindness. Current treatments use drugs or surgery to reduce intraocular pressure (IOP). In this study, a drug-free, nonsurgical method is developed that lowers IOP for 4 months without requiring daily patient adherence. The approach involves expanding the suprachoroidal space (SCS) of the eye with an in situ-forming hydrogel injected using a microneedle. This study tests the hypothesis that SCS expansion increases the drainage of aqueous humor from the eye via the unconventional pathway, which thereby lowers IOP. SCS injection of a commercial hyaluronic acid (HA) hydrogel reduces the IOP of normotensive rabbits for more than 1 month and an optimized HA hydrogel formulation enables IOP reduction for 4 months. Safety assessment by clinical ophthalmic examinations indicate the treatment is well tolerated. Histopathology shows minor hemorrhage and fibrosis at the site of injection. Further analysis by ultrasound biomicroscopy demonstrates a strong correlation of IOP reduction with SCS expansion. Outflow facility measurements show no difference in pressure-dependent outflow by the conventional pathway between treated and untreated eyes, supporting the hypothesis. In conclusion, SCS expansion with an in situ-forming hydrogel can enable extended IOP reduction for treating ocular hypertension and glaucoma without drugs or surgery.

A Small Molecule Inhibitor of VE-PTP Activates Tie2 in Schlemm's Canal Increasing Outflow Facility and Reducing Intraocular Pressure

Purpose

Tyrosine kinase with immunoglobulin-like and EGF-like domains 2 (Tie2) activation in Schlemm's canal (SC) endothelium is required for the maintenance of IOP, making the angiopoietin/Tie2 pathway a target for new and potentially disease modifying glaucoma therapies. The goal of the present study was to examine the effects of a Tie2 activator, AKB-9778, on IOP and outflow function.

Methods

AKB-9778 effects on IOP was evaluated in humans, rabbits, and mice. Localization studies of vascular endothelial protein tyrosine phosphatase (VE-PTP), the target of AKB-9778 and a negative regulator of Tie2, were performed in human and mouse eyes. Mechanistic studies were carried out in mice, monitoring AKB-9778 effects on outflow facility, Tie2 phosphorylation, and filtration area of SC.

Results

AKB-9778 lowered IOP in patients treated subcutaneously for diabetic eye disease. In addition to efficacious, dose-dependent IOP lowering in rabbit eyes, topical ocular AKB-9778 increased Tie2 activation in SC endothelium, reduced IOP, and increased outflow facility in mouse eyes. VE-PTP was localized to SC endothelial cells in human and mouse eyes. Mechanistically, AKB-9778 increased the filtration area of SC for aqueous humor efflux in both wild type and in Tie2+/- mice.

Conclusions

This is the first report of IOP lowering in humans with a Tie2 activator and functional demonstration of its action in remodeling SC to increase outflow facility and lower IOP in fully developed mice. Based on these studies, a phase II clinical trial is in progress to advance topical ocular AKB-9778 as a first in class, Tie2 activator for treatment for ocular hypertension and glaucoma.

The Latest Drugs in Development That Reduce Intraocular Pressure in Ocular Hypertension and Glaucoma

Glaucoma causes irreversible vision loss, with elevated intraocular pressure (IOP) being the only known modifiable risk factor. There are a variety of medical and interventional options for lowering IOP; however, despite these treatments, glaucoma continues to be a leading cause of visual impairment. Further research continues to strive for treatment options with improved side effect profiles, additional IOP-lowering effects, and ease of use. This review provides a brief summary of current IOP-lowering therapies and then outlines pipeline ocular hypotensive agents, their mechanisms of action, benefits, and side effect profiles. Advancements are seen within currently used eye drop classes such as prostaglandin analogues, Rho kinase inhibitors and nitric oxide donors, whilst there are also new drug classes, such as tyrosine protein kinase activators. Most developing drugs are topical drop formulations, with a number already having entered Phase III trials. Alternative drug delivery methods are also in development and will be briefly discussed. Pharmacological and drug delivery developments continue to provide glaucoma patients and clinicians with new options and the promise of better outcomes, particularly in terms of improved tolerance and reduced frequency of dosing.

Increased episcleral venous pressure in a mouse model of circumlimbal suture induced ocular hypertension

PURPOSE

To investigate changes in aqueous humor dynamics during intraocular pressure (IOP) elevation induced by circumlimbal suture in mice.

METHODS

Ocular hypertension (OHT) was induced by applying a circumlimbal suture behind the limbus in male adult C57BL6/J mice. In the OHT group, the suture was left in place for an average of 8 weeks (n = 10, OHT group). In the sham control group the suture was cut at 2 days (n = 9, sham group) and in the naïve control group (n = 5) no suture was implanted. IOP was measured at baseline across 3 days, 1 h post-suture implantation, and at the chronic endpoint. Anterior segments were assessed using optical coherence tomography (OCT). Episcleral venous pressure (EVP), total outflow facility (C), uveoscleral outflow (Fu) and aqueous humor flow rate (Fin) were determined using a constant-flow infusion model.

RESULTS

All aqueous dynamic and chronic IOP outcome measures showed no difference between sham and naïve controls (p > 0.05) and thus these groups were combined into a single control group. IOP was elevated in OHT group compared with controls (p < 0.01). Chronic suture implantation did not change pupil size, anterior chamber depth or iridocorneal angles (p > 0.05). EVP was significantly higher in OHT eyes compared to control eyes (p < 0.01). There was no statistical difference in C, Fu and Fin between groups (p > 0.05). A significant linear correlation was found between IOP and EVP (R² = 0.35, p = 0.001).

CONCLUSIONS

Circumlimbal suture implantation in mouse eyes results in chronic IOP elevation without angle closure. Chronic IOP elevation is likely to reflect higher EVP.

Aqueous humour outflow imaging: seeing is believing

Elevated intraocular pressure (IOP) is the primary risk factor for blindness in glaucoma. IOP is determined by many factors including aqueous humour production and aqueous humour outflow (AHO), where AHO disturbance represents the primary cause of increased IOP. With the recent development of new IOP lowering drugs and Minimally Invasive Glaucoma Surgeries (MIGS), renewed interest has arisen in shedding light on not only how but where AHO is occurring for the trabecular/conventional, uveoscleral/unconventional, and subconjunctival outflow pathways. Historical studies critical to understanding outflow anatomy will be presented, leading to the development of modern imaging methods. New biological behaviours uncovered by modern imaging methods will be discussed with relevance to glaucoma therapies emphasized.

Minimally Invasive Surgery, Implantable Sensors, and Personalized Therapies

Glaucoma management has changed dramatically over the last decades, through clinical advances and technological revolutions. This review discusses the latest innovations and challenges faced in the field around three major axes: minimally-invasive glaucoma surgery (MIGS), implantable sensors and injectable therapeutics. Indeed, the vast number of recently developed MIGS techniques has not only provided clinicians with a wide range of therapeutic options, but they have also enabled them to adjust their therapies more finely which may have contributed a more patient-centric decision-making process. Yet, despite considerable advances in the field, the wide heterogeneity in clinical trial designs blurs the surgical outcomes, specificities and indications. Thus, more high-quality data are required to make the choice of a specific MIGS procedure more than an educated guess. Beyond the scope of MIGS, the potential of IOP telemetry for self-assessment of IOP-control through implantable sensors is developing into a real option for clinicians and an empowering opportunity for patients. Indeed, providing patients with direct feedback enables them to take control and have a clearer representation of their care, in turn leading to a better control of the disease. However, there are potential issues with self-monitoring of IOP, such as increased anxiety levels induced by measured IOP fluctuations and peaks, leading to patients self-treating during IOP spikes and additional office visits. Furthermore, the advent of implantable therapeutics may soon provide yet another step towards personalized glaucoma treatment, by offering not only an efficient alternative to current treatments, but also a therapeutic option that may better adapt to patients' lifestyle. After several decades of relative stagnation through the last century, glaucoma has now entered what many view as a golden age for the specialty. Like every revolution, this one brings its fair share of uncertainty, clinical questioning and uneasy periods of adaptation to ever-changing expectations. Yet, while it is impossible to guess what the landscape of glaucoma surgery will be like in ten or fifteen years, data suggest a bright outlook both for patients and clinicians.

Eye lymphatic defects induced by bone morphogenetic protein 9 deficiency have no functional consequences on intraocular pressure

Aqueous humor drainage is essential for the regulation of intraocular pressure (IOP), a major risk factor for glaucoma. The Schlemm's canal and the non-conventional uveoscleral pathway are known to drain aqueous humor from the eye anterior chamber. It has recently been reported that lymphatic vessels are involved in this process, and that the Schlemm's canal responds to some lymphatic regulators. We have previously shown a critical role for bone morphogenetic protein 9 (BMP9) in lymphatic vessel maturation and valve formation, with repercussions in drainage efficiency. Here, we imaged eye lymphatic vessels and analyzed the consequences of Bmp9 (Gdf2) gene invalidation. A network of lymphatic vessel hyaluronan receptor 1 (LYVE-1)-positive lymphatic vessels was observed in the corneolimbus and the conjunctiva. In contrast, LYVE-1-positive cells present in the ciliary bodies were belonging to the macrophage lineage. Although enlarged conjunctival lymphatic trunks and a reduced valve number were observed in Bmp9-KO mice, there were no morphological differences in the Schlemm's canal compared to wild type animals. Moreover, there were no functional consequences on IOP in both basal control conditions and after laser-induced ocular hypertonia. Thus, the BMP9-activated signaling pathway does not constitute a wise target for new glaucoma therapeutic strategies.

Importance of the Use of Oxidative Stress Biomarkers and Inflammatory Profile in Aqueous and Vitreous Humor in Diabetic Retinopathy

Diabetic retinopathy is one of the leading causes of visual impairment and morbidity worldwide, being the number one cause of blindness in people between 27 and 75 years old. It is estimated that ~191 million people will be diagnosed with this microvascular complication by 2030. Its pathogenesis is due to alterations in the retinal microvasculature as a result of a high concentration of glucose in the blood for a long time which generates numerous molecular changes like oxidative stress. Therefore, this narrative review aims to approach various biomarkers associated with the development of diabetic retinopathy. Focusing on the molecules showing promise as detection tools, among them we consider markers of oxidative stress (TAC, LPO, MDA, 4-HNE, SOD, GPx, and catalase), inflammation (IL-6, IL-1ß, IL-8, IL-10, IL-17A, TNF-α, and MMPs), apoptosis (NF-kB, cyt-c, and caspases), and recently those that have to do with epigenetic modifications, their measurement in different biological matrices obtained from the eye, including importance, obtaining process, handling, and storage of these matrices in order to have the ability to detect the disease in its early stages.

Choroidal vascularity index and choriocapillary changes in retinal vein occlusions

PURPOSE

To evaluate the changes in the choroidal structure in the setting of retinal vein occlusion (RVO).

METHODS

Changes in the structure of the choroid were studied in sixty-four eyes with unilateral central or branch RVO using optical coherence tomography (OCT) with enhanced depth imaging and OCT-angiography (OCT-A). Choroidal vascularity index (CVI), Haller layer/choroidal thickness (H/C) ratio, and choriocapillaris flow density were used to compare the structural characteristics of the choroid with fellow eyes and the eyes of thirty-four age-, gender-, and systemic co-morbidity-matched controls.

RESULTS

Eyes with RVO had a higher H/C ratio but a lower choriocapillaris flow density compared to both fellow and control eyes (p < 0.001). CVI was significantly lower in both eyes of the patients with RVO compared with control eyes (p < 0.05) with a more robust decrease in the eye that had developed RVO (p < 0.001). The H/C ratio (r = 0.303 p < 0.001), CVI (r = - 0.268, p = 0.001), and choriocapillaris flow density (r = - 0.237, p = 0.003) were all correlated with logMAR visual acuity, and other clinical features.

CONCLUSION

Retinal vein occlusions alter the hemodynamic properties of the choroid leading to structural changes. These changes may be secondary to a compensatory mechanism to supply oxygen to hypoxic retina.

A European Study of the Performance and Safety of MINIject in Patients With Medically Uncontrolled Open-angle Glaucoma (STAR-II)

PRECIS

In this European study (STAR-II), MINIject, a novel, ab-interno, supraciliary minimally invasive glaucoma surgery device, effectively lowered intraocular pressure (IOP) and the need for IOP-lowering medications in patients with primary open-angle glaucoma.

PURPOSE

This study evaluates the safety and performance of a minimally invasive supraciliary glaucoma drainage device (MINIject DO627) for surgical treatment of primary open-angle glaucoma in patients refractory to topical hypotensive medications.

METHODS

In a prospective, interventional, single-arm, multicenter, European study (STAR-II), MINIject was successfully implanted in a stand-alone procedure in 29 of 31 patients in 8 sites in 3 countries. The primary endpoint was the success rate 6 months after surgery >60% (defined as diurnal IOP ≤21 and >5 mm Hg with ≥20% IOP reduction from baseline, with/without glaucoma hypotensive medication). ClinicalTrials.gov: NCT03624361.

RESULTS

At the 6-month follow-up, the primary endpoint was fulfilled, with 75.9% of patients reaching prospectively defined success. The mean IOP was reduced by 40.2% (9.9 mm Hg) to 14.7±6.0 mm Hg at 6 months from 24.6±3.8 mm Hg at baseline. The use of IOP-lowering medication ingredients was reduced by 63.4% from 2.9±1.2 at baseline to 1.0±1.3. Furthermore, 79.3% of the patients had mean IOP ≤18 mm Hg, 82.8% achieved a ≥20% IOP reduction, and 55.2% were medication free at 6 months. Six device-related serious adverse events were reported in the study eye: IOP increase (3/31 patients, 9.7%), and single reports of eye pain, corneal erosion, and chorioretinal folds (1/31, 3.2%), all of which resolved. There was minimal change to corneal endothelial cell density.

CONCLUSION

Ab-interno supraciliary surgical implantation using MINIject DO627 in a stand-alone procedure significantly lowers IOP by 40% at the 6-month follow-up, while reducing the need for IOP-lowering medication.

Potential Therapeutic Usage of Nanomedicine for Glaucoma Treatment

Glaucoma is a group of diseases characterized by progressive degeneration of retinal ganglion cells, leading to irreversible blindness. Currently, intraocular pressure reduction is the only established treatment available for glaucoma. With this treatment, the progression of the disease can only be delayed and there is no recovery. In addition, the commercially available eye drops have the disadvantage of low compliance and short therapeutic time, while glaucoma surgery always has the risk of failure due to wound fibrosis. Nanotechnology can overcome the limitations of the current treatment through the encapsulation and conjugation of drugs used for lowering intraocular pressure and antifibrotic agents using biodegradable or biocompatible nanoparticles for the sustained release of the drugs to protect the damaged ocular cells. Furthermore, using nanotechnology, treatment can be administered in various forms, including eye drops, contact lens, and ocular inserts, according to the convenience of the patients. Despite the promising results of delaying the progression of glaucoma, the regeneration of damaged ocular cells, including trabecular meshwork and retinal ganglion cells, is another critical hurdle to overcome. Bone marrow-derived mesenchymal stem cells and Müller glia cells can secrete neurogenic factors that trigger the regeneration of associated cells, including trabecular meshwork and retinal ganglion cells. In conclusion, this review highlights the potential therapeutic applications of nanotechnology- and stem cell-based methods that can be employed for the protection and regeneration of ocular cells.

Aqueous Angiography in Normal Canine Eyes

Purpose

To conduct aqueous angiography (AA) using a clinically applicable technique in normal dogs and to compare findings to intravenous scleral angiography (SA).

Methods

We examined 10 canine cadaver eyes and 12 eyes from live normal dogs. A gravity-fed trocar system delivered 2% sodium fluorescein and 0.25% indocyanine green (ICG) intracamerally (IC) in cadaver eyes. In vivo AA was subsequently performed in one eye of each of the 12 dogs via IC bolus of ICG under sedation. The same 12 dogs received SA via intravenous ICG (mean ± SD) 10.7 ± 3.3 days later. Identical scleral sectors were imaged using a Spectralis confocal scanning laser ophthalmoscope.

Results

The gravity-fed trocar system permitted visualization of the conventional aqueous humor outflow (CAHO) pathways in cadaver eyes, but not in vivo. Fluorescence was observed superonasally in four of the 10 cadaver eyes within 24.0 ± 3.6 seconds. A single IC bolus of ICG showed CAHO pathways in vivo, demonstrating sectoral outflow patterns in the superotemporal sclera in 10 of the 12 eyes within 35.0 ± 4.3 seconds; four of the 12 eyes exhibited pulsatile aqueous movement. SA exhibited fluorescence patterns comparable to AA with weak pulsatile aqueous humor outflow.

Conclusions

Angiography (AA or SA) in dogs permits visualization of the CAHO pathway and its vascular components in vivo. AA may be a more useful modality to assess aqueous humor outflow.

Translational Relevance

Intracameral AA has potential utility for evaluating CAHO in vivo in dogs, an important animal model species.

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.

Canaloplasty with Suprachoroidal Drainage in Patients with Pseudoexfoliation Glaucoma - Four Years Results

Purpose: To evaluate the long-term efficacy and safety of the modified canaloplasty technique with additional suprachoroidal drainage in patients with pseudoexfoliation glaucoma. Methods: This retrospective clinical trial included 131 patients with pseudoexfoliation glaucoma who underwent canaloplasty with suprachoroidal drainage (scD). One hundred and eleven of 131 patients underwent the follow-up examination after 48 months. Primary endpoints were the pressure-lowering and drug-sparing effect as well as the success rates. Secondary endpoints were intra- and postoperative complications and the occurrence of secondary surgical interventions. Results: The IOP was reduced by 45.8% after 12 months (from baseline 23.4 ± 5.1 mmHg to 12.7 ± 2.2 mmHg) and by 45.1% after four years (12.8 ± 2.2 mmHg) (p < .001). The mean number of IOP-lowering medication decreased significantly from 3.4 at baseline to 0.6 after 12 months and to 1.0 after four years. The IOP-reducing effect was even stronger in those cases in whom canaloplasty+ scD was combined with cataract surgery (49.4% after 12 months and 47.6% after four years). No serious complications were observed. Conclusions: The modified surgery technique of canaloplasty with suprachoroidal drainage is a safe and effective way of reducing IOP in patients with pseudoexfoliation glaucoma.

Rho-kinase (ROCK) Inhibitors - A Neuroprotective Therapeutic Paradigm with a Focus on Ocular Utility

BACKGROUND

Glaucoma is a progressive optic neuropathy causing visual impairment and Retinal Ganglionic Cells (RGCs) death gradually posing a need for neuroprotective strategies to minimize the loss of RGCs and visual field. It is recognized as a multifactorial disease, Intraocular Pressure (IOP) being the foremost risk factor. ROCK inhibitors have been probed for various possible indications, such as myocardial ischemia, hypertension, kidney diseases. Their role in neuroprotection and neuronal regeneration has been suggested to be of value in the treatment of neurological diseases, like spinal-cord injury, Alzheimer's disease and multiple sclerosis but recently Rho-associated Kinase inhibitors have been recognized as potential antiglaucoma agents.

EVIDENCE SYNTHESIS

Rho-Kinase is a serine/threonine kinase with a kinase domain which is constitutively active and is involved in the regulation of smooth muscle contraction and stress fibre formation. Two isoforms of Rho-Kinase, ROCK-I (ROCK β) and ROCK-II (ROCK α) have been identified. ROCK II plays a pathophysiological role in glaucoma and hence the inhibitors of ROCK may be beneficial to ameliorate the vision loss. These inhibitors decrease the intraocular pressure in the glaucomatous eye by increasing the aqueous humour outflow through the trabecular meshwork pathway. They also act as anti-scarring agents and hence prevent post-operative scarring after the glaucoma filtration surgery. Their major role involves axon regeneration by increasing the optic nerve blood flow which may be useful in treating the damaged optic neurons. These drugs act directly on the neurons in the central visual pathway, interrupting the RGC apoptosis and therefore serve as a novel pharmacological approach for glaucoma neuroprotection.

CONCLUSION

Based on the results of high-throughput screening, several Rho kinase inhibitors have been designed and developed comprising of diverse scaffolds exhibiting Rho kinase inhibitory activity from micromolar to subnanomolar ranges. This diversity in the scaffolds with inhibitory potential against the kinase and their SAR development will be intricated in the present review. Ripasudil is the only Rho kinase inhibitor marketed to date for the treatment of glaucoma. Another ROCK inhibitor AR-13324 has recently passed the clinical trials whereas AMA0076, K115, PG324, Y39983 and RKI-983 are still under trials. In view of this, a detailed and updated account of ROCK II inhibitors as the next generation therapeutic agents for glaucoma will be discussed in this review.

The Effect of High-Intensity Focused Ultrasound on Aqueous Humor Dynamics in Patients with Glaucoma

PURPOSE

To investigate the effects of high-intensity focused ultrasound (HiFU) on aqueous humor dynamics in patients with glaucoma.

DESIGN

Comparative, nonrandomized, interventional study.

PARTICIPANTS

Adult patients with a diagnosis of open-angle glaucoma or ocular hypertension with suboptimal intraocular pressure (IOP) control despite maximum medical treatment who required further IOP optimization.

METHODS

All patients underwent comprehensive ophthalmic examination before aqueous humor dynamics study measurements, including fluorophotometry and digital Schiøtz tonography. All patients received 6 seconds of HiFU therapy. Aqueous humor dynamics studies were repeated 3 months after the treatment (patients had 4-week washout from their glaucoma medication before their aqueous humor dynamics study measurements at baseline and the 3-month visit).

MAIN OUTCOME MEASURES

Intraocular pressure, facility of topographic outflow, aqueous flow rate, and uveoscleral outflow.

RESULTS

Thirty eyes of 30 patients were included in the study. At the 3-month postoperative visit, the mean postwashout IOP was reduced by 16% (31.7±5.3 vs. 26.6±4.8 mmHg, P = 0.004), and aqueous flow rate was decreased by 15% (2.07±0.73 vs. 1.77±0.55 μl/min, P = 0.05) from baseline. Neither the tonographic outflow facility nor the uveoscleral outflow was significantly different from baseline. There is a 20% risk of treatment failure (those who needed further glaucoma surgical intervention) within 1 month after a single HiFU treatment (n = 6). Only 25 patients (80%) were able to undergo post-treatment washout measurements, and in these eyes, only 26.6% of eyes achieved >20% IOP reduction at 3 months compared with baseline.

CONCLUSIONS

We investigated the aqueous humor dynamics effects of a cyclodestructive procedure and specifically HiFU in patients with uncontrolled open-angle glaucoma on maximum tolerated medical therapy. High-intensity focused ultrasound reduced IOP 3 months postoperatively by 16% and aqueous flow decreased by 15% without any significant effect on tonographic outflow facility and uveoscleral outflow.

Impact of Visual Field Testing on Intraocular Pressure Change Trends in Healthy People and Glaucoma Patients

PURPOSE

To compare the impact of visual field (VF) testing on intraocular pressure (IOP) change trends between healthy subjects and glaucoma patients.

METHODS

We recruited healthy volunteer subjects who did not have previous ocular diseases and open-angle glaucoma patients who were medically controlled well. IOP in both eyes of each participant was measured by using a noncontact tonometer at five time points: before, immediately after (0 minute), and 10, 30, and 60 minutes after the standard automated perimetry. Repeated measures ANOVA was used to analyze the effect of VF testing on IOP change trends in healthy and glaucoma eyes.

RESULTS

Forty healthy subjects (80 eyes) and 31 open-angle glaucoma patients (62 eyes) were included for the study. The baseline IOP of healthy and glaucoma eyes was 16.11 ± 3.01 mmHg and 15.78 ± 3.57 mmHg, respectively. After the VF testing, the IOP in healthy eyes was decreased by 1.5% at 0 minute, 6.5% at 10 minutes (P < 0.001), 6.6% at 30 minutes (P < 0.001), and 7.0% at 1 hour (P < 0.001), indicating that this reduction was sustained for at least 1 hour. However, the IOP in glaucoma eyes was increased by 12.7% at 0 minute (P < 0.001) and, then, returned towards initial values 1 hour after the VF testing.

CONCLUSIONS

IOP change trends after VF field testing between healthy subjects and glaucoma patients were quite different. VF testing led to a mild and relatively sustained IOP decrease in healthy subjects, whereas IOP in open-angle glaucoma patients tended to significantly increase immediately after VF testing and, then, returned to pretest values after 1 hour. These findings indicate that the factors of VF testing should be considered in the clinical IOP measurements.

Tissue-Engineered Models for Glaucoma Research

Glaucoma is a group of optic neuropathies characterized by the progressive degeneration of retinal ganglion cells (RGCs). Patients with glaucoma generally experience elevations in intraocular pressure (IOP), followed by RGC death, peripheral vision loss and eventually blindness. However, despite the substantial economic and health-related impact of glaucoma-related morbidity worldwide, the surgical and pharmacological management of glaucoma is still limited to maintaining IOP within a normal range. This is in large part because the underlying molecular and biophysical mechanisms by which glaucomatous changes occur are still unclear. In the present review article, we describe current tissue-engineered models of the intraocular space that aim to advance the state of glaucoma research. Specifically, we critically evaluate and compare both 2D and 3D-culture models of the trabecular meshwork and nerve fiber layer, both of which are key players in glaucoma pathophysiology. Finally, we point out the need for novel organ-on-a-chip models of glaucoma that functionally integrate currently available 3D models of the retina and the trabecular outflow pathway.

Minimally Invasive Glaucoma Surgery: Where Is the Evidence?

PURPOSE

The last decade has witnessed an unprecedented growth in glaucoma treatment options through the introduction of minimally invasive glaucoma surgeries (MIGS). The aim of the present review is to provide an understanding of the currently available MIGS and to examine what data are currently available to guide treatment choice.

DESIGN

Meta-analysis and systematic review of randomized and non-randomized control trials.

METHODS

Out of 2567 articles identified, a total of 77 articles were retained for analysis, including 28 comparative studies and 12 randomized control trials. Overall, 7570 eyes were included. When data permitted, the weighted mean difference in intraocular pressure reduction was calculated for comparison purposes.

RESULTS

Weighted mean intraocular pressure reductions from all analyzed studies were: 15.3% (iStent), 29.1% (iStent inject), 36.2% (ab interno canaloplasty), 34.4% (Hydrus), 36.5% (gonioscopically-assisted transluminal trabeculotomy), 24.0% (trabectome), 25.1% (Kahook dual blade), 30.2% (Cypass), 38.8% (XEN), and 50.0% (Preserflo).

CONCLUSIONS

One of the advantages of the heterogenous range of available MIGS options is the chance to tailor therapy in an individualized manner. However, high-quality data are required to make this choice more than an educated guess. Overall, this review confirms the efficiency of assessed MIGS compared with standalone phacoemulsification, but it highlights that only few studies compare different MIGS techniques and even fewer assess MIGS against criterion standard treatments. Current evidence, while non-negligible, is mostly limited to heterogenous nonrandomized studies and uncontrolled retrospective comparisons, with few quality randomized control trials. We suggest that future research should be comparative and include relevant comparators, standardized to report key outcome features, long-term to assess sustainability and late complications, and ideally randomized.

Matrix Metalloproteinases and Glaucoma Treatment

Matrix metalloproteinases (MMPs) are a family of proteolytic enzymes that degrade extracellular matrix (ECM) components such as collagen and have important roles in multiple biological processes, including development and tissue remodeling, both in health and disease. The activity of MMPs is influenced by the expression of MMPs and tissue inhibitors of metalloproteinase (TIMPs). In the eye, MMP-mediated ECM turnover in the juxtacanalicular region of the trabecular meshwork (TM) reduces outflow resistance in the conventional outflow pathway and helps maintain intraocular pressure (IOP) homeostasis. An imbalance in the MMP/TIMP ratio may be involved in the elevated IOP often associated with glaucoma. The prostaglandin analog/prostamide (PGA) class of topical ocular hypotensive medications used in glaucoma treatment reduces IOP by increasing outflow through both conventional and unconventional (uveoscleral) outflow pathways. Evidence from in vivo and in vitro studies using animal models and anterior segment explant and cell cultures indicates that the mechanism of IOP lowering by PGAs involves increased MMP expression in the TM and ciliary body, leading to tissue remodeling that enhances conventional and unconventional outflow. PGA effects on MMP expression are dependent on the identity and concentration of the PGA. An intracameral sustained-release PGA implant (Bimatoprost SR) in development for glaucoma treatment can reduce IOP for many months after expected intraocular drug bioavailability. We hypothesize that the higher concentrations of bimatoprost achieved in ocular outflow tissues with the implant produce greater MMP upregulation and more extensive, sustained MMP-mediated target tissue remodeling, providing an extended duration of effect.

Age-related decline of lymphatic drainage from the eye: A noninvasive in vivo photoacoustic tomography study

We aim to determine whether lymphatic drainage from the eye changes with age. Using quantitative photoacoustic tomography, groups of young and older mice were studied in the live state. 10 CD-1 mice of 2-3 months (5M/5F) were studied in addition to 13 older mice of 12-13 months (6M/7F). In each of 23 mice, near-infrared tracer (a near-infrared dye, QC-1 conjugated with Bovine Serum Albumin) was injected into the right eye, and imaging of ipsilateral cervical lymph nodes was performed with laser pulses at 11 different wavelengths prior to and 20 min, 2, 4 and 6 h after injection. Mean pixel intensities (MPIs) of nodes were calculated at each imaging session. The areas under the curves (AUC) were calculated for both groups of mice and compared using the t-test. The slopes of MPI of each region of interest were compared using the linear mixed model before and after adjusting for sex, body weight and intraocular pressure of the right eye. The mean intraocular pressure of right eyes before injection was similar in older and younger groups (12.77 ± 2.01 mmHg and 12.90 ± 2.38 mmHg, respectively; p = 0.888). In each mouse, the photoacoustic signal was detected in the right cervical lymph nodes at the 2-h time point following tracer injection into the right eye. At the 4 and 6 h imaging times, a steady increase of tracer signal was observed. Areas under the curve in the right cervical nodes were decreased significantly in older mice compared to younger mice (p = 0.007). The slopes of MPI in the nodes were significantly decreased in old mice compared to young mice both before and after adjusting for sex, body weight and intraocular pressure of the right eye (p = 0.003). In conclusion, lymphatic drainage from the eye is significantly reduced in older eyes. This finding suggests that impaired lymphatic clearance of aqueous humor, proteins and antigens from the eye may contribute to age-related disease of the eye such as glaucoma and inflammatory eye disease.

Circadian Regulation of IOP Rhythm by Dual Pathways of Glucocorticoids and the Sympathetic Nervous System

Purpose

Elevated IOP can cause the development of glaucoma. The circadian rhythm of IOP depends on the dynamics of the aqueous humor and is synchronized with the circadian rhythm pacemaker, that is, the suprachiasmatic nucleus. The suprachiasmatic nucleus resets peripheral clocks via sympathetic nerves or adrenal glucocorticoids. However, the detailed mechanisms underlying IOP rhythmicity remain unclear. The purpose of this study was to verify this regulatory pathway.

Methods

Adrenalectomy and/or superior cervical ganglionectomy were performed in C57BL/6J mice. Their IOP rhythms were measured under light/dark cycle and constant dark conditions. Ocular administration of corticosterone or norepinephrine was also performed. Localization of adrenergic receptors, glucocorticoid receptors, and clock proteins Bmal1 and Per1 were analyzed using immunohistochemistry. Period2::luciferase rhythms in the cultured iris/ciliary bodies of adrenalectomized and/or superior cervical ganglionectomized mice were monitored to evaluate the effect of the procedures on the local clock. The IOP rhythm of retina and ciliary epithelium-specific Bmal1 knockout mice were measured to determine the significance of the local clock.

Results

Adrenalectomy and superior cervical ganglionectomy disrupted IOP rhythms and the circadian clock in the iris/ciliary body cultures. Instillation of corticosterone and norepinephrine restored the IOP rhythm. β2-Adrenergic receptors, glucocorticoid receptors, and clock proteins were strongly expressed within the nonpigmented epithelia of the ciliary body. However, tissue-specific Bmal1 knock-out mice maintained their IOP rhythm.

Conclusions

These findings suggest direct driving of the IOP rhythm by the suprachiasmatic nucleus, via the dual corticosterone and norepinephrine pathway, but not the ciliary clock, which may be useful for chronotherapy of glaucoma.

No flow through the vitreous humor: How strong is the evidence?

When analyzing vitreal drug delivery, or the pharmacological effects of drugs on intraocular pressure, or when interpreting outflow facility measurements, it is generally accepted that the fluid in the vitreous humor is stagnant. It is accepted that for all practical purposes, the aqueous fluid exits the eye via anterior pathways only, and so there is negligible if any posteriorly directed flow of aqueous through the vitreous humor. This assumption is largely based on the interpretation of experimental data from key sources including Maurice (1957), Moseley (1984), Gaul and Brubaker (1986), Maurice (1987) and Araie et al. (1991). However, there is strong independent evidence suggesting there is a substantial fluid flow across the retinal pigment epithelium from key sources including Cantrill and Pederson (1984), Chihara and Nao-i, Tsuboi (1985), Dahrouj et al. (2014), Smith and Gardiner (2017) and Smith et al. (2019). The conflicting evidence creates a conundrum-how can both interpretations be true? This leads us to re-evaluate the evidence. We demonstrate that the data believed to be supporting no aqueous flow through the vitreous are in fact compatible with a significant normal aqueous flow. We identify strong and independent lines of evidence supporting fluid flow across the RPE, including our new outflow model for the eye. On balance it appears the current evidence favors the view that there is normally a significant aqueous flow across the RPE in vivo. This finding suggests that past and future analyses of outflow facility, interpretations of some drug distributions and the interpretation of some drug effects on eye tissues, may need to be revised.

Nanotechnology for Medical and Surgical Glaucoma Therapy-A Review

Glaucoma is the second leading cause of blindness worldwide. Even though significant advances have been made in its management, currently available antiglaucoma therapies suffer from considerable drawbacks. Typically, the success and efficacy of glaucoma medications are undermined by their limited bioavailability to target tissues and the inadequate adherence demonstrated by patients with glaucoma. The latter is due to a gradual decrease in tolerability of lifelong topical therapies and the significant burden to patients of prescribed stepwise antiglaucoma regimens with frequent dosing which impact quality of life. On the other hand, glaucoma surgery is restricted by the inability of antifibrotic agents to efficiently control the wound healing process without causing severe collateral damage and long-term complications. Evolution of the treatment paradigm for patients with glaucoma will ideally include prevention of retinal ganglion cell degeneration by the successful delivery of neurotrophic factors, anti-inflammatory drugs, and gene therapies. Nanotechnology-based treatments may surpass the limitations of currently available glaucoma therapies through optimized targeted drug delivery, increased bioavailability, and controlled release. This review addresses the recent advances in glaucoma treatment strategies employing nanotechnology, including medical and surgical management, neuroregeneration, and neuroprotection.

Ocular Accommodation, Intraocular Pressure, Development of Myopia and Glaucoma: Role of Ciliary Muscle, Choroid and Metabolism

Ocular accommodation is not just a mechanism for altering curvature of the crystalline lens of the eye, it also enables aqueous humor outflow through the trabecular meshwork, influencing intraocular pressure (IOP). Long term stress on the ciliary muscle from sustained near focusing may initiate myopic eye growth in children and primary open angle glaucoma in presbyopic adults. Multi-factorial studies of ocular accommodation that include measures of IOP, ciliary muscle morphology, anterior chamber depth and assessment of nutritional intake and metabolic markers may elucidate etiology and novel strategies for management of both myopia and chronic glaucoma. Anatomy of the ciliary fibers from anterior insertion in the fluid drainage pathway to their posterior consanguinity with the vascular choroid, alters ocular parameters such as micro-fluctuations of accommodation and pulsatile ocular blood flow that are driven by cardiac contractions conveyed by carotid arteries. Stretching of the choroid has consequences for thinning of the peripheral retina, sclera and lamina cribrosa with potential to induce retinal tears and optic nerve cupping. Early metabolic interventions may lead to prevention or reduced severity of myopia and glaucoma. Finally, it might improve quality of life of patients and decrease disability from visual impairment and blindness.

Canaloplasty in the Treatment of Primary Open-Angle Glaucoma: Patient Selection and Perspectives

Canaloplasty is a surgical procedure that has undergone a number of developments since its introduction in 2005. Many thousands of canaloplasties have been performed around the world since then and is, by definition, a blebless procedure. It does not necessitate the use of any antifibrotic agents and results in safe and effective IOP reductions in patients with open-angle glaucoma (OAG) with minimal complications and no bleb-related adverse events. When considering the surgical management of patients with early and medium stages of the disease, canaloplasty can be considered as a first line option. This paper will overview the theoretical effectiveness of canal surgery, the fundamental aspects of aqueous outflow resistance with particular emphasis on the role of the trabecular meshwork, Schlemm's canal, and the collector channels, and the methods available for the clinical evaluation of the outflow pathways in relation to the ocular anatomy. Further, the paper will detail the surgical technique itself and how this has developed over time together with the clinical aspects that should be accounted for when selecting patients for this surgery.