Measurement of aqueous humor flow

In vivo quantification of anterior and posterior chamber volumes in mice: implications for aqueous humor dynamics

Purpose

Aqueous humor inflow rate, a key parameter influencing aqueous humor dynamics, is typically measured by fluorophotometery. Analyzing fluorophotometric data depends, inter alia, on the volume of aqueous humor in the anterior, but not the posterior, chamber. Previous fluorophotometric studies of aqueous inflow rate in mice have assumed the ratio of anterior:posterior volumes in mice to be similar to those in humans. Our goal was to measure anterior and posterior chamber volumes in mice to facilitate better estimates of aqueous inflow rates.

Methods

We used standard near-infrared optical coherence tomography (OCT) and robotic visible-light OCT (vis-OCT) to visualize, reconstruct and quantify the volumes of the anterior and posterior chambers of the mouse eye in vivo. We used histology and micro-CT scans to validate relevant landmarks from ex vivo tissues to facilitate in vivo measurement.

Results

Posterior chamber volume is 1.1 times the anterior chamber volume in BALB/cAnNCrl mice, i.e. the anterior chamber constitutes about 47% of the total aqueous humor volume, which is very dissimilar to the situation in humans. Anterior chamber volumes in 2-month-old BALB/cAnNCrl and 7-month-old C57BL6/J mice were 1.55 ± 0.36 μL (n=10) and 2.41 ± 0.29 μL (n=8), respectively. This implies that previous studies likely over-estimated aqueous inflow rate by approximately two-fold.

Conclusions

It is necessary to reassess previously reported estimates of aqueous inflow rates, and thus aqueous humor dynamics in the mouse. For example, we now estimate that only 0-15% of aqueous humor drains via the pressure-independent (unconventional) route, similar to that seen in humans and monkeys.

Assessing the Impact of PRESERFLO MicroShunt on Intraocular Pressure in Porcine Eyes Ex Vivo Using Infusion Pump System

To evaluate the effectiveness of the PRESERFLO MicroShunt (PFM) in reducing intraocular pressure (IOP) ex vivo in porcine eyes using an infusion pump system and to simulate various IOP conditions, In this study, porcine eyes received increasing flows between 2 and 20 μL/min. IOP measurements were taken under conditions with and without the PFM [PFM (+) and PFM (-), respectively]. In the PFM (-) group, IOP increased from 7.4 mmHg to 46.3 mmHg as the flow rate increased from 2 μL/min to 20 μL/min. The rate of IOP reduction (%ΔIOP) rose with increasing flow rates, although the absolute IOP values achieved with the PFM insertion also increased. The correlation between IOPs in the PFM (-) conditions and the %ΔIOP was modeled as %ΔIOP = 22.4 Ln [PFM(-) IOP] - 41.7. According to this equation, IOP reduction by PFM insertion is 0% at IOPs of 6.4 mmHg or lower. IOP reductions of 10%, 20%, 30%, and 40% were observed when the pre-insertion IOPs were 10.1, 15.7, 24.6, and 38.4 mmHg, respectively. Achievable post-insertion IOP levels of ≤21 mmHg, ≤18 mmHg, ≤15 mmHg, and ≤12 mmHg corresponded to the initial IOPs of 33 mmHg, 26 mmHg, 20 mmHg, and 14.8 mmHg, respectively. In conclusion, the PFM effectively reduced IOP within a specific range of IOP values in an ex vivo experimental system. In clinical situations, the PFM is unlikely to be effective at low IOP levels. At higher levels, the PFM reduces IOP, but it may be insufficient to achieve the target IOP.

Aqueous humor as eye lymph: A crossroad between venous and lymphatic system

Aqueous humor (AQH) is a transparent fluid with characteristics similar to those of the interstitial fluid, which fills the eyeball posterior and anterior chambers and circulates in them from the sites of production to those of drainage. The AQH volume and pressure homeostasis is essential for the trophism of the ocular avascular tissues and their normal structure and function. Different AQH outflow pathways exist, including a main pathway, quite well defined anatomically and referred to as the conventional pathway, and some accessory pathways, more recently described and still not fully morphofunctionally understood, generically referred to as unconventional pathways. The conventional pathway is based on the existence of a series of conduits starting with the trabecular meshwork and Schlemm's Canal and continuing with a system of intrascleral and episcleral venules, which are tributaries to veins of the anterior segment of the eyeball. The unconventional pathways are mainly represented by the uveoscleral pathway, in which AQH flows through clefts, interstitial conduits located in the ciliary body and sclera, and then merges into the aforementioned intrascleral and episcleral venules. A further unconventional pathway, the lymphatic pathway, has been supported by the demonstration of lymphatic microvessels in the limbal sclera and, possibly, in the uvea (ciliary body, choroid) as well as by the ocular glymphatic channels, present in the neural retina and optic nerve. It follows that AQH may be drained from the eyeball through blood vessels (TM-SC pathway, US pathway) or lymphatic vessels (lymphatic pathway), and the different pathways may integrate or compensate for each other, optimizing the AQH drainage. The present review aims to define the state-of-the-art concerning the structural organization and the functional anatomy of all the AQH outflow pathways. Particular attention is paid to examining the regulatory mechanisms active in each of them. The new data on the anatomy and physiology of AQH outflow pathways is the key to understanding the pathophysiology of AQH outflow disorders and could open the way for novel approaches to their treatment.

Secondary glaucoma: Toward interventions based on molecular underpinnings

Glaucoma is a heterogeneous group of progressive diseases that leads to irreversible blindness. Secondary glaucoma refers to glaucoma caused by a known underlying condition. Pseudoexfoliation and pigment dispersion syndromes are common causes of secondary glaucoma. Their respective deposits may obstruct the trabecular meshwork, leading to aqueous humor outflow resistance, ocular hypertension, and optic neuropathy. There are no disease-specific interventions available for either. Pseudoexfoliation syndrome is characterized by fibrillar deposits (pseudoexfoliative material) on anterior segment structures. Over a decade of multiomics analyses taken together with the current knowledge on pseudoexfoliative glaucoma warrant a re-think of mechanistic possibilities. We propose that the presence of nucleation centers (e.g., vitamin D binding protein), crosslinking enzymes (e.g., transglutaminase 2), aberrant extracellular matrix, flawed endocytosis, and abnormal aqueous-blood barrier contribute to the formation of proteolytically resistant pseudoexfoliative material. Pigment dispersion syndrome is characterized by abnormal iridolenticular contact that disrupts iris pigment epithelium and liberates melanin granules. Iris melanogenesis is aberrant in this condition. Cytotoxic melanogenesis intermediates leak out of melanosomes and cause iris melanocyte and pigment epithelium cell death. Targeting melanogenesis can likely decrease the risk of pigmentary glaucoma. Skin and melanoma research provides insights into potential therapeutics. We propose that specific prostanoid agonists and fenofibrates may reduce melanogenesis by inhibiting cholesterol internalization and de novo synthesis. Additionally, melatonin is a potent melanogenesis suppressor, antioxidant, and hypotensive agent, rendering it a valuable agent for pigmentary glaucoma. In pseudoexfoliative glaucoma, where environmental insults drive pseudoexfoliative material formation, melatonin's antioxidant and hypotensive properties may offer adjunct therapeutic benefits. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology.

Measurement of vitreous humor pressure in vivo using an optic fiber pressure sensor

We conducted a study to assess the pressure difference between the aqueous and vitreous humors in rabbit eyes using a direct intraocular pressure (IOP) measurement method. A micro-optic-fiber pressure sensor was utilized for this purpose. Preliminary experiments with enucleated porcine eyes confirmed the sensor's accuracy in measuring both aqueous and vitreous humor pressure. The main study involved six healthy albino rabbits, where the sensor measured the pressure in the anterior chamber (aIOP) and posterior vitreous-cavity (pIOP). These measurements were compared to aIOP values obtained through rebound tonometry. Additionally, pre- and postoperative pressure comparisons were made after performing a vitrectomy. Results revealed a significant disparity between aqueous and vitreous humor pressures. Prior to vitrectomy, pIOP was 22.8 mmHg, over twice as high as aIOP (11.0 mmHg), but decreased to a similar level following the procedure. Comparison between the sensor measurements and rebound tonometry showed agreement in aIOP values. In conclusion, our study demonstrates that vitreous humor pressure is consistently higher than aqueous humor pressure, reaching the upper limit of normal IOP. Furthermore, vitrectomy effectively reduces pIOP, aligning it with aIOP. These findings contribute valuable insights into intraocular pressure dynamics and have implications for clinical interventions targeting ocular pressure regulation.

Vascular endothelial cell-derived exosomal miR-1246 facilitates posterior capsule opacification development by targeting GSK-3β in diabetes mellitus

Posterior capsule opacification (PCO) is a serious complication after cataract surgery. Diabetes could increase the occurrence of PCO, but the mechanism is still unclear. The purpose of this study is to investigate the role of small extracellular vesicles (sEVs) derived from diabetic aqueous humor in PCO process. Intraoperatively-derived aqueous humor sEVs from patients with diabetic related cataract (DRC) promoted the epithelial-mesenchymal transition (EMT) and metastasis of human lens epithelial cells (LECs). Via mouse PCO surgical model and DiI labeled fluorescence detection of sEVs, the sEVs derived from vascular endothelium were discovered directly contacting with LECs. Furthermore, we demonstrated that high-glucose-cultured human umbilical vein endothelial cells (HUVEC) -derived sEVs facilitated EMT process of HLE-B3 using co-culture model in vitro. MiRNA-seq data and GEO datasets analysis revealed that miR-1246 was essential in EMT process with diabetes. The miR-1246 was highly expressed in diabetic aqueous humor sEVs and high-glucose-treated vascular-endothelial-cell-derived sEVs. Moreover, miR-1246 promoted the metastasis and EMT process of HLE-B3 cells by directly targeting GSK-3β. Inhibiting miR-1246 could negatively regulated EMT. This finding might serve as a potential therapy for diabetic PCO.

Intraocular Pressure and Medication Changes Associated with Xen Gel Stent: A Systematic Review of the Literature

The Xen gel stent (Allergan Inc, an AbbVie company; Dublin, Ireland) was conceived as an option for patients requiring modest IOP reduction but for whom trabeculectomy was not yet indicated. As with any glaucoma surgery, establishing criteria for patient selection and identifying factors that contribute to a high likelihood of success are important. To help guide clinical decision-making, a systematic review of published studies on the gel stent was performed, with the goal of understanding postoperative outcomes based on clinical and patient factors. Results were organized around a series of pertinent clinical questions based on scenarios encountered in clinical practice. Criteria for including studies were intentionally broad, with the objective of simulating the diverse population of glaucoma patients encountered in real-world practice. Outcomes for IOP and medication reduction postoperatively were assessed in various analyses, including in eyes with various glaucoma types and severity; in eyes naïve to surgery as well as those with a history of prior incisional glaucoma surgery; and when surgery was performed as a standalone procedure or at the time of cataract surgery. The results of each of the various analyses were consistent in demonstrating that successful gel stent surgery achieved a postoperative IOP of approximately 14.0 mm Hg and reduction to fewer than 1 glaucoma medication. Additional data are shown on outcomes by method of implant (ab interno vs ab externo); intraoperative use of antifibrotics; and rates of needling in published studies.

Theoretic scientific rationale of double XEN 45 Gel Stent implant in severe glaucomatous ocular hypertension

PURPOSE

To investigate the theoretical hydrodynamic effects of a single or a double XEN 45 Gel Stent (XEN45) implant in glaucomatous eyes.

METHODS

A prospective non-randomized clinical study on 63 glaucomatous eyes of 63 patients. The preoperative intraocular pressure (IOP) values after 14 days of medical therapy washout was correlated to the postoperative IOP values 7 and 45 days after the implant of a XEN45. A mathematical model based on the Hagen-Poiseuille law was designed to assess the residual aqueous humor outflow (AHO) facility in glaucomatous eyes in function of the postoperative IOP reduction. Using XEN45 as unit of measurement, we transformed through equations the residual preoperative AHO in XEN45 equivalent in order to establish theoretical number of stents needed to reach the target IOP.

RESULTS

The mean preoperative washout IOP was 28.9 ± 5.4 mmHg, after 7 and 45 days the mean postoperative IOP was 12.6 ± 4.3 and 15.3 ± 4.4 mmHg, respectively (p < 0.001). A significant positive correlation was found between preoperative and postoperative IOP values. We obtained a mathematical relationship to estimate the theoretical number of XEN45 stents needed to reach the target IOP.

CONCLUSION

The basal IOP (after medical therapy washout) is a predictive factor for the surgical success after a XEN45 implant. Establishing the residual preoperative AHO it is possible to predict the theoretical number of stents to implant. A double XEN45 implant could be considered a possible further tool to reach the target IOP in patients with severe AHO deficiency.

Impact of silicone oil tamponade on intravitreally injected vancomycin pharmacokinetics in cynomolgus monkey eyes

Intravitreal injections of vancomycin (VCM) and ceftazidime (CAZ) are commonly used to treat infectious endophthalmitis. When patient cases require retinal detachment with silicone oil (SO) tamponade, the antibiotic doses are empirically reduced to 25 %. Currently, there is no scientific evidence for these empirical dose reductions. The purpose of the present study is to determine the quantitative impact that SO tamponades have on intraocular VCM pharmacokinetics. Because of high invasiveness of frequent sampling of intraocular VCM concentrations in human, this pharmacokinetic study was performed in cynomolgus monkey's eyes. Population pharmacokinetic modeling and simulation were performed using 75 different intraocular VCM concentrations obtained from 8 male cynomolgus monkeys. A one-compartment model with a first-order diffusion rate was used as a structural pharmacokinetic model. From the covariate analysis, SO tamponade significantly decreased the volume of distribution while pars plana vitrectomy with lensectomy (PPV) significantly increased the clearance and diffusion rate constants. From the Monte Carlo simulation (n = 1,000), the median time above minimum inhibitory concentration (T_>MIC, a therapeutic effect index) durations of SO and normal eyes at clinical doses of 1,000 µg were 2.6 and 11.0 days, respectively. Using intravitreal injections of VCM with SO tamponade or PPV may reduce the therapeutic effect.

A Retrospective Analysis of Safety and Efficacy of XEN 45 Microstent Combined Cataract Surgery in Open-Angle Glaucoma over 24 Months

Objectives:

To evaluate the effect on intraocular pressure (IOP) reduction and safety of ab interno gelatin microstent (XEN 45 Gel Stent; Aquesys, Inc, Aliso Viejo, CA, USA) microincisional glaucoma/cataract surgery in open-angle glaucoma (OAG).

Materials and Methods:

In this retrospective study, 30 eyes of 25 patients with OAG which underwent XEN 45 implantation combined with simultaneous phacoemulsification were clinically evaluated. Clinical outcomes analyzed included IOP, percent of IOP reduction, medication use, complications, best corrected visual acuity, and surgical outcomes at 24-month follow-up.

Results:

After the XEN 45 combined cataract surgery procedure, IOP dropped from 20.37±4.80 mmHg with a mean of 3.07±1.04 medication classes preoperatively to 14.83±1.91 mmHg with a mean of 0.94±1.11 medication classes at 24 months (p=0.001 for both). At 24 months, 55.6% of patients had IOP ≤18 mmHg without medication, 94.4% of patients had IOP ≤18 mmHg with or without medication, and 61.1% of patients reached ≥20% IOP reduction from baseline.

Conclusion:

XEN 45 is an effective minimally invasive surgical treatment for OAG with significant reduction in IOP and glaucoma medications and minimal complications in long-term follow-up.

Intermediate Outcomes of a Novel Standalone Ab Externo SIBS Microshunt With Mitomycin C

PURPOSE

To determine the efficacy, risk factors for failure, and adverse events of a standalone novel ab externo SIBS microshunt with mitomycin C (MMC) during 1-year follow-up.

DESIGN

Retrospective, interventional case series.

SUBJECTS

Glaucomatous eyes with an intraocular pressure (IOP) above target and/or progressing on maximally tolerated medical therapy.

METHODS

Consecutive patients with open-angle glaucoma (OAG) and no previous filtering surgery received an ab externo SIBS microshunt with MMC from July 2015 to November 2017. Main outcome measures were proportion of eyes at 1-year with (1) no 2 consecutive IOP readings >17 mm Hg or clinical hypotony without (complete) or with glaucoma medications (qualified); and (2) at least a 20% reduction from decision IOP. Secondary outcomes included upper IOP thresholds of 14 mm Hg and 21 mm Hg with and without a 20% IOP reduction from baseline, median IOP, medications, risk factors for failure, interventions, complications, and reoperations.

RESULTS

A total of 164 eyes in 132 patients were included. Complete success was achieved in 76.9% of eyes, qualified success in 92.5%. Complete success was 75.6% for an upper IOP cut-off of 14 mm Hg and 76.9% for 21 mm Hg, and qualified success was 91.9% and 92.5%. MMC dose of 0.2 vs 0.4-0.5 mg/mL (hazard ratio [HR] 2.51; 95% CI 1.12-5.65) and primary open-angle glaucoma vs secondary open-angle glaucoma (SOAG) (HR 2.51; 95% CI 1.01-6.23) represented the only risk factors for failure in multivariable analysis. Needling was performed in 8.5% of eyes. Two eyes received surgical revision, and 1 a reoperation.

CONCLUSIONS

One-year results of the ab externo SIBS microshunt demonstrated promising rates of qualified and complete success, decreased drop use, few complications, and infrequent postoperative interventions.

Nonclinical Pharmacokinetic and Pharmacodynamic Assessment of Bimatoprost Following a Single Intracameral Injection of Sustained-Release Implants

Purpose

To assess the pharmacokinetic (PK)/pharmacodynamic (PD) relationship following intracameral Bimatoprost sustained-release (SR) implants (8, 15, 30, and 60 µg) in dogs to determine the optimal investigative dose in humans.

Methods

Forty-four male normotensive beagle dogs were assigned to 1 of 8 groups receiving 8-, 15-, 30-, and 60-µg implants (PD assessment [n = 8/group, 4 groups]; PK assessment [n = 3/group, 4 groups]). Intraocular pressure (IOP) in PD animals and aqueous humor/blood concentrations of bimatoprost and its acid in PK animals were assessed. PK/PD correlation analysis was performed using steady-state data. Residual implants were recovered to assess polymer degradation.

Results

Dose-dependent IOP lowering was observed for all dose groups for at least 3 months postdose. Blood concentrations of bimatoprost and bimatoprost acid were below the limit of quantification (<0.25 ng/mL), whereas dose-dependent concentration-time profiles were observed in the aqueous humor. At steady state, observed and predicted correlation between aqueous humor drug concentration and IOP lowering was similar and translatable to findings in humans following topical bimatoprost eyedrop administration. Implants at all doses were well tolerated and polymer degradation was apparent.

Conclusions

Dose-dependent IOP lowering with Bimatoprost SR was maintained for at least 3 months in dogs, and the implants were well tolerated. The established PK/PD relationship appears to translate to humans. Doses between 8 and 15 µg appear to provide the best benefit/risk profile for clinical development of the implants.

Translational Relevance

The close PK/PD relationship between dog and human helps inform which bimatoprost dose should be investigated in clinical studies.

An injectable peptide hydrogel for reconstruction of the human trabecular meshwork

Glaucoma is a leading cause of irreversible blindness worldwide. Current treatments of glaucoma involve lowering the IOP by means of decreasing aqueous humor production or increasing non-trabecular aqueous humor outflow with the help of IOP-lowering eye drops, nanotechnology enabled glaucoma drainage implants, and trabeculectomy. However, there is currently no effective and permanent cure for this disease. In order to investigate new therapeutic strategies, three dimensional (3D) biomimetic trabecular meshwork (TM) models are in demand. Therefore, we adapted MAX8B, a peptide hydrogel system to bioengineer a 3D trabecular meshwork scaffold. We assessed mechanical and bio-instructive properties of this engineered tissue matrix by using rheological analysis, 3D cell culture and imaging techniques. The scaffold material exhibited shear-thinning ability and biocompatibility for proper hTM growth and proliferation indicating a potential utilization as an injectable implant. Additionally, by using a perfusion system, MAX8B scaffold was tested as an in vitro platform for investigating the effect of Dexamethasone (Dex) on trabecular meshwork outflow facility. The physiological response of hTM cells within the scaffold to Dex treatment clearly supported the effectiveness of this 3D model as a drug-testing platform, which can accelerate discovery of new therapeutic targets for glaucoma. STATEMENT OF SIGNIFICANCE: Artificial 3D-TM (3-dimentional Trabecular Meshwork) developed here with hTM (human TM) cells seeded on peptide-hydrogel scaffolds exhibits the mechanical strength and physiological properties mimicking the native TM tissue. Besides serving a novel and effective 3D-TM model, the MAX8B hydrogel could potentially function as an injectable trabecular meshwork implant.

Translating Minimally Invasive Glaucoma Surgery Devices

Glaucoma is the leading cause of irreversible blindness with over 70 million people affected worldwide. The surgical management of glaucoma aims to lower intraocular pressure by increasing aqueous outflow facility. The latest manufacturing techniques have allowed for the development of a number of novel implantable devices to improve safety and outcomes of glaucoma surgery. These are collectively referred to as minimally invasive glaucoma surgery (MIGS) devices and are among the smallest devices implanted in the human body. This review discusses the design criterion and constraints as well as the user requirements for MIGS devices. We review how recent devices have attempted to meet these challenges and give our opinion as to the necessary characteristics for the development of future devices.

A Mathematical Model of Corneal Metabolism in the Presence of an Iris-Fixated Phakic Intraocular Lens

Purpose

Corneal endothelial cell loss is one of the possible complications associated with phakic iris-fixated intraocular lens (PIOL) implantation. We postulate that this might be connected to the alteration of corneal metabolism secondary to the lens implantation.

Methods

A mathematical model of transport and consumption/production of metabolic species in the cornea is proposed, coupled with a model of aqueous flow and transport of metabolic species in the anterior chamber.

Results

Results are presented both for open and closed eyelids. We showed that, in the presence of a PIOL, glucose availability at the corneal endothelium decreases significantly during sleeping.

Conclusions

Implantation of a PIOL significantly affects nutrient transport processes to the corneal endothelium especially during sleep. It must still be verified whether this finding has a clinical relevance.

Selective permeability of mouse blood-aqueous barrier as determined by 15N-heavy isotope tracing and mass spectrometry

The blood-aqueous barrier plays a key role in regulating aqueous humor homeostasis by selectively restricting passage of proteins into the eye. The kinetics of aqueous flow are traditionally measured using artificial markers; however, these marker molecules do not address the barrier's selective permeability to plasma proteins. Here we applied stable isotope labeling of all serum proteins with nitrogen-15 (¹⁵N) atoms. Following systemic injection of this "heavy" serum in mice, the ¹⁵N-to-endogenous nitrogen-14 (¹⁴N) ratio of each protein in aqueous was measured by mass spectrometry. By monitoring the kinetic changes in these ratios, we determined the permeability profiles of hundreds of serum proteins. Meanwhile, we subjected one of the eyes to neoangiogenic wound healing by inflicting injury to the corneal limbus and compared the ¹⁵N proteomes between the normal eyes and the recovering eyes at 2 weeks after injury. In the injured eye, we detected markedly enhanced permeability to inhibitory complement regulator proteins, such as Cfh, Cfhr, Cfb, Cfi, Cfd, and Vtn. Many of the proteins in this group are implicated in age-related macular degeneration associated with leakage of the blood-retinal barrier due to inflammation. To rule out the possibility that the observed leakage was due simply to physical damage of the blood vessels, we separately created a neovascularization model using an alkali burn of the avascular cornea. In this latter model, elevated levels of Cfh and Cfb were evident. These findings suggest that ocular neovascularization is associated with enhanced permeability to serum complement regulators.

LC-MS analysis to determine the biodistribution of a polymer coated ilomastat ocular implant

Ilomastat is a matrix metalloproteinase inhibitor (MMPi) that has shown the potential to inhibit scarring (fibrosis) by mediating healing after injury or surgery. A long lasting ocular implantable pharmaceutical formulation of ilomastat is being developed to mediate the healing process to prevent scarring after glaucoma filtration surgery. The ilomastat implant was coated with water permeable and biocompatible phosphoryl choline polymer (PC1059) displayed extended slow release of ilomastat in vitro and in vivo. The ocular distribution of ilomastat from the implant in rabbits at day 30 post surgery was determined by the extraction of ilomastat and its internal standard marimastat from the ocular tissues, plasma, aqueous humour and vitreous fluid followed by capillary-flow liquid chromatography (cap-LC), the column effluent was directed into a triple quadrupole mass spectrometer operating in product scan mode. The lower limits of quantification (LLOQs) were 0.3 pg/μL for ocular fluids and plasma, and 3 pg/mg for ocular tissues. The extraction recoveries were 90-95% for ilomastat and its internal standard from ocular tissues. Ilomastat was found in ocular fluids and tissues at day 30 after surgery. The level of ilomastat was 18 times higher in the aqueous humour than vitreous humour. The concentration ranking of ilomastat in the ocular tissues was sclera > bleb conjunctiva > conjunctiva (rest of the eye) > cornea. Mass spectrometry analysis to confirm the presence of ilomastat in the ocular tissues and fluids at day 30 post-surgery establishes the extended release of ilomastat can be achieved in vivo, which is crucial information for optimisation of the ilomastat coated implant.

A generalised porous medium approach to study thermo-fluid dynamics in human eyes

The present work describes the application of the generalised porous medium model to study heat and fluid flow in healthy and glaucomatous eyes of different subject specimens, considering the presence of ocular cavities and porous tissues. The 2D computational model, implemented into the open-source software OpenFOAM, has been verified against benchmark data for mixed convection in domains partially filled with a porous medium. The verified model has been employed to simulate the thermo-fluid dynamic phenomena occurring in the anterior section of four patient-specific human eyes, considering the presence of anterior chamber (AC), trabecular meshwork (TM), Schlemm's canal (SC), and collector channels (CC). The computational domains of the eye are extracted from tomographic images. The dependence of TM porosity and permeability on intraocular pressure (IOP) has been analysed in detail, and the differences between healthy and glaucomatous eye conditions have been highlighted, proving that the different physiological conditions of patients have a significant influence on the thermo-fluid dynamic phenomena. The influence of different eye positions (supine and standing) on thermo-fluid dynamic variables has been also investigated: results are presented in terms of velocity, pressure, temperature, friction coefficient and local Nusselt number. The results clearly indicate that porosity and permeability of TM are two important parameters that affect eye pressure distribution. Graphical abstract Velocity contours and vectors for healthy eyes (top) and glaucomatous eyes (bottom) for standing position.

The role and therapeutic potential of melatonin in age-related ocular diseases

The eye is continuously exposed to solar UV radiation and pollutants, making it prone to oxidative attacks. In fact, oxidative damage is a major cause of age-related ocular diseases including cataract, glaucoma, age-related macular degeneration, and diabetic retinopathy. As the nature of lens cells, trabecular meshwork cells, retinal ganglion cells, retinal pigment epithelial cells, and photoreceptors is postmitotic, autophagy plays a critical role in their cellular homeostasis. In age-related ocular diseases, this process is impaired, and thus, oxidative damage becomes irreversible. Other conditions such as low-grade chronic inflammation and angiogenesis also contribute to the development of retinal diseases (glaucoma, age-related macular degeneration and diabetic retinopathy). As melatonin is known to have remarkable qualities such as antioxidant/antinitridergic, mitochondrial protector, autophagy modulator, anti-inflammatory, and anti-angiogenic, it can represent a powerful tool to counteract all these diseases. The present review analyzes the role and therapeutic potential of melatonin in age-related ocular diseases, focusing on nitro-oxidative stress, autophagy, inflammation, and angiogenesis mechanisms.

The impact of ocular hemodynamics and intracranial pressure on intraocular pressure during acute gravitational changes

Exposure to microgravity causes a bulk fluid shift toward the head, with concomitant changes in blood volume/pressure, and intraocular pressure (IOP). These and other factors, such as intracranial pressure (ICP) changes, are suspected to be involved in the degradation of visual function and ocular anatomical changes exhibited by some astronauts. This is a significant health concern. Here, we describe a lumped-parameter numerical model to simulate volume/pressure alterations in the eye during gravitational changes. The model includes the effects of blood and aqueous humor dynamics, ICP, and IOP-dependent ocular compliance. It is formulated as a series of coupled differential equations and was validated against four existing data sets on parabolic flight, body inversion, and head-down tilt (HDT). The model accurately predicted acute IOP changes in parabolic flight and HDT, and was satisfactory for the more extreme case of inversion. The short-term response to the changing gravitational field was dominated by ocular blood pressures and compliance, while longer-term responses were more dependent on aqueous humor dynamics. ICP had a negligible effect on acute IOP changes. This relatively simple numerical model shows promising predictive capability. To extend the model to more chronic conditions, additional data on longer-term autoregulation of blood and aqueous humor dynamics are needed.NEW & NOTEWORTHY A significant percentage of astronauts present anatomical changes in the posterior eye tissues after spaceflight. Hypothesized increases in ocular blood volume and intracranial pressure (ICP) in space have been considered to be likely factors. In this work, we provide a novel numerical model of the eye that incorporates ocular hemodynamics, gravitational forces, and ICP changes. We find that changes in ocular hemodynamics govern the response of intraocular pressure during acute gravitational change.

24-h monitoring devices and nyctohemeral rhythms of intraocular pressure

Intraocular pressure (IOP) is not a fixed value and varies over both the short term and periods lasting several months or years. In particular, IOP is known to vary throughout the 24-h period of a day, defined as a nyctohemeral rhythm in humans. In clinical practice, it is crucial to evaluate the changes in IOP over 24 h in several situations, including the diagnosis of ocular hypertension and glaucoma (IOP is often higher at night) and to optimize the therapeutic management of glaucoma. Until recently, all evaluations of 24-h IOP rhythm were performed using repeated IOP measurements, requiring individuals to be awakened for nocturnal measurements. This method may be imperfect, because it is not physiologic and disturbs the sleep architecture, and also because it provides a limited number of time point measurements not sufficient to finely asses IOP changes. These limitations may have biased previous descriptions of physiological IOP rhythm. Recently, extraocular and intraocular devices integrating a pressure sensor for continuous IOP monitoring have been developed and are available for use in humans. The objective of this article is to present the contributions of these new 24-h monitoring devices for the study of the nyctohemeral rhythms. In healthy subjects and untreated glaucoma subjects, a nyctohemeral rhythm is consistently found and frequently characterized by a mean diurnal IOP lower than the mean nocturnal IOP, with a diurnal bathyphase - usually in the middle or at the end of the afternoon - and a nocturnal acrophase, usually in the middle or at the end of the night.

Functional Anatomy of the Outflow Facilities

In order to understand the pathophysiology, select optimal therapeutic options for patients and provide clients with honest expectations for cases of canine glaucoma, clinicians should be familiar with a rational understanding of the functional anatomy of the ocular structures involved in this group of diseases. The topographical extension and the structural and humoral complexity of the regions involved with the production and the outflow of aqueous humor undergo numerous changes with aging and disease. Therefore, the anatomy relative to the fluid dynamics of aqueous has become a pivotal yet flexible concept to interpret the different phenotypes of glaucoma.

NMR spectroscopy of human eye tissues: a new insight into ocular biochemistry

Background. The human eye is a complex organ whose anatomy and functions has been described very well to date. Unfortunately, the knowledge of the biochemistry and metabolic properties of eye tissues varies. Our objective was to reveal the biochemical differences between main tissue components of human eyes. Methods. Corneas, irises, ciliary bodies, lenses, and retinas were obtained from cadaver globes 0-1/2 hours postmortem of 6 male donors (age: 44–61 years). The metabolic profile of tissues was investigated with HR MAS ¹H NMR spectroscopy. Results. A total of 29 metabolites were assigned in the NMR spectra of the eye tissues. Significant differences between tissues were revealed in contents of the most distant eye-tissues, while irises and ciliary bodies showed minimal biochemical differences. ATP, acetate, choline, glutamate, lactate, myoinositol, and taurine were identified as the primary biochemical compounds responsible for differentiation of the eye tissues. Conclusions. In this study we showed for the first time the results of the analysis of the main human eye tissues with NMR spectroscopy. The biochemical contents of the selected tissues seemed to correspond to their primary anatomical and functional attributes, the way of the delivery of the nutrients, and the location of the tissues in the eye.

Molecular dynamic simulations of ocular tablet dissolution

Small tablets for implantation into the subconjunctival space in the eye are being developed to inhibit scarring after glaucoma filtration surgery (GFS). There is a need to evaluate drug dissolution at the molecular level to determine how the chemical structure of the active may correlate with dissolution in the nonsink conditions of the conjunctival space. We conducted molecular dynamics simulations to study the dissolution process of tablets derived from two drugs that can inhibit fibrosis after GFS, 5-fluorouracil (5-FU) and the matrix metalloprotease inhibitor (MMPi), ilomastat. The dissolution was simulated in the presence of simple point charge (SPC) water molecules, and the liquid turnover of the aqueous humor in the subconjunctival space was simulated by removal of the dissolved drug molecules at regular intervals and replacement by new water molecules. At the end of the simulation, the total molecular solvent accessible surface area of 5-FU tablets increased by 60 times more than that of ilomastat as a result of tablet swelling and release of molecules into solution. The tablet dissolution pattern shown in our molecular dynamic simulations tends to correlate with experimental release profiles. This work indicates that a series of molecular dynamic simulations can be used to predict the influence of the molecular properties of a drug on its dissolution profile and could be useful during preformulation where sufficient amounts of the drug are not always available to perform dissolution studies.

Experimental research on intraocular aqueous flow by PIV method

BACKGROUND

Aqueous humor flows regularly from posterior chamber to anterior chamber, and this flow much involves intraocular pressure, the eye tissue nutrition and metabolism.

PURPOSE

To visualize and measure the intraocular flow regular pattern of aqueous humor.

METHOD

Intraocular flow in the vitro eyeball is driven to simulate the physiological aqueous humor flow, and the flow field is measured by Particle Image Velocimetry(PIV). Fluorescent particle solution of a certain concentration was infused into the root of Posterior Chamber(PC) of vitro rabbit eye to simulate the generation of aqueous and was drained out at a certain hydrostatic pressure from the angle of Anterior Chamber(AC) to represent the drainage of aqueous. PIV method was used to record and calculate the flow on the midsagittal plane of the eyeball.

RESULTS

Velocity vector distribution in AC has been obtained, and the distribution shows symmetry feature to some extent. Fluorescent particle solution first fills the PC as it is continuously infused, then surges into AC through the pupil, flows upwards toward the central cornea, reflecting and scattering, and eventually converges along the inner cornea surface towards the outflow points at the periphery of the eyeball. Velocity values around the pupillary margin are within the range of 0.008-0.012 m/s, which are close to theoretical values of 0.0133 m/s, under the driving rate of 100 μl/min.

CONCLUSIONS

Flow field of aqueous humor can be measured by PIV method, which makes it possible to study the aqueous humor dynamics by experimental method. Our study provides a basis for experimental research on aqueous humor flow; further, it possibly helps to diagnose and treat eye diseases as shear force damage of ocular tissues and destructions on corneal endothelial cells from the point of intraocular flow field.

Recreating a human trabecular meshwork outflow system on microfabricated porous structures

Glaucoma is the leading cause of irreversible blindness, resulting from an increase in intraocular pressure (IOP). IOP is the only modifiable risk factor of glaucoma and is controlled by the outflow of the aqueous humor through the human trabecular meshwork (HTM). Currently, the lack of a proper in vitro HTM model impedes advances in understanding outflow physiology and discovering effective IOP-lowering anti-glaucoma therapeutics. Therefore, we designed and constructed an in vitro HTM model using micropatterned, porous SU-8 scaffolds, which support cells to recapitulate functional HTM morphology and allow the study of outflow physiology. The pore size of SU-8 scaffolds, surface coating, cell seeding density, and culture duration were evaluated for HTM cell growth. The bioengineered HTM was characterized by F-actin staining and immunocytochemistry of HTM markers. A stand-alone perfusion chamber with an integrated pressure sensing system was further constructed and used for the investigation of the outflow facility of the bioengineered HTM treated with latrunculin B-an IOP lowering agent. Cells in the in vitro model exhibited HTM-like morphology, expression of α-smooth muscle actin, myocilin, and αß-crystallin, outflow characteristics and drug responsiveness. Altogether, we have developed an in vitro HTM model system for understanding HTM cell biology and screening of pharmacological or biological agents that affect trabecular outflow facility, expediting discovery of IOP-lowering, anti-glaucoma therapeutics.

Melatonin and its analog 5-methoxycarbonylamino-N-acetyltryptamine potentiate adrenergic receptor-mediated ocular hypotensive effects in rabbits: significance for combination therapy in glaucoma

Melatonin is currently considered a promising drug for glaucoma treatment because of its ocular hypotensive and neuroprotective effects. We have investigated the effect of melatonin and its analog 5-methoxycarbonylamino-N-acetyltryptamine, 5-MCA-NAT, on β₂/α(2A)-adrenergic receptor mRNA as well as protein expression in cultured rabbit nonpigmented ciliary epithelial cells. Quantitative polymerase chain reaction and immunocytochemical assays revealed a significant β₂-adrenergic receptor downregulation as well as α(2A)-adrenergic receptor up-regulation of treated cells (P < 0.001, maximal significant effect). In addition, we have studied the effect of these drugs upon the ocular hypotensive action of a nonselective β-adrenergic receptor (timolol) and a selective α₂-adrenergic receptor agonist (brimonidine) in normotensive rabbits. Intraocular pressure (IOP) experiments showed that the administration of timolol in rabbits pretreated with melatonin or 5-MCA-NAT evoked an additional IOP reduction of 14.02% ± 5.8% or 16.75% ± 5.48% (P < 0.01) in comparison with rabbits treated with timolol alone for 24 hours. Concerning brimonidine hypotensive action, an additional IOP reduction of 29.26% ± 5.21% or 39.07% ± 5.81% (P < 0.001) was observed in rabbits pretreated with melatonin or 5-MCA-NAT when compared with animals treated with brimonidine alone for 24 hours. Additionally, a sustained potentiating effect of a single dose of 5-MCA-NAT was seen in rabbits treated with brimonidine once daily for up 4 days (extra IOP decrease of 15.57% ± 5.15%, P < 0.05, compared with brimonidine alone). These data confirm the indirect action of melatoninergic compounds on adrenergic receptors and their remarkable effect upon the ocular hypotensive action mainly of α₂-adrenergic receptor agonists but also of β-adrenergic antagonists.

Characterisation of ilomastat for prolonged ocular drug release

We are developing tablet dosage forms for implantation directly into the subconjunctival space of the eye. The matrix metalloproteinase inhibitor, ilomastat, has previously been shown to be efficacious at suppressing scarring following glaucoma filtration surgery (GFS). We report on the physical characterisation of ilomastat which is being developed for ocular implantation. Since ilomastat is being considered for implantation it is necessary to examine its polymorphs and their influence on aspects of the in vitro drug release profile. X-ray powder diffraction identified two polymorphs of ilomastat from different commercial batches of the compound. Tablets were prepared from the two different polymorphs. Isothermal perfusion calorimetry was used to show that amorphous content is not increased during tablet formulation. The melting points of the two polymorphs are 188 and 208°C as determined by differential scanning calorimetry. Utilising single crystal X-ray diffraction, the structural conformations and packing arrangements of the different polymorphs were determined. The orthorhombic crystal crystallised as a monohydrate while the second monoclinic crystal form is non-solvated. Ilomastat tablets prepared from the two different solid forms exhibited similar drug release profiles in vitro under conditions mimicking the aqueous composition, volume and flow of the subconjunctival space after GFS. This suggests that a reproducible dose at each time point during release after implantation should be achievable in vivo with ilomastat tablets prepared from the two polymorphs identified.

Simulating intravitreal injections in anatomically accurate models for rabbit, monkey, and human eyes

Purpose

To develop models for rabbit, monkey, and human that enable prediction of the clearance after intravitreal (IVT) injections in one species from experimental results obtained in another species.

Methods

Anatomically accurate geometric models were constructed for rabbit, monkey, and human that enabled computational fluid dynamic simulation of clearance of an IVT injected bolus. Models were constructed with and without the retrozonular space of Petit. Literature data on clearance after IVT injection of substances spanning a range of molecular weight up to 157 kDa were used to validate the rabbit model.

Results

The space of Petit had a significant increase on the clearance of slowly diffusing substances cleared by the anterior pathway by reducing the bottleneck for drug efflux. Models that did not include this zone could not accurately predict the clearance of slowly diffusing substances whose clearance was accelerated by intraocular pressure-driven convection.

Conclusions

The ocular anatomy must be carefully reconstructed in the model to enable accurate predictions of clearance. This method offers an alternative means for scaling experimental data from one species to another that may be more appropriate than other simple approaches based entirely upon scaling of compartment volumes and flow rates.

Involvement of carbonic anhydrases in the ocular hypotensive effect of melatonin analogue 5-MCA-NAT

We have previously demonstrated that melatonin and its analogue, 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT), reduce intraocular pressure (IOP) in New Zealand rabbits. More recently, we have shown that 5-MCA-NAT can also regulate ciliary adrenoceptor gene expression. Like adrenoceptors, carbonic anhydrase (CA) enzymes are involved in aqueous humour secretion by the ocular ciliary epithelium. Moreover, CA enzymes have been reported to be regulated by melatonin. Hence, the aim of this study was to investigate whether the hypotensive effect of 5-MCA-NAT is also because of a regulation of CA genes and enzymes. Time course of 5-MCA-NAT effect on rabbit IOP was followed for 7 hr every day for up to 144 hr (6 days). 5-MCA-NAT reduced IOP, maximally by 51.30 ± 2.41% (at 3 hr), and the hypotensive effect was maintained for up to 96 hr with a single application. IOP studies with 5-MCA-NAT plus Trusopt(®) and immunohistochemical analysis confirmed that CA are molecular targets of 5-MCA-NAT. In addition, real-time quantitative PCR (qPCR) and immunocytochemical assays were performed to determine changes in CA2 (CAII) and CA12 (CAXII) expression in cultured rabbit nonpigmented ciliary epithelial cells (NPE) treated with 5-MCA-NAT. NPE cells showed a prominent decrease in both CA, at the mRNA and protein levels. These data confirm that the long-term hypotensive effect of 5-MCA-NAT is also due, to a down-regulation of CA2 (CAII) and CA12 (CAXII) expression.

Regulation of ocular adrenoceptor genes expression by 5-MCA-NAT: implications for glaucoma treatment

We have demonstrated that 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT), reduces intraocular pressure (IOP) in rabbits. In addition, we have reported a link between hypotensive effect of 5-MCA-NAT and sympathetic nervous system. Moreover, it is known that aqueous humour production is controlled by the activation of adrenoceptors (ADRs) present in the ocular ciliary epithelium. Thus, the aim of this study is to investigate if the hypotensive effect of 5-MCA-NAT is due to a regulation of ciliary ADR genes expression. To confirm this we followed the effect of 5-MCA-NAT on rabbit IOP for 144 consecutive hours. A sustained IOP reduction for up to 72 h (P<0.01) was seen. In addition, changes in ADRB2 and ADRA2A mRNA were measured in cultured rabbit nonpigmented ciliary epithelial cells. After 5-MCA-NAT treatment, a significant downregulation of ADRB2 and upregulation of ADRA2A was observed. These results provide the regulation of ADRs mRNA by 5-MCA-NAT.

Type 2 helper T-cell cytokines induce morphologic and molecular characteristics of atopic dermatitis in human skin equivalent

Both the immune system and the epidermis likely have an important role in the pathogenesis of atopic dermatitis (AD). The objective of the present study was to develop a human skin equivalent model exhibiting morphologic and molecular characteristics of AD in a controlled manner. Skin equivalents generated from normal adult human keratinocytes were stimulated with type 2 T-helper cell (Th2) cytokines IL-4 and IL-13, and morphologic features and gene expression of the epidermis were studied. Th2 cytokines induced intercellular edema similar to spongiotic changes observed in lesional AD as assessed at histopathologic analysis and electron microscopy. Furthermore, genes known to be specifically expressed in epidermis of patients with AD such as CAII and NELL2 were induced. In contrast, expression of psoriasis-associated genes such as elafin and hBD2 was not changed. Th2 cytokines caused DNA fragmentation in the keratinocytes, which could be inhibited by the caspase inhibitor Z-VAD, which suggests that apoptosis was induced. In addition, up-regulation of the death receptor Fas was observed in keratinocytes after Th2 cytokine stimulation. IL-4 and IL-13 induced phosphorylation of the signaling molecule STAT6. It was concluded that the skin equivalent model described herein may be useful in investigation of the epidermal aspects of AD and for study of drugs that act at the level of keratinocyte biology.

Intraocular pressure and aqueous humor flow during a euglycemic-hyperinsulinemic clamp in patients with type 1 diabetes and microvascular complications

BACKGROUND

Microvascular complications, including retinopathy and nephropathy are seen with type 1 diabetes. It is unknown whether functional changes in aqueous humor flow or intraocular pressure (IOP) develop in parallel with these complications. This study was designed to test the hypothesis that clinical markers of microvascular complications coexist with the alteration in aqueous humor flow and IOP.

METHODS

Ten patients with type 1 diabetes and ten healthy age- and weight-matched controls were studied. Aqueous flow was measured by fluorophotometry during a hyperinsulinemic-euglycemic clamp (insulin 2 mU/kg/min). Intraocular pressure was measured by tonometry at -10, 90 and 240 minutes from the start of the clamp, and outflow facility was measured by tonography at 240 minutes.

RESULTS

During conditions of identical glucose and insulin concentrations, mean aqueous flow was lower by 0.58 microl/min in the diabetes group compared to controls (2.58 +/- 0.65 versus 3.16 +/- 0.66 microl/min, respectively, mean +/- SD, p = 0.07) but statistical significance was not reached. Before the clamp, IOP was higher in the diabetes group (22.6 +/- 3.0 mm Hg) than in the control group (19.3 +/- 1.8 mm Hg, p = 0.01) but at 90 minutes into the clamp, and for the remainder of the study, IOP was reduced in the diabetes group to the level of the control group. Ocular pulse amplitude and outflow facility were not different between groups. Systolic blood pressure was significantly higher in the diabetes group, but diastolic and mean arterial pressures were not different.

CONCLUSIONS

We conclude that compared to healthy participants, patients with type 1 diabetes having microalbuminuria and retinopathy have higher IOPs that are normalized by hyperinsulinemia. During the clamp, a reduction in aqueous flow was not statistically significant.

Inhibition of aldose reductase attenuates endotoxin signals in human non-pigmented ciliary epithelial cells

Chronic inflammatory diseases such as autoimmune and bacterial infections are associated with an elevated risk of ocular inflammation. Ciliary epithelial cells that play an important role in maintaining aqueous humor dynamics and homeostasis of anterior segment of eye are continuously exposed to inflammatory markers during infections and injury. Lipopolysacchharide (LPS), a Gram-negative bacterial endotoxin, dysregulates aqueous humor (AqH) homeostasis by inducing inflammatory changes. We have investigated how inhibition of a polyol pathway enzyme, aldose reductase (AR), alters LPS-induced inflammatory changes in human non-pigmented ciliary epithelial cells (hNPECs). The stimulation of hNPECs with LPS (1 microg/ml) caused increased secretion of inflammatory markers such as PGE(2) and NO in the culture medium as well as increased expression of COX-2 and iNOS proteins in cell extracts. LPS also increased phosphorylation of MAPKs (ERK1/2) and SAPK/JNK and activation of redox-sensitive transcription factors NF-kappaB and AP-1 in hNPECs and inhibition of AR by zopolrestat and sorbinil ameliorated these changes. Further, LPS-induced decrease in the expression of Na/K-ATPase in hNPECs was restored by AR inhibitors. Similar results were observed in ciliary bodies of LPS-injected rats. Taken together, our results suggest that AR plays an important role in the LPS-induced inflammatory changes in hNPECs and that inhibition of AR could be a novel therapeutic approach for ocular inflammation.