Frequency-aware optical coherence tomography image super-resolution via conditional generative adversarial neural network.

Optical coherence tomography (OCT) has stimulated a wide range of medical image-based diagnosis and treatment in fields such as cardiology and ophthalmology. Such applications can be further facilitated by deep learning-based super-resolution technology, which improves the capability of resolving morphological structures. However, existing deep learning-based method only focuses on spatial distribution and disregards frequency fidelity in image reconstruction, leading to a frequency bias. To overcome this limitation, we propose a frequency-aware super-resolution framework that integrates three critical frequency-based modules (i.e., frequency transformation, frequency skip connection, and frequency alignment) and frequency-based loss function into a conditional generative adversarial network (cGAN). We conducted a large-scale quantitative study from an existing coronary OCT dataset to demonstrate the superiority of our proposed framework over existing deep learning frameworks. In addition, we confirmed the generalizability of our framework by applying it to fish corneal images and rat retinal images, demonstrating its capability to super-resolve morphological details in eye imaging.

Frequency-aware optical coherence tomography image super-resolution via conditional generative adversarial neural network.

Optical coherence tomography (OCT) has stimulated a wide range of medical image-based diagnosis and treatment in fields such as cardiology and ophthalmology. Such applications can be further facilitated by deep learning-based super-resolution technology, which improves the capability of resolving morphological structures. However, existing deep learning-based method only focuses on spatial distribution and disregard frequency fidelity in image reconstruction, leading to a frequency bias. To overcome this limitation, we propose a frequency-aware super-resolution framework that integrates three critical frequency-based modules (i.e., frequency transformation, frequency skip connection, and frequency alignment) and frequency-based loss function into a conditional generative adversarial network (cGAN). We conducted a large-scale quantitative study from an existing coronary OCT dataset to demonstrate the superiority of our proposed framework over existing deep learning frameworks. In addition, we confirmed the generalizability of our framework by applying it to fish corneal images and rat retinal images, demonstrating its capability to super-resolve morphological details in eye imaging.

Biodegradable Microspheres and Hydrogel Drug Delivery System of Tumor Necrosis Factor (TNF) Inhibitor and Growth Differentiation Factor 5 (GDF5) Reduces Disk Inflammation in the Rabbit Model.

STUDY DESIGN: Preclinical study. OBJECTIVE: Develop and test a drug delivery system (DDS) composed of anti-inflammatories and growth factors in the rabbit disk injury model. SUMMARY OF BACKGROUND DATA: Biological therapies that inhibit inflammation or enhance cell proliferation can alter intervertebral disk (IVD) homeostasis to favor regeneration. As biological molecules have short half-lives and one molecule may not cover multiple disease pathways, effective treatments may require a combination of growth factors and anti-inflammatory agents delivered in a sustained manner. MATERIALS AND METHODS: Biodegradable microspheres were generated separately to encapsulate tumor necrosis factor alpha (TNFα) inhibitors [etanercept (ETN)] or growth differentiation factor 5 (GDF5) and were embedded into a thermoresponsive hydrogel. Release kinetics and activity of ETN and GDF5 were measured in vitro . For in vivo testing, New Zealand White rabbits (n=12) underwent surgery for disk puncture and treatment with blank-DDS, ETN-DDS, or ETN+GDF5-DDS at levels L34, L45, and L56. Radiographic and magnetic resonance images of the spines were obtained. The IVDs were isolated for histologic and gene expression analyses. RESULTS: ETN and GDF5 were encapsulated into poly (L-lactide-co-glycolide) microspheres and had average initial bursts of 2.4±0.1 and 11.2±0.7 µg from DDS, respectively. In vitro studies confirmed that ETN-DDS inhibited TNFα-induced cytokine release and GDF5-DDS induced protein phosphorylation. In vivo studies showed that rabbit IVDs treated with ETN+GDF5-DDS had better histologic outcomes, higher levels of extracellular, and lower levels of inflammatory gene expression than IVDs treated with blank-DDS or ETN-DDS. CONCLUSIONS: This pilot study demonstrated that DDS can be fabricated to deliver sustained and therapeutic dosages of ETN and GDF5. In addition, ETN+GDF5-DDS may have greater anti-inflammatory and regenerative effects than ETN-DDS alone. Thus, intradiscal injection of controlled release TNF-α inhibitors and growth factors may be a promising treatment to reduce disk inflammation and back pain.

Fluorescein videoangiography data analysis protocol for mapping retinal vascular permeability in humans.

Fluorescein video angiographies (FVAs) are a diagnostic tool for eye diseases, such as diabetic retinopathy (DR). Currently, kinetic tracer model methods based on indicator-dilutions theory use FVAs to extract biomarkers (e.g., volumetric blood flow and retinal vascular permeability) via pixel mapping using two-step non-linear least square fitting. Prior to biomarker extraction, the FVAs must attain optimal quality. The objective of this research is to create a program to remove all frames experiencing signal drops (causes include blinking, squinting, and head movement). 15 FVAs (6 healthy control subjects, 6 diabetes mellitus no DR (DMnoDR) subjects, and 3 mild non-proliferative DR (NPDR) subjects) were analyzed for low quality frames. The average signal of each frame was analyzed as top, middle, and bottom thirds. The frame with maximum average signal up to the final frame of a created "Gold Standard" was compared with the raw AVI's frame with maximum average signal and subsequent frames. All frames before maximum average signal and any remaining frames were compared with the previous good-quality raw frame to determine if the frame of interest was of good quality. All remaining frames were subsequently re-evaluated and flagged if they had a local minimum prominence of 10% of the maximum average signal. The flagged frames', as well as former and subsequent frames', quality were subjectively determined. The AVI quality was subsequently tested via pre-DTKM processing and biomarker extraction via DTKM methods. Results displayed that the semi-automated frame removal process provides sufficient quality AVIs.

Simultaneous Release of Aflibercept and Dexamethasone from an Ocular Drug Delivery System.

PURPOSE: Intravitreal injections of anti-vascular endothelial growth factors (anti-VEGF) are the current standard of care for patients with choroidal neovascularization (CNV) secondary to age-related macular degeneration (AMD). There is a growing subset of patients that does not respond to anti-VEGF monotherapy treatment. Some patients, however, do respond to combination therapy of corticosteroids and anti-VEGF. This treatment requires monthly/bimonthly injections of anti-VEGF and semi-annual injections of corticosteroid. A drug delivery system (DDS) that simultaneously releases multiple drugs could benefit these patients by reducing the number of injections. The purpose of this study was to characterize the simultaneous release of aflibercept and dexamethasone from a biodegradable microparticle- and nanoparticle-hydrogel DDS. METHODS: Dexamethasone-loaded nanoparticles and aflibercept-loaded microparticles were created using modified single- and double-emulsion techniques, respectively. Then, microparticles and nanoparticles were embedded into a thermoresponsive, biodegradable poly(ethylene glycol)-co-(L-lactic acid) diacrylate (PEG-PLLA-DA)-N-isopropylacrylamide (NIPAAm) hydrogel DDS. Drug release studies and characterization of DDS were conducted with varying doses of microparticles and nanoparticles. RESULTS: The combination aflibercept-loaded microparticle- and dexamethasone-loaded nanoparticle- hydrogel (Combo-DDS) achieved a total release time of 224 days. Small decreases were seen in swelling ratio and equilibrium water content for Combo-DDS compared to monotherapy aflibercept-loaded microparticle-hydrogel DDS (AFL-DDS) and monotherapy dexamethasone-loaded nanoparticle-hydrogel DDS (DEX-DDS). Bioactivity of aflibercept was maintained in Combo-DDS compared to AFL-DDS. CONCLUSIONS: The Combo-DDS was able to extend and control the release of both aflibercept and dexamethasone simultaneously from a single DDS. This may eliminate the need for separate dosing regiments of anti-VEGF and corticosteroids for wet AMD patients.

Optimizing the protocol for retinal vascular permeability mapping from fluorescein videoangiography data.

An intact blood-retinal barrier is critical to maintaining the function of the retina. Many diseases of the eye have been directly associated with impairment in vascular permeability, and methods to measure vascular permeability could offer a window into early detection of disease; however, there exist no direct measures of vascular permeability that have be translated to the clinic. This work details a complete clinical workflow to quantify vascular permeability and volumetric blood flow from fluorescein videoangiography data, with validation through realistic simulations. For optimizing the protocol, this study carried on frame rate of fluorescein videoangiography to generate a high-resolution image while minimizing the error.

Compartmental and COMSOL Multiphysics 3D Modeling of Drug Diffusion to the Vitreous Following the Administration of a Sustained-Release Drug Delivery System.

The purpose of this study was to examine antibiotic drug transport from a hydrogel drug delivery system (DDS) using a computational model and a 3D model of the eye. Hydrogel DDSs loaded with vancomycin (VAN) were synthesized and release behavior was characterized in vitro. Four different compartmental and four COMSOL models of the eye were developed to describe transport into the vitreous originating from a DDS placed topically, in the subconjunctiva, subretinally, and intravitreally. The concentration of the simulated DDS was assumed to be the initial concentration of the hydrogel DDS. The simulation was executed over 1500 and 100 h for the compartmental and COMSOL models, respectively. Based on the MATLAB model, topical, subconjunctival, subretinal and vitreous administration took most (~500 h to least (0 h) amount of time to reach peak concentrations in the vitreous, respectively. All routes successfully achieved therapeutic levels of drug (0.007 mg/mL) in the vitreous. These models predict the relative build-up of drug in the vitreous following DDS administration in four different points of origin in the eye. Our model may eventually be used to explore the minimum loading dose of drug required in our DDS leading to reduced drug use and waste.

Treatment Efficacy and Biocompatibility of a Biodegradable Aflibercept-Loaded Microsphere-Hydrogel Drug Delivery System.

Purpose: To evaluate the in vivo treatment efficacy and biocompatibility of a biodegradable aflibercept-loaded microsphere-hydrogel drug delivery system (DDS) in a laser-induced choroidal neovascularization (CNV) rat model. Methods: Two weeks after CNV induction, animals were randomly assigned into four experimental groups: (1) no treatment, (2) single intravitreal (IVT) injection of blank DDS, (3) bimonthly bolus IVT aflibercept injections, and (4) single IVT injection of aflibercept-DDS. CNV lesion sizes were monitored longitudinally using fluorescence angiography and multi-Otsu thresholding for 6 months. For safety and biocompatibility assessment, an additional three non-CNV animals received a blank DDS injection. Electroretinogram, intraocular pressure, and clinical ophthalmoscopic examinations were performed. Results: The average lesion areas at week 0 (treatment intervention) were (1) 8693 ± 628 µm2 for no treatment, (2) 8261 ± 709 µm2 for blank DDS, (3) 10,368 ± 885 µm2 for bolus, and (4) 10,306 ± 1212 µm2 for aflibercept-DDS. For the nontreated groups, CNV lesion size increased by week 2 and remained increased throughout the study. The treated groups exhibited CNV size reduction after week 2 and remained for 6 months. At week 22, the average percent changes in CNV lesion area were +38.87% ± 7.08%, +34.19% ± 9.93%, -25.95% ± 3.51%, and -32.69% ± 5.40% for the above corresponding groups. No signs of chronic inflammation and other ocular abnormalities were found. Conclusions: The aflibercept-DDS was effective in treating CNV lesions for 6 months and is safe, well tolerated, and biocompatible. Translational Relevance: The proposed DDS is a promising system to reduce IVT injection frequency for anti-vascular endothelial growth factor treatment.

Safety and Biocompatibility of Aflibercept-Loaded Microsphere Thermo-Responsive Hydrogel Drug Delivery System in a Nonhuman Primate Model.

Purpose: To evaluate the safety and tolerability of a microsphere thermo-responsive hydrogel drug delivery system (DDS) loaded with aflibercept in a nonhuman primate model. Methods: A sterile 50 µL of aflibercept-loaded microsphere thermo-responsive hydrogel-DDS (aflibercept-DDS) was injected intravitreally into the right eye of 10 healthy rhesus macaques. A complete ophthalmic examination, intraocular pressure (IOP) measurement, fundus photography, spectral-domain optical coherence tomography (SD-OCT), and electroretinogram were performed monthly for 6 months. One macaque was euthanized monthly, and the enucleated eyes were submitted for measurement of bioactive aflibercept concentrations. Four eyes were submitted for histopathology. Results: Injected aflibercept-DDS was visualized in the vitreous until 6 months postinjection. No abnormalities were observed in the anterior segment, and IOP remained within normal range during the study period. A small number of cells were observed in the vitreous of some macaques, but otherwise the remainder of the posterior segment examination was normal. No significant changes in retinal architecture or function as assessed by SD-OCT and histology or full-field electroretinography, respectively, were observed. A mild, focal foreign body reaction around the injectate was observed with histology at 6 months postinjection. A mean of 2.1 ng/µL of aflibercept was measured in the vitreous. Conclusions: Intravitreally injected aflibercept-DDS achieved controlled, sustained release of aflibercept with no adverse effects for up to 6 months in the eyes of healthy rhesus macaques. Translational Relevance: Aflibercept-DDS may be a more effective method to deliver bioactive antivascular endothelial growth factor agents than current practice by reducing the frequency of intravitreal injections and providing controlled drug release.

Advances in ocular drug delivery systems.

Recent advances in pharmacological agents have led to successful treatment of a variety of retinal diseases such as neovascular age-related macular degeneration (AMD), diabetic macular oedema (DMO), and retinal vascular occlusions (RVO). These treatments often require repeated drug injections for an extended period of time. To reduce these repeated treatment burdens, minimally invasive drug delivery systems are needed. An ideal therapy should maintain effective levels of drug for the intended duration of treatment following a single application, recognising that a significant number of months of therapy may be required. There are numerous approaches under investigation to improve treatment options. This review will highlight the advantages and limitations of selected drug delivery systems of novel biomaterial implants and depots. The main emphasis will be placed on less invasive, longer acting, sustained release formulations for the treatment of retinal disorders.

Controlled Release of Vancomycin From a Thermoresponsive Hydrogel System for the Prophylactic Treatment of Postoperative Acute Endophthalmitis.

PURPOSE: To investigate the efficacy of a poly(ethylene glycol) diacrylate and poly(N-isopropylacrylamide) based thermo-responsive hydrogel drug delivery system (DDS) to deliver prophylactic vancomycin (VAN) following ocular surgery. METHODS: VAN was encapsulated in a hydrogel DDS and characterized in terms of initial burst, release kinetics, bioactivity, and cytotoxicity. Long-Evans rats received an intravitreal injection of Staphylococcus aureus to produce acute endophthalmitis in four experimental groups. One of four treatments were then applied: (1) bolus subconjunctival injection of VAN, (2) blank DDS, (3) saline treatment, and (4) subconjunctival injection of VAN DDS. Animals were scored for infection (0-3) at 12, 24, 48, and 72 hours, and eyes were harvested at 24 and 48 hours for histology. RESULTS: Following a 36% initial burst, VAN release from the DDS continued at a steady rate for 2 weeks plateauing at 84% after 504 hours. Bioactivity was maintained for all release samples and cytotoxicity analysis for the DDS revealed cell viability >90%. Not until after 12 hours did any of the groups show evidence of infection; however, at 24 hours, animals that received the VAN DDS had significantly lower infection scores (0 ± 0) than those that received a bolus VAN injection, blank DDS, or saline (1.5 ±1.5, 2.3 ± 0.87, and 2.9 ± 0.25; respectively). At 48 and 72 hours, the VAN DDS and bolus VAN treatment groups performed comparably and showed significantly better infection scores than the control groups. CONCLUSIONS: This DDS appears to have promise as a vehicle for short term, prophylactic antibiotic delivery. TRANSLATIONAL RELEVANCE: This DDS may prevent the development of postoperative endophthalmitis.

Characterization of Biodegradable Microsphere-Hydrogel Ocular Drug Delivery System for Controlled and Extended Release of Ranibizumab.

PURPOSE: To characterize a biodegradable microsphere-hydrogel drug delivery system (DDS) for controlled and extended release of ranibizumab. METHODS: The degradable microsphere-hydrogel DDSs were fabricated by suspending ranibizumab-loaded or blank poly(lactic-co-glycolic acid) microspheres within a poly(ethylene glycol)-co-(L-lactic-acid) diacrylate/N-isopropylacrylamide (PEG-PLLA-DA/NIPAAm) hydrogel. The thermal responsive behavior of various DDS formulations was characterized in terms of volume phase transition temperature (VPTT) and swelling ratios changes from 22°C to 42°C. The mechanical properties were characterized using rheological methods. Degradability of hydrogels were also examined via wet weight loss. Finally, Iodine-125 was used to radiolabel ranibizumab for characterization of encapsulation efficiency and in vitro release. RESULTS: All DDS formulations investigated were injectable through a 28-gauge needle at room temperature. The VPTT increased with increase of cross-linker concentration. The swelling ratios decreased as temperature increased and were not influenced by presence of microspheres. Rheology data confirmed that increase of cross-linker concentration and microsphere loading made DDS stiffer. Increase of degradable cross-linker concentration facilitated hydrogel in vitro degradation. Controlled release of ranibizumab were achieved for investigated DDS formulations for 6 months; and increased degradable cross-linker concentration produced faster and more complete release. CONCLUSIONS: The biodegradable DDSs are suitable for sustained release of ranibizumab. Considering ease of injection, degradability and release of ranibizumab, DDS with 3 mM cross-linker concentration and less than 20 mg/mL microsphere loadings is more favorable for future application. TRANSLATIONAL RELEVANCE: The investigated DDS is promising for controlled and extended release of anti-VEGF therapeutics to achieve better treatment regimen in ocular neovascularizations.

Biodegradable Microsphere-Hydrogel Ocular Drug Delivery System for Controlled and Extended Release of Bioactive Aflibercept In Vitro.

PURPOSE: Current standard of care for neovascular eye diseases require repeated intravitreal bolus injections of anti-vascular endothelial growth factors (anti-VEGFs). The purpose of this study was to validate a degradable microsphere-thermoresponsive hydrogel drug delivery system (DDS) capable of releasing bioactive aflibercept in a controlled and extended manner for 6 months. MATERIALS AND METHODS: The DDS was fabricated by suspending aflibercept-loaded poly(lactic-co-glycolic acid) microspheres within a biodegradable poly(ethylene glycol)-co-(l-lactic acid) diacrylate/N-isopropylacrylamide (PEG-PLLA-DA/NIPAAm) thermoresponsive hydrogel. Encapsulation efficiency of DDSs and in vitro release profiles were characterized by iodine-125 radiolabeled aflibercept. The degradation of hydrogel was determined by dry weight changes. The cytotoxicity from degraded DDS byproducts was investigated by quantifying cell viability using LIVE/DEAD® assay. In addition, dot blot and enzyme-linked immunosorbent assay were used to determine the bioactivity of released drug. Finally, morphology of microspheres and hydrogel were investigated by cryo-scanning electron microscopy before and after thermal transformation. RESULTS: The microsphere-hydrogel DDS was capable of releasing bioactive aflibercept in a controlled and extended manner for 6 months. The amount and rate of aflibercept release can be controlled by both the cross-linker concentration and microspheres load amount. The initial burst (release within 24 h) was from 37.35 ± 4.92 to 74.56 ± 6.16 µg (2 and 3 mM hydrogel, each loaded with 10 and 20 mg/ml of microspheres, respectively), followed by controlled drug release of 0.07-0.15 µg/day. Higher PEG-PLLA-DA concentration (3 mM) degraded faster than the lower concentration (2 mM). No significant cytotoxicity from degraded DDS byproducts was found for all investigated time points. Bioactivity of released drug was maintained at therapeutic level over entire release period. CONCLUSIONS: The microsphere-hydrogel DDS is safe and can deliver bioactive aflibercept in a controlled manner. This may provide a significant advantage over current bolus injection therapies in the treatment of ocular neovascularization.

In Vivo Efficacy of an Injectable Microsphere-Hydrogel Ocular Drug Delivery System.

PURPOSE: Demonstrate in vivo that controlled and extended release of a low dose of anti-vascular endothelial growth factor (anti-VEGF) from a microsphere-hydrogel drug delivery system (DDS) has a therapeutic effect in a laser-induced rat model of choroidal neovascularization (CNV). METHODS: Anti-VEGF (ranibizumab or aflibercept) was loaded into poly(lactic-co-glycolic acid) microspheres that were then suspended within an injectable poly(N-isopropylacrylamide)-based thermo-responsive hydrogel DDS.The DDS was shown previously to release bioactive anti-VEGF for ~200 days. CNV was induced using an Ar-green laser. The four experimental groups were as follows: (i) non-treated, (ii) drug-free DDS, (iii) anti-VEGF-loaded DDS, and (iv) bolus injection of anti-VEGF. CNV lesion areas were measured based on fluorescein angiograms and quantified using a multi-Otsu thresholding technique. Intraocular pressure (IOP) and dark-adapted electroretinogram (ERG) were also obtained pre- and post-treatment (1, 2, 4, 8, and 12 weeks). RESULTS: The anti-VEGF-loaded DDS group had significantly smaller (60%) CNV lesion areas than non-treated animals throughout the study. A small transient increase in IOP was seen immediately after injection; however, all IOP measurements at all time points were within the normal range. There were no significant changes in ERG maximal response compared to pre-treatment measurements for the drug-loaded DDS, which suggests no adverse effects on retinal cellular function. CONCLUSIONS: The current study demonstrates that the DDS can effectively decrease laser-induced CNV lesions in a murine model. Controlled and extended release from our DDS achieved greater treatment efficacy using an order of magnitude less drug than what is required with bolus administration. This suggests that our DDS may provide a significant advantage in the treatment of posterior segment eye diseases.

Extended ocular drug delivery systems for the anterior and posterior segments: biomaterial options and applications.

INTRODUCTION: The development of new therapies for treating various eye conditions has led to a demand for extended release delivery systems, which would lessen the need for frequent application while still achieving therapeutic drug levels in the target tissues. Areas covered: Following an overview of the different ocular drug delivery modalities, this article surveys the biomaterials used to develop sustained release drug delivery systems. Microspheres, nanospheres, liposomes, hydrogels, and composite systems are discussed in terms of their primary materials. The advantages and disadvantages of each drug delivery system are discussed for various applications. Recommendations for modifications and strategies for improvements to these basic systems are also discussed. Expert opinion: An ideal sustained release drug delivery system should be able to encapsulate and deliver the necessary drug to the target tissues at a therapeutic level without any detriment to the drug. Drug encapsulation should be as high as possible to minimize loss and unless it is specifically desired, the initial burst of drug release should be kept to a minimum. By modifying various biomaterials, it is possible to achieve sustained drug delivery to both the anterior and posterior segments of the eye.

Controlled and Extended In Vitro Release of Bioactive Anti-Vascular Endothelial Growth Factors from a Microsphere-Hydrogel Drug Delivery System.

PURPOSE: To demonstrate controlled and extended release of bioactive anti-vascular endothelial growth factor (VEGF) agents (ranibizumab or aflibercept) from an injectable microsphere-hydrogel drug delivery system (DDS). METHODS: Anti-VEGF agents were radiolabeled with iodine-125 and loaded into poly(lactic-co-glycolic acid) (PLGA) 75:25 microspheres using a modified double-emulsion, solvent evaporation technique. Microspheres were then suspended in an injectable poly(N-isopropylacrylamide)-based thermo-responsive hydrogel to create a microsphere-hydrogel DDS. Release profiles were performed in phosphate buffered saline at 37°C and at predetermined intervals, release samples were collected. Microspheres were also made using non-radiolabeled anti-VEGFs to determine the bioactivity of the DDS throughout release. Bioactivity and cytotoxicity of release samples were determined using human umbilical vascular endothelial cells (HUVECs) under VEGF-induced proliferation. RESULTS: The DDS is capable of releasing either ranibizumab or aflibercept for 196 days with an initial burst (first 24 h) of 22.2 ± 2.2 and 13.1 ± 0.5 µg, respectively, followed by controlled release of 0.153 and 0.065 µg/day, respectively. Release samples showed no toxicity in HUVECs at any time. Both anti-VEGFs remained bioactive throughout release with significant inhibition of HUVEC proliferation compared to the drug-free DDS, which showed no inhibitory effect on HUVEC proliferation. CONCLUSIONS: Controlled, extended, and bioactive release for approximately 200 days was achieved for both ranibizumab and aflibercept in vitro. The use of anti-VEGF-loaded microspheres suspended within an injectable, thermo-responsive hydrogel may be an advantageous ocular DDS with the potential to improve upon current therapies.

Thermo-Responsive Hydrogels for Ocular Drug Delivery.

Hydrogel as an ocular drug delivery platform holds great potential. Hydrogels are three-dimensional, hydrophilic, polymeric networks capable of absorbing large amounts of water or biological fluids. They have the ability to swell in an aqueous solvent system, holding solvents within a cross-linked gel system for potential sustained delivery. Through manipulation of permeation and diffusion characteristics, they can retain hydrophobic and hydrophilic agents, small molecules and macromolecules. In addition, hydrogel can be combined with nano- or microspheres to enhance delivery capacity. The aqueous environment of hydrogels can also protect cells and pharmacological agents. Depending on the specific structure, they can be nondegradable or degradable in their application.

Quantitative retinal blood flow mapping from fluorescein videoangiography using tracer kinetic modeling.

Abnormal retinal blood flow (RBF) has been associated with numerous retinal pathologies, yet existing methods for measuring RBF predominantly provide only relative measures of blood flow and are unable to quantify volumetric blood flow, which could allow direct patient to patient comparison. This work presents a methodology based on linear systems theory and an image-based arterial input function to quantitatively map volumetric blood flow from standard fluorescein videoangiography data, and is therefore directly translatable to the clinic. Application of the approach to fluorescein retinal videoangiography in rats (4 control, 4 diabetic) demonstrated significantly higher RBF in 4-5 week diabetic rats as expected from the literature.

Controlled and Extended Release of a Model Protein from a Microsphere-Hydrogel Drug Delivery System.

In extended ocular drug delivery applications, it is necessary to exert control over the release characteristics of the drug. Design considerations must be made to limit the initial burst (IB) and ensure complete release of drug from the drug delivery system (DDS). In this study, ovalbumin was used as a model protein to explore the effects on release of polymer formulation and fabrication technique in poly(lactic-co-glycolic acid) (PLGA) microspheres. Furthermore, the effect on release of suspending these microspheres in an injectable, thermo-responsive poly(N-isopropylacrylamide)-based hydrogel was determined. To characterize release, ovalbumin was radiolabeled with iodine-125. Regardless of polymer formulation or fabrication technique, pulsatile release was achieved with a second burst occurring after ~70 days for microspheres alone. Suspending PLGA 75:25 microspheres within hydrogel reduced the IB by ~75%, delayed the second burst by 28 days, and extended release out to ~200 days with steadier, consistent release throughout compared to microspheres alone. The combined microsphere-hydrogel DDS remains injectable through small-gauge needles and may have many applications, namely ocular drug delivery to the posterior segment.

Inverse relationship between the intraretinal concentration of bioavailable nitric oxide and blood glucose in early experimental diabetic retinopathy.

PURPOSE: To directly measure in vivo retinal nitric oxide (NO) concentration in experimental early diabetic retinopathy and correlate measurements with blood glucose to determine how intraretinal NO changes with severity of diabetes. METHODS: Long-Evans rats were made diabetic with streptozotocin (STZ). Three weeks post STZ injection, intraretinal NO concentration profiles were recorded using a dual NO/electroretinogram microelectrode. Diabetic profiles were compared with profiles from healthy controls, healthy rats injected with the NO synthase inhibitor L-NG-nitroarginine methyl ester (L-NAME), and healthy rats that received acute glucose injections ("acute hyperglycemia"). The NO values at the retina/RPE boundary (100% retinal depth) and retinal surface (0% depth) were analyzed for correlation with blood glucose. RESULTS: The average NO concentrations in the outer retina, inner retina, and vitreous humor of mild diabetic rats (250-400 mg/dL) were significantly higher than controls by 73%, 47%, and 70%, respectively. The average NO concentrations in the outer retina, inner retina, and vitreous humor of severe diabetic rats (500-600 mg/dL) were lower than controls, with NO at 41%, 36%, and 36% of controls, respectively, similar to L-NAME-treated eyes (38%, 36%, 20% of control). The NO levels in moderate diabetic rats (400-500 mg/dL) and acute hyperglycemia rats were similar to controls. The NO was significantly and inversely correlated with blood glucose for diabetic rats at 100% depth (R = -0.91) and 0% depth (R = -0.79) but not for acute hyperglycemia rats. CONCLUSIONS: The higher-than-control level of NO in mild diabetic rats and lower-than-control level in severe diabetic rats show that severity of diabetes is an important factor when measuring the bioavailability of NO in diabetic retinopathy.