A Mechanistic Model of the Intravitreal Pharmacokinetics of Large Molecules and the Pharmacodynamic Suppression of Ocular Vascular Endothelial Growth Factor Levels by Ranibizumab in Patients with Neovascular Age-Related Macular Degeneration

Barcode lipids for absolute quantitation of liposomes in ocular tissues

Lipid-based drug formulations are promising systems for improving delivery of drugs to ocular tissues, such as retina. To develop lipid-based systems further, an improved understanding of their pharmacokinetics is required, but high-quality in vivo experiments require a large number of animals, raising ethical and economic questions. In order to expedite in vivo kinetic testing of lipid-based systems, we propose a barcode approach that is based on barcoding liposomes with non-endogenous lipids. We developed and evaluated a liquid-chromatography-mass spectrometry method to quantify many liposomes simultaneously in aqueous humor, vitreous, and neural retina at higher than ±20% precision and accuracy. Furthermore, we showed in vivo suitability of the method in pharmacokinetic evaluation of six different liposomes after their simultaneous injection into the rat vitreal cavity. We calculated pharmacokinetic parameters in vitreous and aqueous humor, quantified liposome concentrations in the retina, and quantitated retinal distribution of the liposomes in the rats. Compared to individual injections of the liposome formulations, the barcode-based study design enabled reduction of animal numbers from 72 to 12. We believe that the proposed approach is reliable and will reduce and refine ocular pharmacokinetic experiments with liposomes and other lipid-based systems.

Topical Solution for Retinal Delivery: Bevacizumab and Ranibizumab Eye Drops in Anti-Aggregation Formula (AAF) in Rabbits

PURPOSE

Wet age-related macular degeneration (AMD) is a blinding retinal disease. Monthly intravitreal anti-VEGF antibody injections of bevacizumab (off-label) and ranibizumab (FDA approved) are the standard of care. Antibody aggregation may interfere with ocular absorption/distribution. This study assessed topical delivery of dilute antibodies to the posterior segment of rabbit eyes using a novel anti-aggregation formula (AAF).

METHODS

Bevacizumab, or biosimilar ranibizumab was diluted to 5 mg/ml in AAF. All rabbits were dosed twice daily. Substudy 1 rabbits (bevacizumab, 100 µl eye drops): Group 1 (bevacizumab/AAF, n = 6); Group 2 (bevacizumab/PBS, n = 7) and Vehicle control (AAF, n = 1). Substudy 2 rabbits (ranibizumab biosimilar/AAF, 50 µl eye drops): (ranibizumab biosimilar/AAF, n = 8). At 14.5 days, serum was drawn from rabbits. Aqueous, vitreous and retina samples were recovered from eyes and placed into AAF aliquots. Tissue analyzed using AAF as diluent.

RESULTS

Bevacizumab in AAF permeated/accumulated in rabbit aqueous, vitreous and retina 10 times more, than when diluted in PBS. AAF/0.1% hyaluronic acid eye drops, dosed twice daily, provided mean tissue concentrations (ng/g) in retina (29.50), aqueous (12.34), vitreous (3.46), and serum (0.28 ng/ml). Additionally, the highest concentration (ng/g) of ranibizumab biosimilar was present in the retina (18.0), followed by aqueous (7.82) and vitreous (1.47). Serum concentration was negligible (< 0.04 ng/ml). No irritation was observed throughout the studies.

CONCLUSIONS

Bevacizumab and ranibizumab, in an AAF diluent eye drop, can be delivered to the retina, by the twice daily dosing of a low concentration mAb formulation. This may prove to be an adjunct to intravitreal injections.

Drug Distribution After Intravitreal Injection: A Mathematical Model

Purpose

Intravitreal injection of drugs is commonly used for treatment of chorioretinal ocular pathologies, such as age-related macular degeneration. Injection causes a transient increase in the intraocular volume and, consequently, of the intraocular pressure (IOP). The aim of this work is to investigate how intravitreal flow patterns generated during the post-injection eye deflation influence the transport and distribution of the injected drug.

Methods

We present mathematical and computational models of fluid motion and mass transport in the vitreous chamber during the transient phase after injection, including the previously unexplored effects of globe deflation as ocular volume decreases.

Results

During eye globe deflation, significant fluid velocities are generated within the vitreous chamber, which can possibly contribute to drug transport. Pressure variations within the eye globe are small compared to IOP.

Conclusions

Even if significant fluid velocities are generated in the vitreous chamber after drug injection, these are found to have negligible overall effect on drug distribution.

Computer Modeling of Bevacizumab Drug Distribution After Intravitreal Injection in Rabbit and Human Eyes

Age-related macular degeneration (AMD) is a condition brought on by macular deterioration caused primarily by inflammation and cell death in the retina. There is no cure for the disease and current treatments for advanced (wet) AMD rely on intravitreal injections of anti-vascular endothelial growth factor (anti-VEGF) therapeutics. One common off-label anti-VEGF drug used in AMD treatment is bevacizumab. There have been experimental efforts to investigate the pharmacokinetic (PK) behavior of bevacizumab in the vitreous and aqueous humor. Still the quantitative effect of elimination routes and drug concentration in the macula are not well understood. In our study, we developed two spatial models representing rabbit and human vitreous humor to better understand the PK behavior of bevacizumab. We explored convective effects on the vitreous while considering the anterior elimination alone or coupled with posterior elimination. We compared our models with available experimental data and calculated an approximate macula concentration. Our results show that both anterior and posterior elimination play a role in bevacizumab clearance from the eye. Furthermore, an effective bevacizumab concentration close to the macula region is maintained for shorter time periods when compared to the whole vitreous region. This model can improve knowledge and understanding of AMD treatment.

Experimental and mathematical approaches for drug delivery for the treatment of wet age-related macular degeneration

Several chronic eye diseases affect the posterior segment of the eye. Among them age-related macular degeneration can cause vision loss if left untreated and is one of the leading causes of visual impairment in the world. Most treatments are based on intravitreally injected therapeutics that inhibit the action of vascular endothelial growth factor. However, due to the need for monthly injections, this method is associated with poor patient compliance. To address this problem, numerous drug delivery systems (DDSs) have been developed. This review covers a selection of particulate systems, non-stimuli responsive hydrogels, implants, and composite systems that have been developed in the last few decades. Depending on the type of DDS, polymer material, and preparation method, different mechanical properties and drug release profiles can be achieved. Furthermore, DDS development can be optimized by implementing mathematical modeling of both drug release and pharmacokinetic aspects. Several existing mathematical models for diffusion-controlled, swelling-controlled, and erosion-controlled drug delivery from polymeric systems are summarized. Compartmental and physiologically based models for ocular drug transport and pharmacokinetics that have studied drug concentration profiles after intravitreal delivery or release from a DDS are also reviewed. The coupling of drug release models with ocular pharmacokinetic models can lead to obtaining much more efficient DDSs for the treatment of age-related macular degeneration and other diseases of the posterior segment of the eye.

Quantitative intravitreal pharmacokinetics in mouse as a step towards inter-species translation

Although mouse models are widely used in retinal drug development, pharmacokinetics in mouse eye is poorly understood. In this study, we applied non-invasive in vivo fluorophotometry to study pharmacokinetics of intravitreal fluorescein sodium (molecular weight 0.38 kDa) and fluorescein isothiocyanate-dextran (FD-150; molecular weight 150 kDa) in mice. Intravitreal half-lives of fluorescein and FD-150 in mouse eyes were 0.53 ± 0.06 h and 2.61 ± 0.86 h, respectively. These values are 8-230 times shorter than the elimination half-lives of similar compounds in the human vitreous. The apparent volumes of distribution in the mouse vitreous were close to the anatomical volume of the mouse vitreous (FD-150, 5.1 μl; fluorescein, 9.6 μl). Dose scaling factors were calculated from mouse to rat, rabbit, monkey and human translation. Based on pharmacokinetic modelling and compound concentrations in the vitreous and anterior chamber, fluorescein is mainly eliminated posteriorly across blood-retina barrier, but FD-150 is cleared via aqueous humour outflow. The results of this study improve the knowledge of intravitreal pharmacokinetics in mouse and facilitate inter-species scaling in ocular drug development.

Selective drug delivery to the retinal cells: Biological barriers and avenues

Retinal drug delivery is a challenging, but important task, because most retinal diseases are still without any proper therapy. Drug delivery to the retina is hampered by the anatomical and physiological barriers resulting in minimal bioavailability after topical ocular and systemic administrations. Intravitreal injections are current method-of-choice in retinal delivery, but these injections show short duration of action for small molecules and low target bioavailability for many protein, gene based drugs and nanomedicines. State-of-art delivery systems are based on prolonged retention, controlled drug release and physical features (e.g. size and charge). However, drug delivery to the retina is not cell-specific and these approaches do not facilitate intracellular delivery of modern biological drugs (e.g. intracellular proteins, RNA based medicines, gene editing). In this focused review we highlight biological factors and mechanisms that form the basis for the selective retinal drug delivery systems in the future. Therefore, we are presenting current knowledge related to retinal membrane transporters, receptors and targeting ligands in relation to nanomedicines, conjugates, extracellular vesicles, and melanin binding. These issues are discussed in the light of retinal structure and cell types as well as future prospects in the field. Unlike in some other fields of targeted drug delivery (e.g. cancer research), selective delivery technologies have been rarely studied, even though cell targeted delivery may be even more feasible after local administration into the eye.

Translational physiologically-based pharmacokinetic model for ocular disposition of monoclonal antibodies

We have previously published a PBPK model comprising the ocular compartment to characterize the disposition of monoclonal antibodies (mAbs) in rabbits. While rabbits are commonly used preclinical species in ocular research, non-human primates (NHPs) have the most phylogenetic resemblance to humans including the presence of macula in the eyes as well as higher sequence homology. However, their use in ocular research is limited due to the strict ethical guidelines. Similarly, in humans the ocular samples cannot be collected except for the tapping of aqueous humor (AH). Therefore, we have translated this rabbit model to monkeys and human species using literature-reported datasets. Parameters describing the tissue volumes, physiological flows, and FcRn-binding were obtained from the literature, or estimated by fitting the model to the data. In the monkey model, the values for the rate of lysosomal degradation for antibodies (Kdeg), intraocular reflection coefficients (σaq, σret, σcho), bidirectional rate of fluid circulation between the vitreous chamber and the aqueous chamber (QVA), and permeability-surface area product of lens (PSlens) were estimated; and were found to be 31.5 h-1, 0.7629, 0.6982, 0.9999, 1.64 × 10-5 L/h, and 4.62 × 10-7 L/h, respectively. The monkey model could capture the data in plasma, aqueous humor, vitreous humor and retina reasonably well with the predictions being within twofold of the observed values. For the human model, only the value of Kdeg was estimated to fit the model to the plasma pharmacokinetics (PK) of mAbs and was found to be 24.4 h-1 (4.14%). The human model could also capture the ocular PK data reasonably well with the predictions being within two- to threefold of observed values for the plasma, aqueous and vitreous humor. Thus, the proposed framework can be used to characterize and predict the PK of mAbs in the eye of monkey and human species following systemic and intravitreal administration. The model can also facilitate the development of new antibody-based therapeutics for the treatment of ocular diseases as well as predict ocular toxicities of such molecules following systemic administration.

Pseudophakia as a surprising protective factor in neovascular age-related macular degeneration

PURPOSE

To assess the impact of lens status on macular function among patients treated for neovascular age-related macular degeneration (nvAMD) in whom scheduled intravitreal injections were delayed.

METHODS

We reviewed demographic and clinical data as well as macular optical coherence tomographic images of 34 patients (48 eyes) who did not follow their injection schedule during the first wave of COVID-19 in Israel. Functional worsening was defined as a loss of at least 0.1 in decimal best-corrected visual acuity (BCVA). Morphological worsening was defined as new or increased subretinal/intraretinal fluid or a new hemorrhage. OCT indices of quality were used as a measure for cataract density and progression.

RESULTS

Pseudophakia was associated with a better functional outcome than phakic status: there was a loss of 0.06±0.12 vs. 0.15±0.10 decimal BCVA in the pseudophakic and phakic eyes, respectively (P=.001). A similar trend was observed for morphological changes over the same period: there was an increase in macular thickness of 9±26% vs.12±40%, respectively (P=0.79). During the first wave of COVID-19, the index of OCT quality remained stable for phakic eyes (26±3.6 before the first wave of COVID-19, 26±2.9 afterward; P=1) and pseudophakic eyes (30±2.4 before the first wave of COVID-19, 30±2.6 afterward; P=1).

CONCLUSION

Pseudophakic eyes with nvAMD that missed their scheduled intravitreal injections experienced fewer morphological and functional complications than phakic eyes with nvAMD.

How can machine learning and multiscale modeling benefit ocular drug development?

The eyes possess sophisticated physiological structures, diverse disease targets, limited drug delivery space, distinctive barriers, and complicated biomechanical processes, requiring a more in-depth understanding of the interactions between drug delivery systems and biological systems for ocular formulation development. However, the tiny size of the eyes makes sampling difficult and invasive studies costly and ethically constrained. Developing ocular formulations following conventional trial-and-error formulation and manufacturing process screening procedures is inefficient. Along with the popularity of computational pharmaceutics, non-invasive in silico modeling & simulation offer new opportunities for the paradigm shift of ocular formulation development. The current work first systematically reviews the theoretical underpinnings, advanced applications, and unique advantages of data-driven machine learning and multiscale simulation approaches represented by molecular simulation, mathematical modeling, and pharmacokinetic (PK)/pharmacodynamic (PD) modeling for ocular drug development. Following this, a new computer-driven framework for rational pharmaceutical formulation design is proposed, inspired by the potential of in silico explorations in understanding drug delivery details and facilitating drug formulation design. Lastly, to promote the paradigm shift, integrated in silico methodologies were highlighted, and discussions on data challenges, model practicality, personalized modeling, regulatory science, interdisciplinary collaboration, and talent training were conducted in detail with a view to achieving more efficient objective-oriented pharmaceutical formulation design.

Preclinical modeling of intravitreal suspensions

There are a number of obstacles that complicate the development of intravitreal delivered small molecules therapies. One serious complication is the potential need for complex polymer depot formulations early in the drug discovery process. The development of such formulations often requires substantial investment of time and material which may not be readily available in preclinical development. Herein I present a diffusion limited pseudo-steady state model to provide prediction of drug release from an intravitreally administered suspension formulation. By using such a model, a preclinical formulator may be able to more confidently determine if development of a complex formulation is required or if a simple suspension may work to support a study design. In this report, the model is used to predict the intravitreal preformance of two different molecules (triamcinolone acetonide and GNE-947) at multiple dose levels in rabbit eyes as well as provide a prediction for the performance of a marketed formulation of Trimacinolone Acetonide in humans.

Quantitative systems pharmacology of the eye: Tools and data for ocular QSP

Good eyesight belongs to the most-valued attributes of health, and diseases of the eye are a significant healthcare burden. Case numbers are expected to further increase in the next decades due to an aging society. The development of drugs in ophthalmology, however, is difficult due to limited accessibility of the eye, in terms of drug administration and in terms of sampling of tissues for drug pharmacokinetics (PKs) and pharmacodynamics (PDs). Ocular quantitative systems pharmacology models provide the opportunity to describe the distribution of drugs in the eye as well as the resulting drug-response in specific segments of the eye. In particular, ocular physiologically-based PK (PBPK) models are necessary to describe drug concentration levels in different regions of the eye. Further, ocular effect models using molecular data from specific cellular systems are needed to develop dose-response correlations. We here describe the current status of PK/PBPK as well as PD models for the eyes and discuss cellular systems, data repositories, as well as animal models in ophthalmology. The application of the various concepts is highlighted for the development of new treatments for postoperative fibrosis after glaucoma surgery.

Six-month sustained delivery of anti-VEGF from in-situ forming hydrogel in the suprachoroidal space

Patients with wet age-related macular degeneration (AMD) require intravitreal injections of bevacizumab (Bev) or other drugs, often on a monthly basis, which is a burden on the healthcare system. Here, we developed an in-situ forming hydrogel comprised of Bev and hyaluronic acid (HA) crosslinked with poly(ethylene glycol) diacrylate for slow release of Bev after injection into the suprachoroidal space (SCS) of the eye using a microneedle. Liquid Bev formulations were cleared from SCS within 5 days, even when formulated with high viscosity, unless Bev was conjugated to a high molecular-weight HA (2.6 MDa), which delayed clearance until 1 month. To extend release to 6 months, we synthesized in-situ forming Bev-HA hydrogel initially as a low-viscosity mixture suitable for injection and flow in the SCS to cover a large area extending to the posterior pole of the eye where the macula is located in humans. Within 1 h after injection, Bev and HA were crosslinked, which retained Bev for slow release as the hydrogel biodegraded. In vivo studies in the rabbit eye reported Bev release for >6 months, depending on gel formulation and Bev assay. The in-situ forming Bev-HA hydrogel was well tolerated, as assessed by clinical exam, fundus imaging, histological analysis, and intraocular pressure measurement. We conclude that Bev released from an in-situ forming hydrogel may enable long-acting treatments of AMD and other posterior ocular indications.

Evaluation of Molecular Properties versus In Vivo Performance of Aflibercept, Brolucizumab, and Ranibizumab in a Retinal Vascular Hyperpermeability Model

Purpose

To evaluate the molecular, pharmacokinetic, and pharmacological properties of three anti-vascular endothelial growth factor (VEGF) agents—aflibercept, brolucizumab, and ranibizumab—and to provide a prediction of the optimal design of an intravitreal VEGF challenge in rabbits to assess the preclinical in vivo activity of the different anti-VEGF agents.

Methods

Biochemical analyses and cellular and animal models of retinopathy were used to characterize anti-VEGF efficacy. Anti-VEGF biochemical binding affinity was determined through a kinetic exclusion assay. The in vitro potency was investigated by a calcium mobilization assay. Pharmacokinetic parameters were estimated for each drug to predict intraocular exposure relationships among the agents. The in silico modeling efforts informed the design of an in vivo rabbit model of VEGF-induced retinal hyperpermeability to determine the extent of VEGF neutralization in vivo. Consequently, data generated from the in vivo study enabled pharmacokinetic analysis and the generation of a logistical model describing the impact of the anti-VEGF agents on the VEGF-induced vascular leakage in rabbits.

Results

The three anti-VEGF agents ranked from most efficacious to least efficacious as aflibercept, brolucizumab, and ranibizumab, with results consistent and significant within each individual characterization experiment.

Conclusions

This composite study demonstrated how the molecular properties of aflibercept, brolucizumab, and ranibizumab translate into differences of in vivo efficacy, with results in line with the reported literature.

Translational Relevance

In silico, in vitro, and in vivo integrated studies provide information that enables the enhanced characterization of translational properties of anti-VEGF agents currently used for the treatment of retinal diseases.

Alternative Routes of Administration for Therapeutic Antibodies—State of the Art

Background: For the past two decades, there has been a huge expansion in the development of therapeutic antibodies, with 6 to 10 novel entities approved each year. Around 70% of these Abs are delivered through IV injection, a mode of administration allowing rapid and systemic delivery of the drug. However, according to the evidence presented in the literature, beyond the reduction of invasiveness, a better efficacy can be achieved with local delivery. Consequently, efforts have been made toward the development of innovative methods of administration, and in the formulation and engineering of novel Abs to improve their therapeutic index. Objective: This review presents an overview of the routes of administration used to deliver Abs, different from the IV route, whether approved or in the clinical evaluation stage. We provide a description of the physical and biological fundamentals for each route of administration, highlighting their relevance with examples of clinically-relevant Abs, and discussing their strengths and limitations. Methods: We reviewed and analyzed the current literature, published as of the 1 April 2022 using MEDLINE and EMBASE databases, as well as the FDA and EMA websites. Ongoing trials were identified using clinicaltrials.gov. Publications and data were identified using a list of general keywords. Conclusions: Apart from the most commonly used IV route, topical delivery of Abs has shown clinical successes, improving drug bioavailability and efficacy while reducing side-effects. However, additional research is necessary to understand the consequences of biological barriers associated with local delivery for Ab partitioning, in order to optimize delivery methods and devices, and to adapt Ab formulation to local delivery. Novel modes of administration for Abs might in fine allow a better support to patients, especially in the context of chronic diseases, as well as a reduction of the treatment cost.

Quantitative pharmacokinetic analyses of anterior and posterior elimination routes of intravitreal anti-VEGF macromolecules using published human and rabbit data

The purpose of this study was to evaluate the contribution of the anterior elimination route for four anti-vascular endothelial growth factor (anti-VEGF) macromolecules (aflibercept, bevacizumab, pegaptanib and ranibizumab) after intravitreal injection using published human and rabbit data and three previously described pharmacokinetic (PK) modeling methods. A PubMed search was used to identify published studies with concentration-time data. The data were utilized only if the intravitreally injected drugs were used as plain solutions and several criteria for a well-performed PK study were fulfilled. The three methods to analyze rabbit data were (1) the equation for vitreal elimination half-life based molecular size assuming anterior elimination, (2) Maurice equation and plot for the ratio of aqueous humor (AH) to vitreal concentration assuming anterior elimination, and (3) the equation for amount of macromolecule eliminated anteriorly based on the area under the curve in AH. The first and third methods were used for human data. In the second and third methods, AH flow rate is a key model parameter, and it was varied between 2 and 3 μl/min. The methods were applied to data from 9 rabbit studies (1 for aflibercept, 5 for bevacizumab, and 3 for ranibizumab) and 5 human studies (1 for aflibercept, 3 for bevacizumab, and 1 for ranibizumab). Experimental half-lives of anti-VEGF macromolecules in both vitreous and aqueous humor were close to those calculated with the equations for vitreal elimination half-life in humans and rabbits. Rabbit data analyzed with Maurice plot indicated that the contribution of anterior elimination was usually at least 75%. In most human and rabbit studies, the calculated percentage of anterior elimination was at least 51%. Variability between studies was extensive for bevacizumab and ranibizumab. The results suggest that the anterior elimination route dominates after intravitreal injection of anti-VEGF macromolecules. However, the clinical data are sparse and variability is extensive, the latter emphasizing the need of proper experimental design.

Effects of Flow Hydrodynamics and Eye Movements on Intraocular Drug Clearance

New in vitro prototypes (PK-Eye™) were tested with and without eye movement to understand diffusion and convection effects on intraocular clearance. Port placement in front ((i) ciliary inflow model) and behind the model lens ((ii) posterior inflow model) was used to study bevacizumab (1.25 mg/50 µL) and dexamethasone (0.1 mg/100 µL) in phosphate-buffered saline (PBS, pH 7.4) and simulated vitreal fluid (SVF). Dexamethasone was studied in a (iii) retinal-choroid-sclera (RCS) outflow model (with ciliary inflow and two outflow pathways). Ciliary vs. posterior inflow placement did not affect the half-life for dexamethasone at 2.0 µL/min using PBS (4.7 days vs. 4.8 days) and SVF (4.9 days with ciliary inflow), but it did decrease the half-life for bevacizumab in PBS (20.4 days vs. 2.4 days) and SVF (19.2 days vs. 10.8 days). Eye movement only affected the half-life of dexamethasone in both media. Dexamethasone in the RCS model showed approximately 20% and 75% clearance from the RCS and anterior outflows, respectively. The half-life of the protein was comparable to human data in the posterior inflow model. Shorter half-life values for a protein in a ciliary inflow model can be achieved with other eye movements. The RCS flow model with eye movement was comparable to human half-life data for dexamethasone.

Characterization and Validation of In Vitro and In Vivo Models to Investigate TNF-α-Induced Inflammation in Retinal Diseases

Purpose

Inflammation is implicated in the etiology of diverse retinopathies including uveitis, age-related macular degeneration or diabetic retinopathy. Tumor necrosis factor alpha (TNF-α) is a well-known proinflammatory cytokine that is described as a biomarker for inflammation in diverse retinopathies and therefore emerged as an interesting target to treat inflammation in the eye by neutralizing anti-TNF-α antibodies.

Methods

Recently, we have demonstrated that Adeno-associated virus (AAV)–mediated expression of human TNF-α in the murine eye induces retinal inflammation including vasculitis and fibrosis, thereby mimicking human disease-relevant pathologies. In a proof-of-mechanism study, we now tested whether AAV-TNF-α induced pathologies can be reversed by neutralizing TNF-α antibody treatment.

Results

Strikingly, a single intravitreal injection of the TNF-α antibody golimumab reduced AAV-TNF-α–induced retinal inflammation and retinal thickening. Furthermore, AAV-TNF-α–mediated impaired retinal function was partially rescued by golimumab as revealed by electroretinography recordings. Finally, to study TNF-α-induced vasculitis in human in vitro cell culture assays, we established a monocyte-to-endothelium adhesion co-culture system. Indeed, also in vitro TNF-α induced monocyte adhesion to human retinal endothelial cells, which was prevented by golimumab.

Conclusions

Overall, our study describes valuable in vitro and in vivo approaches to study the function of TNF-α in retinal inflammation and demonstrated a preclinical proof-of-mechanism treatment with golimumab.

Translational Relevance

The AAV-based model expressing human TNF-α allows us to investigate TNF-α–driven pathologies supporting research in mechanisms of retinal inflammation.

Yeast-produced fructosamine-3-kinase retains mobility after ex vivo intravitreal injection in human and bovine eyes as determined by Fluorescence Correlation Spectroscopy

Globally, over 2 billion people suffer from vision impairment. Despite complex multifactorial etiology, advanced glycation end products are involved in the pathogenesis of many causative age- and diabetes-related eye diseases. Deglycating enzyme fructosamine-3-kinase (FN3K) was recently proposed as a potential therapeutic, but for further biopharmaceutical development, knowledge on its manufacturability and stability and mobility in the vitreous fluid of the eye is indispensable. We evaluated recombinant production of FN3K in two host systems, and its diffusion behavior in both bovine and human vitreous. Compared to Escherichia coli, intracellular production in Pichia pastoris yielded more and higher purity FN3K. The yeast-produced enzyme was used in a first attempt to use fluorescence correlation spectroscopy to study protein mobility in non-sonicated bovine vitreous, human vitreous, and intact bovine eyes. It was demonstrated that FN3K retained mobility upon intravitreal injection, although a certain delay in diffusion was observed. Alkylation of free cysteines was tolerated both in terms of enzymatic activity and vitreous diffusion. Ex vivo diffusion data gathered and the availability of yeast-produced high purity enzyme now clear the path for in vivo pharmacokinetics studies of FN3K.

Translating pharmacology models effectively to predict therapeutic benefit

Many in vitro and in vivo models are used in pharmacological research to evaluate the role of targeted proteins in a disease. Understanding the translational relevance and limitation of these models for analyzing the disposition, pharmacokinetic/pharmacodynamic (PK/PD) profile, mechanism, and efficacy of a drug, is essential when selecting the most appropriate model of the disease of interest and predicting clinically efficacious doses of the investigational drug. Here, we review selected animal models used in ophthalmology, infectious diseases, oncology, autoimmune diseases, and neuroscience. Each area has specific challenges around translatability and determination of an efficacious dose: new patient-specific dosing methods could help overcome these limitations.

Anti-VEGF Drugs Dynamics: Relevance for Clinical Practice

Background: A drug and disease assessment model was used to evaluate the impact of different treatment regimens on intravitreal ranibizumab, bevacizumab, aflibercept, and brolucizumab concentrations and the proportion of free vascular endothelial growth factor (VEGF) to total VEGF. Methods: A time-dependent mathematical model using Wolfram Mathematica software was used. The pharmacokinetic and pharmacodynamic data for anti-VEGFs were obtained from published reports. The model simulated drug concentration after single and multiple doses of ranibizumab, bevacizumab, aflibercept, and brolucizumab, and it extrapolated time-dependent intraocular free VEGF proportion values. Various fixed treatment regimens (q4, q8, q10, q12) were simulated and evaluated as candidates for clinical utilization. Results: Our mathematical model shows good correlation between intraocular VEGF proportion values and clinical data. Simulations suggest that each anti-VEGF agent would allow for distinct treatment intervals to keep the proportion of free VEGF under threshold levels. Regimens scheduling q8 ranibizumab, q8 bevacizumab, q12 aflibercept, and q10 brolucizumab administration permit to maintain the proportion of unbound VEGF below 0.001%. Conclusions: Fixed q8 ranibizumab, q8 bevacizumab, q12 aflibercept, or q10 brolucizumab regimens may produce adequate intraocular VEGF inhibition.

Targeting Ocular Drug Delivery: An Examination of Local Anatomy and Current Approaches

Ocular drug delivery remains the focus of much modern research. Primary routes of administration include the surface, the intravitreal space, the subretinal space, and the subconjunctival space, each with its own series of unique challenges, limitations, and advantages. Each of these approaches requires careful consideration of the local anatomy, physical barriers, and key cells as well as the interface between the anatomy and the drug or drug system being delivered. While least invasive, the topical route poses a challenge with the many physical barriers that prevent drug penetration into the eye; while injection into the intravitreal, subretinal, and subconjunctival spaces are direct and targeted but limited due to the many internal clearance mechanisms and potential for damage to the eye. Polymeric-based, sustained-release drug delivery systems have been identified as a potential solution to many of these challenges; however, the design and successful implementation of a sustained-release system that is well-tolerated, bioactive, biocompatible, and degradable remains, in many cases, only in the early stages. The drugs and biomaterials in question also require special attention as small chemical changes could result in vastly different outcomes. This paper explores the anatomy and key cells of these four primary drug delivery routes as well as the interface between drug and drug delivery systems and the anatomy, reviewing the recent developments and current state of research in each area. Finally, this paper also examines the frequently used drugs and biomaterials found in ocular drug delivery and summarizes the primary interactions observed.

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.

Pharmacokinetics of Pullulan-Dexamethasone Conjugates in Retinal Drug Delivery

The treatment of retinal diseases by intravitreal injections requires frequent administration unless drug delivery systems with long retention and controlled release are used. In this work, we focused on pullulan (≈67 kDa) conjugates of dexamethasone as therapeutic systems for intravitreal administration. The pullulan-dexamethasone conjugates self-assemble into negatively charged nanoparticles (average size 326 ± 29 nm). Intravitreal injections of pullulan and pullulan-dexamethasone were safe in mouse, rat and rabbit eyes. Fluorescently labeled pullulan particles showed prolonged retention in the vitreous and they were almost completely eliminated via aqueous humor outflow. Pullulan conjugates also distributed to the retina via Müller glial cells when tested in ex vivo retina explants and in vivo. Pharmacokinetic simulations showed that pullulan-dexamethasone conjugates may release free and active dexamethasone in the vitreous humor for over 16 days, even though a large fraction of dexamethasone may be eliminated from the eye as bound pullulan-dexamethasone. We conclude that pullulan based drug conjugates are promising intravitreal drug delivery systems as they may reduce injection frequency and deliver drugs into the retinal cells.

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.

Mechanistic Model for the Prediction of Small-Molecule Vitreal Clearance Combining Diffusion-Limited and Permeability-Limited Clearance

The discovery of new small-molecule drugs for intravitreal administration would benefit from simple models to predict vitreal clearance (CL). The current models available have limitations in their applicability to small-molecule drugs and translatability to humans. We developed a mechanistic model combining the diffusion rate of the molecule in the vitreous and permeability across posterior segment tissues and applied it to 30 small molecules with observed CL available mostly from literature. We used Caco-2 permeability as a surrogate for ocular tissue permeability. The model predicted rabbit vitreal CL well, with 80% of the predictions being within a 2-fold range of the observed CL. For an accurate prediction, it was crucial to consider the anterior diffusion CL from the vitreous to the aqueous and a limiting diffusion CL for the whole eye. We observed no major differences in model accuracy when using literature permeability values from retinal pigment epithelial cell models. Importantly, by adopting the specific dimensions of the human eye, the model was able to accurately predict vitreal CL of four compounds for which human vitreal CL data are available. In summary, this mechanistic model enables a simple, accurate, and translatable estimation of small-molecule vitreal CL.

Ocular Pharmacokinetics of Intravitreally Injected Protein Therapeutics: Comparison among Standard-of-Care Formats

The current standard of care for antivascular endothelial growth factor (VEGF) treatment requires frequent intravitreal (IVT) injections of protein therapeutics, as a result of limited retention within the eye. A thorough understanding of the determinants of ocular pharmacokinetics (PK) and its translation across species is an essential prerequisite for developing more durable treatments. In this work, we studied the ocular PK in macaques of the protein formats that comprise today's anti-VEGF standard of care. Cynomolgus monkeys received a single IVT injection of a single-chain variable fragment (scFv, brolucizumab), antigen-binding fragment (Fab, ranibizumab), fragment crystallizable-fusion protein (Fc-fusion, aflibercept), or immunoglobulin G monoclonal antibody (IgG, VA2 CrossMAb). Drug concentrations were determined in aqueous humor samples collected up to 42 days postinjection using immunoassay methods. The ocular half-life (t_1/2) was 2.28, 2.62, 3.13, and 3.26 days for scFv, Fab, Fc-fusion, and IgG, respectively. A correlation with human t_1/2 values from the literature confirmed the translational significance of the cynomolgus monkey as an animal model for ocular research. The relation between ocular t_1/2 and molecular size was also investigated. Size was inferred from the molecular weight (MW) or determined experimentally by dynamic light scattering. The MW and hydrodynamic radius were found to be good predictors for the ocular t_1/2 of globular proteins. The analysis showed that molecular size is a determinant of ocular disposition and may be used in lieu of dedicated PK studies in animals.

Pharmacokinetics of intravitreal macromolecules: Scaling between rats and rabbits

Rats are widely used to study ocular drug responses, whereas rabbits are the most widely used preclinical model of ocular pharmacokinetics. Despite their wide use in evaluation of intravitreally injected drugs, translational information about pharmacokinetics and dose scaling between rats and rabbits is missing. In this study, we investigated intravitreal pharmacokinetics in rats and rabbits using non-invasive ocular fluorophotometry. Fluorescein and fluorescently labeled molecules (dextrans) with different molecular weights (376 Da, 10, 150 and 500 kDa), were injected into the vitreous of rabbits and rats. Intravitreal concentrations of the compounds were determined and pharmacokinetic parameters were calculated. Overall, the elimination half-lives of the macromolecules in rat vitreous were 5-6 times shorter than in rabbits, and the half-lives were prolonged at increasing molecular weights. The apparent volumes of distribution for tested compounds in rats and rabbits were in the range of the anatomical vitreal volumes. In both species, anterior route of elimination was predominant for the dextrans, whereas fluorescein was mainly eliminated via posterior route. Rabbit-to-rat ratios for intravitreal clearance were in the range of 2 to 5 for dextrans. Therefore, 2-5 times higher doses are needed for similar drug exposure in rabbits than in rats. Also, the shorter half-lives of macromolecules in the rat vitreous must be taken into account in translation to rabbit and human studies. The scaling factors presented herein will augment translational drug development for eye diseases.

Nonclinical Safety Assessment of FHTR2163, An Antigen-Binding Fragment Against HTRA1 for the Treatment of Geographic Atrophy

FHTR2163 is an antigen-binding fragment of a humanized immunoglobulin G1 monoclonal antibody directed against high-temperature requirement A serine peptidase 1 (HTRA1) that is being developed as a potential intravitreal (ITV) treatment for patients with geographic atrophy (GA), an advanced form of dry age-related macular degeneration. The nonclinical toxicology program was designed to assess the safety and tolerability of HTRA1 inhibition following ITV administration of FHTR2163 to support ITV administration in patients with GA. FHTR2163 was well tolerated in a single-dose ITV-administered 8-day toxicity study in cynomolgus monkeys following a 50 µL high (>700 mOsm/kg) osmolality formulation up to 12.5 mg/eye; however, 100 µL (2× 50 µL injections) of a high-osmolality formulation resulted in transient retinal detachment. Repeat-dose ITV administration every 2 weeks of FHTR2163 was well tolerated in 8- and 26-week studies with ITV injection of 100 µL (2× 50 μL) of iso-osmolar formulation up to 15 mg/eye, or 50 µL of the high-osmolality formulation up to 12.5 mg/eye. Observed transient and reversible ocular effects included inflammation and perivascular infiltrates, consistent with an immune response attributed to the administration of heterologous (humanized) protein. Overall, FHTR2163 was well tolerated, and the nonclinical package supported the continued clinical development of FHTR2163 in patients with GA.

Screening of chemical linkers for development of pullulan bioconjugates for intravitreal ocular applications

The treatment of posterior segment disorders of the eye requires therapeutic strategies to achieve drug effects over prolonged times. Innovative colloidal delivery systems can be designed to deliver drugs to the retina and prolong their intravitreal permanence. In order to exploit pullulan (Pull) as polymeric drug carrier for intravitreal drug delivery, derivatives of hydrophobic model molecule rhodamine B (RhB) were conjugated to the pullulan backbone through linkers with different stability, namely ether (Et), hydrazone (Hy) or ester (Es) bond to obtain Pull-Et-RhB, Pull-Hy-RhB and Pull-Es-RhB, respectively. Dynamic light scattering and transmission electron microscopy analyses showed that the polymer conjugates self-assembled into 20-25 nm particles. Pull-Et-RhB was fairly stable at all tested pH values. At the vitreal pH of 7.4, 50% of RhB was released from Pull-Hy-RhB and Pull-Es-RhB in 11 and 6 days, respectively. At endosomal pH (5.5), 50% of RhB was released from Pull-Hy-RhB and Pull-Es-RhB in 4 and 1 days, respectively. Multiple particle tracking analyses in ex vivo porcine eye model showed that the diffusivity of the bioconjugates in the vitreous was about 10³ times lower than in water. Flow cytometry and confocal microscopy analyses showed that bioconjugates are remarkably taken up by the retinal RPE cells. In vivo studies showed that after intravitreal injection to mice, the bioconjugates localize in the ganglion cell layer and in the inner and outer plexiform layers. Pull-Hy-RhB particles were detected also inside the retinal blood vessels. These results demonstrate that pullulan with tailored linkers for drug conjugation is a promising vehicle for long-acting intravitreal injectables that are capable to permeate to the retina.

Diffusion and Protein Corona Formation of Lipid-Based Nanoparticles in the Vitreous Humor: Profiling and Pharmacokinetic Considerations

The vitreous humor is the first barrier encountered by intravitreally injected nanoparticles. Lipid-based nanoparticles in the vitreous are studied by evaluating their diffusion with single-particle tracking technology and by characterizing their protein coronae with surface plasmon resonance and high-resolution proteomics. Single-particle tracking results indicate that the vitreal mobility of the formulations is dependent on their charge. Anionic and neutral formulations are mobile, whereas larger (>200 nm) neutral particles have restricted diffusion, and cationic particles are immobilized in the vitreous. PEGylation increases the mobility of cationic and larger neutral formulations but does not affect anionic and smaller neutral particles. Convection has a significant role in the pharmacokinetics of nanoparticles, whereas diffusion drives the transport of antibodies. Surface plasmon resonance studies determine that the vitreal corona of anionic formulations is sparse. Proteomics data reveals 76 differentially abundant proteins, whose enrichment is specific to either the hard or the soft corona. PEGylation does not affect protein enrichment. This suggests that protein-specific rather than formulation-specific factors are drivers of protein adsorption on nanoparticles in the vitreous. In summary, our findings contribute to understanding the pharmacokinetics of nanoparticles in the vitreous and help advance the development of nanoparticle-based treatments for eye diseases.

Ocular Drug Delivery to the Retina: Current Innovations and Future Perspectives

Treatment options for retinal diseases, such as neovascular age-related macular degeneration, diabetic retinopathy, and retinal vascular disorders, have markedly expanded following the development of anti-vascular endothelial growth factor intravitreal injection methods. However, because intravitreal treatment requires monthly or bimonthly repeat injections to achieve optimal efficacy, recent investigations have focused on extended drug delivery systems to lengthen the treatment intervals in the long term. Dose escalation and increasing molecular weight of drugs, intravitreal implants and nanoparticles, hydrogels, combined systems, and port delivery systems are presently under preclinical and clinical investigations. In addition, less invasive techniques rather than intravitreal administration routes, such as topical, subconjunctival, suprachoroidal, subretinal, and trans-scleral, have been evaluated to reduce the treatment burden. Despite the latest advancements in the field of ophthalmic pharmacology, enhancing drug efficacy with high ocular bioavailability while avoiding systemic and local adverse effects is quite challenging. Consequently, despite the performance of numerous in vitro studies, only a few techniques have translated to clinical trials. This review discusses the recent developments in ocular drug delivery to the retina, the pharmacokinetics of intravitreal drugs, efforts to extend drug efficacy in the intraocular space, minimally invasive techniques for drug delivery to the retina, and future perspectives in this field.

Extended Pharmacokinetic Model of the Intravitreal Injections of Macromolecules in Rabbits. Part 2: Parameter Estimation Based on Concentration Dynamics in the Vitreous, Retina, and Aqueous Humor

PURPOSE

To estimate the diffusion coefficients of an IgG antibody (150 kDa) and its antigen-binding fragment (Fab; 50 kDa) in the neural retina (Dret) and the combined retinal pigment epithelium-choroid (DRPE-cho) with a 3-dimensional (3D) ocular pharmacokinetic (PK) model of the rabbit eye.

METHODS

Vitreous, retina, and aqueous humor concentrations of IgG and Fab after intravitreal injection in rabbits were taken from Gadkar et al. (2015). A least-squares method was used to estimate Dret and DRPE-cho with the 3D finite element model where mass transport was defined with diffusion and convection. Different intraocular pressures (IOP), initial distribution volumes (Vinit), and neural retina/vitreous partition coefficients (Kret/vit) were tested. Sensitivity analysis was performed for the final model.

RESULTS

With the final IgG model (IOP 10.1 Torr, Vinit 400 μl, Kret/vit 0.5), the estimated Dret and DRPE-cho were 36.8 × 10-9 cm2s-1 and 4.11 × 10-9 cm2s-1, respectively, and 76% of the dose was eliminated via the anterior chamber. Modeling of Fab revealed that a physiological model parameter "aqueous humor formation rate" sets constraints that need to be considered in the parameter estimation.

CONCLUSIONS

This study extends the use of 3D ocular PK models for parameter estimation using simultaneously macromolecule concentrations in three ocular tissues.

Mathematical models for the effect of anti-vascular endothelial growth factor on visual acuity

The standard of care treatment for neovascular age-related macular degeneration, delivered as ocular injection, is based on anti-vascular endothelial growth factor (anti-VEGF). The course of treatment may need to be modified quickly for certain patients based on their response. Models that track both the concentration and the response to an anti-VEGF treatment are presented. The specific focus is to assess the existence of analytical solutions for the different types of models. Both an ODE-based model and a map-based model illustrate the dependence of the solution on various biological parameters and allow the measurement of patient-specific parameters from experimental data. A PDE-based model incorporates diffusive effects. The results are consistent with observed values, and could provide a framework for practitioners to understand the effect of the therapy on the progression of the disease in both responsive and non-responsive patients.

Development of a physiologically-based pharmacokinetic model for ocular disposition of monoclonal antibodies in rabbits

Development of protein therapeutics for ocular disorders, particularly age-related macular degeneration (AMD), is a highly competitive and expanding therapeutic area. However, the application of a predictive and translatable ocular PK model to better understand ocular disposition of protein therapeutics, such as a physiologically-based pharmacokinetic (PBPK) model, is missing from the literature. Here, we present an expansion of an antibody platform PBPK model towards rabbit and incorporate a novel anatomical and physiologically relevant ocular component. Parameters describing all tissues, flows, and binding events were obtained from existing literature and fixed a priori. First, translation of the platform PBPK model to rabbit was confirmed by evaluating the model's ability to predict plasma PK of a systemically administered exogenous antibody. Then, the PBPK model with the new ocular component was validated by estimation of serum and ocular (i.e. aqueous humor, retina, and vitreous humor) PK of two intravitreally administered monoclonal antibodies. We show that the proposed PBPK model is capable of accurately (i.e. within twofold) predicting ocular exposure of antibody-based drugs. The proposed PBPK model can be used for preclinical-to-clinical translation of antibodies developed for ocular disorders, and assessment of ocular toxicity for systemically administered antibody-based therapeutics.

Intravitreal hydrogels for sustained release of therapeutic proteins

This review highlights how hydrogel formulations can improve intravitreal protein delivery to the posterior segment of the eye in order to increase therapeutic outcome and patient compliance. Several therapeutic proteins have shown excellent clinical successes for the treatment of various intraocular diseases. However, drug delivery to the posterior segment of the eye faces significant challenges due to multiple physiological barriers preventing drugs from reaching the retina, among which intravitreal protein instability and rapid clearance from the site of injection. Hence, frequent injections are required to maintain therapeutic levels. Moreover, because the world population ages, the number of patients suffering from ocular diseases, such as age-related macular degeneration (AMD) and diabetic retinopathy (DR) is increasing and causing increased health care costs. Therefore, there is a growing need for suitable delivery systems able to tackle the current limitations in retinal protein delivery, which also may reduce costs. Hydrogels have shown to be promising delivery systems capable of sustaining release of therapeutic proteins and thus extending their local presence. Here, an extensive overview of preclinically developed intravitreal hydrogels is provided with attention to the rational design of clinically useful intravitreal systems. The currently used polymers, crosslinking mechanisms, in vitro/in vivo models and advancements are discussed together with the limitations and future perspective of these biomaterials.

PET study of ocular and blood pharmacokinetics of intravitreal bevacizumab and aflibercept in rats

Intravitreal injections are the standard procedure in the treatment of retinal pathologies, such as the administration of the anti-VEGF antibodies in age-related macular degeneration. The aim of this study is to evaluate the intraocular and blood pharmacokinetics after an intravitreal injection of ⁸⁹Zr-labelled bevacizumab and ⁸⁹Zr-labelled aflibercept in Sprague-Dawley rats using Positron Emission Tomography. First, both antibodies were radiolabelled to zirconium-89 with a maximum specific activity of 15 Mbq/mg for bevacizumab and 10 Mbq/mg for aflibercept. Four µL containing 1-1.2 Mq of ⁸⁹Zr-labelled compound were injected into the vitreous through a 35 G needle. A microPET acquisition was carried out immediately after the injection and at different time points through a 12-day study and blood samples were obtained through the tail vein. Radiolabelling was successfully performed with a radiochemical purity after ultrafiltration above 95% for both agents. Both antibodies ocular curves followed a two-compartment model in which an intraocular elimination half-life of 16.44 h was found for ⁸⁹Zr-bevacizumab and 4.51 h for ⁸⁹Zr-aflibercept, considering the alpha phase as the elimination phase. Regarding the beta phase, a half-life of 3.23 days for ⁸⁹Zr-bevacizumab and 4.69 days for ⁸⁹Zr-aflibercept were observed. With regards to blood concentration, ⁸⁹Zr-bevacizumab showed a blood half-life of 7.08 days, whereas ⁸⁹Zr-aflibercept's was 3.18 days, by a one-compartment model with first-order absorption kinetics. In conclusion, this study shows for the first time the ocular and blood pharmacokinetic analysis after intravitreal injection of aflibercept and bevacizumab in rats.

Ocular Half-Life of Intravitreal Biologics in Humans and Other Species: Meta-Analysis and Model-Based Prediction

Therapeutic antibodies administered intravitreally are the current standard of care to treat retinal diseases. The ocular half-life (t_1/2) is a key determinant of the duration of target suppression. To support the development of novel, longer-acting drugs, a reliable determination of t_1/2 is needed together with an improved understanding of the factors that influence it. A model-based meta-analysis was conducted in humans and nonclinical species (rat, rabbit, monkey, and pig) to determine consensus values for the ocular t_1/2 of IgG antibodies and Fab fragments. Results from multiple literature and in-house pharmacokinetic studies are presented within a mechanistic framework that assumes diffusion-controlled drug elimination from the vitreous. Our analysis shows, both theoretically and experimentally, that the ocular t_1/2 increases in direct proportion to the product of the hydrodynamic radius of the macromolecule (3.0 nm for Fab and 5.0 nm for IgG) and the square of the radius of the vitreous globe, which varies approximately 24-fold from the rat to the human. Interspecies differences in the proportionality factors are observed and discussed in mechanistic terms. In addition, mathematical formulae are presented that allow prediction of the ocular t_1/2 for molecules of interest. The utility of these formulae is successfully demonstrated in case studies of aflibercept, brolucizumab, and PEGylated Fabs, where the predicted ocular t_1/2 values are found to be in reasonable agreement with the experimental data available for these molecules.

Influence of Charge, Hydrophobicity, and Size on Vitreous Pharmacokinetics of Large Molecules

Purpose

Development of therapeutics for retinal disease with improved durability is hampered by inadequate understanding of pharmacokinetic (PK) drivers following intravitreal injection. Previous work shows that hydrodynamic radius is correlated with vitreal half-life over the range of 3 to 7 nm, and that charge and hydrophobicity influence systemic clearance. Better understanding the molecular attributes affecting vitreal elimination half-life enables improved design of therapeutics and enhances clinical translatability.

Methods

Impacts of charge and hydrophobicity on vitreal PK in the rabbit were systematically assessed using antibody and antibody fragment (Fab) variant series, including ranibizumab, altered through amino acid changes in hypervariable regions of the light chain. The impact of molecule size on vitreal PK was assessed in the rabbit, nonhuman primate, and human for a range of molecules (1–45 nm, net charge −1324 to +22.9 in rabbit), including published and internal data.

Results

No correlation was observed between vitreal PK and charge or hydrophobicity. Equivalent rabbit vitreal PK was observed for ranibizumab and its variants with isoelectric points (pI) in the range of 6.8 to 10.2, and hydrophobicities of the variable domain unit (FvHI) between 1009 and 1296; additional variant series had vitreal PK similarly unaffected by pI (5.4–10.2) and FvHI (1004–1358). Strong correlations were observed between vitreal half-life and hydrodynamic radius for preclinical species (R² = 0.8794–0.9366).

Conclusions

Diffusive properties of soluble large molecules, as quantified by hydrodynamic radius, make a key contribution to vitreal elimination, whereas differences in charge or hydrophobicity make minor or negligible contributions.

Translational Relevance

These results support estimation of vitreal elimination rates based on molecular size in relevant preclinical species and humans.

Hyaluronic Acid-Antibody Fragment Bioconjugates for Extended Ocular Pharmacokinetics

Treatment of ocular diseases associated with neovascularization currently requires frequent intravitreal injections of antivascular endothelial growth factor (anti-VEGF) therapies. Reducing the required frequency of anti-VEGF injections and associated clinical visits may improve patient adherence to the prescribed treatment regimen and improve outcomes. Herein, we explore conjugation of rabbit and fragment antibodies (Fab) to the biopolymer hyaluronic acid (HA) as a half-life modifying strategy, and assess the impact on Fab biophysical properties and vitreal pharmacokinetics. HA-Fab conjugates of three distinct molecular weights and hydrodynamic radii (R_H) were assessed for in vivo pharmacokinetic performance relative to unconjugated Fab after intravitreal injection in rabbits. Covalent conjugation to HA did not significantly alter the thermal stability or secondary or tertiary structure, or diminish the potency of the Fab, thereby preserving its pharmacological properties. Conjugation to HA did significantly slow the in vivo clearance of Fab from the rabbit vitreous in an R_H-dependent manner. Compared to free Fab (observed vitreal half-life of 2.8 days), HA-Fab conjugates cleared with observed half-lives of 7.6, 10.2, and 18.3 days for 40 kDa, 200 kDa, and 600 kDa HA conjugates, respectively. This work elucidates a possible strategy for long-acting delivery of proteins intended for the treatment of chronic posterior ocular diseases.

Photoablation of Human Vitreous Opacities by Light-Induced Vapor Nanobubbles

Myopia, diabetes, and aging are the main causes of progressive vitreous collagen aggregation, resulting in vitreous opacities, which can significantly disturb vision. As vitreous opacities, which induce the visual phenomenon of "floaters", are accessible with nanomaterials and light, we propose a nanotechnology-based approach to locally ablate them with highly reduced light energy compared to the more traditional YAG laser therapy. Our strategy relies on the plasmon properties of gold nanoparticles that generate vapor nanobubbles upon pulsed-laser illumination whose mechanical force can ablate vitreous opacities. We designed gold nanoparticles coated with hyaluronic acid (HA), which have excellent diffusional mobility in human vitreous, an essential requirement to reach the vitreous opacities. In addition, we found that HA-coated gold nanoparticles can accumulate extensively on human vitreous opacities that were obtained by vitrectomy from patients with vision-degrading myodesopsia. When subsequently applying nanosecond laser pulses, the collagen aggregates were efficiently destroyed with ∼1000 times less light energy than typically used in YAG laser therapy. This low-energy "floater-specific destruction", which is due to the accumulation of the small gold nanoparticles on the opacities, is attractive, as it may be safer to the surrounding ocular tissues while at the same time being easier and faster to apply compared to YAG laser therapy, where the opacities need to be ablated piece by piece by a tightly focused laser beam. Gold nanoparticle-assisted photoablation may therefore provide a safer, faster, and more reliable destruction of vitreous opacities in the treatment of ophthalmologic diseases.

Impact of angiogenic activation and inhibition on miRNA profiles of human retinal endothelial cells

BACKGROUND

Human retinal microvascular endothelial cells (HRMVECs) are involved in the pathogenesis of retinopathy of prematurity. In this study, the microRNA (miRNA) expression profiles of HRMVECs were investigated under resting conditions, angiogenic stimulation (VEGF treatment) and anti-VEGF treatment.

MATERIALS AND METHODS

The miRNA profiles of HRMVECs under resting and angiogenic conditions (VEGF treatment), as well as after addition of aflibercept, bevacizumab or ranibizumab were evaluated by analyzing the transcriptome of small non-coding RNAs. Differentially expressed miRNAs were validated using qPCR and classified using Gene Ontology enrichment analysis.

RESULTS

Ten miRNAs were found to be significantly changed more than 2-fold. Seven of these miRNAs were changed between resting conditions and angiogenic stimulation. Four of these miRNAs (miR-139-5p/-3p and miR-335-5p/-3p) were validated by qPCR in independent experiments and were found to be associated with angiogenesis and cell migration in Gene Ontology analysis. In addition, analysis of the most abundant miRNAs in the HRMVEC miRNome (representing at least 1% of the miRNome) was conducted and identified miR-21-5p, miR-29a-3p, miR-100-5p and miR-126-5p/-3p to be differently expressed by at least 15% between resting conditions and angiogenic conditions. These miRNAs were found to be associated with apoptotic signaling, regulation of kinase activity, intracellular signal transduction, cell surface receptor signaling and positive regulation of cell differentiation in Gene Ontology analysis. No differentially regulated miRNAs between angiogenic stimulation and angiogenic stimulation plus anti-VEGF treatment were identified.

CONCLUSION

In this study we characterized the miRNA profile of HRMVECs under resting, angiogenic and anti-angiogenic conditions and identified several miRNAs of potential pathophysiologic importance for angioproliferative retinal diseases. Our results have implications for possible miRNA-targeted angiomodulatory approaches in diseases like diabetic retinopathy or retinopathy of prematurity.

Pharmacokinetic Simulations of Intravitreal Biologicals: Aspects of Drug Delivery to the Posterior and Anterior Segments

Biologicals are important ocular drugs that are be delivered using monthly and bimonthly intravitreal injections to treat retinal diseases, such as age-related macular degeneration. Long acting delivery systems are needed for prolongation of their dosing interval. Intravitreal biologicals are eliminated from the eye via the aqueous humor outflow. Thus, the anterior and posterior segments are exposed to the drug. We utilized a kinetic simulation model to estimate protein drug concentrations in the vitreous and aqueous humor after bolus injection and controlled release administration to the vitreous. The simulations predicted accurately the experimental levels of 5 biologicals in the vitreous and aqueous humor. The good match between the simulations and experimental data demonstrated almost complete anterior segment bioavailability, and major dose sparing with ocular controlled release systems. Overall, the model is a useful tool in the design of intraocular delivery of biologicals.

Strategies for modifying drug residence time and ocular bioavailability to decrease treatment frequency for back of the eye diseases

INTRODUCTION

Treating posterior eye diseases has become a major area of focus for pharmaceutical and biotechnology companies. Current standard of care for treating posterior eye diseases relies on administration via intravitreal injection. Although effective, this is not without complications and there is great incentive to develop longer-acting therapeutics and/or sustained release delivery systems. Here, we present an overview of emerging technologies for delivery of biologics to the back of the eye.

AREAS COVERED

Posterior eye diseases, intravitreal injection, age-related macular degeneration, anti-VEGF, ocular pharmacokinetics, novel technologies to extend half-life, in vivo models, translation to the clinic, and hurdles to effective patient care.

EXPERT OPINION

Posterior eye diseases are a worldwide public health issue. Although anti-VEGF molecules represent a major advance for treating diseases involving choroidal neovascularization, frequent injection can be burdensome for patients and clinicians. There is a need for effective and patient-friendly treatments for posterior eye diseases. Many technologies that enable long-acting delivery to the back of the eye are being evaluated. However, successful development of novel therapies and delivery technologies is hampered by a multitude of factors, including patient education, translatability of in vitro/in vivo preclinical data to the clinic, and regulatory challenges associated with novel technologies.