Near-Real Surgical Specimens (NRSS): A Novel Platform for Standardized Assessment of Suprachoroidal Drug Delivery

Abstract

Purpose

Suprachoroidal (SC) drug delivery is a promising avenue for treating posterior segment ocular diseases. Current ex vivo models, primarily human cadaveric eyes, are limited by tissue variability and altered post-mortem fluid dynamics. We introduce near-real surgical specimens (NRSS), an engineered ocular tissue platform, to overcome these limitations and provide standardized, reproducible evaluation of SC drug delivery.

Methods

NRSS ocular models were engineered with biomimetic properties calibrated to fresh human tissue. Suprachoroidal injections of a contrast-enhanced balanced salt solution (100 µL) were performed in NRSS specimens (n = 8) using the Everads Suprachoroidal Injector, which features a 30-gauge needle and a non-sharp nitinol tissue separator for tangential blunt dissection into the suprachoroidal space (SCS). An integrated contrast imaging system enabled real-time visualization. Key parameters, including SC coverage patterns, interspecimen variability (coefficient of variation [CV]), and injection dynamics, were quantified and compared to published human cadaveric eye data.

Results

The NRSS platform demonstrated consistent SC coverage patterns. With the Everads Suprachoroidal Injector, injectate exhibited an initial posterior diffusion from the injection site, followed by postero-circumferential spread. NRSS specimens showed significantly reduced inter-specimen variability in SC coverage compared to reported cadaveric studies (CV: 6.8% for NRSS vs. 17.7% for cadaveric tissue; P < 0.01). Real-time visualization provided detailed insights into the dynamic characteristics of SC injection, including fluid flow patterns and tissue interactions not readily observable in static cadaveric models.

Conclusions

The NRSS platform offers a reproducible and standardized method for evaluating SC drug delivery, closely mimicking clinically relevant biomechanical properties. It enables direct visualization of injection dynamics and holds potential for customization to model pathologic conditions, providing distinct advantages over traditional cadaveric models for device testing, therapeutic assessment, and surgical training. The Everads Suprachoroidal Injector demonstrated effective and reliable access to the SCS in NRSS models with a characteristic posterior-first spread pattern.

Translational Relevance

NRSS technology addresses critical limitations in current SC injection assessment models. It provides a robust, standardized platform for optimizing delivery devices, quantitatively evaluating novel therapeutics, and enhancing surgical training for SC procedures, thereby potentially accelerating the clinical translation of SC therapies.

Introduction

Suprachoroidal (SC) drug delivery is emerging as an auspicious approach for treating a spectrum of posterior segment diseases. This route offers the potential for targeted drug delivery to the choroid and retina, potentially enhancing efficacy and improving safety profiles compared to conventional intravitreal injections.^1,2 However, the suprachoroidal space (SCS)—a potential space between the sclera and choroid—presents unique anatomical and physiological challenges for reliable and predictable drug administration. Effective access requires precise technical execution, and the dynamics of injectate spread within this potential space are not yet fully elucidated.³

Pioneering quantitative work by Hedgeland et al.³ using their TS-Micro Device in human cadaveric eyes provided foundational data on SC injection spread. Their study reported that a 100 µL injection covered approximately 33.3% (±5.9%) of the SCS and, importantly, that injectate spread circumferentially before posterior movement. This work also highlighted significant inter-donor variability in SC coverage, despite consistency between eyes from the same donor, and a nonlinear relationship between injection volume and achieved coverage.

Although invaluable, human cadaveric models possess inherent limitations: unpredictable post-mortem tissue properties, altered fluid dynamics, significant donor-to-donor variability, and an inability to directly visualize injection dynamics in real-time.^4,5 H Sourcing cadaveric eyes with specific pathologies to investigate disease-state effects on SC delivery also remains a considerable challenge. These factors collectively impede the systematic development and optimization of SC delivery systems and constrain standardized evaluations of novel therapeutics.

To address these critical gaps, we have developed near-real surgical specimens (NRSS), an engineered tissue platform meticulously designed to replicate the biomechanical properties of human ocular tissues. The NRSS platform aims to provide a consistent and customizable ex vivo system for the standardized assessment of suprachoroidal drug delivery. Key advantages of NRSS include highly reproducible tissue characteristics, integrated real-time visualization of injection dynamics, the potential for simulating various pathological conditions, and standardized metrics for quantitative performance evaluation.

This study validates the NRSS platform for SC injection assessment, using the Everads Suprachoroidal Injector.^6,7 We hypothesized that NRSS models would yield SC distribution patterns and biomechanical responses comparable to those reported for human tissue, but with significantly improved reproducibility and enhanced visualization capabilities that can reveal novel insights into injection dynamics. Furthermore, we investigated how device characteristics, such as those of the Evereads injector, impact these dynamics within the NRSS model.

Material and Methods

NRSS Specimen Engineering

NRSS ocular models were engineered using a proprietary methodology to create biological substrate analogs with biomechanical properties closely mimicking fresh-frozen human ocular tissue. The models were fabricated using a combination of biocompatible hydrogels to simulate the multilayered structure of the eye, including the sclera and chorioretinal complex. Manufacturing was performed under standardized conditions to ensure high inter-specimen reproducibility.

Validation of Tissue Mechanical Properties

The mechanical properties of NRSS tissue components (e.g., modulus of elasticity, puncture resistance) were characterized using uniaxial tensile testing, compression testing, and puncture resistance measurements (Instron mechanical testing system). These properties were calibrated against published data for fresh human ocular tissues. Fluid dynamics within the engineered SCS were characterized by measuring pressure-volume relationships, which were then compared to available data from ex vivo human eyes to ensure physiological relevance.

SC Injection Protocol

SC injections were performed in eight (n = 8) standard NRSS specimens. All injections used the Everads Suprachoroidal Injector (Everads Therapy Ltd., Tel Aviv, Israel) (Fig. 1). This novel, nonsurgical device uses a 30-gauge needle in conjunction with a non-sharp tissue separator made of nitinol. The injector is designed to achieve tangential blunt dissection, creating a channel from the sclera to the choroid to access the SCS. The injection procedure involves inserting the needle tip at an approximately 30° angle up to a sleeve stopper, which controls entry depth, ensuring the needle bevel remains within the sclera. Upon actuator depression, the nitinol tissue separator extends beyond the sharp needle tip, performing the tangential blunt dissection. After the tissue separator retracts upon actuator release, the drug is delivered. This injector has been reported as tolerable and effective in human studies, and has demonstrated rapid posterior segment delivery in nonhuman primates.^2,4,5 Injections were performed 3.5 mm posterior to the limbus in the superotemporal location. A volume of 100 µL of contrast-enhanced balanced salt solution was injected over 10 seconds.

Figure 1.

The Everads Suprachoroidal Injector. (A) Overview of the injector, a non-surgical device designed for accessing the suprachoroidal space (SCS). Image courtesy of Everads Therapy Ltd. (B) Detailed schematic illustrating the mechanism of SCS access. (C) The key mechanism of action: upon depressing the actuator, the non-sharp nitinol tissue separator extends tangentially beyond the sharp needle tip. This performs blunt dissection, creating a channel between the sclera and choroid directly into the SCS. Upon release of the actuator, the tissue separator retracts, and the therapeutic agent is injected into the newly created channel. (Adapted from Everads Therapy Ltd. materials).

Quantification Methodology

Real-time visualization of SC injection spread was achieved using an integrated imaging system within the NRSS platform. Digital image analysis software (custom software) with calibrated area and distribution pattern measurements was used for quantification. Key parameters measured included:

• 1.Percent SC coverage: (total area of distribution/total available SC area) × 100%
• 2.Injectate distribution pattern: Characterization of initial spread (posterior vs. circumferential) and subsequent progression
• 3.Injection thickness: Expansion of the SC space
• 4.Temporal dynamics of distribution: Rate and evolution of spread over time
• 5.Center of mass migration during injection

Measurements from NRSS specimens were compared to published data from Hedgeland et al.³ for human cadaveric eyes to evaluate correlation and reproducibility.

Statistical Analysis

Data are presented as mean ± standard deviation (SD) or coefficient of variation (CV) where appropriate. Statistical significance of differences in CV between NRSS and published cadaveric data was assessed using appropriate statistical tests (e.g., F-test for equality of variances). Paired t-tests were used for direct comparisons within the NRSS group if applicable, and analysis of variance for multi-group analyses if pathology models were included in future expansions of this work. A P value < 0.05 was considered statistically significant. Comparison of mean coverage was performed if data allowed.

Results

SC Coverage Patterns and Injectate Distribution in NRSS

Suprachoroidal injections of 100 µL using the Everads Suprachoroidal Injector in standard NRSS specimens consistently resulted in diffuse posterior coverage. A key observation was the injectate distribution pattern: an initial, rapid posterior spread from the injection site, which was then followed by a gradual posterocircumferential progression throughout the accessible SCS (Fig. 2). This finding contrasts with the circumferential-then-posterior spread reported by Hedgeland et al.,³ who used a different injection device (TS-Micro Device; Topsky Technologies, San Jose, CA, USA). When the injection volume was doubled to 200 µL in a subset of NRSS specimens, SC coverage increased proportionally with the volume.

Figure 2.

Real-time visualization of suprachoroidal injection dynamics in an NRSS specimen. Sequential images (or a composite image) from the integrated imaging system demonstrating the spread of 100 µL contrast-enhanced solution injected via the Everads Suprachoroidal Injector. Note the initial posterior flow (arrow indicating direction from injection site) followed by subsequent postero-circumferential progression of the injectate within the suprachoroidal space. Images captured using an integrated real-time camera and intraoperative ocular coherence tomography.

Reproducibility and Standardization

The NRSS platform demonstrated a marked improvement in inter-specimen reproducibility compared to data from human cadaveric eye studies. The CV for percent SC coverage in NRSS specimens was 6.8%. This is significantly lower than the 17.7% CV reported by Hedgeland et al.³ for 100 µL injections in human cadaveric eyes (P < 0.01 when comparing variances). The standard deviation for percent coverage in NRSS was 2.4%, compared to 5.9% in the aforementioned cadaveric tissue study.

The thickness of the SC space created by the injection in NRSS specimens ranged from 0.2 to 0.9 mm. This is consistent with the 0.1 to 1.0 mm range reported in cadaveric tissue.³ The distribution of SC space thickness in NRSS models also followed similar patterns, with maximal expansion near the injection site, gradually tapering with increasing distance. Injection thickness measurements showed a CV of 8.2% in NRSS specimens, comparing favorably to the approximately 25% CV estimated from published cadaveric data.

Real-Time Visualization of Injection Dynamics

The integrated real-time visualization system was a critical component of the NRSS platform, allowing for detailed monitoring and characterization of SC injection dynamics not typically observable in cadaveric models. This system confirmed the rapid posterior spread of the injectate immediately upon delivery via the Everads Injector, followed by the slower postero-circumferential progression (Fig. 2). Real-time observation indicated that the SC fluid initially follows paths of least resistance along natural tissue planes, influenced by intraocular pressure gradients simulated in the NRSS, before more gradually filling the potential space. The trajectory of the center of mass during SC injection showed remarkably consistent patterns across all standard NRSS specimens, with a maximum deviation of only 1.7 mm between specimens at any given time point during or after injection. This high consistency contrasts sharply with the significant inter-donor variability in final distribution patterns reported in cadaveric studies.

Discussion

This study introduces and validates the NRSS platform as a novel, engineered tissue system for the standardized assessment of suprachoroidal drug delivery. Our findings demonstrate that NRSS models not only replicate key biomechanical properties and SC coverage parameters observed in human cadaveric eyes but also offer significant advantages in terms of reproducibility and the ability to visualize injection dynamics in real-time.

A crucial advantage of the NRSS platform is the significant reduction in inter-specimen variability. The CV for SC coverage in NRSS (6.8%) was substantially lower than that reported for human cadaveric eyes. This enhanced consistency addresses a significant limitation of cadaveric studies—the inherent biological variability among donors, which complicates standardized comparisons of different delivery devices, formulations, or injection techniques. The high reproducibility of NRSS makes it a more reliable tool for discerning subtle but significant differences in performance.

The integrated real-time visualization system represents another significant advancement. Unlike traditional cadaveric models that often rely on static, post-injection imaging or destructive tissue sectioning, NRSS allows for direct observation of the entire injection process. This capability was instrumental in characterizing the injection dynamics of the Everads Suprachoroidal Injector. We observed an initial posterior spread of the injectate, followed by postero-circumferential progression. This pattern differs from the circumferential-then-posterior spread reported by Hedgeland et al.,³ who used the TS-Micro Device. This difference is likely attributable to the distinct design and mechanism of action of the Everads Injector, specifically its non-sharp nitinol tissue separator designed for tangential blunt dissection to access the posterior aspect of the SCS directly. The real-time visualization offered by NRSS is invaluable for understanding such device-tissue interactions. It can significantly aid in the design and optimization of future SC delivery systems, allowing engineers to directly observe how parameters such as needle design, injection speed, or formulation viscosity influence distribution.

The reliable performance of the Everads Injector in consistently accessing the SCS and achieving posterior distribution across multiple NRSS specimens supports its potential clinical utility. The distinct posterior-first spread pattern observed with this injector may have implications for targeting specific posterior segment structures and could be advantageous for certain therapeutic applications. Confirming this specific spread pattern underscores the importance of considering both device choice and injection site selection when aiming for optimal therapeutic delivery to the posterior pole.

Beyond device testing, the NRSS platform shows considerable promise for surgical training and education. The consistent tissue feedback and real-time visual confirmation of injectate placement can provide a standardized and effective learning environment for clinicians acquiring skills in subcutaneous injection. This can accelerate the learning curve and potentially improve clinical outcomes. Preliminary experiences with NRSS-based training at the Centers for Advanced Surgical Exploration suggest improved technique consistency among trainees. Furthermore, the potential to customize NRSS models to simulate specific ocular pathologies (e.g., varying scleral thickness, altered choroidal properties) could provide unique training scenarios for managing challenging clinical situations, a feature not readily available with cadaveric tissues.

Limitations

Although NRSS addresses many limitations of cadaveric models, it is not without its constraints. First, the engineered tissues, while biomechanically analogous, lack the full cellular and molecular complexity of living tissue. This may limit their utility for studying biological interactions such as drug clearance mechanisms, cellular uptake, or immunogenicity. Second, while the current integrated visualization system provides excellent real-time qualitative and quantitative data on spread dynamics, future iterations could benefit from incorporating higher-resolution imaging modalities, such as intraoperative ocular coherence tomography, for more detailed quantitative analysis of tissue layers during injection. Last, although our NRSS results for parameters such as SC space thickness and overall coverage variability correlate well with published cadaveric findings where applicable, further validation against in vivo human data, as it becomes available, will be necessary to solidify the platform's translational relevance. Future development of the NRSS platform will focus on enhancing its biological fidelity, expanding the range of customizable pathological conditions, and potentially integrating with other advanced imaging technologies and minimally invasive surgical applications.

Conclusions

The NRSS platform provides a reproducible, standardized, and biomechanically relevant ex vivo system for the comprehensive evaluation of suprachoroidal drug delivery. Our results demonstrate that NRSS generates SC injectate coverage patterns with significantly improved reproducibility compared to traditional human cadaveric tissue, while also offering the unique advantage of real-time visualization of injection dynamics. Specifically, using the Everads Suprachoroidal Injector, a distinct initial posterior spread followed by postero-circumferential progression was observed, highlighting the capability of NRSS to elucidate device-specific delivery characteristics.

The NRSS technology addresses key limitations inherent in current SC injection assessment models. It offers novel capabilities for accelerating the development of delivery devices, optimizing therapeutic formulations, and enhancing clinical training. As suprachoroidal delivery continues to gain prominence as a therapeutic route for ocular diseases, robust and standardized evaluation platforms like NRSS will be increasingly vital for translating this promising technology safely and effectively into widespread clinical practice.