Contrast solution properties and scan parameters influence the apparent diffusivity of computed tomography contrast agents in articular cartilage

The inability to detect early degenerative changes to the articular cartilage surface that commonly precede bulk osteoarthritic degradation is an obstacle to early disease detection for research or clinical diagnosis. Leveraging a known artefact that blurs tissue boundaries in clinical arthrograms, contrast agent (CA) diffusivity can be derived from computed tomography arthrography (CTa) scans. We combined experimental and computational approaches to study protocol variations that may alter the CTa-derived apparent diffusivity. In experimental studies on bovine cartilage explants, we examined how CA dilution and transport direction (absorption versus desorption) influence the apparent diffusivity of untreated and enzymatically digested cartilage. Using multiphysics simulations, we examined mechanisms underlying experimental observations and the effects of image resolution, scan interval and early scan termination. The apparent diffusivity during absorption decreased with increasing CA concentration by an amount similar to the increase induced by tissue digestion. Models indicated that osmotically-induced fluid efflux strongly contributed to the concentration effect. Simulated changes to spatial resolution, scan spacing and total scan time all influenced the apparent diffusivity, indicating the importance of consistent protocols. With careful control of imaging protocols and interpretations guided by transport models, CTa-derived diffusivity offers promise as a biomarker for early degenerative changes.

Assessment of joint pharmacokinetics and consequences for the intraarticular delivery of biologics

Intraarticular (IA) injections provide the opportunity to deliver biologics directly to their site of action for a local and efficient treatment of osteoarthritis. However, the synovial joint is a challenging site of administration since the drug is rapidly eliminated across the synovial membrane and has limited distribution into cartilage, resulting in unsatisfactory therapeutic efficacy. In order to rationally develop appropriate drug delivery systems, it is essential to thoroughly understand the unique biopharmaceutical environments and kinetics in the joint to adequately simulate them in relevant experimental models. This review presents a detailed view on articular kinetics and drug-tissue interplay of IA administered drugs and summarizes how these can be translated into reasonable formulation strategies by identification of key factors through which the joint residence time can be prolonged and specific structures can be targeted. In this way, pros and cons of the delivery approaches for biologics will be evaluated and the extent to which biorelevant models are applicable to gain mechanistic insights and ameliorate formulation design is discussed.

Effects of solvent osmolarity and viscosity on cartilage energy dissipation under high-frequency loading

Articular cartilage is a spatially heterogeneous, dissipative biological hydrogel with a high fluid volume fraction. Although energy dissipation is important in the context of delaying cartilage damage, the dynamic behavior of articular cartilage equilibrated in media of varied osmolarity and viscosity is not widely understood. This study investigated the mechanical behaviors of cartilage when equilibrated to media of varying osmolarity and viscosity. Dynamic moduli and phase shift were measured at both low (1 Hz) and high (75-300 Hz) frequency, with cartilage samples compressed to varied offset strain levels. Increasing solution osmolarity and viscosity both independently resulted in larger energy dissipation and decreased dynamic modulus of cartilage at both low and high frequency. Mechanical property alterations induced by varying osmolarity are likely due to the change in permeability and fluid volume fraction within the tissue. The effects of solution viscosity are likely due to frictional interactions at the solid-fluid interface, affecting energy dissipation. These findings highlight the significance of interstitial fluid on the energy dissipation capabilities of the tissue, which can influence the onset of cartilage damage.

Mechanics of 3D Cell-Hydrogel Interactions: Experiments, Models, and Mechanisms

Hydrogels are highly water-swollen molecular networks that are ideal platforms to create tissue mimetics owing to their vast and tunable properties. As such, hydrogels are promising cell-delivery vehicles for applications in tissue engineering and have also emerged as an important base for ex vivo models to study healthy and pathophysiological events in a carefully controlled three-dimensional environment. Cells are readily encapsulated in hydrogels resulting in a plethora of biochemical and mechanical communication mechanisms, which recapitulates the natural cell and extracellular matrix interaction in tissues. These interactions are complex, with multiple events that are invariably coupled and spanning multiple length and time scales. To study and identify the underlying mechanisms involved, an integrated experimental and computational approach is ideally needed. This review discusses the state of our knowledge on cell-hydrogel interactions, with a focus on mechanics and transport, and in this context, highlights recent advancements in experiments, mathematical and computational modeling. The review begins with a background on the thermodynamics and physics fundamentals that govern hydrogel mechanics and transport. The review focuses on two main classes of hydrogels, described as semiflexible polymer networks that represent physically cross-linked fibrous hydrogels and flexible polymer networks representing the chemically cross-linked synthetic and natural hydrogels. In this review, we highlight five main cell-hydrogel interactions that involve key cellular functions related to communication, mechanosensing, migration, growth, and tissue deposition and elaboration. For each of these cellular functions, recent experiments and the most up to date modeling strategies are discussed and then followed by a summary of how to tune hydrogel properties to achieve a desired functional cellular outcome. We conclude with a summary linking these advancements and make the case for the need to integrate experiments and modeling to advance our fundamental understanding of cell-matrix interactions that will ultimately help identify new therapeutic approaches and enable successful tissue engineering.

Sliding contact accelerates solute transport into the cartilage surface compared to axial loading

OBJECTIVE

The objectives of this study were: first, to compare solute uptake driven by sliding to cyclic uniaxial compression. And secondly, to evaluate the role of the superficial region on passive diffusion to determine if mechanical action is merely overcoming the low permeability of the superficial region or exceeding equilibrium capacity of the tissue.

DESIGN

Tests were performed on osteochondral plugs under two types of conditions: cyclic loading (sliding vs axial compression) and unloaded passive diffusion (intact vs superficial zone removed). The articular surfaces were exposed to a fluorescent bath and uptake was quantified from the surface to the subchondral bone using fluorescent microscopy. Primary outcome measures were total mass transfer, mass transfer rate, and surface partition factor.

RESULTS

Mass transfer was 2.1-fold higher at 0.5 h for sliding compared to uniaxial compression (p = 0.004). This increased to 4.4-fold at 2 h (p = 0.002). Solute transport for both loading conditions at 2 h had reached or exceeded intact passive diffusion at 12 h. Total mass transport and mass transport per hour was higher in samples without the superficial region compared to intact samples at equilibrium. Rate of mass transfer was not declining for samples subject to sliding indicating solute uptake induced by sliding would exceed passive tissue capacity.

CONCLUSIONS

These results are the first to quantify solute uptake between two components of joint articulation. The study demonstrates that sliding is a larger driver of solute transport compared to cyclic uniaxial compression. This has implications for cell nutrition, tissue engineering and biochemical signaling.

WNT3A-loaded exosomes enable cartilage repair

Cartilage defects repair poorly. Recent genetic studies suggest that WNT3a may contribute to cartilage regeneration, however the dense, avascular cartilage extracellular matrix limits its penetration and signalling to chondrocytes. Extracellular vesicles actively penetrate intact cartilage. This study investigates the effect of delivering WNT3a into large cartilage defects in vivo using exosomes as a delivery vehicle. Exosomes were purified by ultracentrifugation from conditioned medium of either L-cells overexpressing WNT3a or control un-transduced L-cells, and characterized by electron microscopy, nanoparticle tracking analysis and marker profiling. WNT3a loaded on exosomes was quantified by western blotting and functionally characterized in vitro using the SUPER8TOPFlash reporter assay and other established readouts including proliferation and proteoglycan content. In vivo pathway activation was assessed using TCF/Lef:H2B-GFP reporter mice. Wnt3a loaded exosomes were injected into the knees of mice, in which large osteochondral defects were surgically generated. The degree of repair was histologically scored after 8 weeks. WNT3a was successfully loaded on exosomes and resulted in activation of WNT signalling in vitro. In vivo, recombinant WNT3a failed to activate WNT signalling in cartilage, whereas a single administration of WNT3a loaded exosomes activated canonical WNT signalling for at least one week, and eight weeks later, improved the repair of osteochondral defects. WNT3a assembled on exosomes, is efficiently delivered into cartilage and contributes to the healing of osteochondral defects.

Size-Dependent Effective Diffusivity in Healthy Human and Porcine Joint Synovium

Intra-articular drug delivery can be effective in targeting a diseased joint but is hampered by rapid clearance times from the diarthrodial joint. The synovium is a multi-layered tissue that surrounds the diarthrodial joint and governs molecular transport into and out of the joint. No models of drug clearance through synovium exist to quantify diffusivity across solutes, tissue type and disease pathology. We previously have developed a finite element model of synovium as a porous, permeable, fluid-filled tissue and used an inverse method to determine urea's effective diffusivity (D_eff) in de-vitalized synovium explants.²² Here we apply this method to determine D_eff from unsteady diffusive transport of model solutes and confirm the role of molecular weight in solute transport. As molecular weight increased, D_eff decreased in both human and porcine tissues, with similar behavior across the two species. Unsteady transport was well-described by a single exponential transient decay in concentration, yielding solute half-lives (t_1/2) that compared favorably with the D_eff determined from the finite element model fit. Determined values for D_eff parallel prior observations of size-dependent in vivo drug clearance and provide an intrinsic parameter with greater ability to resolve size-dependence in vitro. Thus, this work forms the basis for understanding the influence of size on drug transport in synovium and can guide future studies to elucidate the role of charge and tissue pathology on the transport of therapeutics in healthy and pathological human synovium.

Combined Experimental Approach and Finite Element Modeling of Small Molecule Transport Through Joint Synovium to Measure Effective Diffusivity

Trans-synovial solute transport plays a critical role in the clearance of intra-articularly (IA) delivered drugs. In this study, we present a computational finite element model (FEM) of solute transport through the synovium validated by experiments on synovial explants. Unsteady diffusion of urea, a small uncharged molecule, was measured through devitalized porcine and human synovium using custom-built diffusion chambers. A multiphasic computational model was constructed and optimized with the experimental data to extract effective diffusivity for urea within the synovium. A monotonic decrease in urea concentration was observed in the donor bath over time, with an effective diffusivity found to be an order of magnitude lower in synovium versus that measured in free solution. Parametric studies incorporating an intimal cell layer with varying thickness and varying effective diffusivities were performed, revealing a dependence of drug clearance kinetics on both parameters. The findings of this study indicate that the synovial matrix impedes urea solute transport out of the joint with little retention of the solute in the matrix.

Molecular transport in articular cartilage - what have we learned from the past 50 years?

Developing therapeutic molecules that target chondrocytes and locally produced inflammatory factors within arthritic cartilage is an active area of investigation. The extensive studies that have been conducted over the past 50 years have enabled the accurate prediction and reliable optimization of the transport of a wide variety of molecules into cartilage. In this Review, the factors that can be used to tune the transport kinetics of therapeutics are summarized. Overall, the most crucial factor when designing new therapeutic molecules is solute size. The diffusivity and partition coefficient of a solute both decrease with increasing solute size as indicated by molecular mass or by hydrodynamic radius. Surprisingly, despite having an effective pore size of ~6 nm, molecules of ~16 nm radius can diffuse through the cartilage matrix. Alteration of the shape or charge of a solute and the application of physiological loading to cartilage can be used to predictably improve solute transport kinetics, and this knowledge can be used to improve the development of therapeutic agents for osteoarthritis that target the cartilage.

The Effect of Charge and Mechanical Loading on Antibody Diffusion Through the Articular Surface of Cartilage

Molecular transport of osteoarthritis (OA) therapeutics within articular cartilage is influenced by many factors, such as solute charge, that have yet to be fully understood. This study characterizes how solute charge influences local diffusion and convective transport of antibodies within the heterogeneous cartilage matrix. Three fluorescently tagged solutes of varying isoelectric point (pI) (4.7–5.9) were tested in either cyclic or passive cartilage loading conditions. In each case, local diffusivities were calculated based on local fluorescence in the cartilage sample, as observed by confocal microscopy. In agreement with past research, local solute diffusivities within the heterogeneous cartilage matrix were highest around 200–275 μm from the articular surface, but 3–4 times lower at the articular surface and in the deeper zones of the tissue. Transport of all 150 kDa solutes was significantly increased by the application of mechanical loading at 1 Hz, but local transport enhancement was not significantly affected by changes in solute isoelectric point. More positively charged solutes (higher pI) had significantly higher local diffusivities 200–275 μm from the tissue surface, but no other differences were observed. This implies that there are certain regions of cartilage that are more sensitive to changes in solute charge than others, which could be useful for future development of OA therapeutics.

How can 50 years of solute transport data in articular cartilage inform the design of arthritis therapeutics?

OBJECTIVE

For the last half century, transport of nutrients and therapeutics in articular cartilage has been studied with various in vitro systems that attempt to model in vivo conditions. However, experimental technique, tissue species, and tissue storage condition (fresh/frozen) vary widely and there is debate on the most appropriate model system. Additionally, there is still no clear overarching framework with which to predict solute transport properties based on molecular characteristics. This review aims to develop such a framework, and to assess whether experimental procedure affects trends in transport data.

METHODS

Solute data from 31 published papers that investigated transport in healthy articular cartilage were obtained and analyzed for trends.

RESULTS

Here, we show that diffusivity of spherical and globular solutes in cartilage can be predicted by molecular weight (MW) and hydrodynamic radius via a power-law relationship. This relationship is robust for many solutes, spanning 5 orders of magnitude in MW and was not affected by variations in cartilage species, age, condition (fresh/frozen), and experimental technique. Traditional models of transport in porous media exhibited mixed effectiveness at predicting diffusivity in cartilage, but were good in predicting solute partition coefficient.

CONCLUSION

Ultimately, these robust relationships can be used to accurately predict and improve transport of solutes in adult human cartilage and enable the development of better optimized arthritis therapeutics.

Measurement of local diffusion and composition in degraded articular cartilage reveals the unique role of surface structure in controlling macromolecular transport

Developing effective therapeutics for osteoarthritis (OA) necessitates that such molecules can reach and target chondrocytes within articular cartilage. However, predicting how well very large therapeutic molecules diffuse through cartilage is often difficult, and the relationship between local transport mechanics for these molecules and tissue heterogeneities in the tissue is still unclear. In this study, a 150 kDa antibody diffused through the articular surface of healthy and enzymatically degraded cartilage, which enabled the calculation of local diffusion mechanics in tissue with large compositional variations. Local cartilage composition and structure was quantified with Fourier transform infrared (FTIR) spectroscopy and second harmonic generation (SHG) imaging techniques. Overall, both local concentrations of aggrecan and collagen were correlated to local diffusivities for both healthy and surface-degraded samples (0.3 > R² < 0.9). However, samples that underwent surface degradation by collagenase exhibited stronger correlations (R² > 0.75) compared to healthy samples (R² < 0.46), suggesting that the highly aligned collagen at the surface of cartilage acts as a barrier to macromolecular transport.