Multi-scale imaging techniques to investigate solute transport across articular cartilage

Drug Delivery Strategies and Nanozyme Technologies to Overcome Limitations for Targeting Oxidative Stress in Osteoarthritis

Oxidative stress is an important, but elusive, therapeutic target for osteoarthritis (OA). Antioxidant strategies that target oxidative stress through the elimination of reactive oxygen species (ROS) have been widely evaluated for OA but are limited by the physiological characteristics of the joint. Current hallmarks in antioxidant treatment strategies include poor bioavailability, poor stability, and poor retention in the joint. For example, oral intake of exogenous antioxidants has limited access to the joint space, and intra-articular injections require frequent dosing to provide therapeutic effects. Advancements in ROS-scavenging nanomaterials, also known as nanozymes, leverage bioactive material properties to improve delivery and retention. Material properties of nanozymes can be tuned to overcome physiological barriers in the knee. However, the clinical application of these nanozymes is still limited, and studies to understand their utility in treating OA are still in their infancy. The objective of this review is to evaluate current antioxidant treatment strategies and the development of nanozymes as a potential alternative to conventional small molecules and enzymes.

Cellular therapy and tissue engineering for cartilage repair

Articular cartilage (AC) has limited capacity for repair. The first attempt to repair cartilage using tissue engineering was reported in 1977. Since then, cell-based interventions have entered clinical practice in orthopaedics, and several tissue engineering approaches to repair cartilage are in the translational pipeline towards clinical application. Classically, these involve a scaffold, substrate or matrix to provide structure, and cells such as chondrocytes or mesenchymal stromal cells to generate the tissue. We discuss the advantages and drawbacks of the use of various cell types, natural and synthetic scaffolds, multiphasic or gradient-based scaffolds, and self-organizing or self-assembling scaffold-free systems, for the engineering of cartilage constructs. Several challenges persist including achieving zonal tissue organization and integration with the surrounding tissue upon implantation. Approaches to improve cartilage thickness, organization and mechanical properties include mechanical stimulation, culture under hypoxic conditions, and stimulation with growth factors or other macromolecules. In addition, advanced technologies such as bioreactors, biosensors and 3D bioprinting are actively being explored. Understanding the underlying mechanisms of action of cell therapy and tissue engineering approaches will help improve and refine therapy development. Finally, we discuss recent studies of the intrinsic cellular and molecular mechanisms of cartilage repair that have identified novel signals and targets and are inspiring the development of molecular therapies to enhance the recruitment and cartilage reparative activity of joint-resident stem and progenitor cells. A one-fits-all solution is unrealistic, and identifying patients who will respond to a specific targeted treatment will be critical.

A Cationic Contrast Agent in X-ray Imaging of Articular Cartilage: Pre-Clinical Evaluation of Diffusion and Attenuation Properties

The aim of this study was the preliminary assessment of a new cationic contrast agent, the CA4+, via the analysis of spatial distribution in cartilage of ex vivo bovine samples, at micrometer and millimeter scale. Osteochondral plugs (n = 18) extracted from bovine stifle joints (n = 2) were immersed in CA4+ solution up to 26 h. Planar images were acquired at different time points, using a microCT apparatus. The CA4+ distribution in cartilage and saturation time were evaluated. Tibial plates from bovine stifle joints (n = 3) were imaged with CT, before and after 24 h-CA4+ bath immersion, at different concentrations. Afterward, potential CA4+ washout from cartilage was investigated. From microCT acquisitions, the CA4+ distribution differentiated into three distinct layers inside the cartilage, reflecting the spatial distribution of proteoglycans. After 24 h of diffusion, the iodine concentration reached in cartilage was approximately seven times that of the CA4+ bath. The resulting saturation time was 1.9 ± 0.9 h and 2.6 ± 2.9 h for femoral and tibial samples, respectively. Analysis of clinical CT acquisitions confirmed overall contrast enhancement of cartilage after 24 h immersion, observed for each CA4+ concentration. Distinct contrast enhancement was reached in different cartilage regions, depending on tissue’s local features. Incomplete but remarkable washout of cartilage was observed. CA4+ significantly improved cartilage visualization and its qualitative analysis.

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.

Estimation Parameters of Soil Solute Transport Processes by Using the Electric Resistivity Method

Preferential solute transport is a common phenomenon in soil, and it is of great significance to accurately describe the mechanism of pollutant transport and water and soil environmental governance. However, the description of preferential solutes still relies on applying solute breakthrough curves for model parameters fitting. At present, most of the solute breakthrough curves are obtained indoors, and with some limitations. Therefore, this study established a method for securing solute breakthrough curves based on the electrical resistivity method. The research results show that the change in soil concentration during the tracer infiltration process can be captured by establishing the fitting relationship between soil resistivity and solute concentration. Then the solute breakthrough curve can be found. Through a time moment analysis, the difference between the breakthrough curve parameters obtained by the traditional method and the resistivity method is slight; the average error is less than 10%. On this basis, the sensitive response of the parameters of the “mobile–immobile” model to concentration was elucidated through different concentration tracer experiments, among which β and D are more sensitive, and w is less sensitive. The suitable tracer concentration range should be 50–120 mg/L. Therefore, the established method could obtain the breakthrough curves and describe the transport of preferential solutes at the field scale.

Cationic Contrast Agents for Computed Tomography of Cartilage for Early Diagnosis of Osteoarthritis

Here we describe methods for synthesizing cationic contrast agents for computed tomography (CT) of cartilage for early diagnosis of tissue degeneration. CT imaging of soft tissues like cartilage is possible only if radio-opaque contrast agents (e.g., ioxaglate) can penetrate through the full thickness of tissue in sufficient concentrations. Ioxaglate (IOX), however, is anionic and is repelled by the negatively charged cartilage matrix resulting in poor CT attenuation. Here we demonstrate cartilage penetrating cationic contrast agents using multi-arm Avidin (mAv) conjugated to ioxaglate (mAv-IOX). mAv-IOX rapidly penetrates through the full thickness of cartilage in high concentrations owing to weak-reversible nature of electrostatic interactions resulting in high CT attenuation even with low doses unlike IOX. The technology has the potential for enabling clinical CT of cartilage and other negatively charged soft tissues.

Charge-Based Multiarm Avidin Nanoconstruct as a Platform Technology for Applications in Drug Delivery

Here we describe methods for synthesizing cationic multiarm Avidin (mAv) nanoconstruct that has a wide range of applications in drug delivery and imaging for a variety of negatively charged tissues. The multiarm structure provides multiple sites for covalent conjugation of drugs. We use avidin-biotin reaction that gives the flexibility for conjugating any desired biotinylated drug to mAv by simple mixing at room temperature. We also describe methods to control hydrolysis rates of ester linkers to enable sustained (and tunable) drug release rates in therapeutic doses.

Resveratrol and Curcumin Attenuate Ex Vivo Sugar-Induced Cartilage Glycation, Stiffening, Senescence, and Degeneration

OBJECTIVE

Advanced glycation end-product (AGE) accumulation is implicated in osteoarthritis (OA) pathogenesis in aging and diabetic populations. Here, we develop a representative nonenzymatic glycation-induced OA cartilage explant culture model and investigate the effectiveness of resveratrol, curcumin, and eugenol in inhibiting AGEs and the structural and biological hallmarks of cartilage degeneration.

DESIGN

Bovine cartilage explants were treated with AGE-bovine serum albumin, threose, and ribose to determine the optimal conditions that induce physiological levels of AGEs while maintaining chondrocyte viability. AGE crosslinks, tissue stiffness, cell viability, metabolism and senescence, nitrite release and loss of glycosaminoglycans were assessed. Explants were cotreated with resveratrol, curcumin, or eugenol to evaluate their anti-AGE properties. Blind docking analysis was conducted to estimate binding energies of drugs with collagen II.

RESULTS

Treatment with 100 mM ribose significantly increased AGE crosslink formation and tissue stiffness, resulting in reduced chondrocyte metabolism and enhanced senescence. Blind docking analysis revealed stronger binding energies of both resveratrol and curcumin than ribose, with glycation sites along a human collagen II fragment, indicating their increased likelihood of competitively inhibiting ribose activity. Resveratrol and curcumin, but not eugenol, successfully inhibited AGE crosslink formation and its associated downstream biological response.

CONCLUSIONS

We establish a cartilage explant model of OA that recapitulates several aspects of aged human cartilage. We find that resveratrol and curcumin are effective anti-AGE therapeutics with the potential to decelerate age-related and diabetes-induced OA. This in vitro nonenzymatic glycation-induced model provides a tool for screening OA drugs, to simultaneously evaluate AGE-induced biological and mechanical changes.

Topographic features of nano-pores within the osteochondral interface and their effects on transport properties -a 3D imaging and modeling study

Recent insights suggest that the osteochondral interface plays a central role in maintaining healthy articulating joints. Uncovering the underlying transport mechanisms is key to the understanding of the cross-talk between articular cartilage and subchondral bone. Here, we describe the mechanisms that facilitate transport at the osteochondral interface. Using scanning electron microscopy (SEM), we found a continuous transition of mineralization architecture from the non-calcified cartilage towards the calcified cartilage. This refurbishes the classical picture of the so-called tidemark; a well-defined discontinuity at the osteochondral interface. Using focused-ion-beam SEM (FIB-SEM) on one osteochondral plug derived from a human cadaveric knee, we elucidated that the pore structure gradually varies from the calcified cartilage towards the subchondral bone plate. We identified nano-pores with radius of 10.71 ± 6.45 nm in calcified cartilage to 39.1 ± 26.17 nm in the subchondral bone plate. The extracted pore sizes were used to construct 3D pore-scale numerical models to explore the effect of pore sizes and connectivity among different pores. Results indicated that connectivity of nano-pores in calcified cartilage is highly compromised compared to the subchondral bone plate. Flow simulations showed a permeability decrease by about 2000-fold and solute transport simulations using a tracer (iodixanol, 1.5 kDa with a free diffusivity of 2.5 × 10^-10 m²/s) showed diffusivity decrease by a factor of 1.5. Taken together, architecture of the nano-pores and the complex mineralization pattern in the osteochondral interface considerably impacts the cross-talk between cartilage and bone.

Overcoming negatively charged tissue barriers: Drug delivery using cationic peptides and proteins

Negatively charged tissues are ubiquitous in the human body and are associated with a number of common diseases yet remain an outstanding challenge for targeted drug delivery. While the anionic proteoglycans are critical for tissue structure and function, they make tissue matrix dense, conferring a high negative fixed charge density (FCD) that makes drug penetration through the tissue deep zones and drug delivery to resident cells extremely challenging. The high negative FCD of these tissues is now being utilized by taking advantage of electrostatic interactions to create positively charged multi-stage delivery methods that can sequentially penetrate through the full thickness of tissues, create a drug depot and target cells. After decades of work on attempting delivery using strong binding interactions, significant advances have recently been made using weak and reversible electrostatic interactions, a characteristic now considered essential to drug penetration and retention in negatively charged tissues. Here we discuss these advances using examples of negatively charged tissues (cartilage, meniscus, tendons and ligaments, nucleus pulposus, vitreous of eye, mucin, skin), and delve into how each of their structures, tissue matrix compositions and high negative FCDs create barriers to drug entry and explore how charge interactions are being used to overcome these barriers. We review work on tissue targeting cationic peptide and protein-based drug delivery, compare and contrast drug delivery designs, and also present examples of technologies that are entering clinical trials. We also present strategies on further enhancing drug retention within diseased tissues of lower FCD by using synergistic effects of short-range binding interactions like hydrophobic and H-bonds that stabilize long-range charge interactions. As electrostatic interactions are incorporated into design of drug delivery materials and used as a strategy to create properties that are reversible, tunable and dynamic, bio-electroceuticals are becoming an exciting new direction of research and clinical work.

Molecular Signaling Interactions and Transport at the Osteochondral Interface: A Review

Articular joints are comprised of different tissues, including cartilage and bone, with distinctive structural and mechanical properties. Joint homeostasis depends on mechanical and biological integrity of these components and signaling exchanges between them. Chondrocytes and osteocytes actively sense, integrate, and convert mechanical forces into biochemical signals in cartilage and bone, respectively. The osteochondral interface between the bone and cartilage allows these tissues to communicate with each other and exchange signaling and nutritional molecules, and by that ensure an integrated response to mechanical stimuli. It is currently not well known how molecules are transported between these tissues. Measuring molecular transport in vivo is highly desirable for tracking cartilage degeneration and osteoarthritis progression. Since transport of contrast agents, which are used for joint imaging, also depend on diffusion through the cartilage extracellular matrix, contrast agent enhanced imaging may provide a high resolution, non-invasive method for investigating molecular transport in the osteochondral unit. Only a few techniques have been developed to track molecular transport at the osteochondral interface, and there appear opportunities for development in this field. This review will describe current knowledge of the molecular interactions and transport in the osteochondral interface and discuss the potential of using contrast agents for investigating molecular transport and structural changes of the joint.

Avidin-biotin technology to synthesize multi-arm nano-construct for drug delivery

Here we describe methods for synthesizing a cationic, multi-arm Avidin (mAv) nano-construct that has a wide range of applications in drug delivery and imaging of negatively charged tissues. We use Avidin-biotin technology that gives the flexibility for conjugating biotinylated Dexamethasone to mAv by simple mixing at room temperature. We also describe methods to control hydrolysis rates of ester linkers to enable sustained (and tunable) drug release rates in therapeutic doses.•

Multi-arm structure provides multiple sites for covalent conjugation of drugs

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Use of Avidin-biotin reaction gives multi-arm nano-construct a modular design enabling conjugation and delivery of similar sized biotinylated drugs.

Bioreactor for mobilization of mesenchymal stem/stromal cells into scaffolds under mechanical stimulation: Preliminary results

Introduction

Articular cartilage (AC) is a viscoelastic tissue with a limited regenerative capability because of the lack of vasculature. Mechanical stimulation contributes to the homeostasis of functional AC since it promotes the delivery of nutrients, cytokines and growth factors between the distant chondrocytes. We hypothesized that biomechanical stimulation might enhance mobilization of endogenous mesenchymal stem/stromal cells (MSCs) from neighboring niches as the bone marrow.

Aim

This study aimed to introduce a bioreactor for inducing mobilization of MSCs from one compartment to another above by mechanical stimulation in vitro.

Methods

A novel mechanical system for evaluating mobilization of cells in a 3D context in vitro is presented. The system consists of a compression bioreactor able to induce loading on hydrogel-based scaffolds, custom-made software for settings management and data recording, and image based biological evaluation. Intermittent load was applied under a periodic regime with frequency of 0.3 Hz and unload phases of 10 seconds each 180 cycles over 24 hours. The mechanical stimulation acted on an alginate scaffold and a cell reservoir containing MSCs below it. The dynamic compression exerted amplitude of 200 μm as 10% strain regarding the original height of the scaffold.

Results

The bioreactor was able to stimulate the scaffolds and the cells for 24.4 (±1.7) hours, exerting compression with vertical displacements of 185.8 (±17.8) μm and a force-amplitude of 1.87 (±1.37; min 0.31, max 4.42) N. Our results suggest that continuous mechanical stimulation hampered the viability of the cells located at the cell reservoir when comparing to intermittent mechanical stimulation (34.4 ± 2.0% vs. 66.8 ± 5.9%, respectively).

Functionalizing alginate scaffolds with laminin-521 (LN521) seemed to enhance the mobilization of cells from 48 (±21) to 194 (±39) cells/mm³ after applying intermittent mechanical loading.

Conclusion

The bioreactor presented here was able to provide mechanical stimulation that seemed to induce the mobilization of MSCs into LN521-alginate scaffolds under an intermittent loading regime.

Multi-arm Avidin nano-construct for intra-cartilage delivery of small molecule drugs

Targeted drug delivery to joint tissues like cartilage remains a challenge that has prevented clinical translation of promising osteoarthritis (OA) drugs. Local intra-articular (IA) injections of drugs suffer from rapid clearance from the joint space and slow diffusive transport through the dense, avascular cartilage matrix comprised of negatively charged glycosaminoglycans (GAGs). Here we apply drug carriers that leverage electrostatic interactions with the tissue's high negative fixed charge density (FCD) for delivering small molecule drugs to cartilage cell and matrix sites. We demonstrate that a multi-arm cationic nano-construct of Avidin (mAv) with 28 sites for covalent drug conjugation can rapidly penetrate through the full thickness of cartilage in high concentration and have long intra-cartilage residence time in both healthy and arthritic cartilage via weak-reversible binding with negatively charged aggrecans. mAv's intra-cartilage mean uptake was found to be 112× and 33× the equilibration bath concentration in healthy and arthritic (50% GAG depleted) cartilage, respectively. mAv was conjugated with Dexamethasone (mAv-Dex), a broad-spectrum glucocorticoid, using a combination of hydrolysable ester linkers derived from succinic anhydride (SA), 3,3-dimethylglutaric anhydride (GA) and phthalic anhydride (PA) in 2:1:1 M ratio that enabled 50% drug release within 38.5 h followed by sustained release in therapeutic doses over 2 weeks. A single 10 μM low dose of controlled release mAv-Dex (2:1:1) effectively suppressed IL-1α-induced GAG loss, cell death and inflammatory response significantly better than unmodified Dex over 2 weeks in cartilage explant culture models of OA. With this multi-arm design, <1 μM Avidin was needed - a concentration which has been shown to be safe, preventing further GAG loss and cytotoxicity. A charge-based cartilage homing drug delivery platform like this can elicit disease modifying effects as well as facilitate long-term symptomatic pain and inflammation relief by enhancing tissue specificity and prolonging intra-cartilage residence time of OA drugs. This nano-construct thus has high translational potential for enabling intra-cartilage delivery of a broad array of small molecule OA drugs and their combinations to chondrocytes, enabling OA treatment with a single injection of low drug doses and eliminating toxicity issues associated with multiple high dose injections.

Contrast enhanced computed tomography for real-time quantification of glycosaminoglycans in cartilage tissue engineered constructs

Tissue engineering and regenerative medicine are two therapeutic strategies to treat, and to potentially cure, diseases affecting cartilaginous tissues, such as osteoarthritis and cartilage defects. Insights into the processes occurring during regeneration are essential to steer and inform development of the envisaged regenerative strategy, however tools are needed for longitudinal and quantitative monitoring of cartilage matrix components. In this study, we introduce a contrast-enhanced computed tomography (CECT)-based method using a cationic iodinated contrast agent (CA4+) for longitudinal quantification of glycosaminoglycans (GAG) in cartilage-engineered constructs. CA4+ concentration and scanning protocols were first optimized to ensure no cytotoxicity and a facile procedure with minimal radiation dose. Chondrocyte and mesenchymal stem cell pellets, containing different GAG content were generated and exposed to CA4+. The CA4+ content in the pellets, as determined by micro computed tomography, was plotted against GAG content, as measured by 1,9-dimethylmethylene blue analysis, and showed a high linear correlation. The established equation was used for longitudinal measurements of GAG content over 28 days of pellet culture. Importantly, this method did not adversely affect cell viability or chondrogenesis. Additionally, the CA4+ distribution accurately matched safranin-O staining on histological sections. Hence, we show proof-of-concept for the application of CECT, utilizing a positively charged contrast agent, for longitudinal and quantitative imaging of GAG distribution in cartilage tissue-engineered constructs. STATEMENT OF SIGNIFICANCE: Tissue engineering and regenerative medicine are promising therapeutic strategies for different joint pathologies such as cartilage defects or osteoarthritis. Currently, in vitro assessment on the quality and composition of the engineered cartilage mainly relies on destructive methods. Therefore, there is a need for the development of techniques that allow for longitudinal and quantitative imaging and monitoring of cartilage-engineered constructs. This work harnesses the electrostatic interactions between the negatively-charged glycosaminoglycans (GAGs) and a positively-charged contrast agent for longitudinal and non-destructive quantification of GAGs, providing valuable insight on GAG development and distribution in cartilage engineered constructs. Such technique can advance the development of regenerative strategies, not only by allowing continuous monitoring but also by serving as a pre-implantation screening tool.

Interleukin-1 receptor antagonist (IL-1Ra) is more effective in suppressing cytokine-induced catabolism in cartilage-synovium co-culture than in cartilage monoculture

Background

Most in vitro studies of potential osteoarthritis (OA) therapies have used cartilage monocultures, even though synovium is a key player in mediating joint inflammation and, thereby, cartilage degeneration. In the case of interleukin-1 (IL-1) inhibition using its receptor antagonist (IL-1Ra), like chondrocytes, synoviocytes also express IL-1 receptors that influence intra-articular IL-1 signaling and IL-1Ra efficacy. The short residence time of IL-1Ra after intra-articular injection requires the application of frequent dosing, which is clinically impractical and comes with increased risk of infection; these limitations motivate the development of effective drug delivery strategies that can maintain sustained intra-articular IL-1Ra concentrations with only a single injection. The goals of this study were to assess how the presence of synovium in IL-1-challenged cartilage-synovium co-culture impacts the time-dependent biological response of single and sustained doses of IL-1Ra, and to understand the mechanisms underlying any co-culture effects.

Methods

Bovine cartilage explants with or without synovium were treated with IL-1α followed by single or multiple doses of IL-1Ra. Effects of IL-1Ra in rescuing IL-1α-induced catabolism in cartilage monoculture and cartilage-synovium co-culture were assessed by measuring loss of glycosaminoglycans (GAGs) and collagen using DMMB (dimethyl-methylene blue) and hydroxyproline assays, respectively, nitric oxide (NO) release using Griess assay, cell viability by fluorescence staining, metabolic activity using Alamar blue, and proteoglycan biosynthesis by radiolabel incorporation. Day 2 conditioned media from mono and co-cultures were analyzed by mass spectrometry and cytokine array to identify proteins unique to co-culture that contribute to biological crosstalk.

Results

A single dose of IL-1Ra was ineffective, and a sustained dose was necessary to significantly suppress IL-1α-induced catabolism as observed by enhanced suppression of GAG and collagen loss, NO synthesis, rescue of chondrocyte metabolism, viability, and GAG biosynthesis rates. The synovium exhibited a protective role as the effects of single-dose IL-1Ra were significantly enhanced in cartilage-synovium co-culture and were accompanied by release of anti-catabolic factors IL-4, carbonic anhydrase-3, and matrilin-3. A total of 26 unique proteins were identified in conditioned media from co-cultures, while expression levels of many additional proteins important to cartilage homeostasis were altered in co-culture compared to monocultures; principal component analysis revealed distinct clustering between co-culture and cartilage and synovium monocultures, thereby confirming significant crosstalk.

Conclusions

IL-1Ra suppresses cytokine-induced catabolism in cartilage more effectively in the presence of synovium, which was associated with endogenous production of anti-catabolic factors. Biological crosstalk between cartilage and synovium is significant; thus, their co-cultures should better model the intra-articular actions of potential OA therapeutics. Additionally, chondroprotective effects of IL-1Ra require sustained drug levels, underscoring the need for developing drug delivery strategies to enhance its joint residence time following a single intra-articular injection.

SOLUTE TRANSPORT IN ARTICULAR CARTILAGE UNDER ROLLING-COMPRESSION LOAD

Solute transport is one of the important aspects involved in maintaining the physiological activity of tissues. The mechanical environment drives nutrition in and waste out in articular cartilage due to its avascularity, which plays a key role in the biological activity of articular cartilage. The human knee joint motion is a complex interaction between different bones including relative rolling and/or sliding movements. Rolling-compression process is a typical physiological load in knee joint motion. To investigate solute transport behavior in articular cartilage under rolling-compression load, fluorescence tracers with molecular weights of 40kDa and 0.43kDa were used respectively to mark the transport in fresh articular cartilage of mature pigs. Solute fluorescence intensity changing with time and depth of cartilage layer was measured under rolling-compression load and static state, respectively, and the distribution of corresponding relative concentration was calculated by the fluorescence microscope imaging method. The experiment results show that the solute relative concentration in articular cartilage under rolling-compression load increases significantly, even up to 62.4%, comparing with that under static state, and the changes of concentration vary in different layers and that small molecular weight solute is easier to transport than relatively large molecular weight solute in articular cartilage. Therefore, rolling-compression load can promote the solute transport in cartilage, and the mechanical loading may have application in functional cartilage tissue engineering.

Cartilage penetrating cationic peptide carriers for applications in drug delivery to avascular negatively charged tissues

Drug delivery to avascular, negatively charged tissues like cartilage remains a challenge. The constant turnover of synovial fluid results in short residence time of administered drugs in the joint space and the dense negatively charged matrix of cartilage hinders their diffusive transport. Drugs are, therefore, unable to reach their cell and matrix targets in sufficient doses, and fail to elicit relevant biological response, which has led to unsuccessful clinical trials. The high negative fixed charge density (FCD) of cartilage, however, can be used to convert cartilage from a barrier to drug entry into a depot by making drugs positively charged. Here we design cartilage penetrating and binding cationic peptide carriers (CPCs) with varying net charge, spatial distribution and hydrophobicity to deliver large-sized therapeutics and investigate their electro-diffusive transport in healthy and arthritic cartilage. We showed that CPC uptake increased with increasing net charge up to +14 but dropped as charge increased further due to stronger binding interactions that hindered CPC penetrability and uptake showing that weak-reversible binding is key to enable their penetration through full tissue thickness. Even after 90% GAG depletion, while CPC +14 uptake reduced by over 50% but still had a significantly high value of 148× showing that intra-tissue long-range charge-based binding is further stabilized by short-range H-bond and hydrophobic interactions. The work presents an approach for rational design of cationic carriers based on tissue FCD and properties of macromolecules to be delivered. These design rules can be extended to drug delivery for other avascular, negatively charged tissues. STATEMENT OF SIGNIFICANCE: Osteoarthritis (OA) remains an untreatable disease partly due to short joint residence time of drugs and a lack of delivery methods that can effectively target the dense, avascular, highly negatively charged cartilage tissue. In this study, we designed cartilage penetrating and binding cationic peptide carriers (CPCs) that, due to their optimal charge provide adequate electrical driving force to rapidly transport OA drugs into cartilage and reach their cell and matrix targets in therapeutic doses before drugs exit the joint space. This way cartilage is converted from being a barrier to drug entry into a drug depot that can provide sustained drug release for several weeks. This study also investigates synergistic effects of short-range H-bond and hydrophobic interactions in combination with long-range electrostatic interactions on intra-cartilage solute transport. The work provides rules for rational design of cartilage penetrating charge-based carriers depending on the net charge of tissue (normal versus arthritic), macromolecule to be delivered and whether the application is in drug delivery or tissue imaging.