Arthritis and joint pain plague aging people worldwide. Presently, there are no drugs that directly treat osteoarthritis, one of the most prevalent disabling conditions. Available treatments only address the pain associated with the condition, rather than the underlying causes.

Osteoarthritis causes the cartilage that cushions the joints to break down, which also leads to changes in the underlying bone, causing intense pain and stiffness in the joints. Cartilage damage leads to inflammation in the joint, which further exacerbates cartilage loss and bone alterations. For years, researchers and clinicians thought osteoarthritis was simply the result of overusing the joints over time since osteoarthritis occurs most commonly as people age.

“It really was thought to be just a wear and tear process. You've gotten old. Your joints wore out. And now we know that that's not really the case. It's an active disease,” said Farshid Guilak, a bioengineer at Washington University in St. Louis. “Cartilage, which gets so affected, is a really complex tissue that looks really simple.”

Unlike other tissues in the body, cartilage is completely avascular. It has no blood supply, no nerves, and no connection to the lymphatic system. Because cartilage lacks these connections, it cannot repair itself if it gets damaged. Osteoarthritis progresses slowly over the course of decades, but once the damage is done, it only gets worse.

Osteoarthritis doesn’t only affect older people. Young people who tear their anterior cruciate ligaments (ACL) are at risk for developing osteoarthritis within 10 to 15 years after their injury (1). If “you’re an athlete in your early 30s, and you get osteoarthritis and you need to get your knee or hip replaced, it's only going to be a few years before you need it replaced again. And each time it's replaced, it has more significant consequences in terms of quality of life and the invasiveness of the procedure,” said Fergal O’Brien, a bioengineer at the Royal College of Surgeons in Ireland.

Over the past 20 or so years , researchers have identified multiple drug targets and potential therapeutics for osteoarthritis, but a major roadblock facing these novel treatments has been how to deliver them to the joints. If scientists deliver drugs systemically in the blood, the dose of drug that finally reaches the joint is often not high enough to have a therapeutic effect. Even drugs injected directly into the joint diffuse away too quickly to be effective.

To overcome these challenges, osteoarthritis researchers and engineers have developed new ways to deliver osteoarthritis treatments to the joints. From scaffolds with gene therapy-carrying nanoparticles to protein-based gels and “smart cells” that sense the state of the joint and release drugs autonomously, osteoarthritis treatments are finally getting where patients need them most. 

Release the nanoparticles

One way to treat the damage associated with osteoarthritis is to target the underlying tissue itself: bone. By using nanoparticles to deliver microRNAs that block inhibitors of bone repair genes to stem cells in the bone, O’Brien hopes to halt osteoarthritis progression and help bones heal. Delivering nanoparticles to cells in the joint, however, suffers from the same challenges other researchers face when trying to deliver arthritis drugs there.

“The problem is, you’re throwing a whole bunch of particles into an inflamed environment, and due to the nature of that joint, those particles are going to get cleared very, very rapidly. So, you'll be very lucky to get any particles to the site of interest,” O’Brien said.

Rather than injecting the nanoparticles into the joint, O’Brien and his team developed a flexible scaffold made of collagen and hyaluronic acid to carry the nanoparticles that surgeons could place directly into the diseased joint (2). By using the scaffold, the researchers ensured that the nanoparticles remained at the injured joint, reducing the potential for off-target effects and preventing the nanoparticles from being cleared by the body.

The researchers found that stem cells in the bone migrated onto the scaffold over a few weeks. By loading the scaffold with nanoparticles, the cells there “gobble up those particles,” said O’Brien. “What we've been able to do now is to both target the prochondrogenic genes and also to target switching off the ones that are associated with inflammation.” The cells expressed the microRNA gene therapy for at least six to eight weeks.

O’Brien and his team tested these micro-RNA-nanoparticle-loaded scaffolds in rats and observed an increase in bone volume and calcium deposition (3). They hope to begin studies in larger animals soon.                                                                        

“I've been fortunate enough to see two technologies from my lab already go all the way to humans and know now that they worked effectively in those human patients,” said O’Brien. “We will be looking to continue that story and bring it into humans.”

Cartilage healing gel

While a scaffold fit to a damaged joint is much smaller than an entire hip replacement, researchers at New York University wondered if they could design an osteoarthritis therapy that would be even less invasive to deliver. Arthritis researcher Chuan-ju Liu teamed up with bioengineer Jin Montclare to find out.

Liu and his team previously developed the therapeutic protein Atsttrin, which protects against the loss of cartilage-forming cells in joints with osteoarthritis (4, 5). But when researchers tried injecting Atsttrin into arthritic joints, they saw the same problem: it simply diffused away too quickly to be effective. When another research group delivered it to a mouse joint on a scaffold, it did not repair bone injury as well as expected (6).

To find a more effective way to deliver Atsttrin, Liu collaborated with Montclare, whose team engineers proteins that behave like synthetic polymers. Because the polymers are made of protein, they are biocompatible and safe to use in the body. 

A useful polymer for treating osteoarthritis would need to be injectable but also form a more solid structure once inside the joint. Montclare and her team engineered a protein mix made of a domain of the elastin protein (a key protein component of connective tissues) and the coiled-coil domain of a cartilage matrix protein that is liquid at room temperature. Once they added Atsttrin to the mix, they injected the solution into the affected joint.

“Once it hits the body temperature, it forms this very porous gel like Jello,” said Montclare. “This gel that is encapsulating the therapeutic then slowly releases it over time.”

When Montclare and Liu injected their Atsttrin-loaded gel into rabbits, they found that it prevented post-traumatic osteoarthritis from developing after an injury, and if osteoarthritis was already present in the joint, it halted cartilage degradation and started to reverse the cartilage damage (7).

“We didn’t anticipate [that], but we were super excited,” said Montclare. “If we did this in human models, they might behave differently, but hopefully, the idea is to make it a single injection that can revert the process.”

Montclare and Liu are now working on optimizing the dose of Atsttrin and better understanding its pharmacokinetics as it moves out of the gel. They are also tweaking the properties of the protein gel to see if they can further reverse the osteoarthritis damage.

“We were able to identify what the problem was and then systematically engineer our system to apply it for post-traumatic osteoarthritis in this case, but this opens up the possibility of other diseases to apply our materials,” said Montclare. “This opens up a whole new area of doing biomaterials research.”

Autonomous smart cells

Scaffolds and gels in joints get osteoarthritis therapies right where they need to be, but what if the cartilage cells in joints throughout the body could fix themselves at the first sign of damage? By engineering cells to sense inflammation and release anti-inflammatory medicines in response, Guilak uses synthetic biology to create an autonomous therapy for osteoarthritis.

“We can create cells that sense anything that a cell can normally sense, and it turns out, cells are pretty smart. If you tap into their systems, they can certainly sense inflammation,” said Guilak.

As a proof of concept, Guilak and his team tested their hypothesis using a drug for rheumatoid arthritis, which is a less common form of arthritis but one that has available treatments. The researchers deconstructed the genetic pathway that cells use to sense inflammation, and they used CRISPR to create an artificial gene circuit in cartilage cells to produce and release the anti-inflammatory protein IL-1 receptor antagonist (generic name Anakinra) when the cells sense inflammation (8).

Because arthritis occurs in joints throughout the body, Guilak and his team inserted their engineered cartilage cells onto a woven scaffold that they injected just under the skin of a mouse’s back. Guilak compared this to the hormone-releasing implant that people often use for birth control. In this case, the cartilage cells in the implanted scaffold sense systemic inflammation and release anti-inflammatory drugs into the bloodstream to make their way to the affected joints. But the researchers could insert these cell-loaded scaffolds directly into injured joints as well.

Looking for other stimuli that cells might sense to help treat osteoarthritis, Guilak and his team realized that cells in the joint constantly sense the weight of the body as it moves through the world.

“You are putting five times your body weight on this cartilage. What if you could make a cell that sensed loading, and cells would make drugs every time you loaded the joint?” said Guilak.

Repeating the same approach as with the inflammation-sensing cells, the researchers deconstructed how cartilage-forming cells sense weight loading, and they designed synthetic gene circuits so that when those cells sensed more loading, they produced Ankarina (9). Guilak and his team are investigating how much of the anti-inflammatory drug the cells should produce and how much loading should trigger the production and release of the drug.

Guilak and his team hope that one day they can combine their engineered “smart” cartilage cells with the scaffolds made by the company Guilak cofounded, CytexOrtho. The hip-shaped scaffold is an alternative to a hip replacement in that it resurfaces a hip with cartilage and delays the need for a hip replacement (10). CytexOrtho will begin testing this scaffold in clinical trials later this year.

“If we have systems like that, then we can use cells that have been modified on top of regrowing the cartilage. We can actually have anti-inflammatory drugs delivered directly to the joint,” said Guilak. “You're going to have cells that automatically sense disease, whatever it is.”

References

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3. Castaño, I.M. et al. Rapid bone repair with the recruitment of CD206+M2-like macrophages using non-viral scaffold-mediated miR-133a inhibition of host cells. Acta Biomaterialia  109, 267-279 (2020).
4. Tang, W. et al. The Growth Factor Progranulin Binds to TNF Receptors and Is Therapeutic Against Inflammatory Arthritis in Mice. Science  332, 478-484 (2011).
5. Zhao, Y. et al. Progranulin protect against osteoarthritis through interacting with TNF-α and β-Catenin signalling. Ann Rheum Dis  74, 2244-2253 (2015).
6. Wang, Q. et al. 3D-Printed Atsttrin-Incorporated Alginate/Hydroxyapatite Scaffold Promotes Bone Defect Regeneration with TNF/TNFR Signaling Involvement. Adv Healthcare Mater  4, 1701-1708 (2015).
7. Katyal, P. et al. Injectable recombinant block polymer gel for sustained delivery of therapeutic protein in post traumatic osteoarthritis. Biomaterials  281, 121370 (2022).
8. Choi, Y.R. et al. A genome-engineered bioartificial implant for autoregulated anticytokine drug delivery. Sci Adv  7, eabj1414 (2021).
9. Nims, R.J. et al. A synthetic mechanogenetic gene circuit for autonomous drug delivery in engineered tissues. Sci Adv  7, eabd9858 (2021).
10. Moutos, F.T., Freed, L.E. and Guilak, F. A biomimetic three-dimensional woven composite scaffold for functional tissue engineering of cartilage. Nature Mater  6, 162-167 (2007).