APS

Understanding this exciting yet complex field of regenerative medicine

The idea, of course, is brilliant: Collect, enhance, and reintroduce horses’ own natural healing powers, so they can heal better.

Such is the concept behind orthobiologics—the use of biological products such as stem cells and blood components to treat damaged musculoskeletal systems. It merges orthopedics—the branch of medicine concerned with the skeleton and associated structures, such as tendons and ligaments—and biologics, which in this context are naturally derived substances with healing properties.

Despite its growing popularity, the field of orthobiologics remains fraught with confusion, contradiction, and questions. Fortunately, researchers are addressing those issues relentlessly, having published nearly 100 studies and reviews in the past year. Our sources share updates from the equine world to help you understand these therapies.

Mother Nature’s Sustainable Regenerators

Musculoskeletal injuries heal with scar tissue, which is tough, rigid, and inferior to the more malleable tissue the horse once had, says Maarten Oosterlinck, DVM, PhD, Dipl. ECVSMR, ECVS, of the Department of Large Animal Surgery, Anaesthesia and Orthopaedics at Ghent University, in Belgium.

The initial idea behind orthobiologics was they would regenerate tissue that’s more like the original—hence the term regenerative medicine, he says.

The latest research suggests orthobiologics don’t copy original tissue that leads to scar-free healing. However, they do seem to have a remarkable capacity to stop disease progression while reconstructing tissue in a more usable, elastic, and sustainable way, says Lauren Schnabel, DVM, PhD, Dipl. ACVS, ACVSMR, associate professor of equine orthopedic surgery at North Carolina State College of Veterinary Medicine, in Raleigh.

“We’ve shown in studies … that the tendon healing you get is superior to those not treated with orthobiologics, because you get more normal tissue (with) more normal functional properties that are less prone to re-injury,” she says.

Orthobiologics harness the body’s natural defense mechanisms and antimicrobial properties, with reduced side effects compared to traditional pharmaceutical products, Schnabel explains. “It’s really taking what already happens naturally in the body and sort of magnifying it or making it optimal,” she says.

 

APS, orthobiologics, regenerative medicine

Stem/Stromal Cells

Named for their ability to “branch out” to become cells that build multiple kinds of tissue, mesenchymal stem cells (MSC) and mesenchymal stromal cells appear in embryos and adult bone marrow, respectively. In 2019 scientists suggested redefining the MSC acronym as “medicinal signaling cells,” because the therapeutic cells usually come from bone marrow rather than the mesenchyme—a part of embryos. In this article we’ll refer to all these similar cell types as stem cells.

Twenty years ago biologists had successfully developed stem cells into cartilage, tendon, heart, bone, and other kinds of tissues in their laboratories—spurring hope for unprecedented tissue repair therapies in humans and animals.

Scientists now know stem cells use a very different healing mechanism than previously suspected, Oosterlinck says. In fact, recent research has revealed it’s not what these cells become that matters, but what they do—and, more specifically, how they do it, in large part through a process known as paracrine signaling.

“Basically, there’s a lot of crosstalk that happens between stem cells and the injured cells,” Schnabel says. That crosstalk seems to “recruit” special cells within the injured tissue called progenitor cells—and those appear to develop into the original local tissue cells, such as tendon cells.

Some research groups are looking at ways to predifferentiate cells in a ­laboratory—preparing them to go into tendon versus cartilage tissue, for example—to help encourage them to be more effective in their therapeutic environment, Oosterlinck says.

Meanwhile, other research teams are showing how stem cells trigger specific cytokines (cell-signaling proteins) and growth factors that contribute to better tissue healing, he says.

The cells also encourage vascularization, says Schnabel. “They actually bring blood vessels into the area, promoting angiogenesis,” she says.

These recent discoveries about stem cell functions are major breakthroughs, says Schnabel. Her team currently focuses on optimizing the use of these cells. One thing they’re homing in on is dosing, which has always been complex, especially in horses, because it’s impossible, she says, to achieve the per-pound dose of cells recommended in human medicine.

They’re also fine-tuning the question of treatment timing, she says. Traditionally, clinicians have treated horses with stem cells once the initial inflammatory response from the injury subsides. “The idea was that you don’t want to cause more inflammation, potentially, and have an even lamer horse, and you don’t want the stem cells to get killed by the inflammation,” she explains.

Ultimately, that might not be the right approach. “All the studies we’ve been doing actually suggest the opposite, that having them in an inflammatory environment is good, because it further primes the cells to secrete the things you want,” she says.

“Then that also begs the question, if you get a horse after the time of acute inflammation, could you prime the cells first in the lab so that they’re ready to go when you put them in the horse?” she adds. “And that’s been a major focus of our work. We have a lot of strong preliminary data suggesting that that’s true.”

Schnabel’s team, as well as other research groups, have looked specifically at tendon healing, revealing that horses treated with stem cells have significantly reduced re-injury rates, especially in the superficial digital flexor tendon (SDFT)—which has a traditional re-injury rate of up to 70% in racehorses (RK Smith, et al.). “This is huge,” she says.

Platelet-Rich Plasma (PRP)

Unique to mammals, platelets are sphere-shaped cell fragments that circulate in the blood and play major roles in clotting. Scientists have discovered that platelets release growth factors from their internal structures; these growth factors promote healing, says Schnabel. When injected into tendons and ligaments, PRP appears to help build collagen Type 1, which is more elastic than Type 3—the scar tissue kind.

To harness that power, clinicians draw blood from the horse’s jugular veins and centrifuge it in a way that doubles or even triples the concentration of platelets before injecting the resulting product into the injured tissue, she says.

Platelet-rich plasma isn’t effective unless the platelets get activated, Schnabel explains. Often, the injured tissue activates them, but new research suggests shock wave therapy to the treated tissue might give an even better boost, she says. 

In the past couple of years, Schnabel’s team has also been testing platelets’ antimicrobial properties, which appear to be related to tiny proteins called antimicrobial peptides. These peptides “are capable of really great things”—namely, attacking pathogenic microorganisms such as bacteria, viruses, parasites, and fungi in a nonpharmaceutical way, she says.

“They can kill microorganisms directly and may also make them more susceptible to pharmaceuticals by exposing them to the immune system,” she explains.

This is particularly useful when it comes to eliminating biofilms—­­­extra-resistant layers of pathogenic bacteria, fungi, or protists that form on wounds and hinder healing. “They can actually bore through those biofilms and kill the bacteria,” Schnabel says.

Her group recently published their research showing how a specialized version of PRP could eradicate biofilms in synovial (joint) fluid in the laboratory. She says another study, showing this specialized PRP treats hock infections more effectively than traditional antibiotics, is currently under review.

Getting those antimicrobial properties requires an extra-rich formulation of PRP—with platelets concentrated to about 50 times greater than in normal blood, she says. “It’s an extremely specialized product, but it’s based on the same principle that platelets contain all these amazing peptides and growth factors, and then it’s just streamlining that.”

Autologous Conditioned Serum and Autologous Protein Solution

White blood cells secrete a valuable healing component called interleukin-1 receptor antagonist protein (IRAP). This works as an anti-inflammatory by blocking the effects of a potent cytokine called interleukin-1 (IL-1), which drives ­inflammation—especially in joint osteoarthritis. “IRAP basically sits on the receptor of IL-1,” says Schnabel.

Autologous conditioned serum (ACS) and autologous protein solution (APS) are chock full of IRAP because the ­production process stimulates white blood cells to release high concentrations of it, Oosterlinck says.

Because ACS and APS start with whole blood—including platelets—they’re full of growth factors, as well, Schnabel adds. Combined, the effects of these biologics halt disease progression to allow for better healing, unlike non-steroidal anti-inflammatories (NSAIDs) and analgesics, which essentially just treat the clinical signs (e.g., pain).

“That’s why these products are so attractive, because they’re focused on stopping the disease process, not just masking the effects of the disease,” she says.

While they can encourage healthy tissue growth, it’s important to have realistic expectations about what these products can do, cautions Schnabel. “We’re not talking about reversing cartilage damage,” she says. “But they can have some really powerful effects on structures like the lining of the joint, the synovial capsule, and soft tissues within the joint.”

APS and ACS are similar products, she says, differing mainly in how they’re made and how frequently they’re injected. ACS requires overnight incubation, often up to 24 hours, whereas APS can be centrifuged stall-side during the veterinary visit.

APS has become the clinician’s go-to biologic compared to ACS, “just because of the practicalities,” says Oosterlinck. “You can make it while you’re standing next to the horse, and 20 minutes later you have your product and can inject it wherever you like.”

But ACS processing yields a much greater volume of product, says Schnabel. And while it requires multiple injections (typically three to five, seven to 10 days apart) as opposed to just one for APS, the sheer volume of ACS could make it the better option when it comes to multiple joints or large joints like the stifle. “ACS is often more cost-effective,” she says.

Both products seem equally effective at treating cartilage and synovial cells in the laboratory, Schnabel says. So far, no one has published comparisons of ACS and APS in live horses.

Who’s the Donor?

Traditionally, orthobiologics come from the patient, making them autologous (self-supplied). But recently, scientists have been developing biologics sourced from other horses. As long as the allogeneic products are cell-free—­meaning PRP, ACS, and APS, but not stem cells—they’re far more practical because they can be prepared in advance in ready-to-use formats, without risking immune reactions to donor products, says Oosterlinck.

Allogeneic biologics also benefit from the good of the masses—essentially, increasing the chances of a higher-quality product, adds Schnabel. That’s because quality is highly variable from one individual to the next—with some having much higher amounts and different types of antimicrobial peptides, for example, than others. “Studies have shown that if you pool them, you have much better efficacy,” she says. “You’re getting the best of all the properties.”

Even so, allogeneic acellular products require strict quality control and regular donor horse screening for transmissible diseases, including equine hepatitis viruses, says Schnabel. Such screening is critical for product safety, she says.

As for stem cells, those derived from blood (not bone marrow) can be prepared from donors, with benefits such as not having to take the patient’s cells to the lab and culture them before injection, says Oosterlinck.

Take-Home Message

Orthobiologics might not work the way people once thought, but many clinicians continue to find significant therapeutic benefits—likely related to the release of growth factors and cytokines as well as cell signaling that seems to trigger healing. Horses treated with orthobiologics often heal better, with more elastic and usable tissue, than those treated without. Our sources say ongoing research should focus on standardized, controlled experiments to better understand the effects of different doses and timing regimens.