Current Equine Autologous Blood–Derived Orthobiologics for Treatment of Musculoskeletal Injuries

Use of autologous blood–derived orthobiologics in clinical equine practice continues to increase as practitioners learn more about their ability to improve healing of musculoskeletal tissues and advantages over conventional therapies, such as steroids that simply decrease inflammation and mask pain.^1-4 Horse owners have been educated about this advantage and, despite the fact that orthobiologics may take longer than steroids to have a positive clinical effect, clients have widely accepted use of orthobiologics to preserve the longevity of their equine athletes and to avoid the potential complications of steroid use. Client acceptance is particularly relevant given the recent heightened awareness of equine metabolic disorders and the sensitivity of equine patients with these disorders to the effects of steroids, which puts them at increased risk for severe complications, including laminitis.⁵

Kits for autologous blood–derived orthobiologics are available. Although using kits will never be as simple or quick as drawing up a steroid dose out of a vial, they are generally feasible for stall-side use by practitioners with a centrifuge that is typically kit specific. Kits must be equine specific due to species variation in how platelets and leukocytes are processed through centrifugation, and all orthobiologics are affected by inherent patient variability and environmental factors. Although a centrifuge can be cumbersome and take up valuable space in an ambulatory truck, most equine practitioners are used to transporting imaging, shockwave, and laser equipment of similar sizes. In addition, use of autologous orthobiologics is minimally invasive for the patient, requiring only venipuncture compared with the more invasive techniques needed to concentrate or conduct long-term culture of autologous mesenchymal stem cells harvested from bone marrow or adipose tissue, making autologous orthobiologics attractive alternatives. Several recent systematic reviews and
meta-analyses have highlighted the value of orthobiologics as well as the critical need for further research to standardize and optimize their use.^1,2,6

This article reviews the most common autologous blood–derived orthobiologics for which kits are commercially available (TABLE 1) in terms of what each orthobiologic is, the premise behind its use, and the evidence that currently exists for its efficacy in the treatment of equine musculoskeletal injuries.

Common Autologous Blood–Derived Orthobiologics

Platelet-Rich Plasma

Platelet-rich plasma (PRP) is a broad term that has been used to describe plasma products containing a higher concentration of platelets than that of whole blood and varying amounts of leukocytes, red blood cells (RBCs), and plasma proteins. Platelets are a vital source of growth factors (e.g., platelet-derived growth factor [PDGF], insulin-like growth factor [IGF], transforming growth factor β1 [TGF-β1], fibroblast growth factor [FGF], vascular endothelial growth factor [VEGF]) as well as numerous cytokines and chemokines that are released from their storage α-granules during activation and that modulate healing by promoting angiogenesis, cellular migration, cellular proliferation, cellular differentiation, and matrix synthesis. Although PRP classification systems are now well defined, the field of equine medicine has lagged in adopting them and characterizing equine PRP products in the literature and daily clinical practice, making it difficult to draw conclusions about their efficacy.^7,8

The 2 most commonly used types of PRP in equine practice are pure, or leukocyte-poor, PRP (P-PRP) and leukocyte-rich PRP (L-PRP). Commercially available kits can be found in TABLE 1.

• P-PRP contains plasma and platelets with minimal leukocytes and no RBCs and is most commonly generated through single soft-spin centrifugation systems, which keep the platelets above the buffy coat containing the leukocytes. More recently, commercially available kits that use sequential centrifugation techniques have come on the market to produce P-PRP with more concentrated platelets than can be generated through single soft-spin centrifugation systems.
• L-PRP contains platelets, leukocytes, some RBCs, and a small amount of plasma and is generated through single or sequential hard-spin centrifugation systems or gravity filtration systems. The equine-specific commercially available kits currently used to generate L-PRP are all centrifugation systems.

In addition, a few equine-specific commercially available kits are marketed as adjustable/variable to produce either P-PRP or L-PRP, depending on how they are manipulated. P-PRP and L-PRP kits are generally processed with anticoagulant citrate dextrose solution, solution A (ACD-A), which preserves platelet morphology and function, takes less than 20 minutes to process, and generates approximately 6 to 10 mL of final product. Of the kits previously listed in TABLE 1, ProVet APC is unique in that its specific centrifuge is much smaller and easier to transport than others. The Rebound PRP kit does not require a specific centrifuge; thus, most standard centrifuges can be used.

Recent systematic reviews and meta-analyses support the overall use of equine PRP products for the treatment of osteoarthritis and tendon and ligament injuries (specifically superficial digital flexor tendon [SDFT] injuries) but highlight the fact that PRP classification and standardization are needed to determine the optimal PRP formulation for each specific injury being treated.^1,2,6 A large amount of research is needed to determine optimal platelet concentration and dosing per volume as well as optimal leukocyte concentration. Although there are currently no consensus statements in equine medicine, several large meta-analyses and consensus statements from human medicine have found P-PRP to be superior to L-PRP for the treatment of osteoarthritis.^9,10 Similar studies for soft tissue injuries in humans have been variable and site specific^11,12; some studies also have suggested a role for L-PRP in the treatment of more chronic soft tissue injuries.^13,14

Autologous Conditioned Serum

Autologous conditioned serum (ACS) is an acellular product that is produced by incubating whole blood with medical-grade chromium sulfate–etched glass beads or borosilicate beads for 18 to 24 hours at 37 °C (98.6 °F), followed by centrifugation. The beads are used to condition or stimulate the blood components, primarily monocytes, to release cytokines and growth factors into the serum, which is then isolated for use. When first described in the 1990s, the method was largely focused on conditioning monocytes to release interleukin-1 (IL-1) receptor antagonist (IL-1Ra) protein (IRAP), a competitive receptor antagonist to the major inflammatory cytokine IL-1, for which some of the commercially available kits are named.¹⁵ Although IL-1 receptor blockade by IL-1Ra is still viewed as one of the main mechanisms by which ACS exerts its disease-modifying effect, it is now known that this method of conditioning increases the concentration of additional disease-modifying anti-inflammatory cytokines (IL-4, IL-10, and IL-13) and growth factors (IGF-1, TGF-β1, FGF, PDGF, VEGF, and HGF [hepatocyte growth factor]) as well as several proinflammatory cytokines (IL-1β and TNF-α [tumor necrosis factor α]).¹⁶ For this reason, research studies have evaluated the ratio of IL-1Ra to IL-1β in both ACS and autologous protein solution to ensure that IL-1Ra is being concentrated to a greater extent than IL-1β, and several studies of humans have linked this ratio to outcomes.^17,18

Commercially available kits for equine ACS processing  can be found in TABLE 1. The ACS systems produce a relatively large volume of final product (average 20 mL) from which aliquots can be frozen, and the manufacturer recommends treating osteoarthritis with a series of 3 intra-articular injections every 7 to 14 days. Few studies have evaluated the effect of ACS on osteoarthritis, and several of these studies had very small numbers or lacked control groups.¹⁹ Nevertheless, mixed results have been reported and are potentially reflective of osteoarthritis severity or the inherent variability of this autologous orthobiologic.^19-23

A study examining the clinical effect of ACS in 20 horses with naturally occurring lameness found that outcome was associated with the ACS cytokine profile.²³ Specifically, concentrations of IGF-1 and IL-1Ra in their ACS product were higher in responder horses in which lameness was improved or abolished than among nonresponder horses in which lameness was not improved.²³

Although not marketed as such, ACS is also being used in equine practice to treat tendon and ligament injuries. A single clinical study in this regard has been published, in which 15 horses with 17 naturally occurring SDFT lesions received either a single intralesional injection of ACS or saline as a control.²⁴ Compared with controls, lameness scores for ACS-treated horses were reduced, ultrasonography scores were improved at certain time points throughout the study, and collagen type I expression on tendon needle biopsy samples were increased, suggestive of improved healing.²⁴

Autologous Protein Solution

Autologous protein solution (APS) is a highly cellular product produced through a 20-minute point-of-care technique by using a single commercially available kit (TABLE 1). In a 2-step process, blood mixed with ACD-A is first processed to separate out L-PRP, which is then filtered through polyacrylamide beads to produce the final small-volume APS product (average 3 to 4 mL) for which the manufacturer recommends a single injection for treatment. This method combines the beneficial effects of increased IL-1Ra with additional potential therapeutic effects from the L-PRP containing cytokines and growth factors. There is controversial evidence, however, as to use of such a highly concentrated leukocyte product (up to 12× whole blood) for the treatment of osteoarthritis, as discussed previously in the PRP section. If desired, the platelet-poor plasma produced in the first APS processing step that is normally discarded may be used to dilute and increase the volume of the final APS product. Recent evidence indicates that the platelet-poor plasma generated contains α₂-macroglobulin (α₂M), which may provide anti-inflammatory effects and improve healing.²⁵

Similar to ACS, a limited number of published studies evaluated the use of APS to treat osteoarthritis in horses and only 1 study evaluated the efficacy of APS for the treatment of tendon lesions. A clinical study examining the effects of APS on naturally occurring lameness in 40 horses found significant improvements in lameness grade, peak vertical force, and joint range of motion at 14 days in the APS-treated group compared with baseline values and the control group.²⁶ During follow-up phone surveys, horse owners also reported lameness improvement at 12 and 52 weeks after treatment for horses in the APS treatment group.²⁶ In a more recent study evaluating the effect of APS treatment on experimentally induced acute synovitis in horses, APS did not decrease joint circumference, synovial fluid parameters, or subjective and objective lameness scores in horses.²⁷ However, gross and histopathology scores of APS-treated joints were significantly better than those of control joints and were similar to those of normal joints, suggesting that APS has disease-modifying effects.²⁷ In the only published study to the author’s knowledge that evaluated the use of APS for tendon healing, treatment of collagenase-induced SDFT lesions with a single intralesional injection of APS did not result in any significant differences in ultrasonographic or histopathologic scores compared with saline-treated controls.²⁸ APS-treated tendons did, however, have decreased DNA content, decreased expression of collagen type III, and a trend for higher modulus of elasticity during biomechanical testing compared with saline-treated control tendons, perhaps suggestive of improved healing and warranting further investigation.²⁸

α₂-Macroglobulin

α₂M is a naturally occurring large tetrameric protein that functions as a broad-spectrum protease inhibitor and inflammatory cytokine mediator. α₂M can inhibit proteases involved in inflammation and tissue catabolism through its unique structure in a “bait and trap” mechanism in which the protease is trapped inside the α₂M protein and cleared from circulation via the liver.^25,29-31 In addition, α₂M has been shown to directly bind and limit the action of proinflammatory cytokines responsible for articular cartilage degradation.^25,29-31 Unfortunately, even under inflammatory conditions, α₂M does not diffuse well into the joint from the systemic circulation due to its large size and molecular weight, which is why injecting concentrated α₂M directly into the joint is desirable.

In horses, α₂M is typically processed with a single commercially available kit (TABLE 1) in final volumes of 15 mL or 30 mL, from which aliquots can be frozen for future use. The resultant Alpha2EQ is an acellular product that is generated via a 2-step process in which plasma is first isolated from whole blood with ACD and then placed in a proprietary filtration vial that is centrifuged, allowing α₂M to be filtered. On the basis of its mechanism of action and published studies in other species, it is recommended that α₂M be delivered during the acute inflammatory phase to block the damaging cytokines and proteases present during that time. Although studies in other species, including a clinical trial among humans with knee osteoarthritis, support the use of α₂M,³² there are currently no published studies on Alpha2EQ use in horses; more research is needed to support its widespread clinical use in the treatment of osteoarthritis and other joint and axial skeleton pathology.

Patient Considerations

The composition of all of the autologous blood–derived orthobiologics described above is inherently variable due to the natural variability among patients with regard to cytokine and growth factor concentrations as well as specific patient factors and blood-collection techniques that can affect quality at the time of processing. The general health of the patient must always be considered, as well as recent immune system disturbances, administered medications and supplements, and exercise history. The following recommendations for washout periods before blood collection for orthobiologic processing are based on the available literature^33-39:

• A 24-hour washout period is recommended after NSAID administration, short-acting sedation, general anesthesia, surgical procedures, and bouts of intensive exercise.
• A 1-week washout period is recommended after vaccination.
• A 2-week washout period is recommended after use of long-acting tranquilizers (e.g., reserpine, trazodone), pentosan administration, and fatty acid supplementation.

Washout periods must be selected conservatively on a case-by-case basis after other conditions have been corrected or resolved (e.g., pituitary pars intermedia dysfunction or other metabolic disorders, dehydration, illness/fever, conditions warranting antibiotic or systemic steroid administration). In all circumstances, it is imperative that when the patient is ready for blood collection for orthobiologic processing that the jugular vein undergo sterile preparation before careful, slow, and steady aspiration of blood via venipuncture with an 18-G or 19-G butterfly needle.

Storage

Since many of the autologous blood–based orthobiologics generate final volumes greater than needed for a single treatment and are often used for repeated dosing, freezing of the products is commonly performed by equine practitioners in the clinical setting. Although more research is needed, general recommendations are to avoid automatic defrost freezers that undergo freeze/thaw cycles that degrade proteins and to store sterile aliquots of orthobiologics at the coldest temperatures possible (i.e., at least in a −20 °C [−4 °F] manual defrost freezer but ideally at −80 °C [−112 °F] or in liquid nitrogen).⁴⁰ In addition, although most kit manufacturers recommend that frozen orthobiologics be used within 6 months, kits containing growth factors such as PRP, ACS, and APS should ideally be used within 1 month to avoid loss of IGF-1 as IGF-1 has been linked to positive outcome in 1 study.^23,40

The Future of Orthobiologics

A large amount of research is needed to define the optimal composition of each product as well as the optimal dosing, delivery method, timing of treatment protocols for injury-specific applications, and when the patient can return to work after treatment. Furthermore, future research must focus on interactions of orthobiologics with other commonly used therapeutics (e.g., polyacrylamide hydrogels for joint injury) as well as interactions of orthobiologics with physical rehabilitation protocols and commonly used therapeutic modalities (e.g., shockwave, laser, and pulsed electromagnetic field therapy).

Summary

Equine practitioners are fortunate that a variety of orthobiologics are available and, for the most part, their use is accepted by clients. The autologous blood–derived orthobiologics described in this review are easy to generate and use and show great promise for improving patient outcomes through modulation of the injury environment and superior tissue healing.

Disclosure

Dr. Schnabel has received research support from Astaria Global, which produces a product mentioned in this article.