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First injectable pentosan for equine osteoarthritis is approved by FDA


Appropriate management can help control the clinical signs and improve equine quality of life

Equine Osteoarthritis

Credit: JM Fotografie/Adobe Stock

Officials with the FDA have approved pentosan polysulfate sodium injection (Zycosan; Anzac Animal Health) for the control of clinical signs associated with osteoarthritis (OA) in horses. This therapy is a heparin-like compound and is the first injectable pentosan product to receive FDA approval.1

Pentosan polysulfate sodium injection will be an important addition to the available approved drugs to control the clinical signs of OA in horses, according to the FDA. It is indicated for intramuscular use in horses only and is only available with a veterinary prescription. The drug should be administered at a dose of 3 mg/kg (1.4 mg/lb) by intramuscular injection once weekly for 4 weeks (for a total of 4 doses). Pentosan polysulfate sodium injection is supplied in a 7.5 ml single use vial.1,2

Filling a therapeutic need

OA involves the slow breakdown of joint cartilage, which can result in a variety of clinical signs such as lameness, stiffness, and joint swelling. Although there is no cure for the condition, appropriate management can help control the clinical signs of OA in horses and improve their quality of life.1

Unapproved injectable formulations of pentosan have a long history of use in horses, but the newly approved therapy is the first injectable pentosan product to have met the FDA’s standards. FDA-approved drugs have been demonstrated to be safe and effective for their intended use and have been consistently manufactured to preserve the drugs’ identity, strength, quality, and purity. The FDA is strongly encouraging veterinarians to use the legally marketed FDA-approved pentosan polysulfate sodium injection when pentosan is indicated for use in an equine patient.1

Showing the drug’s efficacy

The drug’s sponsor conducted a field study of 237 client-owned horses of various breeds that were each diagnosed with osteoarthritis in 1 lame leg. In the treatment group, 120 horses received pentosan polysulfate sodium injection by intramuscular administration in the neck once every 7 days for 4 weeks for a total of 4 doses. In the control group, 117 horses were given an equal volume of saline at the same dosage regimen.1,3

Each horse’s lameness was graded on Days 0 and 28 of the study. On Day 28, improved lameness grades were demonstrated in 57% of horses in the group that received pentosan polysulfate sodium injection and 36% in the negative control group.3 The study results indicated that treatment with this injection at the labeled dose benefited horses with single limb lameness due to OA.1

The most common adverse reactions associated with the administration of pentosan polysulfate sodium injection were administration site reactions such as pain, heat, swelling, redness, and neck muscle cramping; and prolonged coagulation parameters,1,3 lethargy, behavior changes, and colic.3

Some injection site reactions initially occurred around the time of dosing and other injection site reactions were initially delayed, up to 3 days after dosing. All peri-dosing reactions resolved without specific treatment. All delayed reactions resolved within 5 days from onset, and most didn’t require treatment. Clinical signs of bleeding were not seen in any horses.1

FDA guidance

In a letter to veterinarians, the FDA strongly encouraged the use of the approved pentosan formulation. In contrast to the approved pentosan polysulfate sodium injection, the absorption and bioavailability of compounded formulations of injectable pentosan polysulfate sodium are unknown and may be inadequate or variable, according to the FDA. Furthermore, although there are many post approval requirements for sponsors of FDA-approved animal drugs, including the mandatory reporting of adverse drug experience reports, there are no such mandatory reporting requirements for compounded preparations.2

The letter also urged veterinary professionals and animal owners to report adverse drug experiences and product defects associated with animal drugs and devices to the agency. FDA officials also vowed to monitor and evaluate reported adverse events associated with pentosan polysulfate sodium, including monitoring for potential human safety risks.2

The FDA has also emphasized that pentosan polysulfate sodium injection is not indicated for use in humans. Because the formulation is a weak anticoagulant, people who take an anticoagulant should use caution when handling or administering the drug.2


  1. FDA approves first injectable pentosan for osteoarthritis in horses. News release. FDA. December 20, 2022. Accessed December 20, 2022. https://www.fda.gov/animal-veterinary/cvm-updates/fda-approves-first-injectable-pentosan-osteoarthritis-horses
  2. Dear veterinarian letter advising veterinarians to use Zycosan (pentosan polysulfate sodium injection) instead of unapproved drugs in their equine patients. News release. FDA. December 20, 2022. Accessed December 20, 2022. https://www.fda.gov/animal-veterinary/product-safety-information/dear-veterinarian-letter-advising-veterinarians-use-zycosan-pentosan-polysulfate-sodium-injection
  3. Freedom of Information Summary: NADA 141-559. FDA. December 20, 2022. Accessed December 20, 2022. https://animaldrugsatfda.fda.gov/adafda/app/search/public/document/downloadFoi/13249
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Renee Schmid, DVM
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