Ins And Outs Of Transdermal NSAID In Horses

If you're fighting tooth and nail to administer oral medications, the introduction of a transdermal option often pleases horses and handlers alike. Transdermal medications are applied to the surface of the skin, typically in the form of an ointment or gel, so as to allow gradual absorption into the body. Are you considering transdermal medications, especially anti-inflammatories? If so, it's important to understand how these drugs are absorbed and eliminated, particularly if your horse is involved in competition and subject to stringent drug withdrawal times.

In the U.S., a transdermal formulation of the nonsteroidal anti-inflammatory (NSAID) diclofenac sodium, sold under the brand name Surpass, is approved for use in horses. Recently, a transdermal product containing flunixin meglumine was approved in cattle and is therefore available for off-label use in horses. Flunixin meglumine is often called by the common brand name Banamine.

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To assess how transdermal delivery of flunixin meglumine behaves in the body compared to other routes of administration (e.g., oral, intravenous, intramuscular), researchers applied a 10-mL dose of transdermal flunixin meglumine along the midline of the horse's back, from the withers to the tailhead. This was equivalent to a standard 500-mg dose of flunixin meglumine per horse. *

Blood and urine samples were intermittently collected for up to 96 hours after administration to measure the intricacies of how the medication is absorbed and cleared, a science known as pharmacokinetics.

“Transdermal application was well tolerated in all six study horses, with no horse showing any negative dermal effects,” said Peter Huntington, B.V.Sc., M.A.C.V.Sc., director of nutrition at Kentucky Equine Research.

In terms of the drug's behavior, here's what the research team found:

  • The maximum concentration of flunixin meglumine in blood was only an average of 515 ng/mL, which is much lower than oral and intramuscular concentrations. Oral administration of a comparable dose of flunixin meglumine results in average concentrations ranging from 3,340 to 7,200 ng/mL. Intramuscular flunixin meglumine reaches average concentrations of 1,799-3,269 ng/mL.
  • Transdermal absorption into the horse's bloodstream was slow, an average of 8.76 hours. Oral flunixin meglumine requires less than an hour to be absorbed.
  • Elimination (clearance) of transdermal flunixin meglumine was also much slower than other routes of administration. It took 22 hours for half of the drug to be eliminated from the bloodstream (called the half-life) when the transdermal formulation was used. In contrast, the intravenous half-life is 3.38-9.68 hours.
  • The data also showed that transdermal flunixin meglumine has unique pharmacokinetics called “flip-flop kinetics” in which the rate of absorption is actually slower than the rate of elimination.

“Based on this data, it is not surprising that flunixin meglumine and a metabolite were still detectable in urine 96 hours after application. This means that the withdrawal time for transdermal flunixin meglumine would be longer than for any other route of administration,” said Huntington.

He added, “Even though the blood concentrations of flunixin meglumine were lower compared to those achieved by other routes, the slow, variable absorption and 'flip- flop kinetics' indicate the medicine applied transdermally persists in the body longer than if given by other routes of administration. More excretion studies in exercising horses are needed before safe withholding periods before competition can be established.”

Despite the low circulating drug concentrations, transdermal flunixin meglumine still elicited an anti-inflammatory effect for up to 72 hours after application.

In terms of safety, Huntington warned, “Considering that transdermal flunixin meglumine is indeed absorbed systemically, this means that horses are at risk of the common side effects associated with NSAIDs administered by other routes: gastric ulceration, dorsal colitis, and disruption of the intestinal microbiome.”

Horses treated with any type of NSAID by any route of administration, including transdermal, would therefore benefit from a digestive tract supplement containing antacids, stomach-coating agents, and a hindgut buffer.

“These supplements reduce the risk of damage to the gut lining or change in the microbiota resulting from NSAID use,” Huntington advised.

*Knych, H.K., R.M. Arthur, S.R. Gretler, D.S. McKemie, S. Goldin, and P.H. Kass. 2021. Pharmacokinetics of transdermal flunixin meglumine and effects on biomarkers of inflammation in horses. Journal of Veterinary Pharmacology and Therapeutics:12993.

Reprinted courtesy of Kentucky Equine Research. Visit ker.com for the latest in equine nutrition and management, and subscribe to Equinews to receive these articles directly.

 

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Lots Of Older Horses Are Dealing With Chronic Pain, But Their Owners May Not Know It

Almost all older horses suffer from chronic pain, and one veterinarian believes many of their owners can't tell. Dr. Alex Bianco, clinical faculty in large animal internal medicine at the University of Minnesota told The Horse that chronic pain often factors into the decision to euthanize a horse but many horsemen fail to recognize it in its early stages.

Bianco cited a 2020 Swiss study that used 182 responses from experienced horse owners who considered their horses sound. When veterinarians examined the horses, they found that 100 of the horses in the study were considered a grade 2 or higher on the AAEP lameness scale, which is scored up to 5.

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Adding to the challenge, older horses are often not given lameness exams, which leaves them at risk of suffering from unmanaged pain. Bianco also pointed out however that the same Swiss study acknowledged horse owners have a high level of trust in their veterinarians.

Bianco reported that options to address chronic pain include non-steroidal anti-inflammatory drugs (NSAIDs), opiates, gabapentin, acetaminophen (though liver toxicity may be a concern) and ketamine. Therapeutic options are also available and include corticosteroid injections into joints, topical diclofenac, therapeutic shoeing, transecting ligament, tendon or nerves to remove pain, or surgical fusing of joints.

Bianco said the key to improved welfare for older horses is making sure owners know they need to have their horses evaluated by their veterinarians regularly.

Read more at The Horse.

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Sore Back? Skip The Bute

Horse owners have been known to take medications prescribed to the animals in their care, despite the possible dangers. While some anti-inflammatories, antibiotics, and dewormers may be the same in both human and equine medicine, but their formulations can be vastly different; because of this, human ingestion of equine drugs isn't recommended.

An American woman experiencing severe back pain attempted to take some phenylbutazone (Bute) that had been prescribed for one of the horses in her care. She took three doses of the non-steroidal anti-inflammatory drug (NSAID), each suitable for a 400-pound horse, reports the journal Clinics and Practice.

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Three days later, the woman went to the hospital complaining of nausea, vomiting and weakness, which she said she had been experiencing for two weeks. She often had back pain that was controlled by use of over-the-counter naproxen, but reported that this episode was exceptionally painful, which is why she ingested the Bute.

She presented to the hospital with nearly low blood pressure, a high heart rate, and normal O2 levels, as well as dry mucous membranes. Blood work showed a low white blood count, low platelet levels, elevated liver enzymes, and acute kidney injury. Her urine blood screen showed use of amphetamines and marijuana.

Poison control and toxicology were engaged once it was discovered she had ingested the Bute, but no antidote was recommended. She was given an N-acetyl cysteine and sodium bicarbonate drip because of the suspected acute liver damage caused by the phenylbutazone.

After three days of in-hospital treatment, the woman was feeling better and checked herself out.

Though phenylbutazone was used in human medicine for the treatment of multiple forms of arthritis, it was removed from the human medicinal arsenal in 1970 because of an increased risk of agranulocytosis, a life-threatening blood disorder.

Read more at HorseTalk.

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Bute: More Isn’t Better, It Can Be Dangerous

Phenylbutazone (“bute”) should have a place in every horse owner's first-aid arsenal to fight pain. However, if a traditional dose is given and the horse finds no relief, research says owners should reach for a different medication, not just administer additional doses of bute.

To test this theory, Dr. Ronald Erkert of Oklahoma State University conducted a recent study using three different pain treatment protocols on nine horses that were chronically lame in a front leg; each protocol lasted for four days. One group had injectable bute administered at two grams per 1,000 pounds. The second group had four grams per 1,000 pounds of bute injected. The third group received a saline injection to serve as a control. Before each injection and at six, 12 and 24 hours after the final dose, the horses were given a lameness exam and trotted on a force plate.

Erkert found no difference in lameness scores whether the horse was given two grams of bute or four grams of bute per 1,000 pounds. Erkert said that though his study was on injectable bute, administering additional grams of oral bute also has no significant benefit. Four grams of bute per 1,000 pounds approaches near-toxic levels. Bute toxicity can show up as diarrhea, gastric ulcers, colic, kidney failure and endotoxic shock.

Erkert recommends owners who give a horse bute and see no significant change in the horse's comfort level consult with their veterinarian to find a different pain control drug rather than administering more bute.

Read more at EQUUS magazine.

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