Researchers say bump kinase inhibitors show promise in treating horses with this neurologic disease

Equine protozoal myeloencephalitis (EPM) presents a major challenge for horse owners and veterinarians. This sometimes debilitating neurologic disease, driven by the protozoan parasite Sarcocystis neurona (and occasionally Neospora hughesi), surfaced in 1964. By the early 1990s, equine practitioners identified S. neurona itself in a horse with clinical signs of EPM. Today, equine researchers have deemed EPM the most common infectious neurologic condition affecting horses in North America. Understanding and addressing the disease remains crucial for equine health.

As far as treatment, the Food and Drug Administration (FDA) has approved only three drugs for EPM, and none are 100% effective. Therefore, great opportunity exists for creating new EPM medications, and one such class of drugs currently in development is the bump kinase inhibitors or BKIs.

In this article we’ll briefly review EPM and the current FDA-approved treatments, laying the foundation for understanding BKIs and where they are in clinical development, providing a ray of hope for those caring for horses with EPM.

Brief Review of EPM

Horses ingest the sporocysts of S. neurona that infected opossums shed in their feces. The horse’s immune system either clears the parasites or they migrate to the central nervous system (CNS), most commonly the spinal cord, invading those cells and causing clinical signs of disease.

The reported seroprevalence of S. neurona in North America can be quite high, reaching 89% in some studies. If a horse is seropositive it means he was infected with S. neurona but does not necessarily exhibit signs of neurologic disease. If the immune system clears the parasite before it migrates to the CNS, the horse typically has antibodies in his bloodstream against S. neurona (i.e., remains seropositive) but shows no signs of disease.

When the horse’s immune system doesn’t clear the infection, S. neurona can essentially “land” anywhere within the CNS, causing trauma and inflammation. Clinical signs vary based on the parasite’s location but could include asymmetric muscle atrophy and weakness, incoordination (ataxia), and stumbling. The signs, which usually involve all four limbs, can be so subtle the horse might originally appear lame rather than neurologic, or he could present with severe acute signs, having difficulty standing. Other clinical signs can include head tilt, facial nerve paralysis, difficulty swallowing, and even seizures.   

Despite the high seroprevalence of 89% mentioned, prevalence of clinical disease remains extremely low at only 0.5-1%. It is unclear why most horses can successfully fight off infection with S. neurona while others cannot and instead develop clinical signs.

Current FDA-Approved Treatment Recommendations

Authors of the most recently published ACVIM Consensus Statement on treating EPM (Reed et al., 2016), report that horses should only be treated with one of the three FDA-approved medications. Although those guidelines are nearly a decade old, no new medications have become available, and the authors say the recommendations from 2016 remain relevant today.

A combination of sulfadiazine and pyrimethamine (ReBalance) This was the first targeted therapy for EPM. Horses treated with this combination receive 1 milligram/kilogram of pyrimethamine and 20 mg/kg sulfadiazine per dose, which is given once daily for 90 to 270 days. This drug must be administered after a two-hour fast, and feed should be withheld for an additional two hours after administration. For ReBalance, practitioners saw treatment success in 60-70% of patients in studies, meaning horses either improved by two or more grades in overall neurologic dysfunction or the cerebrospinal fluid (CSF) reverted to negative status for S. neurona immunoblot after 90 days. Side effects of this medication include bone marrow suppression, anorexia, urticaria (hives), and self-limiting colitis (diarrhea that resolves on its own). Horses often show progressive anemia during the treatment period. Defects in foals of mares treated during pregnancy do occur, so this product should not be administered to pregnant mares.

Diclazuril (Protazil) This benzeneacetonitrile (triazine) is an oral pellet fed once daily. No loading dose is needed, and corn oil does not improve the bioavailability.

Ponazuril (Marquis) Also a benzeneacetonitrile (triazine), this oral paste is administered once daily, and caretakers can give a loading dose of 15 mg/kg (three times the regular dose) the first day. Researchers have found that concurrent administration of ½ cup of corn oil increases the bioavailability up to 15%, increasing both serum and CSF concentrations of ponazuril.

For both diclazuril and ponazuril, the FDA-approved treatment course is 28 days. However, many horses stay on a treatment for six to eight weeks or longer if veterinarians feel the clinical improvement falls short of expectations. Based on published data, successful treatment is achieved in 62-67% of horses, with success defined as improvement of one neurologic grade or negative serum and CSF testing results for S. neurona.

Although reported treatment success rates for all three FDA-approved drugs might seem low, Steve Reed, DVM, Dipl. ACVIM, internal medicine specialist and shareholder at Rood & Riddle Equine Hospital in Lexington, Kentucky, says he believes response rates are higher.

“The studies that were performed to get FDA approval didn’t have the EPM tests that we have today,” he explains. “I believe many of the horses included in those studies didn’t have EPM, which lowered the response rates. In my practice, the treatment success rates are much higher. Still, some horses do relapse.”

He also says ponazuril is his typical go-to drug, but it isn’t unusual for him to also treat a horse after one of the triazines with sulfadiazine and pyrimethamine.

“I will prescribe each medication for 28 days, so I am attacking the protozoan by two different pathways,” says Reed. “Triazines first to attack the apicoplast, then the folate blocker.” In other words, they target organelles in the protozoa responsible for essential metabolic functions and block folate, which is crucial for proliferation.

Evolving Therapies for EPM: Bump Kinase Inhibitors

Researchers are actively seeking alternative therapies to treat horses with EPM. But the process is arduous, says Daniel Howe, PhD, professor in parasitology at the University of Kentucky’s Gluck Equine Research Center, in Lexington.

Back in 2016 and 2017 a group from the University of Washington led by Wesley C. Van Voorhis, MD, PhD, published two studies expanding scientists’ knowledge regarding a class of molecules called bump kinase inhibitors for a variety of parasitic diseases, including EPM.

“These are drugs that block the function of a protein in the parasite called CDPK1, which stands for calcium-dependent protein kinase 1,” explains Howe. “This protein is important for the parasite’s ability to get inside of cells. What is important to understand is that S. neurona and its cousin N. hughesi are both obligate intracellular pathogens … they have to get inside a cell in their host animal to replicate and cause infection. If they can’t get inside a cell they die.”

Kinases are a type of enzyme that adds chemicals called phosphates to other molecules, such as sugars or proteins. A kinase such as CDPK1 regulates function of other proteins by phosphorylating them, which effectively turns those proteins “on” and “off.”

“The BKIs inhibit the kinase CDPK1 by binding to the functional pocket of the kinase and blocking its ability to phosphorylate the normal protein substrates,” Howe says.

“One very important thing to note is that the CDPK1 protein inhibited by the BKIs is pretty parasite-specific,” he adds. “It isn’t found in mammalian cells. So, BKIs shouldn’t cause problems or side effects in the host,” in this case the horse.

Although neither Van Voorhis nor his group have published articles specific to BKIs in EPM since 2017, Howe says, “This doesn’t mean we haven’t been doing anything. We have.”

Each BKI Van Voorhis and his team studied has a number that represents the BKI variations synthesized and tested to date.

“So far, they have synthesized and tested over 2,000 BKIs,” says Howe. “For EPM, the BKI blocking CDPK1 studied in 2016 was BKI-1553. 

 “Since then we’ve tested multiple additional BKIs in cell culture against S. neurona to see how effective they are in blocking S. neurona growth, and we are now working on BKI-1708,” he adds.

Howe says he and his colleagues have found BKI-1708 effective against the parasite in cell culture at “nanomolar level,” which works well for in vitro (in the lab, not in the live animal) work.

“Experiments currently being conducted involve treating horses with BKI-1708 and collecting blood and CSF,” says Howe.

“We are measuring how much of the drug gets into the blood and ultimately the CSF to see if sufficient levels of the drug are getting where it needs to be to inhibit parasite growth.”

He says his team has completed five or six different experiments with both oral and intravenous administration to check levels achieved in plasma and CSF. Further, another pharmacokinetic (how the body affects a specific substance after administration) study was scheduled to begin in May 2024.

Howe and colleagues are hopeful BKI-1708 will reach sufficient levels in horses’ CSF to support its clinical use. The next step, he says, will be to test BKI-1708 in horses diagnosed with EPM.

“But even if the experimental use of BKI-1708 proves effective in clinical cases of EPM, it will still take several years to get FDA approval,” says Howe.

Clinical Use of BKI-1708 in Client-Owned EPM Horses

Reed and Howe have collaborated regularly on EPM research projects for more than a decade. “As soon as we do this next pharmacokinetic trial, then we are going to see if we can get an approval for treating client-owned animals,” explains Reed. “We will offer BK-1708 to clients, or they can elect to stick with a traditional treatment.”

 “This would be a noninferiority trial to say this new drug, BKI-1708, is at least as good as Marquis,” he adds.

At the end of the trial, owners will still be offered a course of Marquis whether the horse responded or not. Reed says he hopes this will encourage owners to try BKI-1708.

The BKI-1708 clinical trial won’t begin unless Howe and his team find positive results in the upcoming pharmacokinetic study. If all goes to plan, the earliest this product would be available to horse owners in such a trial would be the fall of 2024.

Hurdles to overcome during the process include ensuring adequate drug supply, compounding it into an oral formulation, and obtaining future funding. Drug development and the FDA approval process are lengthy and expensive, and EPM isn’t a major focus in the world of pharmaceuticals.

“EPM is important, especially to owners who own a horse with EPM, but it isn’t big business because not that many horses actually have the disease,” Reed explains.

This means there isn’t great incentive for a pharmaceutical company to use its resources developing a medication with a small target population.

Supportive Care for EPM

In addition to medications targeting the parasite, veterinarians recommend using various supportive therapies concurrently.

Howe has reported that non-steroidal anti-inflammatory drugs (NSAIDs) are frequently administered during the first several days of treatment to counteract a worsening condition that occurs presumably due to the mass killing of the parasites within the central nervous system. Corticosteroids can be administered (0.1 mg/kg dexamethasone once or twice daily) and dimethyl sulfoxide (DMSO, 1 mg/kg as a 10% solution IV or by nasogastric tube once or twice daily) could also be administered concurrently.

However, Reed generally hasn’t seen the worries surrounding a big kill. “But still, most of these horses are on Bute (phenylbutazone) and Banamine (flunixin meglumine) during treatment,” he says.

“Other than NSAIDs, the supportive therapies I use the most are levamisole (a drug used to treat parasitic infection) and vitamin E,” he adds. “Less than one-third of my patients are prescribed levamisole in combination with a triazine (ponazuril and diclazuril), but almost all are on vitamin E to help the nervous system heal. Vitamin E is especially important if they aren’t on grass.”

Veterinarians typically recommend administering 20 IU/kg liquid vitamin E daily by mouth and 1 mg/kg levamisole twice daily by mouth for the first two weeks, then for the first week each month thereafter.

Researchers have indicated in studies that veterinarians use other immunomodulators in horses during EPM treatment. These include killed P. acnes (Eqstim), mycobacterial wall extract (Equimune IV), and transfer factor (4Life Transfer Factor).

Take-Home Message

Equine protozoal myeloencephalitis ranks as the most common infectious neurologic disease of horses. However, our sources say the number of clinical cases diagnosed each year is still quite small. Only three FDA-approved drugs are available for treating horses with EPM, and two are related, limiting treatment options. Hence, the discovery of BKIs has been exciting for researchers. “We’re all pretty optimistic that this could be a new alternative method for treating EPM,” Howe says.

Editor’s Note: This article was originally published in the Fall 2024 issue of The Horse: Your Guide to Equine Health Care.