FDA Approves First-Ever Antivenom for Scorpion Stings

FDA Approves First-Ever Antivenom for Scorpion Stings

August 8th, 2011 // 12:43 pm @

The US Food and Drug Administration (FDA) for the first time yesterday approved an antivenom specifically for the treatment of Centruroides scorpion stings.

“This is an historic event,” Leslie Boyer, MD, lead investigator on the antivenom’s clinical trials, said in a press statement. “This is the first-ever drug approved for this use by the FDA; the first-ever drug that we are aware of being developed fully in Latin America and subsequently approved by the FDA; the first-ever scorpion antivenom proved effective under controlled clinical trials; and the first-ever antivenom with so few allergic reactions.”

Clinical trials have demonstrated the product, called Anascorp, to be capable of rapidly reversing the symptoms of scorpion envenomation, which can be particularly dangerous to children and include fluid in the lungs, breathing problems, excess saliva, blurred vision, slurred speech, trouble swallowing, abnormal eye movements, and muscle twitching.

The injectable antivenom, produced in Mexico by Instituto Bioclon and licensed in the United States by Rare Disease Therapeutics of Franklin, Tennessee, is made from the plasma of horses immunized with scorpion venom. It targets scorpion venom that is toxic to the nerves.

“Ninety-eight percent of children treated with this drug have their symptoms from scorpion stings resolved in less than 4 hours,” Dr. Boyer said during a press briefing here yesterday.

“Historically, chart records of the same types of patients showed that 98% of children were still sick and still needing intensive care if they were admitted before the antivenom was available. A record like that is just about as strong as a new drug can have,” said Dr. Boyer, who is director of the University of Arizona’s Venom Immunochemistry, Pharmacology and Emergency Response (VIPER) Institute at the College of Medicine in Tucson.

The efficacy of Anascorp, Centruroides (Scorpion) Immune F(ab)2 (Equine) Injection, was demonstrated in a randomized, double-blind, placebo-controlled trial of 15 children experiencing the neurologic signs of scorpion stings. In the study, the symptoms resolved within 4 hours among all 8 patients who received the antivenom but in only 1 of the patients who received the placebo.

In addition, safety and efficacy data for the FDA approval were collected from 1534 patients in open-label and blinded studies conducted over 12 years at a consortium of 27 hospitals in Arizona and Nevada.

Study sites ranged from urban medical centers to rural settings, where stings can be more common and sometimes require a costly airlift to a regional hospital with pediatric intensive care.

“We did a follow-up study at San Carlos Indian Health Center that proved that we could use this treatment in a small town setting and treat patients just as well as if they’d been in an intensive care in Tucson, Arizona,” Dr. Boyer said.

The most common adverse effects observed with the treatment included vomiting, fever, rash, nausea, itchiness, headache, runny nose, and muscle pain; these were experienced in only about 1% of cases.

In the United States, the majority of scorpion stings occur in Arizona, where about 8000 stings occur each year. Several hundred of those, usually only from the particularly venomous bark scorpion, require medical treatment, and children are the most vulnerable to the stings, explained Andreas A. Theodorou, MD, chief of the Division of Pediatrics at the University of Arizona, at the press briefing.

“A child who has been stung loses control of their body in such a way that it can be almost like a violent convulsion, and it can be extremely stressful for parents to witness the reaction,” he explained.

“As clinicians, we are very concerned that the breathing is affected in a discoordinative way, they have secretions that they can’t handle, their lungs fill with fluid, and they may need to be placed on ventilators. So this is a critical state and a very dangerous situation.”

No Sedation

Before the introduction of the antivenom, children were treated with a substantial amount of sedation. “A fraction of that amount of sedation would allow an adult to be sedated enough for surgery, but even the excessive amount we had to use on these kids barely kept them in the bed,” Dr. Theodorou explained.

“Now, when a child comes in with scorpion sting symptoms, we infuse the drug and 4 hours later the symptoms have subsided and the child can go home. The difference is phenomenal.”

Dr. Boyer added that the new treatment does not even include sedation. “There’s no sedative component with this at all,” she told Medscape Medical News.

“Antibodies go straight to their target and that’s it. We used to have to hold children in the ICU [intensive care unit] for days even after the antivenom wore off because they had basically been overdosed on sedation and we had to wait for that to clear the system.”

While Anascorp is the first FDA-approved scorpion antivenom, it isn’t the first to be investigated in the United States. Significant progress was made on a prior antivenom treatment developed using goat serum at Arizona State University in the 1990s, but development was abruptly halted when the researcher on the drug retired.

Dr. Boyer became involved in the effort to find a replacement while accompanying a National Geographic crew filing venomous creatures in Mexico in 1999.

“We knew that we had only 250 stings a year in the US that were dangerous enough to require treatment with antivenom, but in Mexico, they had 250,000 a year,” she explained.

That’s largely because the vast geographic area in North America where scorpions thrive is in Mexico, with Arizona and a narrow slice of Nevada representing only the most northern reaches of the scorpion habitat, she explained.

Dr. Boyer discovered that researchers at the Cuernavaca campus of the National Autonomous University of Mexico were investigating the effects of the new antivenom, but that they needed support in conducting the clinical trials that would be necessary in demonstrating safety and efficacy.

“We discovered we needed each other as partners in a major collaboration that would use their biotechnology and science together with our translational medical technology and ability to do medical studies.”

The antivenom, which has been available in Mexico since the 1990s, must be administered by a physician and will usually be used in hospital emergency rooms. The product is expected to be available commercially in about a month.

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