ARIZONA DAILY STAR (Tucson) 11 July 05 Tests begin on new antivenin - Ariz. researchers hope remedy has fewer side effects (Carla McClain)
Tests of a new - and perhaps improved - drug to treat potentially fatal rattlesnake bites start this week on Southern Arizona bite victims.
If the new snake antivenin proves superior in human testing this summer and next, it will replace the drug now in widespread use that causes significant problems in some patients.
That drug - known by the brand name CroFab and developed by University of Arizona researchers - puts patients at risk of a relapse of venom poisoning within weeks after treatment with it.
The new drug - called Antivipmyn and used for the past two years in Mexico - so far shows no evidence of that risk, said Dr. Gary Figge, emergency physician at Northwest Medical Center, one of three Tucson hospitals preparing to test it.
Also, Antivipmyn appears more effective for preventing the life-threatening hemorrhaging that untreated rattlesnake venom can cause.
"We're testing this in Tucson because we have such a high rate of snakebites, but also because the biggie here is the Western diamondback rattlesnake, which causes the most clotting problems," Figge said.
"So we will get a very good idea of how well this antivenin handles that effect."
Leading the study is the Arizona Poison and Drug Information Center at the University of Arizona, which handles more snakebite cases than any other poison center in the country - some 300 a year. The center deals with cases statewide except in Maricopa County.
Of those cases, about 150 occur every season, from March to October, in the Southern Arizona region. Last year, 121 Southern Arizonans were bitten by rattlesnakes - a bit lower than average.
The new antivenin will be tested in a study, with only 12 snakebite patients as participants this year, at University Medical Center and St. Mary's Hospital, as well as Northwest Medical Center.
Six patients will receive the current CroFab antivenin and the other six will get the new drug. If Antivipmyn proves safe and effective, it will be tested on more patients next summer.
But, with few bite cases so far this year, the season has gotten off to a slow start, Figge said, possibly due to weeks of extreme heat, when rattlesnakes tend to lay low. The monsoon rains, expected here by next week, typically trigger more snake activity - and unhappy encounters with humans.
Northwest Medical Center, which treats more bite victims than any other Tucson hospital, has seen only one case so far, and that turned out to be a "dry bite" - with no venom injected.
"But I'm sure we'll have enough patients for testing," Figge said. "That will not be a problem."
In use in the United States only for the past four years, the CroFab antivenin was, at first, heralded by doctors for causing fewer side effects than the old horse serum antivenin used since the 1930s.
Made with newer technology, the highly purified CroFab all but eliminated the "serum sickness" that plagued more then 80 percent of patients after treatment with the old antivenin.
But CroFab, made from a small protein molecule, developed problems because it was excreted too quickly by the kidneys - sometimes before it fully neutralized the snake venom.
"So, we started seeing this re-envenomation phenomenon two weeks later - there was enough venom left to re-envenomate the patient," Figge said. "People would start bruising again, they'd have clotting problems, and in some cases, the pain would return, and they'd be back in the hospital."
In one case last summer, a Tucson woman treated for a severe rattlesnake bite on the leg ended up back in the hospital two weeks later. Her leg was massively swollen and bruised, causing her a lot of pain and bleeding problems, Figge said.
"It was as if she'd been bitten all over again. She was back in the ICU and needed a lot more antivenin," he said.
"Most patients do fine on CroFab, but when this kind of thing does happen, it is a significant problem."
Made with a larger protein molecule, the new Antivipmyn stays in the body longer to combat the venom, he said.
Although deaths from rattlesnake bites are now rare, averaging only 8 to 12 a year in the United States, about a fourth of victims do suffer some functional disability, scarring, or even loss of a limb, according to the Poison Center.
Snake bites
Southern Arizona accounts for about half of statewide snakebites.
300
The number of snakebite cases a year the University of Arizona handles, more than any other poison center in the country
121
The number of times Southern Arizonans were bitten by rattlesnakes last year
Tests begin on new antivenin