Comparison of F(ab') 2 and Fab antivenoms in rattlesnake envenomation: First year's post-marketing experience with F(ab') 2 in New Mexico.

Two antivenoms are available for rattlesnake envenomations in the U.S., Fab (CroFab®, BTG, UK), and F(ab') 2 (Anavip®, Bioclon, Mexico) antivenom (AV) with F(ab') 2 AV released in October 2018. The F(ab') 2 AV Phase 3 comparative clinical trial demonstrated similar efficacy in treating venom-caused hematologic toxicity, similar rates of Types I and III hypersensitivity reactions, and a lower rate of recurrent hematological effects than FabAV. We hypothesized that a post-marketing, comparative study of effectiveness and rates of hypersensitivity reactions in treating rattlesnake envenomations in New Mexico would demonstrate similar outcomes. Patients eligible for the study presented to a New Mexico healthcare facility between May and October 2019 and were known/suspected to have a rattlesnake bite. Exclusion criteria for antivenom comparison were those with a dry bite, lost to follow-up, or late presentation. All cases were included for patient/bite demographics, initial local control, hematological control, number of maintenance/control doses, development of persistent, recurrent or late-, new-onset hematologic effects, and hypersensitivity reactions. We used Fisher's exact tests for analysis and 0.05 cutoff to determine significance. There were 54 rattlesnake-bitten patients in New Mexico with 17 excluded for comparison of antivenom because of dry bites, loss to follow-up, and one case of late presentation. Thirty-seven patients remained for comparative analysis between F(ab') 2 AV (n = 11) and FabAV (n = 26). There were no significant demographic differences between F(ab') 2 and Fab-treated patients. No patient had a Type I hypersensitivity reaction. No rescue doses were given. The rate of recurrent, persistent or late-, new-onset of hematologic effects was 0% with F(ab') 2 AV and 29% with FabAV. No patient was readmitted. No patient had bleeding complications. Type III hypersensitivity reactions were similar between F(ab') 2 AV (36%) and FabAV (25%). The results of our study are consistent with the Phase 3 clinical comparative trial and indicate no significant differences in safety or effectiveness between FabAV and F(ab') 2 AV. F(ab') 2 AV offers the advantages of not requiring maintenance doses and may have a lower rate of late hematologic effects in treating rattlesnake envenomations.
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