Fenbendazole for Humans

Fenbendazole (FZ) is a broad spectrum anthelmintic drug with greater efficacy against adult and immature stages of parasites than early generation benzimidazoles, such as thiabendazole. The FDA approves fenbendazole for feed administration to poultry at 16 parts per million (ppm). A similar formulation is available for administration to captive game birds in the United States and Turkey, where it is metabolized to a pharmacologicly active molecule, fenbendazole sulfoxide (FZ sulfoxide), which has shown good clinical efficacy for controlling worm infestations in these animals (10) and a high survival rate of sick and injured rabbits when given with other anthelmintics such as dexamethasone (0.1-0.2 mg/kg oral q 24 h x 28 days) (12).

Since fenbendazole interferes with tubulin polymerization by binding to its microtubule domains and disrupting the equilibrium of this polymer, it has been suggested that it acts as a radiosensitizer in tumor cells. To test this hypothesis, EMT6 tumors were grown in mice and treated three times i.p. with FZ (Table 1). The growth of unirradiated or irradiated tumors was evaluated. The time to four-fold tumor volume was stratified and the results did not differ between fenbendazole treated groups or irradiated versus unirradiated tumors.

The estimated amount of pheasant tissue with FZ sulfone residues that could be consumed daily without adverse effects was calculated using Equation (3) and the food consumption values established by the FDA/EMA for different poultry species and tissues. This estimate was then compared with the NOAELs in rats and dogs for repeat-dose toxicity, reproductive toxicity, carcinogenicity, and teratogenicity. This resulted in a conclusion that consumption of pheasant meat with residue-containing tissues would not pose a health risk for humans, based on the NOAELs and the limits established by the US FDA/EMA and Belgium (Table 2). fenbendazole for humans

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