Background: Flubendazole, originally developed to treat infections with intestinal nematodes, has been shown to be efficacious in animal models of filarial infections. For treatment of filarial nematodes, systemic exposure is needed. For this purpose, an orally bioavailable amorphous solid dispersion (ASD) formulation was developed. As this formulation results in improved systemic absorption, the pharmacokinetic and toxicological profile of flubendazole administered with this formulation have been assessed to ensure human safety before clinical trials could be initiated. Methods & Findings: Safety pharmacology, toxicity and genotoxicity studies have been conducted with the flubendazole ASD formulation. In animals, flubendazole has good oral bioavailability from an ASD formulation ranging from 15 % in dogs, 276 % in rats to more than 100% in jirds. In in vivo toxicity studies with the ASD formulation, high systemic exposure to flubendazole and its main metabolites was reached. Flubendazole, up to high peak plasma concentrations, does not induce Cmax related effects in the CNS or cardiovascular system. In repeated dose toxicity studies in rats and dogs, flubendazole-induced changes were observed in haematological, lymphoid and gastrointestinal systems and in testes. In dog, the liver was an additional target organ. Upon treatment cessation, at least partial recovery was observed for these changes in the dog. In rat, the low dose was the No Observed Adverse Effect Level (NOAEL) was , i.e. 5 mg (as base)/kg body weight/day (mg eq./kg/day)mg eq./kg/day in males and 2.5 mg eq./kg/day in females. In dog, the NOAEL was lower than the low dose, 20 mg eq./kg/day. Regarding genotoxicity, flubendazole was negative in the Ames test but positive in the in vivo micronucleus test. Conclusions: Based on these results, in combination with previously described genotoxicity and reproductive toxicity data and the outcome of the preclinical efficacy studies, it was concluded that no flubendazole treatment regimen can be selected that would provide efficacy in humans at safe exposure.