Fexinidazole--a new oral nitroimidazole drug candidate entering clinical development for the treatment of sleeping sickness

Abstract

Background: Human African trypanosomiasis (HAT), also known as sleeping sickness, is a fatal parasitic disease caused by trypanosomes. Current treatment options for HAT are scarce, toxic, no longer effective, or very difficult to administer, in particular for the advanced, fatal stage of the disease (stage 2, chronic HAT). New safe, effective and easy-to-use treatments are urgently needed. Here it is shown that fexinidazole, a 2-substituted 5-nitroimidazole rediscovered by the Drugs for Neglected Diseases initiative (DNDi) after extensive compound mining efforts of more than 700 new and existing nitroheterocycles, could be a short-course, safe and effective oral treatment curing both acute and chronic HAT and that could be implemented at the primary health care level. To complete the preclinical development and meet the regulatory requirements before initiating human trials, the anti-parasitic properties and the pharmacokinetic, metabolic and toxicological profile of fexinidazole have been assessed.

Methods and findings: Standard in vitro and in vivo anti-parasitic activity assays were conducted to assess drug efficacy in experimental models for HAT. In parallel, a full range of preclinical pharmacology and safety studies, as required by international regulatory guidelines before initiating human studies, have been conducted. Fexinidazole is moderately active in vitro against African trypanosomes (IC₅₀ against laboratory strains and recent clinical isolates ranged between 0.16 and 0.93 µg/mL) and oral administration of fexinidazole at doses of 100 mg/kg/day for 4 days or 200 mg/kg/day for 5 days cured mice with acute and chronic infection respectively, the latter being a model for the advanced and fatal stage of the disease when parasites have disseminated into the brain. In laboratory animals, fexinidazole is well absorbed after oral administration and readily distributes throughout the body, including the brain. The absolute bioavailability of oral fexinidazole was 41% in mice, 30% in rats, and 10% in dogs. Furthermore, fexinidazole is rapidly metabolised in vivo to at least two biologically active metabolites (a sulfoxide and a sulfone derivative) that likely account for a significant portion of the therapeutic effect. Key pharmacokinetic parameter after oral absorption in mice for fexinidazole and its sulfoxide and sulfone metabolites are a C(max) of 500, 14171 and 13651 ng/mL respectively, and an AUC₀₋₂₄ of 424, 45031 and 96286 h.ng/mL respectively. Essentially similar PK profiles were observed in rats and dogs. Toxicology studies (including safety pharmacology and 4-weeks repeated-dose toxicokinetics in rat and dog) have shown that fexinidazole is well tolerated. The No Observed Adverse Event Levels in the 4-weeks repeated dose toxicity studies in rats and dogs was 200 mg/kg/day in both species, with no issues of concern identified for doses up to 800 mg/kg/day. While fexinidazole, like many nitroheterocycles, is mutagenic in the Ames test due to bacterial specific metabolism, it is not genotoxic to mammalian cells in vitro or in vivo as assessed in an in vitro micronucleus test on human lymphocytes, an in vivo mouse bone marrow micronucleus test, and an ex vivo unscheduled DNA synthesis test in rats.

Conclusions: The results of the preclinical pharmacological and safety studies indicate that fexinidazole is a safe and effective oral drug candidate with no untoward effects that would preclude evaluation in man. The drug has entered first-in-human phase I studies in September 2009. Fexinidazole is the first new clinical drug candidate with the potential for treating advanced-stage sleeping sickness in thirty years.

Conflict of interest statement

DT, GM and MAB are paid consultants to DNDi. All other authors declare that they have no competing interests.

Figures

Figure 1. Effect of fexinidazole and its two main metabolites on T. b. rhodesiense (STIB 900).

Parasite viability was measured in vitro after 72-h drug exposure. Fexinidazole - open circles (n = 11). Fexinidazole sulfoxide - open squares (n = 4). Fexinidazole sulfone - open diamonds (n = 4).

Figure 2. Chemical structure of fexinidazole and its main metabolites , including 14C-labeled fexinidazole indicating which carbon atom was labelled.

Figure 3. Plasma concentrations of fexinidazole and its two main metabolites after 5 days of oral administration.

200 mg/kg fexinidazole was administered to mice (n = 3). Fexinidazole - open circles. Fexinidazole sulfoxide - open squares. Fexinidazole sulfone - open diamonds.

Figure 4. Mutagenic activity of fexinidazole in the Ames test.

Salmonella typhymurium strains TA98 (A) and TA100 (B) and their nitroreductase-deficient variants TA98NR and TA100NR were used, in the presence and absence of metabolic activation (+/− S9). A: Solid circles: TA98 +S9; Open circles: TA98 -S9; Solid squares: TA98NR +S9; Open squares: TA98NR -S9; Negative control: Mean number of revertants per plate were TA98 (−S9): 21; TA98 (+S9): 34; TA98NR (−S9): 29; TA98 (+S9): 18. B: Solid circles: TA100 +S9; Open circles: TA100 −S9; Solid squares: TA100NR +S9; Open squares: TA100NR −S9; Negative control: Mean number of revertants per plate were TA100 (−S9): 104; TA100 (+S9): 116; TA100NR (−S9): 90; TA100NR (+S9): 111.