The discovery of artemisinin more than 30 years ago provided a completely new antimalarial structural prototype; that is, a molecule with a pharmacophoric peroxide bond in a unique 1,2,4-trioxane heterocycle. Available evidence suggests that artemisinin and related peroxidic antimalarial drugs exert their parasiticidal activity subsequent to reductive activation by haem, released as a result of haemoglobin digestion by the malaria-causing parasite. This irreversible redox reaction produces carbon-centred free radicals, leading to alkylation of haem and proteins (enzymes), one of which--the sarcoplasmic-endoplasmic reticulum ATPase PfATP6 (ref. 7)--may be critical to parasite survival. Notably, there is no evidence of drug resistance to any member of the artemisinin family of drugs. The chemotherapy of malaria has benefited greatly from the semi-synthetic artemisinins artemether and artesunate as they rapidly reduce parasite burden, have good therapeutic indices and provide for successful treatment outcomes. However, as a drug class, the artemisinins suffer from chemical (semi-synthetic availability, purity and cost), biopharmaceutical (poor bioavailability and limiting pharmacokinetics) and treatment (non-compliance with long treatment regimens and recrudescence) issues that limit their therapeutic potential. Here we describe how a synthetic peroxide antimalarial drug development candidate was identified in a collaborative drug discovery project.

Ozonide OZ439 is a synthetic peroxide antimalarial drug candidate designed to provide a single-dose oral cure in humans. OZ439 has successfully completed Phase I clinical trials, where it was shown to be safe at doses up to 1,600 mg and is currently undergoing Phase IIa trials in malaria patients. Herein, we describe the discovery of OZ439 and the exceptional antimalarial and pharmacokinetic properties that led to its selection as a clinical drug development candidate. In vitro, OZ439 is fast-acting against all asexual erythrocytic Plasmodium falciparum stages with IC 50 values comparable to those for the clinically used artemisinin derivatives. Unlike all other synthetic peroxides and semisynthetic artemisinin derivatives, OZ439 completely cures Plasmodium berghei -infected mice with a single oral dose of 20 mg/kg and exhibits prophylactic activity superior to that of the benchmark chemoprophylactic agent, mefloquine. Compared with other peroxide-containing antimalarial agents, such as the artemisinin derivatives and the first-generation ozonide OZ277, OZ439 exhibits a substantial increase in the pharmacokinetic half-life and blood concentration versus time profile in three preclinical species. The outstanding efficacy and prolonged blood concentrations of OZ439 are the result of a design strategy that stabilizes the intrinsically unstable pharmacophoric peroxide bond, thereby reducing clearance yet maintaining the necessary Fe(II)-reactivity to elicit parasite death.

Pyronaridine was synthesized in 1970 at the Institute of Chinese Parasitic Disease and has been used in China for over 30 years for the treatment of malaria. Pyronaridine has high potency against Plasmodium falciparum, including chloroquine-resistant strains. Studies in various animal models have shown pyronaridine to be effective against strains resistant to other anti-malarials, including chloroquine. Resistance to pyronaridine appears to emerge slowly and is further retarded when pyronaridine is used in combination with other anti-malarials, in particular, artesunate. Pyronaridine toxicity is generally less than that of chloroquine, though evidence of embryotoxicity in rodents suggests use with caution in pregnancy. Clinical pharmacokinetic data for pyronaridine indicates an elimination T1/2 of 13.2 and 9.6 days, respectively, in adults and children with acute uncomplicated falciparum and vivax malaria in artemisinin-combination therapy. Clinical data for mono or combined pyronaridine therapy show excellent anti-malarial effects against P. falciparum and studies of combination therapy also show promise against Plasmodium vivax. Pyronaridine has been developed as a fixed dose combination therapy, in a 3:1 ratio, with artesunate for the treatment of acute uncomplicated P. falciparum malaria and blood stage P. vivax malaria with the name of Pyramax® and has received Positive Opinion by European Medicines Agency under the Article 58 procedure.

Aims To assess the safety and pharmacokinetics of a new synthetic ozonide antimalarial, OZ439, in a first‐in‐man, double‐blind study in healthy volunteers. Methods OZ439 was administered as single oral daily doses of a capsule formulation (50–1200 mg) or an oral dispersion (400–1600 mg, fed and fasted states) and for up to 3 days as an oral dispersion (200–800 mg day−1). Plasma concentrations of OZ439 and its metabolites were measured by LC‐MS. Results The pharmacokinetic (PK) profile of OZ439 was characterized by a tmax of around 3 h, followed by a multiphasic profile with a terminal half‐life of 25–30 h. The PK parameters were approximately dose proportional for each group and profiles of the metabolites followed a similar pattern to that of the parent compound. Following dosing for 3 days, accumulation was less than two‐fold but steady‐state was not achieved. In the presence of food, no effect was observed on the t1/2 of OZ439 while the exposure was increased by 3 to 4.5‐fold. Exposure was higher and inter‐subject variability was reduced when OZ439 was administered as an oral dispersion compared with a capsule. The urinary clearance of OZ439 and its metabolites was found to be negligible and OZ439 did not induce CYP3A4. The antimalarial activity profiles of a subset of serum samples suggested that the major antimalarial activity originated from OZ439 rather than from any of the metabolites. Conclusion The safety and pharmacokinetic profile of OZ439 merits progression to phase 2a proof of concept studies in the target population of acute uncomplicated malaria.