Genz-644442 became the focus of medicinal chemistry optimization; 321 analogs were synthesized and were tested for in vitro potency against P. falciparum and for in vitro absorption, distribution, metabolism, and excretion (ADME) properties. This yielded compounds with IC 50 s of approximately 30 nM. The lead compound, Genz-668764, has been characterized in more detail. It is a single enantiomer with IC 50 s of 28 to 65 nM against P. falciparum in vitro. In the 4-day P. berghei model, when it was dosed at 100 mg/kg of body weight/day, no parasites were detected on day 4 postinfection. However, parasites recrudesced by day 9. Dosing at 200 mg/kg/day twice a day resulted in cures of 3/5 animals. The compound had comparable activity against P. falciparum blood stages in a humanengrafted NOD-scid mouse model. Genz-668764 had a terminal half-life of 2.8 h and plasma trough levels of 41 ng/ml when it was dosed twice a day orally at 55 mg/kg/day. Seven-day rat safety studies showed a no-observableadverse-effect level (NOAEL) at 200 mg/kg/day; the compound was not mutagenic in Ames tests, did not inhibit the hERG channel, and did not have potent activity against a broad panel of receptors and enzymes. Employing allometric scaling and using in vitro ADME data, the predicted human minimum efficacious dose of Genz-668764 in a 3-day once-daily dosing regimen was 421 mg/day/70 kg, which would maintain plasma trough levels above the IC 90 against P. falciparum for at least 96 h after the last dose. The predicted human therapeutic index was approximately 3, on the basis of the exposure in rats at the NOAEL. We were unable to select for parasites with >2-fold decreased sensitivity to the parent compound, Genz-644442, over 270 days of in vitro culture under drug pressure. These characteristics make Genz-668764 a good candidate for preclinical development.Malaria continues to be a major global health burden, endemic in 87 countries with 2.5 billion people at risk (11). Widespread resistance to current antimalarials such as chloroquine (29, 40), atovaquone (33), pyrimethamine (39), and sulfadoxine (37) and, more recently, reduced efficacy of artemisinin derivatives (7, 38) emphasize the urgent need for new antimalarial drugs. At least in part because the majority of people affected live in the poorest parts of the world, the response of the pharmaceutical industry has been sparse or irregular. New collaborations combining the expertise of academia, industry, and public-private partnerships have been suggested to overcome these obstacles, and it is in that spirit