Antimalarial drug discovery - approaches and progress towards new medicines

Abstract

Malaria elimination has recently been reinstated as a global health priority but current therapies seem to be insufficient for the task. Elimination efforts require new drug classes that alleviate symptoms, prevent transmission and provide a radical cure. To develop these next-generation medicines, public-private partnerships are funding innovative approaches to identify compounds that target multiple parasite species at multiple stages of the parasite life cycle. In this Review, we discuss the cell-, chemistry- and target-based approaches used to discover new drug candidates that are currently in clinical trials or undergoing preclinical testing.

Figure 1. Drug development strategy for the identification of novel antimalarials

At the screening-stage millions of compounds can be screened (1,000 – 2,000,000 compounds per screening campaign). When hits are identified, (on average, a hit rate of 1.0% is observed) they are ranked based on a number of criteria (such as potency, ease of synthesis, known limitations to their use, novelty) to determine a possible lead compound. These compounds are tested in transmission and radical cure assays which are low throughput, time consuming and expensive and are therefore only applied to a handful of compounds. Following lead selection, multiple chemical derivatives of the lead are synthesized with the goal of maximizing potency and bioavailability while reducing cross-reactivity with possible human targets (lead optimization). The best candidate is selected for preclinical testing, an expensive and time-consuming process that involves assessing safety and finding the optimal doses that can be used in phase I human trials.

Figure 2. Assays available for determining the stage of activity and potency of potential antimalarial compounds

To detect likely prophylactic activity, P. berghei or P. yoelli sporozoites are seeded onto human hepatoma cells and then the infection rate is imaged or detected enzymatically. To detect prophylactic in vivo activity, rodent malaria sporozoites are injected into a mouse shortly before or after the mouse has been treated with the compound. The infection can be visualized with luciferase (with genetically modified parasites) or by measuring reduction in blood stage parasitemia and/or improved survival. The radical cure potential of a compound is tested using the hypnozoite-forming monkey model, P. cynolmolgi. In the in vitro assay, P. cynolmolgi sporozoites are seeded onto primary monkey hepatocytes and imaged to determine the ratio between large, rapidly developing schizonts and dormant “small forms” (thought to be hypnozoites). Radical cure agents will eliminate all parasites including hypnozoites, whereas prophylactic compounds will act against growing schizonts only. In the in vivo model, monkeys are infected with P. cynolmolgi sporozoites followed by treatment with a compound that eliminates all blood stage parasites (e.g. chloroquine) and then by a potential radical cure compound. The monkeys are then monitored over several months to measure reductions in the frequency of hypnozoite-caused relapses. The IC₅₀ of P. falciparum blood stages is typically measured in a parasite proliferation assay. P. vivax blood stage sensitivity is determined using a schizont maturation assay using parasites taken directly from infected patients. The in vivo efficacy of blood-stage compounds is typically measured in mice infected with P. berghei or P. yoelli, although SCID mice can be infected with P. falciparum. Transmission-blocking activity can be assayed in vitro by looking either at the viability or the development of purified P. falciparum gametocytes or ookinetes. The ability of gametocytes to infect mosquitoes is measured using a standard membrane feeding assay (with P. falciparum)^, or by direct feeding from an infected mouse(with P. berghei or P. yoelli). After feeding the number of oocysts per mosquito midgut is counted to determine drug efficacy. Notes: Pf, P. falciparum; Pv, P. vivax; Pb, P. berghei; Py, P. yoelli; Pc, P. cynomolgi.

Figure 3. Chemical structures of new antimalarial candidates

Lead compound chemical structures developed through optimization of chemical classes identified using A) cellular-based screens B) target-based screens or C) chemistry-based approaches. For the cellular screen, the compound classes are novel and all are derived from leads that have potent activity against parasite blood stages. DSM265 and P218 were derived from target-based screens against PfDHOD and PfDHFR respectively. OZ439 was rationally designed to have the parasite killing activities of artemisinins, but with a longer half life. References: TCMDC-139046; NITD609 (KAE609)^, ; GNF156 (KAF156). GSK1057714; MMV390048; DSM265; P218; OZ439.