Novel Conjugated Quinoline−Indoles Compromise Plasmodium falciparum Mitochondrial Function and Show Promising Antimalarial Activity Silvia C. Teguh, †,∥ Nectarios Klonis, †,∥ Sandra Duffy, # Leonardo Lucantoni, # Vicky M. Avery, # Craig A. Hutton, §,∥ Jonathan B. Baell,* ,⊥,▽ and Leann Tilley* ,†,‡,▽ † Department of Biochemistry and Molecular Biology, ‡ ARC Centre of Excellence for Coherent X-ray Science, and § School of Chemistry, ∥ Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne VIC 3010, Australia ⊥ Medicinal Chemistry, Monash Institute of Pharmaceutical Science, Parkville VIC 3052, Australia # Eskitis Institute for Drug Discovery, Brisbane Innovation Park, Griffith University, Nathan QLD 4111, Australia * S Supporting Information ABSTRACT: A novel class of antimalarial compounds, based on an indol-3-yl linked to the 2-position of a 4-aminoquinoline moiety, shows promising activity against the malaria parasite, Plasmodium falciparum. Compounds with a quaternary nitro- gen on the quinoline show improved activity against the chloroquine-resistant K1 strain. Nonquaternerized 4-amino- quinolines retain significant potency but are relatively less active against the K1 strain. Alkylation of the 4-amino group preferentially improves the activity against the chloroquine- sensitive 3D7 strain. The quinoline-indoles show only weak activity as inhibitors of β-hematin formation, and their activities are only weakly antagonized by a hemoglobinase inhibitor. The compounds appear to dissipate mitochondrial potential as an early event in their antimalarial action and therefore may exert their activity by compromising Plasmodium mitochondrial function. Interestingly, we observed a structural relationship between our compounds and the anticancer and anthelminthic compound, pyrvinium pamoate, which has also been proposed to exert its action via compromising mitochondrial function. ■ INTRODUCTION Malaria remains one of the world’s greatest global health challenges. It is estimated that 2.2 billion people live in malaria endemic areas and that the more than 500 million cases of malaria each year result in up to 1.2 million deaths, mostly in young children. 1,2 Plasmodium falciparum is the causative agent of the most severe form of malaria in humans. Drugs have long been a mainstay in the fight against malaria, but their use has been dogged by the development of resistance. While a number of highly successful drugs have been deployed (including quinoline and sulfa drugs), the rapid development and spread of resistance has rendered most of them ineffective in endemic areas. 3 For example until the advent of widespread resistance, the efficacy, long half-life, affordability, and safety profile of chloroquine (3, Figure 1) and other 4-aminoquinolines made this the drug class of choice for combating malaria. 4- Aminoquinolines target the pathway for detoxification of hemoglobin breakdown products. 4−6 Chloroquine, a weak base, is accumulated in the parasite’s acidic digestive vacuole. 7 Here it binds the hematin that is released during hemoglobin degradation and prevents the formation of hemozoin. 8−11 Free hematin has redox and detergent-like activities that are thought to lead to rapid parasite killing. 12−15 Unfortunately chloroquine resistance emerged after decades of successful use of this drug and has since reached all malaria- endemic regions. 16 Chloroquine resistance arises as a con- sequence of mutations in a transporter in the digestive vacuole membrane (called the P. falciparum chloroquine resistance transporter, PfCRT), with the mutant protein mediating the export of the drug out of the vacuole and hence away from its site of action. 17−19 The effectiveness of chloroquine has now declined to the point where it has been officially abandoned in most countries. Nonetheless, newer drugs are too expensive for many and 300−500 million courses of chloroquine are still used each year. 20−23 Other antimalarials such as atovaquone target the mitochondrial cytochrome bc1 complex, while antifolate agents inhibit dihydropteroate synthase or dihydrofolate reductase; 24,25 however, widespread resistance has reached the level where these drugs have only limited use. 26,27 The World Health Organization currently recommends artemisinin-based combination therapies (ACTs) for treating uncomplicated malaria. 1 ACTs combine an artemisinin deriva- tive, with another longer-acting drug such as lumefantrine, Received: May 3, 2013 Published: July 9, 2013 Article pubs.acs.org/jmc © 2013 American Chemical Society 6200 dx.doi.org/10.1021/jm400656s | J. Med. Chem. 2013, 56, 6200−6215