Design, synthesis and structure–activity relationships of (1H-pyridin-4-ylidene)amines as potential antimalarials Tiago Rodrigues a , Rita C. Guedes a , Daniel J. V. A. dos Santos a , Marta Carrasco a , Jiri Gut b , Philip J. Rosenthal b , Rui Moreira a, * , Francisca Lopes a a iMed.UL, Faculty of Pharmacy, University of Lisbon, Av. Prof. Gama Pinto, 1649-003 Lisbon, Portugal b Department of Medicine, San Francisco General Hospital, University of California, San Francisco, Box 0811, CA 94143, USA article info Article history: Received 6 April 2009 Revised 5 May 2009 Accepted 5 May 2009 Available online 9 May 2009 Keywords: Antiplasmodial (1H-Pyridin-4-ylidene)amines abstract (1H-Pyridin-4-ylidene)amines containing lipophilic side chains at the imine nitrogen atom were pre- pared as potential clopidol isosteres in the development of antimalarials. Their antiplasmodial activity was evaluated in vitro against the Plasmodium falciparum W2 (chloroquine-resistant) and FCR3 (atovaqu- one-resistant) strains. The most active of these derivatives, 4m, had an IC 50 of 1 lM against W2 and 3 lM against FCR3. Molecular modeling studies suggest that (1H-pyridin-4-ylidene)amines may bind to the ubiquinol oxidation Q o site of cytochrome bc 1 . Ó 2009 Elsevier Ltd. All rights reserved. Malaria is a serious global health problem, and the problem is complicated by the rapid emergence and spread of multidrug- resistant Plasmodium falciparum. 1 The urgent need for new drugs triggered the search for lead compounds, preferably acting on underexploited parasite targets. 2,3 Targeting the mitochondrial electron transport chain of the human malaria parasite has proved to be a valid chemotherapeutic strategy 4,5 as shown by atovaquone (1, Fig. 1), which is used in combination with proguanil to treat multidrug-resistant P. falciparum infections. 6 Atovaquone is a po- tent inhibitor of the bc 1 complex, a mitochondrial multisubunit en- ergy-transducing membrane protein. 7,8 Atovaquone binds selectively to the Q o site of cytochrome b, close to the site of inter- action with the Rieske iron-sulfur protein (ISP), displacing ubiqui- nol and substantially blocking the conformational change of the ISP that is required for electron transfer to cytochrome c 1 . 8 This inhibi- tion of electron transfer by atovaquone blocks respiration and pro- duces a concomitant collapse of mitochondrial transmembrane potential. 8–10 Despite its effectiveness, atovaquone resistant strains swiftly emerged when it was introduced in clinical therapy. 11 Interest in developing novel inhibitors of parasite respiration as potential antimalarials led to the re-discovery of the anticoccidial drug clop- idol, 2, a 4(1H)-pyridone with antiplasmodial activity that inhibits the mitochondrial respiration. 12,13 The structure of clopidol was modified based on the assumption that it could be acting as ubiqui- none antagonist. 14 Replacing the 5-Cl of clopidol by lipophilic side chains led to the discovery of GW844520, 3, which displays excellent in vitro activ- ity against multi-drug resistant P. falciparum strains coupled with low frequency of spontaneous in vitro resistance. 14 We now report our research on the (1H-pyridin-4-ylidene)amine scaffold, 4, as a potential isostere of clopidol and as a potential antimalarial. Based on the evidence that a lipophilic chain can improve anti- malarial activity of 4(1H)-pyridones relative to that of clopidol, 14 we decided to incorporate an aromatic moiety at the imine nitro- 0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2009.05.017 * Corresponding author. Tel.: +351 217946476; fax: +351 217946470. E-mail address: rmoreira@ff.ul.pt (R. Moreira). O O OH Cl N H O Me Me Cl Cl N H O Me Me Cl O OCF 3 N N R 2 R 3 R 1 R 4 R 5 R 6 1 2 3 4 Figure 1. Structures of atovaquone, 1, clopidol, 2, GW844520, 3, and (1H-pyridin-4- ylidene)amines, 4. Bioorganic & Medicinal Chemistry Letters 19 (2009) 3476–3480 Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl