THIS IS A PROOF 1 Chloroquine resistance in Plasmodium falciparum has recently been shown to result from mutations in the novel vacuolar transporter PfCRT. Field studies have demonstrated the importance of these mutations in clinical resistance. Although a pfcr t homolog has been identified in Plasmodium vivax , there is no association between in vivo c hloroquine resist ance and codon mut ations in the P . vivax gene. [AU:OK?] This is consistent with lines of evidence that suggest alternative mechanisms of chloroquine resistance among various malaria parasite species. Addresses *National Center for Biotechnology Research, National Library of Medicine, National Institutes of Health, Building 45, 45 Center Drive, Bethesda, MD 20892-6510, USA † Department of Microbiology and Immunology, Albert Einstein College of Medicine of Yeshiva University, 1300 Morris Park Avenue, The Bronx, NY 10461, USA ‡ Malaria Research and Training Center, Faculty of Medicine, Pharmacy and Dentistry, University of Mali, PO Box 1805, Bamako, Mali § Malaria Section, Center for Vaccine Development, University of Maryland School of Medicine, 685 West Baltimore Street, HSF 480, Baltimore, MD 21201, USA # Malaria Genetics Section, Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Building 4, 4 Center Drive, MSC 0425, Bethesda, MD 20892-0425, USA; email: tew@helix.nih.gov Current Opinion in Microbiology 2001, 4:XXX–XXX 1369-5274/01/$ — see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations CQ chloroquine PfCRT P. falciparum chloroquine resistance transporter Introduction [AUQ1: I have shortened the title slightly to make it more concise. I understand you want to emphasis the fact that the transporter plays a role in CQ resistance in P. falci- parum and not P. vivax but I think this is covered in the abstract and it’s better to catch the readers attention with a shorter title. Is this OK?] Malaria parasite resistance to the drug chloroquine (CQ) poses a severe and increasing public health threat. This inexpensive and widely consumed drug has been the main line of attack against the parasite, and its increasing failure accompanies a return of malaria-related morbidity and mortality levels not seen for decades [1 • ]. The problem is most acute in Plasmodium falciparum malaria, the species responsible for the most severe form of the disease. The emergence of CQ-resistant P. vivax, a species that causes 75–90 million cases of non-fatal malaria annually [2 • ], has recently become an area of increasing concern. Here, we review recent progress in deciphering CQ resis- tance in malaria parasites. These developments include the identification of mutations in a vacuolar transporter as the basis for CQ resistance in P. falciparum and the finding of absolute selection of these mutations in clinical cases of CQ treatment failure. These results are generating new hypotheses on the molecular mechanism of CQ resistance. Investigations into CQ resistance in other malaria parasites also provide evidence that mechanisms of resistance differ among Plasmodium species. Three distinct evolutionary clades of malaria parasites Malaria parasites are classified in the phylum Apicomplexa, a large protist group consisting of almost 5000 species. All apicomplexans are parasites and contain an organellar structure, the apical complex, involved in host cell invasion. Within the phylum, the genus Plasmodium includes ~200 known malaria species that par- asitize birds, reptiles, and mammals. The genus divides into three distinct and highly diver gent evolutionary clades [3,4]: the first includes P . falciparum and a closely related parasite of apes, P . reichenowi ; the second clade consists of P . vivax and monkey malaria species including P . knowlesi ; and finally , the third clade includes rodent malaria species such as P . berghei and P . chabaudi . [AU:OK?] Major differ- ences in host specificity and disease manifestation occur among species of these clades, as do wide variations in genome composition and codon usage [5,6]. Because of the difficulties of working with P. falciparum in the laboratory, there has been support for the use of many of these relat- ed species as models, for example, in studies of host cell invasion [7], malaria vaccine development [8], and anti- malarial drug resistance (reviewed in [9 • ]). The mechanism of chloroquine action In human erythrocytes, P. falciparum supports its growth by taking up host cell cytoplasm in an acidic digestive food vacuole [10]. Toxic heme, in its hematin ( μ -oxodimer) [AU:OK?] form, is released in the vacuole by hemoglobin digestion and crystallized into innocuous hemozoin, or malaria pigment. CQ is proposed to interfere with this process by complexing with hematin [11,12], thereby cre- ating toxic complexes that cause parasite death. The actual mechanism of toxicity [AUQ2: Is this toxicity of hematin or the CQ—hematin complex? Or both?] is still subject to Conservation of a novel vacuolar transporter in Plasmodium species and its central role in chloroquine resistance of P. falciparum [AU:OK?] Jane MR Carlton*, David A Fidock † , Abdoulaye Djimdé ‡§¶ , Christopher V Plowe § and Thomas E Wellems #