Delayed parasite elimination in human infections treated with clindamycin parallels ‘delayed death’ of Plasmodium falciparum in vitro Dominik Burkhardt a , Jochen Wiesner b , Nicole Stoesser c,d , Michael Ramharter a,c,1 , Anne-Catrin Uhlemann d , Saadou Issifou a,c , Hassan Jomaa b , Sanjeev Krishna d , Peter G. Kremsner a,c , Steffen Borrmann e,f, * a Department of Parasitology, Institute of Tropical Medicine, University of Tu ¨ bingen, Germany b Institute of Biochemistry, University of Giessen, Germany c Medical Research Unit, Albert Schweitzer Hospital, Lambare ´ne ´, Gabon d Department of Cellular and Molecular Medicine, Infectious Diseases, St. George’s Hospital Medical School, London, United Kingdom e Institute of Hygiene, University of Heidelberg School of Medicine, FRG, Germany f Kenya Medical Research Institute, Centre for Geographical Medicine Research – Coast, Kilifi, Kenya Received 2 November 2006; received in revised form 15 December 2006; accepted 17 December 2006 Abstract Clindamycin is safe and effective for the treatment of Plasmodium falciparum malaria, but its use as monotherapy is limited by unac- ceptably slow initial clinical response rates. To investigate whether the protracted action is due to an accumulative, time of exposure- dependent or a delayed effect on parasite growth, we studied the in vivo and in vitro pharmacodynamic profiles of clindamycin against P. falciparum. In vivo, elimination of young, circulating asexual parasite stages during treatment with clindamycin displayed an unusual biphasic kinetic: a plateau phase was followed by a precipitated decline of asexual parasite densities to nearly undetectable levels after 72 and 60 h in adult patients and asymptomatic children, respectively, suggesting an uninhibited capacity to establish a second, but not third, infectious cycle. In vitro, continuous exposure of a laboratory-adapted P. falciparum strain to clindamycin with concentrations of up to 100 lM for two replication cycles (96 h) did not produce inhibitory effects of >50% compared with drug-free controls as measured by the production of P. falciparum histidine-rich protein II (PfHRP2). PfHRP2 production was completely arrested after the second cycle (96–144 h) (>10,000-fold decrease of mean half-inhibitory concentrations measured at 96–144 h compared to 48–96 h). Furthermore, incubation with clindamycin during only the first (0–48 h) versus three (0–144 h) parasite replication cycles led to comparable inhibition of PfHRP2 production in the third infectious cycle (96–144 h) (mean IC 99 of 27 and 22 nM, respectively; P = 0.2). When parasite cultures were exposed to different concentrations of clindamycin ranging from 50 to 1,000 nM for 72 h and followed up in an experiment designed to simulate a typical 3-day treatment regimen, parasitaemia was initially suppressed below the microscopic detection threshold. Nonetheless, parasites reappeared in a dose-dependent manner after removal of drug at 72 h but not in continuously drug-exposed controls. The delayed, but potent, antimalarial effect of clindamycin appears to be of greatest potential benefit in new combinations of clindamycin with rapidly acting antimalarial combination partners. Ó 2007 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Plasmodium falciparum; Malaria; Chemotherapy; Clindamycin; Apicoplast; Chemosensitivity 1. Introduction Widespread resistance to commonly used antimalarial drugs necessitates optimal use of those that are still effica- cious. Combining two or more drugs with independent 0020-7519/$30.00 Ó 2007 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpara.2006.12.010 * Corresponding author. Tel.: +254 733 487242; fax: +254 41 522390. E-mail address: sborrmann@kilifi.mimcom.net (S. Borrmann). 1 Present address: Department of Internal Medicine I, Division of Infectious Diseases, Medical University of Vienna, Austria. www.elsevier.com/locate/ijpara International Journal for Parasitology 37 (2007) 777–785