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JAC Advance Access published online on May 30, 2006
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkl209
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© The Author 2006. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received January 17, 2006
Revised April 15, 2006
Accepted May 2, 2006

Original article

In vitro assessment of the pharmacodynamic properties and the partitioning of OZ277/RBx-11160 in cultures of Plasmodium falciparum

Sonja Maerki 1, Reto Brun 1, Susan A. Charman 2, Arnulf Dorn 3, Hugues Matile 3, and Sergio Wittlin 1 *

1 Swiss Tropical Institute, Socinstrasse 57, CH-4002 Basel, Switzerland
2 Centre for Drug Candidate Optimization, Monash University, Parkville, Australia 3052
3 F. Hoffmann-La Roche Ltd, Grenzacherstrasse 124, CH-4070 Basel, Switzerland

* To whom correspondence should be addressed.
Sergio Wittlin, E-mail: sergio.wittlin{at}unibas.ch


  Abstract

Objectives: Using synchronous cultures of Plasmodium falciparum malaria, the stage sensitivity of the parasite to OZ277 (RBx-11160), the first fully synthetic antimalarial peroxide that has entered Phase II clinical trials, was investigated in vitro over a concentration range of 1x to 100x the IC50. Secondly, partitioning of OZ277 into P. falciparum-infected red blood cells (RBCs) and uninfected RBCs was studied in vitro by measuring its distribution between RBCs and plasma (R/P).

Methods: The effects of timed in vitro exposure (1, 6, 12 or 24 h) to OZ277 were monitored by incorporation of [3H]hypoxanthine into parasite nucleic acids and by light-microscopic analysis of parasite morphology. Partitioning studies were performed with radiolabelled [14C]OZ277.

Results: After 1 h of exposure to OZ277 at the highest concentration (100x the IC50) followed by removal of the compound, the hypoxanthine assay showed that growth of mature stages of P. falciparum was reduced to below 20%. Young ring forms were slightly less sensitive (43% growth). Similar stage-specific profiles were found for the antimalarial reference compounds artemether and chloroquine. Strong inhibition (≤6% growth) of all parasite stages was observed when the parasites were exposed to each of the three compounds for 6 h or longer. After removal of the compounds, the parasites did not recover, indicating that the observed growth inhibitions were cytotoxic rather than cytostatic. Pyrimethamine was confirmed to be active exclusively against young schizonts. Light-microscopic analysis also demonstrated the specificity of pyrimethamine against the schizont forms and showed that OZ277, artemether and chloroquine attenuated parasite growth more rapidly than did pyrimethamine. The R/P for OZ277 was 1.5 for uninfected RBCs and up to 270 for infected RBCs.

Conclusions: The present study indicates similar stage-specific profiles for OZ277 and for the more well-established antimalarial agents artemether and chloroquine. Secondly, the study describes a significant accumulation of radiolabelled OZ277 in P. falciparum-infected RBCs.

Keywords: stage specificity; uptake; peroxides; antimalarials.
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