In Vitro Antimalarial Activity of Azithromycin, Artesunate, and Quinine in Combination and Correlation with Clinical Outcome

Abstract

Azithromycin when used in combination with faster-acting antimalarials has proven efficacious in treating Plasmodium falciparum malaria in phase 2 clinical trials. The aim of this study was to establish optimal combination ratios for azithromycin in combination with either dihydroartemisinin or quinine, to determine the clinical correlates of in vitro drug sensitivity for these compounds, and to assess the cross-sensitivity patterns. Seventy-three fresh P. falciparum isolates originating from patients from the western border regions of Thailand were successfully tested for their drug susceptibility in a histidine-rich protein 2 (HRP2) assay. With overall mean fractional inhibitory concentrations of 0.84 (95% confidence interval [CI] ‫؍‬ 0.77 to 1.08) and 0.78 (95% CI ‫؍‬ 0.72 to 0.98), the interactions between azithromycin and dihydroartemisinin, as well as quinine, were classified as additive, with a tendency toward synergism. The strongest tendency toward synergy was seen with a combination ratio of 1:547 for the combination with dihydroartemisinin and 1:44 with quinine. The geometric mean 50% inhibitory concentration (IC 50 ) of azithromycin was 2,570.3 (95% CI ‫؍‬ 2,175.58 to 3,036.58) ng/ml. The IC 50 s for mefloquine, quinine, and chloroquine were 11.42, 64.4, and 54.4 ng/ml, respectively, suggesting a relatively high level of background resistance in this patient population. Distinct correlations (R ‫؍‬ 0.53; P ‫؍‬ 0.001) between quinine in vitro results and parasite clearance may indicate a compromised sensitivity to this drug. The correlation with dihydroartemisinin data was weaker (R ‫؍‬ 0.34; P ‫؍‬ 0.038), and no such correlation was observed for azithromycin. Our in vitro data confirm that azithromycin in combination with artemisinin derivatives or quinine exerts additive to synergistic interactions, shows no cross-sensitivity with traditional antimalarials, and has substantial antimalarial activity on its own.

The SYBR Green I Malaria Drug Sensitivity Assay: Performance in Low Parasitemia Samples

“…4 are likely to give similar IC values with the SYBR Green I assay as compared with other assays, geometric mean parasite densities as high as this are rarely encountered under field conditions in malaria-endemic countries, e.g., Ref. 9.…”

Antimalarial Drugs – What is in Use and What is in the Pipeline

“…There was no significant difference in the in-vitro azithromycin 27 susceptibility between the isolates from patients who where cured and therapy failures. In contrast, isolates from patients with therapy failures where less sensitive towards quinine 3 or artesunate 11 [167].…”

Global Infectious Disease Surveillance at DoD Overseas Laboratories, 1999–2007

“…21 DoD-GEIS scientists in Asia, East Africa, and South America have documented the continued spread of markers suggesting the potential for malaria drug resistance emergencenotably recent artesunate-class drug resistance-while working to standardize laboratory methodology and microscopy. [22][23][24][25][26][27][28][29][30] A collaboration between the National Aeronautics and Space Administration, USAMRU-K, and DoD-GEIS headquarters has developed an effective, satellite-based early warning tool for rift valley fever in East Africa and is now working to expand its applicability to other climate-dependent epidemic diseases. [31][32][33][34][35][36] The network has also overseen the development and refinement of a series of systems designed to bring real-time disease surveillance to remote or low-resource settings, including Cambodia, Laos, Thailand, the Philippines, and Andean Ridge countries of South America.…”