Scalable preparation and differential pharmacologic and toxicologic profiles of primaquine enantiomers

Abstract

Hematotoxicity in individuals genetically deficient in glucose-6-phosphate dehydrogenase (G6PD) activity is the major limitation of primaquine (PQ), the only antimalarial drug in clinical use for treatment of relapsing Plasmodium vivax malaria. PQ is currently clinically used in its racemic form. A scalable procedure was developed to resolve racemic PQ, thus providing pure enantiomers for the first time for detailed preclinical evaluation and potentially for clinical use. These enantiomers were compared for antiparasitic activity using several mouse models and also for general and hematological toxicities in mice and dogs. (+)-(S)-PQ showed better suppressive and causal prophylactic activity than (-)-(R)-PQ in mice infected with Plasmodium berghei. Similarly, (+)-(S)-PQ was a more potent suppressive agent than (-)-(R)-PQ in a mouse model of Pneumocystis carinii pneumonia. However, at higher doses, (+)-(S)-PQ also showed more systemic toxicity for mice. In beagle dogs, (+)-(S)-PQ caused more methemoglobinemia and was toxic at 5 mg/kg of body weight/day given orally for 3 days, while (-)-(R)-PQ was well tolerated. In a novel mouse model of hemolytic anemia associated with human G6PD deficiency, it was also demonstrated that (-)-(R)-PQ was less hemolytic than (+)-(S)-PQ for the G6PD-deficient human red cells engrafted in the NOD-SCID mice. All these data suggest that while (+)-(S)-PQ shows greater potency in terms of antiparasitic efficacy in rodents, it is also more hematotoxic than (-)-(R)-PQ in mice and dogs. Activity and toxicity differences of PQ enantiomers in different species can be attributed to their different pharmacokinetic and metabolic profiles. Taken together, these studies suggest that (-)-(R)-PQ may have a better safety margin than the racemate in human.

Copyright © 2014, American Society for Microbiology. All Rights Reserved.

Figures

FIG 1

Structures of primaquine, NPC1161C, and their enantiomers.

FIG 2

Effect of treatment with PQ enantiomers on total body weight of Plasmodium berghei-infected mice. The P. berghei-infected mice were treated once daily for 3 days (days 0, 1, and 2) (2 h after infection) with racemic PQ and PQ enantiomers. Chloroquine was tested as a control drug. The results shown here are for the 100-mg/kg dose. The lower doses did not show any effect on total body weights. Each point shows mean ± SD values from 5 animals in each group.

FIG 3

Prophylactic antimalarial activities of (+)- and (−)-primaquine against P. berghei in mice. Mice were inoculated with sporozoites on day 0. Treatment with the drugs (p.o.) was daily on days −1, 0, and 1. Mice normally succumb to infection at about 1 week postinfection. The graph represents the percentage of mice surviving to day 31 (study end). mpk, mg/kg.

FIG 4

Hemolytic toxicities of PQ enantiomers in NOD-SCID mice engrafted with G6PD-deficient human erythrocytes. The data represent the loss of human erythrocytes on day 7 of treatment. Each bar represents the mean values ± SD from four animals. The data were analyzed by using the Student t test. P values, PBS versus PQ (12.5), <0.0001 (statistical significant difference [S]); PBS versus (+)-PQ (12.5), 0.003 (S); PBS versus (−)-PQ (12.5), 0.05 (S); PBS versus (+)-PQ (6.25), 0.0004(S); PBS versus (−)-PQ (6.25), 0.38 (difference statistically not significant [NS]); PQ (12.5) versus (+)-PQ 12.5, 0.35 (NS); PQ (12.5) versus (−)-PQ (12.5), 0.0002 (S); (+)-PQ (12.5) versus (−)-PQ (12.5), 0.007 (S).