8-aminoquinolines as anticoccidials - Part III

8-Aminoquinolines with an Aminoxyalkyl Side Chain Exert in vitro Dual-Stage Antiplasmodial Activity

A series of novel 8-aminoquinolines (8-AQs) with an aminoxyalkyl side chain were synthesized and evaluated for in vitro antiplasmodial properties against asexual blood stages, liver stages, and sexual stages of Plasmodium falciparum. 8-AQs bearing 2-alkoxy and 5-phenoxy substituents on the quinoline ring system were found to be the most promising compounds under study, exhibiting potent blood schizontocidal and moderate tissue schizontocidal in vitro activity.

2-tert-butyl-8-quinolinamines exhibit potent blood schizontocidal antimalarial activity via inhibition of heme crystallization

We have recently reported that the attachment of a bulky metabolically stable tert-butyl group at the C-2 position of a quinoline ring in primaquine results in a tremendous improvement in the blood schizontocidal antimalarial activity of 8-quinolinamine. Because free heme released from hemoglobin catabolism in a malarial parasite is highly toxic, the parasite protects itself mainly by crystallization of heme into insoluble nontoxic hemozoin. We now demonstrate the ability of 2-tert-butylprimaquine to inhibit in vitro beta-hematin formation, to form a complex with heme with a stoichiometry of 1:1, and to enhance heme-induced hemolysis. The results described herein indicate that a major improvement in the blood-schizontocidal antimalarial activity of 2-tert-butylprimaquine might be due to a disturbance of heme catabolism pathway in the malarial parasite.

8-Aminoquinolines: future role as antiprotozoal drugs

PURPOSE OF REVIEW

This review focuses on recent developments on evaluation of 8-aminoquinoline analogs with broader efficacy and reduced toxicity, which would provide better drugs for treatment of protozoal infections.

RECENT FINDINGS

The earlier efforts towards development of 8-aminoquinoline analogs have been directed to extensive derivatization programs. This has led to discovery of tafenoquine for prophylaxis against malaria infections and sitamaquine with utility for treatment of visceral leishmaniasis. Bulaquine, a primaquine pro-drug, has shown reduced methemoglobin toxicity and better malaria-transmission-blocking activity than primaquine. Stereoselective pharmacologic and toxicologic characteristics of chiral 8-aminoquinolines provided the lead for enantiomeric separation of an 8-aminoquinoline analog NPC1161B, with greatly reduced toxicity and potent antimalarial action against blood as well as tissue stages of the parasite. NPC1161B has also shown promising use as an antileishmanial agent. Better understanding of the mechanisms of toxicity and efficacy may help in development of 8-aminoquinoline analogs with superior therapeutic actions, reduced toxicity and broader utility.

SUMMARY

Extensive derivatization approaches followed by better understanding of structure-activity relationships and biotransformation mechanisms of toxicity have provided 8-aminoquinoline analogs with better pharmacologic and reduced toxicologic profiles. The novel 8-aminoquinoline analogs may have broader utility in public health as future antiprotozoals.