Susceptibility of glucose-6-phosphate dehydrogenase deficient red cells to primaquine, primaquine enantiomers, and its two putative metabolites. II. Effect on red blood cell membrane, lipid peroxidation, MC-540 staining, and scanning electron microscopic studies

Primaquine-5,6-Orthoquinone Is Directly Hemolytic to Older G6PD Deficient RBCs in a Humanized Mouse Model

Primaquine and Tafenoquine are the only approved drugs that can achieve a radical cure for Plasmodium vivax malaria but are contraindicated in patients who are deficient in glucose 6-phosphate dehydrogenase (G6PDd) due to risk of severe hemolysis from reactive oxygen species generated by redox cycling of drug metabolites. 5-hydroxyprimaquine and its quinoneimine cause robust redox cycling in red blood cells (RBCs) but are so labile as to not be detected in blood or urine. Rather, the quinoneimine is rapidly converted into primaquine-5,6-orthoquinone (5,6-POQ) that is then excreted in the urine. The extent to which 5,6-POQ contributes to hemolysis remains unclear, although some have suggested that it is a minor toxin that should be used predominantly as a surrogate to infer levels of 5-hydroxyprimaquine. In this report, we describe a novel humanized mouse model of the G6PD Mediterranean variant (hG6PDMed-) that recapitulates the human biology of RBC age-dependent enzyme decay, as well as an isogenic matched control mouse with human nondeficient G6PD hG6PDND In vitro challenge of RBCs with 5,6-POQ causes increased generation of superoxide and methemoglobin. Infusion of treated RBCs shows that 5,6-POQ selectively causes in vivo clearance of older hG6PDMed- RBCs. These findings support the hypothesis that 5,6-POQ directly induces hemolysis and challenges the notion that 5,6-POQ is an inactive metabolic waste product. Indeed, given the extreme lability of 5-hydroxyprimaquine and the relative stability of 5,6-POQ, these data raise the possibility that 5,6-POQ is a major hemolytic primaquine metabolite in vivo. SIGNIFICANCE STATEMENT: These findings demonstrate that 5,6-POQ, which has been considered an inert waste product of primaquine metabolism, directly induces ROS that cause clearance of older G6PDd RBCs. As 5,6-POQ is relatively stable compared with other active primaquine metabolites, these data support the hypothesis that 5,6-POQ is a major toxin in primaquine induced hemolysis. The findings herein also establish a new model of G6PDd and provide the first direct evidence, to our knowledge, that young G6PDd RBCs are resistant to primaquine-induced hemolysis.

Enantioselective Interactions of Anti-Infective 8-Aminoquinoline Therapeutics with Human Monoamine Oxidases A and B

8-Aminoquinolines (8-AQs) are an important class of anti-infective therapeutics. The monoamine oxidases (MAOs) play a key role in metabolism of 8-AQs. A major role for MAO-A in metabolism of primaquine (PQ), the prototypical 8-AQ antimalarial, has been demonstrated. These investigations were further extended to characterize the enantioselective interactions of PQ and NPC1161 (8-[(4-amino-1-methylbutyl) amino]-5-[3, 4-dichlorophenoxy]-6-methoxy-4-methylquinoline) with human MAO-A and -B. NPC1161B, the (R)-(−) enantiomer with outstanding potential for malaria radical cure, treatment of visceral leishmaniasis and pneumocystis pneumonia infections is poised for clinical development. PQ showed moderate inhibition of human MAO-A and -B. Racemic PQ and (R)-(−)-PQ both showed marginally greater (1.2- and 1.6-fold, respectively) inhibition of MAO-A as compared to MAO-B. However, (S)-(+)-PQ showed a reverse selectivity with greater inhibition of MAO-B than MAO-A. Racemic NPC1161 was a strong inhibitor of MAOs with 3.7-fold selectivity against MAO-B compared to MAO-A. The (S)-(+) enantiomer (NPC1161A) was a better inhibitor of MAO-A and -B compared to the (R)-(−) enantiomer (NPC1161B), with more than 10-fold selectivity for inhibition of MAO-B over MAO-A. The enantioselective interaction of NPC1161 and strong binding of NPC1161A with MAO-B was further confirmed by enzyme-inhibitor binding and computational docking analyses. Differential interactions of PQ and NPC1161 enantiomers with human MAOs may contribute to the enantioselective pharmacodynamics and toxicity of anti-infective 8-AQs therapeutics.

Prevalence and Genetic Characterization of Glucose-6-Phosphate Dehydrogenase Deficiency in Anemic Subjects from Uttar Pradesh, India

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is caused by one or more mutations in the G6PD gene on chromosome X. It affects approximately 400 million people worldwide. The purpose of this study was to detect the prevalence of G6PD deficiency and G6PD gene mutations in the hospital-based settings in patients referred for suspected G6PD deficiency. A qualitative fluorescent spot test and dichlorophenol-indolphenol (DCIP) test were performed. G6PD-deficient, positive samples were further processed for mutation analysis by Sanger sequencing. Out of 1,069 cases, 95 (8.8%) were detected as G6PD deficient (by DCIP test) and were sent for molecular analysis. The G6PD Mediterranean mutation (563C > T) is the most common variant among G6PD-deficient individuals followed by the Coimbra (592C→T) and Orissa (131C→G) variants. We concluded that all symptomatic patients (anemic or jaundiced) should be investigated for G6PD deficiency. Our findings will inform our population screening approach and help provide better management for G6PD-deficient patients.

Enantioselective pharmacokinetics of primaquine in healthy human volunteers

Primaquine (PQ), a racemic drug, is the only treatment available for radical cure of relapsing Plasmodium vivax malaria and blocking transmission of P. falciparum malaria. Recent studies have shown differential pharmacologic and toxicologic profiles of individual PQ enantiomers in rodent, dog, and primate animal models. This study was conducted in six healthy adult human volunteers to determine the plasma pharmacokinetic profile of enantiomers of PQ and carboxyprimaquine (cPQ), the major plasma metabolite. The individuals were orally administered PQ diphosphate, equivalent to 45-mg base, 30 minutes after a normal breakfast. Blood samples were collected at different time intervals, and plasma samples were analyzed for enantiomers of PQ and cPQ. Plasma PQ concentrations were low and variable for both parent enantiomers and peaked around 2-4 hours. Peak (-)-(R)-PQ concentrations ranged from 121 ng/ml to 221 ng/ml, and peak (+)-(S)-PQ concentrations ranged from 168 ng/ml to 299 ng/ml. The cPQ concentrations were much higher and were surprisingly consistent from subject to subject. Essentially all the cPQ detected in plasma was (-)-cPQ. The peak concentrations of (-)-cPQ were observed at 8 hours (range: 1104-1756 ng/ml); however, very high concentrations were sustained through 24 hours. (+)-cPQ was two orders of magnitude lower than (-)-cPQ, and in a few subjects it was detected but only under the limit of quantification. In vitro studies with primary human hepatocytes also suggested more rapid metabolism of (-)-PQ compared with (+)-PQ. The results suggest more rapid metabolism of (-)-PQ to (-) cPQ compared with (+)-PQ. Alternatively, (+)-PQ or (+)-cPQ could be rapidly converted to another metabolite(s) or distributed to tissues. This is the first clinical report on enantioselective pharmacokinetic profiles of PQ and cPQ and supports further clinical evaluation of individual PQ enantiomers.

Pharmacokinetics and pharmacodynamics of (+)-primaquine and (-)-primaquine enantiomers in rhesus macaques (Macaca mulatta)

Primaquine (PQ) remains the sole available drug to prevent relapse of Plasmodium vivax malaria more than 60 years after licensure. While this drug was administered as a racemic mixture, prior studies suggested a pharmacodynamic advantage based on differential antirelapse activity and/or toxicities of its enantiomers. Oral primaquine enantiomers prepared using a novel, easily scalable method were given for 7 days to healthy rhesus macaques in a dose-rising fashion to evaluate their effects on the blood, liver, and kidneys. The enantiomers were then administered to Plasmodium cynomolgi-infected rhesus macaques at doses of 1.3 and 0.6 mg/kg of body weight/day in combination with chloroquine. The (-)-PQ enantiomer had higher clearance and apparent volume of distribution than did (+)-PQ and was more extensively converted to the carboxy metabolite. There is evidence for differential oxidative stress with a concentration-dependent rise in methemoglobin (MetHgb) with increasing doses of (+)-PQ greater than that seen for (-)-PQ. There was a marked, reversible hepatotoxicity in 2 of 3 animals dosed with (-)-PQ at 4.5 mg/kg. (-)-PQ in combination with chloroquine was successful in preventing P. cynomolgi disease relapse at doses of 0.6 and 1.3 mg/kg/day, while 1 of 2 animals receiving (+)-PQ at 0.6 mg/kg/day relapsed. While (-)-PQ was also associated with hepatotoxicity at higher doses as seen previously, this has not been identified as a clinical concern in humans during >60 years of use. Limited evidence for increased MetHgb generation with the (+) form in the rhesus macaque model suggests that it may be possible to improve the therapeutic window for hematologic toxicity in the clinic by separating primaquine into its enantiomers.

4(1H)-pyridone and 4(1H)-quinolone derivatives as antimalarials with erythrocytic, exoerythrocytic, and transmission blocking activities

Infectious diseases are the second leading cause of deaths in the world with malaria being responsible for approximately the same amount of deaths as cancer in 2012. Despite the success in malaria prevention and control measures decreasing the disease mortality rate by 45% since 2000, the development of single-dose therapeutics with radical cure potential is required to completely eradicate this deadly condition. Targeting multiple stages of the malaria parasite is becoming a primary requirement for new candidates in antimalarial drug discovery and development. Recently, 4(1H)- pyridone, 4(1H)-quinolone, 1,2,3,4-tetrahydroacridone, and phenoxyethoxy-4(1H)-quinolone chemotypes have been shown to be antimalarials with blood stage activity, liver stage activity, and transmission blocking activity. Advancements in structure-activity relationship and structure-property relationship studies, biological evaluation in vitro and in vivo, as well as pharmacokinetics of the 4(1H)-pyridone and 4(1H)-quinolone chemotypes are discussed.

Stereoselectivity in the pharmacodynamics and pharmacokinetics of the chiral antimalarial drugs

Several of the antimalarial drugs are chiral and administered as the racemate. These drugs include chloroquine, hydroxychloroquine, quinacrine, primaquine, mefloquine, halofantrine, lumefantrine and tafenoquine. Quinine and quinidine are also stereoisomers, although they are given separately rather than in combination. From the perspective of antimalarial activity, most of these agents demonstrate little stereoselectivity in their effects in vitro. Mefloquine, on the other hand, displays in vitro stereoselectivity against some strains of P. falciparum, with a eudismic ratio of almost 2 : 1 in favour of the (+)-enantiomer. Additionally, for some of these agents (e.g. halofantrine, primaquine, chloroquine), stereoselectivity has been noted in the ability of the enantiomers to cause certain adverse effects. In recent years, stereospecific analytical methods capable of measuring the individual enantiomers after the administration of racemic drugs have been reported for a number of chiral antimalarial drugs. These assays have revealed that almost all the studied antimalarial drugs display stereoselectivity in their pharmacokinetics, leading to enantioselectivity in their plasma concentrations. Whereas the oral absorption of these agents appears to be non-stereoselective, stereoselectivity is often seen in their volume of distribution and/or clearance. With regard to distribution, plasma protein binding of some chiral antimalarial drugs exhibits a significant degree of stereoselectivity, leading to stereoselective distribution to blood cells and other tissues. Because of their low hepatic extraction ratios, stereoselective plasma protein binding also contributes to the stereoselectivity in the metabolism of these drugs. Chiral metabolites are formed from some parent antimalarial drugs, although stereoselective aspects of the pharmacokinetics of the metabolites are not well understood. It is concluded that knowledge of the stereoselective aspects of these agents may be helpful in better understanding their mechanisms of action and possibly optimising their clinical safety and/or effectiveness.

Primaquine-induced hemolytic anemia: formation of free radicals in rat erythrocytes exposed to 6-methoxy-8-hydroxylaminoquinoline

Primaquine is an important antimalarial drug that is often dose-limited in therapy by the onset of hemolytic anemia. We have shown recently that an N-hydroxy metabolite of primaquine, 6-methoxy-8-hydroxylaminoquinoline (MAQ-NOH), is a direct-acting hemolytic agent in rat red cells and that the hemolytic activity of this metabolite is associated with GSH oxidation and oxidative damage to both membrane lipids and skeletal proteins. To determine whether the formation of free radicals may be involved in this process, rat red cells (40% suspensions) were incubated with hemolytic concentrations of MAQ-NOH (150-750 microM) and examined by EPR spectroscopy using 2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole-1-oxide (EMPO) as a spin trap. Addition of MAQ-NOH to red cell suspensions containing 10 mM EMPO gave rise to an EPR spectrum with hyperfine constants consistent with those of an EMPO-hydroxyl radical adduct standard. Of interest, formation of EMPO-OH was constant for up to 20 min and dependent on the presence of erythrocytic GSH. Although no other radical adduct signals were detected in the cells by EPR, spectrophotometric analysis revealed the presence of ferrylhemoglobin, which indicates that hydrogen peroxide is generated under these experimental conditions. The data support the hypothesis that oxygen-derived and possibly other free radicals are involved in the mechanism underlying MAQ-NOH-induced hemolytic anemia.

Primaquine-induced hemolytic anemia: effect of 6-methoxy-8-hydroxylaminoquinoline on rat erythrocyte sulfhydryl status, membrane lipids, cytoskeletal proteins, and morphology

Previous studies have shown that 6-methoxy-8-hydroxylaminoquinoline (MAQ-NOH), an N-hydroxy metabolite of the antimalarial drug, primaquine, is a direct-acting hemolytic agent in rats. To investigate the mechanism underlying this hemolytic activity, the effects of hemotoxic concentrations of MAQ-NOH on rat erythrocyte sulfhydryl status, membrane lipids, skeletal proteins, and morphology have been examined. Treatment of rat erythrocytes with a TC(50) concentration of MAQ-NOH (350 microM) caused only a modest and transient depletion of reduced glutathione (GSH) (~30%), which was matched by modest increases in the levels of glutathione disulfide and glutathione-protein mixed disulfides. Lipid peroxidation, as measured by thiobarbituric acid-reactive substances and F(2)-isoprostane formation, was induced in a concentration-dependent manner by MAQ-NOH. However, the formation of disulfide-linked hemoglobin adducts on membrane skeletal proteins and changes in erythrocyte morphology were not observed. These data suggest that hemolytic activity results from peroxidative damage to the lipid of the red cell membrane and is not dependent on skeletal protein thiol oxidation. However, when red cell GSH was depleted (>90%) by titration with diethyl maleate, hemolytic activity of MAQ-NOH was markedly enhanced. Of interest, exacerbation of hemotoxicity was not matched by increases in lipid peroxidation, but by the appearance of hemoglobin-skeletal protein adducts. Collectively, the data are consistent with the concept that MAQ-NOH may operate by more than one mechanism; one that involves lipid peroxidation in the presence of normal amounts of erythrocytic GSH, and one that involves protein oxidation in red cells with low levels of GSH, such as are seen in individuals with glucose-6-phosphate dehydrogenase deficiency.

Oxidative activity of primaquine metabolites on rat erythrocytes in vitro and in vivo

The oxidative activities of primaquine [6-methoxy-8-(4-amino-1-methylbutylamino)quinoline] and its metabolites, the quinone-imine derivatives of 5-hydroxyprimaquine [5-hydroxy-6-methoxy-8-(4-amino-1-methylbutylamino)quinoline] and 5-hydroxydemethylprimaquine [5-hydroxy-6-demethyl-8-(4-amino-1-methylbutylamino)quinoline], 6-methoxy-8-amino quinoline and hydrogen peroxide, were studied on rat erythrocytes in vitro and in vivo. In both cases, the most effective metabolites in oxidizing hemoglobin and depleting non-protein sulfhydryl groups from erythrocytes were the quinone-imine derivatives of the ring-hydroxylated metabolites, 5-hydroxyprimaquine and 5-hydroxydemethyl-primaquine. The latter quinone-imines were shown by light absorption spectroscopy and oxygen consumption studies to be able to oxidize purified rat hemoglobin to methemoglobin but to be unable to react directly with reduced glutathione. In agreement with these results, no radical adduct was detected by electron paramagnetic resonance spectroscopy in incubations of rat erythrocytes with the quinone-imines and the spin-trap 5,5-dimethyl-1-pyrroline-N-oxide; metabolite-derived free radicals were detected instead. Taken together, the results suggest that 5-hydroxyprimaquine and 5-hydroxydemethylprimaquine are important metabolites in the expression of primaquine hemotoxicity, in contrast to 6-methoxy-8-aminoquinoline. Additionally, the results indicate that hydrogen peroxide is the ultimate oxidant formed from the ring-hydroxylated metabolites by redox-cycling of the corresponding quinone-imine derivatives both in vitro and in vivo.