Primaquine-induced hemolytic anemia: susceptibility of normal versus glutathione-depleted rat erythrocytes to 5-hydroxyprimaquine

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.

Population pharmacokinetic modelling of primaquine exposures in lactating women and breastfed infants

Current guidelines advise against primaquine treatment for breastfeeding mothers to avoid the potential for haemolysis in infants with G6PD deficiency. To predict the haemolytic risk, the amount of drug received from the breast milk and the resulting infant drug exposure need to be characterised. Here, we develop a pharmacokinetic model to describe the drug concentrations in breastfeeding women using venous, capillary, and breast milk data. A mother-to-infant model is developed to mimic the infant feeding pattern and used to predict their drug exposures. Primaquine and carboxyprimaquine exposures in infants are <1% of the exposure in mothers. Therefore, even in infants with the most severe G6PD deficiency variants, it is highly unlikely that standard doses of primaquine (0.25-1 mg base/kg once daily given to the mother for 1-14 days) would cause significant haemolysis. After the neonatal period, primaquine should not be restricted for breastfeeding women (Clinical Trials Registration: NCT01780753).

Comparative metabolism and tolerability of racemic primaquine and its enantiomers in human volunteers during 7-day administration

Primaquine (PQ) is an 8-aminoquinoline antimalarial, active against dormant Plasmodium vivax hypnozoites and P. falciparum mature gametocytes. PQ is currently used for P. vivax radical cure and prevention of malaria transmission. PQ is a racemic drug and since the metabolism and pharmacology of PQ's enantiomers have been shown to be divergent, the objectives of this study were to evaluate the comparative tolerability and metabolism of PQ with respect to its two enantiomers in human volunteers in a 7 days' treatment schedule. Fifteen subjects with normal glucose-6-phosphate dehydrogenase (G6PDn) completed four arms, receiving each of the treatments, once daily for 7 days, in a crossover fashion, with a 7-14 days washout period in between: R-(-) enantiomer (RPQ) 22.5 mg; S-(+) enantiomer (SPQ) 22.5 mg; racemic PQ (RSPQ) 45 mg, and placebo. Volunteers were monitored for any adverse events (AEs) during the study period. PQ and metabolites were quantified in plasma and red blood cells (RBCs) by UHPLC-UV-MS/MS. Plasma PQ was significantly higher in SPQ treatment group than for RPQ. Carboxy-primaquine, a major plasma metabolite, was much higher in the RPQ treated group than SPQ; primaquine carbamoyl glucuronide, another major plasma metabolite, was derived only from SPQ. The ortho-quinone metabolites were also detected and showed differences for the two enantiomers in a similar pattern to the parent drugs. Both enantiomers and racemic PQ were well tolerated in G6PDn subjects with the 7 days regimen; three subjects showed mild AEs which did not require any intervention or discontinuation of the drug. The most consistent changes in G6PDn subjects were a gradual increase in methemoglobin and bilirubin, but these were not clinically important. However, the bilirubin increase suggests mild progressive damage to a small fraction of red cells. PQ enantiomers were also individually administered to two G6PD deficient (G6PDd) subjects, one heterozygous female and one hemizygous male. These G6PDd subjects showed similar results with the two enantiomers, but the responses in the hemizygous male were more pronounced. These studies suggest that although the metabolism profiles of individual PQ enantiomers are markedly different, they did not show significant differences in the safety and tolerability in G6PDn subjects.

Simultaneous and enantiospecific quantification of primaquine and carboxyprimaquine in human plasma using liquid chromatography-tandem mass spectrometry

Background

The enantiomers of the 8-aminoquinoline anti-malarial primaquine have different pharmacological properties. Development of an analytical method for simultaneous quantification of the enantiomers of primaquine and its metabolite, carboxyprimaquine, will support clinical pharmacometric assessments.

Methods

A simple and sensitive method consisting of liquid chromatography coupled with tandem mass spectrometry (LC–MS/MS) was developed for simultaneous and enantiospecific determination of primaquine and its metabolite, carboxyprimaquine, in human plasma. Stable isotopes were used as internal standards to compensate for potential interference and matrix effects. Plasma samples (100 µL) were precipitated with 1% formic acid in acetonitrile followed by phospholipid removal solid phase extraction. Primaquine and carboxyprimaquine enantiomers were separated on a Chiralcel OD-3R (150 mm × 4.6 mm; I.D. 3 μm) column using a LC gradient mode. For separation of racemic primaquine and carboxyprimaquine, the LC method was modified and validated using a reverse phase column (Hypersil Gold 100 mm × 4.6 mm; I.D. 3 µm) and a mobile phase composed of 10 mM ammonium acetate buffer, pH 3.5 and acetonitrile in the isocratic mode. Method validation was performed according to regulatory guidelines.

Results

The calibration range was set to 0.571–260 ng/mL and 2.44–2,500 ng/mL for primaquine and carboxyprimaquine enantiomers, respectively, resulting in a correlation coefficient (r²) ≥ 0.0998 for all calibration curves. The intra- and inter-day assay precisions were < 10% and the accuracy was between 94.7 to 103% for all enantiomers of primaquine and carboxyprimaquine. The enantiospecific method was also modified and validated to quantify racemic primaquine and carboxyprimaquine, reducing the total run time from 30 to 8 min. The inter-, intra-day assay precision of the racemic quantification method was < 15%. The absolute recoveries of primaquine and carboxyprimaquine were between 70 and 80%. Stability was demonstrated for up to 2 years in − 80 °C. Both the enantiomeric and racemic LC–MS/MS methods were successfully implemented in pharmacokinetic studies in healthy volunteers.

Conclusions

Simple, sensitive and accurate LC–MS/MS methods for the quantification of enantiomeric and racemic primaquine and carboxyprimaquine in human plasma were validated successfully and implemented in clinical routine drug analysis.

Measurements of 5,6 orthoquinone, surrogate for presumed active primaquine metabolite 5-hydroxyprimaquine, in the urine of Cambodian adults

The active metabolites of primaquine, in particular 5-hydroxyprimaquine, likely responsible for clearance of dormant hypnozoites, are produced through the hepatic CYP450 2D6 (CYP2D6) enzymatic pathway. With the inherent instability of 5-hydroxyprimaquine, a stable surrogate, 5,6 orthoquinone, can now be detected and measured in the urine as part of primaquine pharmacokinetic studies. This study performed CYP450 2D6 genotyping and primaquine pharmacokinetic testing, to include urine 5,6 orthoquinone, in 27 healthy adult Cambodians, as a preliminary step to prepare for future clinical studies assessing primaquine efficacy for Plasmodium vivax infections. The CYP2D6 *10 reduced activity allele was found in 57% of volunteers, and the CYP2D6 genotypes were dominated by *1/*10 (33%) and *10/*10 (30%). Predicted phenotypes were evenly split between Normal Metabolizer (NM) and Intermediate Metabolizer (IM) except one volunteer with a gene duplication and unclear phenotype, classifying as either IM or NM. Median plasma PQ area under the curve (AUC) was lower in the NM group (460 hr*ng/mL) compared to the IM group (561 hr*ng/mL), although not statistically significant. Similar to what has been found in the US study, no 5,6 orthoquinone was detected in the plasma. The urine creatinine-corrected 5,6 orthoquinone AUC in the NM group was almost three times higher than in the IM group, with peak measurements (T max ) at 4 hours. Although there is variation among individuals, future studies examining the relationship between the levels of urine 5,6 orthoquinone and primaquine radical cure efficacy could result in a metabolism biomarker predictive of radical cure.

Quantitative determination of primaquine-5,6-ortho-quinone and carboxyprimaquine-5,6-ortho-quinone in human erythrocytes by UHPLC-MS/MS

The antimalarial drug primaquine (PQ) causes methemoglobinemia and hemolysis in individuals with a genetic deficiency of glucose 6-phosphate dehydrogenase. Reactive oxygen species (ROS) generated by redox cycling of the metabolite primaquine-5,6-orthoquinone (POQ) in erythrocytes has been attributed to be responsible for the toxicity of PQ. Carboxyprimaquine (CPQ), the major human plasma metabolite of PQ, can also form the analogous carboxyprimaquine-5,6-orthoquinone (CPOQ) metabolite, which can also generate ROS in erythrocytes by redox cycling, thus contributing to the hematotoxicity of this drug. In order to study these pathways and characterize such effects in vivo, methods are needed for characterization and quantification of POQ and CPOQ in human erythrocytes. The purpose of this work was to develop a validated method for the quantitative determination of CPOQ and POQ metabolites in human erythrocytes, suitable for clinical studies of PQ metabolism. Several liquid-liquid extraction methods using different organic solvents had been investigated. The solvent mixture of water-methanol-acetonitrile (9:9:5, v/v) was shown to yield the best results for the two analytes. Chromatographic analysis of POQ and CPOQ in human erythrocytes was achieved on a high strength silica (HSS) column and gradient elution (water and acetonitrile, both containing 0.1% formic acid) by ultra-high-performance liquid chromatography coupled with tandem mass spectrometry (UHPLC-MS/MS). Quantitative estimation of POQ and CPOQ was executed by monitoring ion pairs of m/z 260.23 > 175.03 and m/z 275.19 > 175.04, respectively. The method, which was validated for precision, accuracy, selectivity, and linearity, was successfully applied for the quantitative determination of POQ and CPOQ, the key metabolites of PQ in human erythrocytes in PQ clinical study.

Point-of-care tests for malaria: speeding up the diagnostics at the bedside and challenges in malaria cases detection

Malaria remains as one of the major public health problems worldwide. About 228 million cases occurred in 2018 only, with Africa bearing about 93% of the cases. Asymptomatic population carrying the various forms of the parasite Plasmodium in endemic areas plays an important role in the spread of the disease. To tackle this battle, more sensitive and precise detection kits for malaria are crucial to better control the number of new malaria cases. In this review, we not only discuss some of the available approaches to rapidly detect new malaria cases in endemic areas but also shed light on parallel problems that may affect the detection of individuals infected with the parasite, covering kelch 13 mutation, glucose 6-phosphate dehydrogenase deficiency, and hemoglobin disorders. Available approaches for malaria detection covered in this review are focused on point-of-care tests, including portable polymerase chain reaction and aptamers.

Hemolytic Dynamics of Weekly Primaquine Antirelapse Therapy Among Cambodians With Acute Plasmodium vivax Malaria With or Without Glucose-6-Phosphate Dehydrogenase Deficiency

Background

Hemoglobin (Hb) data are limited in Southeast Asian glucose-6-phosphate dehydrogenase (G6PD) deficient (G6PD⁻) patients treated weekly with the World Health Organization–recommended primaquine regimen (ie, 0.75 mg/kg/week for 8 weeks [PQ _0.75]).

Methods

We treated Cambodians who had acute Plasmodium vivax infection with PQ_0.75 and a 3-day course of dihydroartemisinin/piperaquine and determined the Hb level, reticulocyte count, G6PD genotype, and Hb type.

Results

Seventy-five patients (male sex, 63) aged 5–63 years (median, 24 years) were enrolled. Eighteen were G6PD deficient (including 17 with G6PD Viangchan) and 57 were not G6PD deficient; 26 had HbE (of whom 25 were heterozygous), and 6 had α-/β-thalassemia. Mean Hb concentrations at baseline (ie, day 0) were similar between G6PD deficient and G6PD normal patients (12.9 g/dL [range, 9‒16.3 g/dL] and 13.26 g/dL [range, 9.6‒16 g/dL], respectively; P = .46). G6PD deficiency (P = <.001), higher Hb concentration at baseline (P = <.001), higher parasitemia level at baseline (P = .02), and thalassemia (P = .027) influenced the initial decrease in Hb level, calculated as the nadir level minus the baseline level (range, −5.8–0 g/dL; mean, −1.88 g/dL). By day 14, the mean difference from the day 7 level (calculated as the day 14 level minus the day 7 level) was 0.03 g/dL (range, −0.25‒0.32 g/dL). Reticulocyte counts decreased from days 1 to 3, peaking on day 7 (in the G6PD normal group) and day 14 (in the G6PD deficient group); reticulocytemia at baseline (P = .001), G6PD deficiency (P = <.001), and female sex (P = .034) correlated with higher counts. One symptomatic, G6PD-deficient, anemic male patient was transfused on day 4.

Conclusions

The first PQ_0.75 exposure was associated with the greatest decrease in Hb level and 1 blood transfusion, followed by clinically insignificant decreases in Hb levels. PQ_0.75 requires monitoring during the week after treatment. Safer antirelapse regimens are needed in Southeast Asia.

Clinical Trials Registration

ACTRN12613000003774.

Determination of Cytochrome P450 Isoenzyme 2D6 (CYP2D6) Genotypes and Pharmacogenomic Impact on Primaquine Metabolism in an Active-Duty US Military Population

Background

Plasmodium vivax malaria requires a 2-week course of primaquine (PQ) for radical cure. Evidence suggests that the hepatic isoenzyme cytochrome P450 2D6 (CYP2D6) is the key enzyme required to convert PQ into its active metabolite.

Methods

CYP2D6 genotypes and phenotypes of 550 service personnel were determined, and the pharmacokinetics (PK) of a 30-mg oral dose of PQ was measured in 45 volunteers. Blood and urine samples were collected, with PQ and metabolites were measured using ultraperformance liquid chromatography with mass spectrometry.

Results

Seventy-six CYP2D6 genotypes were characterized for 530 service personnel. Of the 515 personnel for whom a single phenotype was predicted, 58% had a normal metabolizer (NM) phenotype, 35% had an intermediate metabolizer (IM) phenotype, 5% had a poor metabolizer (PM) phenotype, and 2% had an ultrametabolizer phenotype. The median PQ area under the concentration time curve from 0 to ∞ was lower for the NM phenotype as compared to the IM or PM phenotypes. The novel 5,6-ortho-quinone was detected in urine but not plasma from all personnel with the NM phenotype.

Conclusion

The plasma PK profile suggests PQ metabolism is decreased in personnel with the IM or PM phenotypes as compared to those with the NM phenotype. The finding of 5,6-ortho-quinone, the stable surrogate for the unstable 5-hydroxyprimaquine metabolite, almost exclusively in personnel with the NM phenotype, compared with sporadic or no production in those with the IM or PM phenotypes, provides further evidence for the role of CYP2D6 in radical cure.

Clinical Trials Registration

NCT02960568.

Evaluating the Role of the Steroid and Xenobiotic Receptor (SXR/PXR) in PCB-153 Metabolism and Protection against Associated Adverse Effects during Perinatal and Chronic Exposure in Mice

BACKGROUND

Polychlorinated biphenyls (PCBs) are environmental toxicants; PCB exposure has been associated with adverse effects on wildlife and humans. However, the mechanisms underlying these adverse effects are not fully understood. The steroid and xenobiotic receptor [SXR; also known as the pregnane X receptor (PXR) and formally known as NR1I2] is a nuclear hormone receptor that regulates inducible metabolism of drugs and xenobiotics and is activated or inhibited by various PCB congeners.

OBJECTIVES

The aim of this study was to investigate the effects of exposure to PCB-153, the most prevalent PCB congener in human tissues, on SXR knockout mice (SXRKO) and to elucidate the role of SXR in PCB-153 metabolism and promotion of its harmful effects.

METHODS

Wild-type (WT) and SXRKO mice were chronically or perinatally exposed to a low dose (54μg/kg/d) of PCB-153. Blood, livers, and spleens were analyzed using transcriptome sequencing (RNA-seq) and molecular techniques to investigate the impacts of exposure on metabolism, oxidative stress, and hematological parameters.

RESULTS

SXRKO mice perinatally exposed to PCB-153 displayed elevated oxidative stress, symptoms of hemolytic anemia, and premature death. Transcriptomal analysis revealed that expression of genes involved in metabolic processes was altered in SXRKO mice. Elevated levels of the PCB-153 metabolite, 3-OH-PCB-153, were found in exposed SXRKO mice compared to exposed WT mice. Blood hemoglobin (HGB) levels were lower throughout the lifespan, and the occurrence of intestinal tumors was larger in SXRKO mice chronically exposed to PCB-153 compared to vehicle and WT controls.

DISCUSSION

Our results suggest that altered metabolism induced by SXR loss of function resulted in the accumulation of hydroxylated metabolites upon exposure to PCB-153, leading to oxidative stress, hemolytic anemia, and tumor development in a mouse model. These results support a major role for SXR/PXR in protection against xenobiotic-induced oxidative stress by maintaining proper metabolism in response to PCB-153 exposure. This role of SXR could be generally applicable to other environmental toxicants as well as pharmaceutical drugs. https://doi.org/10.1289/EHP6262.

Tafenoquine: A Step toward Malaria Elimination

There is a pressing need for compounds with broad-spectrum activity against malaria parasites at various life cycle stages to achieve malaria elimination. However, this goal cannot be accomplished without targeting the tenacious dormant liver-stage hypnozoite that causes multiple relapses after the first episode of illness. In the search for the magic bullet to radically cure Plasmodium vivax malaria, tafenoquine outperformed other candidate drugs and was approved by the U.S. Food and Drug Administration in 2018. Tafenoquine is an 8-aminoquinoline that inhibits multiple life stages of various Plasmodium species. Additionally, its much longer half-life allows for single-dose treatment, which will improve the compliance rate. Despite its approval and the long-time use of other 8-aminoquinolines, the mechanisms behind tafenoquine's activity and adverse effects are still largely unknown. In this Perspective, we discuss the plausible underlying mechanisms of tafenoquine's antiparasitic activity and highlight its role as a cellular stressor. We also discuss potential drug combinations and the development of next-generation 8-aminoquinolines to further improve the therapeutic index of tafenoquine for malaria treatment and prevention.

Enantiospecific pharmacokinetics and drug-drug interactions of primaquine and blood-stage antimalarial drugs

Objectives

Characterization of the pharmacokinetic properties of the enantiomers of primaquine and carboxyprimaquine following administration of racemic primaquine given alone and in combination with commonly used antimalarial drugs.

Methods

Enantiomeric pharmacokinetics were evaluated in 49 healthy adult volunteers enrolled in three randomized cross-over studies in which a single dose of primaquine was given alone and then, after a suitable washout period, in combination with chloroquine, dihydroartemisinin/piperaquine or pyronaridine/artesunate. Non-linear mixed-effects modelling was used to characterize pharmacokinetics and assess the impact of drug-drug interactions.

Results

The volume of distribution of racemic primaquine was decreased by a median (95% CI) of 22.0% (2.24%-39.9%), 24.0% (15.0%-31.5%) and 25.7% (20.3%-31.1%) when co-administered with chloroquine, dihydroartemisinin/piperaquine and pyronaridine/artesunate, respectively. The oral clearance of primaquine was decreased by a median of 19.1% (14.5%-22.8%) when co-administered with pyronaridine/artesunate. These interactions were enantiospecific with a relatively higher effect on (+)-S-primaquine than on (-)-R-primaquine. No drug-drug interaction effects were seen on the pharmacokinetics of either carboxyprimaquine enantiomer.

Conclusions

Population pharmacokinetic models characterizing the enantiospecific properties of primaquine were developed successfully. Exposure to primaquine, particularly to the (+)-S-primaquine but not the carboxy metabolites, increased by up to 30% when co-administered with commonly used antimalarial drugs. A better mechanistic understanding of primaquine metabolism is required for assessment of its efficacy and haematological toxicity in humans.

8-Aminoquinoline Therapy for Latent Malaria

The technical genesis and practice of 8-aminoquinoline therapy of latent malaria offer singular scientific, clinical, and public health insights. The 8-aminoquinolines brought revolutionary scientific discoveries, dogmatic practices, benign neglect, and, finally, enduring promise against endemic malaria. The clinical use of plasmochin-the first rationally synthesized blood schizontocide and the first gametocytocide, tissue schizontocide, and hypnozoitocide of any kind-commenced in 1926. Plasmochin became known to sometimes provoke fatal hemolytic crises. World War II delivered a newer 8-aminoquinoline, primaquine, and the discovery of glucose-6-phosphate dehydrogenase (G6PD) deficiency as the basis of its hemolytic toxicity came in 1956. Primaquine nonetheless became the sole therapeutic option against latent malaria. After 40 years of fitful development, in 2018 the U.S. Food and Drug Administration registered the 8-aminoquinoline called tafenoquine for the prevention of all malarias and the treatment of those that relapse. Tafenoquine also cannot be used in G6PD-unknown or -deficient patients. The hemolytic toxicity of the 8-aminoquinolines impedes their great potential, but this problem has not been a research priority. This review explores the complex technical dimensions of the history of 8-aminoquinolines. The therapeutic principles thus examined may be leveraged in improved practice and in understanding the bright prospect of discovery of newer drugs that cannot harm G6PD-deficient patients.

Formation primaquine-5,6-orthoquinone, the putative active and toxic metabolite of primaquine via direct oxidation in human erythrocytes

BACKGROUND

The activity and haemolytic toxicity associated with primaquine has been linked to its reactive metabolites. The reactive metabolites are thought to be primarily formed through the action of cytochrome P₄₅₀-mediated pathways. Human erythrocytes generally are not considered a significant contributor to drug biotransformation. As erythrocytes are the target of primaquine toxicity, the ability of erythrocytes to mediate the formation of reactive oxidative primaquine metabolites in the absence of hepatic enzymes, was evaluated.

METHODS

Primaquine and its enantiomers were incubated separately with human red blood cells and haemoglobin. Post-incubation analysis was performed with UPLC-MS/MS to identify products of biotransformation.

RESULTS

The major metabolite detected was identified as primaquine-5,6-orthoquinone, reflecting the pathway yielding putative active and haematotoxic metabolites of primaquine, which was formed by oxidative demethylation of 5-hydroxyprimaquine. Incubation of primaquine with haemoglobin in a cell-free system yielded similar results. It appears that the observed biotransformation is due to non-enzymatic processes, perhaps due to reactive oxygen species (ROS) present in erythrocytes or in the haemoglobin incubates.

CONCLUSION

This study presents new evidence that primaquine-5,6-orthoquinone, the metabolite of primaquine reflecting the oxidative biotransformation pathway, is generated in erythrocytes, probably by non-enzymatic means, and may not require transport from the liver or other tissues.

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.

Functional characterization of 50 CYP2D6 allelic variants by assessing primaquine 5-hydroxylation

Cytochrome P450 2D6 (CYP2D6) is responsible for the metabolic activation of primaquine, an antimalarial drug. CYP2D6 is genetically polymorphic, and these polymorphisms are associated with interindividual variations observed in the therapeutic efficacy of primaquine. To further understand this association, we performed in vitro enzymatic analyses of the wild-type CYP2D6.1 and 49 CYP2D6 allelic variants, which were expressed in 293FT cells, using primaquine as a substrate. The concentrations of CYP2D6 variant holoenzymes were measured by using carbon monoxide (CO)-reduced difference spectroscopy, and the wild type and 27 variants showed a peak at 450 nm. The kinetic parameters K_m, V_max, and intrinsic clearance (V_max/K_m) of primaquine 5-hydroxylation were characterized. The kinetic parameters of the wild type and 16 variants were measured, but the values for the remaining 33 variants could not be determined because of low metabolite concentrations. Among the variants, six (i.e., CYP2D6.17, .18, .35, .39, .53, and .70) showed significantly reduced intrinsic clearance compared with that of CYP2D6.1. Three-dimensional structural modeling analysis was performed to elucidate the mechanism of changes in the kinetics of CYP2D6 variants. Our findings provide insights into the allele-specific activity of CYP2D6 for primaquine, which could be clinically useful for malaria treatment and eradication efforts.

Metabolism of primaquine in normal human volunteers: investigation of phase I and phase II metabolites from plasma and urine using ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry

BACKGROUND

Primaquine (PQ), an 8-aminoquinoline, is the only drug approved by the United States Food and Drug Administration for radical cure and prevention of relapse in Plasmodium vivax infections. Knowledge of the metabolism of PQ is critical for understanding the therapeutic efficacy and hemolytic toxicity of this drug. Recent in vitro studies with primary human hepatocytes have been useful for developing the ultra high-performance liquid chromatography coupled with high-resolution mass spectrometric (UHPLC-QToF-MS) methods for simultaneous determination of PQ and its metabolites generated through phase I and phase II pathways for drug metabolism.

METHODS

These methods were further optimized and applied for phenotyping PQ metabolites from plasma and urine from healthy human volunteers treated with single 45 mg dose of PQ. Identity of the metabolites was predicted by MetaboLynx using LC-MS/MS fragmentation patterns. Selected metabolites were confirmed with appropriate standards.

RESULTS

Besides PQ and carboxy PQ (cPQ), the major plasma metabolite, thirty-four additional metabolites were identified in human plasma and urine. Based on these metabolites, PQ is viewed as metabolized in humans via three pathways. Pathway 1 involves direct glucuronide/glucose/carbamate/acetate conjugation of PQ. Pathway 2 involves hydroxylation (likely cytochrome P450-mediated) at different positions on the quinoline ring, with mono-, di-, or even tri-hydroxylations possible, and subsequent glucuronide conjugation of the hydroxylated metabolites. Pathway 3 involves the monoamine oxidase catalyzed oxidative deamination of PQ resulting in formation of PQ-aldehyde, PQ alcohol and cPQ, which are further metabolized through additional phase I hydroxylations and/or phase II glucuronide conjugations.

CONCLUSION

This approach and these findings augment our understanding and provide comprehensive view of pathways for PQ metabolism in humans. These will advance the clinical studies of PQ metabolism in different populations for different therapeutic regimens and an understanding of the role these play in PQ efficacy and safety outcomes, and their possible relation to metabolizing enzyme polymorphisms.

Primaquine pharmacology in the context of CYP 2D6 pharmacogenomics: Current state of the art

Primaquine is the only antimalarial drug available to clinicians for the treatment of relapsing forms of malaria. Primaquine development and usage dates back to the 1940s and has been administered to millions of individuals to treat and eliminate malaria infections. Primaquine therapy is not without disadvantages, however, as it can cause life threatening hemolysis in humans with glucose-6-phosphate dehydrogenase (G6PD) deficiency. In addition, the efficacy of primaquine against relapsing malaria was recently linked to CYP 2D6 mediated activation to an active metabolite, the structure of which has escaped definitive identification for over 75years. CYP 2D6 is highly polymorphic among various human populations adding further complexity to a comprehensive understanding of primaquine pharmacology. This review aims to discuss primaquine pharmacology in the context of state of the art understanding of CYP 2D6 mediated 8-aminoquinoline metabolic activation, and shed light on the current knowledge gaps of 8-aminoquinoline mechanistic understanding against relapsing malaria.

Structure-based de novo design, molecular docking and molecular dynamics of primaquine analogues acting as quinone reductase II inhibitors

Primaquine is a traditional antimalarial drug with low parasitic resistance and generally good acceptance at higher doses, which has been used for over 60 years in malaria treatment. However, several limitations related to its hematotoxicity have been reported. It is believed that this toxicity comes from the hydroxylation of the C-5 and C-6 positions of its 8-aminoquinoline ring before binding to the molecular target: the quinone reductase II (NQO2) human protein. In this study we propose primaquine derivatives, with substitution at position C-6 of the 8-aminoquinoline ring, planned to have better binding to NQO2, compared to primaquine, but with a reduced toxicity related to the C-5 position being possible to be oxidized. On this sense the proposed analogues were suggested in order to reduce or inhibit hydroxylation and further oxidation to hemotoxic metabolites. Five C-6 substituted primaquine analogues were selected by de novo design and further submitted to docking and molecular dynamics simulations. Our results suggest that all analogues bind better to NQO2 than primaquine and may become better antimalarials. However, the analogues 3 and 4 are predicted to have a better activity/toxicity balance.

Development of HepG2-derived cells expressing cytochrome P450s for assessing metabolism-associated drug-induced liver toxicity

The generation of reactive metabolites from therapeutic agents is one of the major mechanisms of drug-induced liver injury (DILI). In order to evaluate metabolism-related toxicity and improve drug efficacy and safety, we generated a battery of HepG2-derived cell lines that express 14 cytochrome P450s (CYPs) (1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9, 2C18, 2C19, 2D6, 2E1, 3A4, 3A5 and 3A7) individually using a lentiviral expression system. The expression/production of a specific CYP in each cell line was confirmed by an increased abundance of the CYP at both mRNA and protein levels. Moreover, the enzymatic activities of representative CYPs in the corresponding cell lines were also measured. Using our CYP-expressed HepG2 cells, the toxicity of three drugs that could induce DILI (amiodarone, chlorpromazine and primaquine) was assessed, and all of them showed altered (increased or decreased) toxicity compared to the toxicity in drug-treated wild-type HepG2 cells. CYP-mediated drug toxicity examined in our cell system is consistent with previous reports, demonstrating the potential of these cells for assessing metabolism-related drug toxicity. This cell system provides a practical in vitro approach for drug metabolism screening and for early detection of drug toxicity. It is also a surrogate enzyme source for the enzymatic characterization of a particular CYP that contributes to drug-induced liver toxicity.

Single-cell evaluation of red blood cell bio-mechanical and nano-structural alterations upon chemically induced oxidative stress

Erythroid cells, specifically red blood cells (RBCs), are constantly exposed to highly reactive radicals during cellular gaseous exchange. Such exposure often exceeds the cells' innate anti-oxidant defense systems, leading to progressive damage and eventual senescence. One of the contributing factors to this process are alterations to hemoglobin conformation and globin binding to red cell cytoskeleton. However, in addition to the aforementioned changes, it is possible that oxidative damage induces critical changes to the erythrocyte cytoskeleton and corresponding bio-mechanical and nano-structural properties of the red cell membrane. To quantitatively characterize how oxidative damage accounts for such changes, we employed single-cell manipulation techniques such as micropipette aspiration and atomic force microscopy (AFM) on RBCs. These investigations demonstrated visible morphological changes upon chemically induced oxidative damage (using hydrogen peroxide, diamide, primaquine bisphosphate and cumene hydroperoxide). Our results provide previously unavailable observations on remarkable changes in red cell cytoskeletal architecture and membrane stiffness due to oxidative damage. Furthermore, we also demonstrate that a pathogen that infects human blood cells, Plasmodium falciparum was unable to penetrate through the oxidant-exposed RBCs that have damaged cytoskeleton and stiffer membranes. This indicates the importance of bio-physical factors pertinent to aged RBCs and it's relevance to malaria infectivity.

Differential cytochrome P450 2D metabolism alters tafenoquine pharmacokinetics

Cytochrome P450 (CYP) 2D metabolism is required for the liver-stage antimalarial efficacy of the 8-aminoquinoline molecule tafenoquine in mice. This could be problematic for Plasmodium vivax radical cure, as the human CYP 2D ortholog (2D6) is highly polymorphic. Diminished CYP 2D6 enzyme activity, as in the poor-metabolizer phenotype, could compromise radical curative efficacy in humans. Despite the importance of CYP 2D metabolism for tafenoquine liver-stage efficacy, the exact role that CYP 2D metabolism plays in the metabolism and pharmacokinetics of tafenoquine and other 8-aminoquinoline molecules has not been extensively studied. In this study, a series of tafenoquine pharmacokinetic experiments were conducted in mice with different CYP 2D metabolism statuses, including wild-type (WT) (reflecting extensive metabolizers for CYP 2D6 substrates) and CYPmouse 2D knockout (KO) (reflecting poor metabolizers for CYP 2D6 substrates) mice. Plasma and liver pharmacokinetic profiles from a single 20-mg/kg of body weight dose of tafenoquine differed between the strains; however, the differences were less striking than previous results obtained for primaquine in the same model. Additionally, the presence of a 5,6-ortho-quinone tafenoquine metabolite was examined in both mouse strains. The 5,6-ortho-quinone species of tafenoquine was observed, and concentrations of the metabolite were highest in the WT extensive-metabolizer phenotype. Altogether, this study indicates that CYP 2D metabolism in mice affects tafenoquine pharmacokinetics and could have implications for human tafenoquine pharmacokinetics in polymorphic CYP 2D6 human populations.

Inability to maintain GSH pool in G6PD-deficient red cells causes futile AMPK activation and irreversible metabolic disturbance

AIMS

Glucose 6-phosphate dehydrogenase (G6PD) is essential for maintenance of nicotinamide dinucleotide hydrogen phosphate (NADPH) levels and redox homeostasis. A number of drugs, such as antimalarial drugs, act to induce reactive oxygen species and hemolytic crisis in G6PD-deficient patients. We used diamide (DIA) to mimic drug-induced oxidative stress and studied how these drugs affect cellular metabolism using a metabolomic approach.

RESULTS

There are a few differences in metabolome between red blood cells (RBCs) from normal and G6PD-deficient individuals. DIA causes modest changes in normal RBC metabolism. In contrast, there are significant changes in various biochemical pathways, namely glutathione (GSH) metabolism, purine metabolism, and glycolysis, in G6PD-deficient cells. GSH depletion is concomitant with a shift in energy metabolism. Adenosine monophosphate (AMP) and adenosine diphosphate (ADP) accumulation activates AMP protein kinase (AMPK) and increases entry of glucose into glycolysis. However, inhibition of pyruvate kinase (PK) reduces the efficacy of energy production. Metabolic changes and protein oxidation occurs to a greater extent in G6PD-deficient RBCs than in normal cells, leading to severe irreversible loss of deformability of the former.

INNOVATION AND CONCLUSION

Normal and G6PD-deficient RBCs differ in their responses to oxidants. Normal cells have adequate NADPH regeneration for maintenance of GSH pool. In contrast, G6PD-deficient cells are unable to regenerate enough NADPH under a stressful situation, and switch to biosynthetic pathway for GSH supply. Rapid GSH exhaustion causes energy crisis and futile AMPK activation. Our findings suggest that drug-induced oxidative stress differentially affects metabolism and metabolite signaling in normal and G6PD-deficient cells. It also provides an insight into the pathophysiology of acute hemolytic anemia in G6PD-deficient patients.

Computational Study on the Effect of Exocyclic Substituents on the Ionization Potential of Primaquine: Insights into the Design of Primaquine-Based Antimalarial Drugs with Less Methemoglobin Generation

The effect of an exocyclic substituent on the ionization potential of primaquine, an important antimalarial drug, was investigated using density functional theory methods. It was found that an electron-donating group (EDG) makes the ionization potential decrease. In contrast, an electron-withdrawing group (EWG) makes the ionization potential increase. Among all the exocyclic positions, a substituent at the 5- or 7-position has the largest effect. This can be explained by the contribution of the atomic orbitals at those positions to the highest occupied molecular orbital (HOMO). In addition, a substituent at the N8-position has a considerably large effect on the ionization potential because this atom makes the second largest contribution to the HOMO. These findings have potential implications for the design of less hemotoxic antimalarial drugs. We suggest that it is worth considering placement of an EWG at the 5-, 7-, or N8-positions of primaquine in future drug discovery attempts.

Differential CYP 2D6 metabolism alters primaquine pharmacokinetics

Primaquine (PQ) metabolism by the cytochrome P450 (CYP) 2D family of enzymes is required for antimalarial activity in both humans (2D6) and mice (2D). Human CYP 2D6 is highly polymorphic, and decreased CYP 2D6 enzyme activity has been linked to decreased PQ antimalarial activity. Despite the importance of CYP 2D metabolism in PQ efficacy, the exact role that these enzymes play in PQ metabolism and pharmacokinetics has not been extensively studied in vivo. In this study, a series of PQ pharmacokinetic experiments were conducted in mice with differential CYP 2D metabolism characteristics, including wild-type (WT), CYP 2D knockout (KO), and humanized CYP 2D6 (KO/knock-in [KO/KI]) mice. Plasma and liver pharmacokinetic profiles from a single PQ dose (20 mg/kg of body weight) differed significantly among the strains for PQ and carboxy-PQ. Additionally, due to the suspected role of phenolic metabolites in PQ efficacy, these were probed using reference standards. Levels of phenolic metabolites were highest in mice capable of metabolizing CYP 2D6 substrates (WT and KO/KI 2D6 mice). PQ phenolic metabolites were present in different quantities in the two strains, illustrating species-specific differences in PQ metabolism between the human and mouse enzymes. Taking the data together, this report furthers understanding of PQ pharmacokinetics in the context of differential CYP 2D metabolism and has important implications for PQ administration in humans with different levels of CYP 2D6 enzyme activity.

Enantioselective metabolism of primaquine by human CYP2D6

Background

Primaquine, currently the only approved drug for the treatment and radical cure of Plasmodium vivax malaria, is still used as a racemic mixture. Clinical use of primaquine has been limited due to haemolytic toxicity in individuals with genetic deficiency in glucose-6-phosphate dehydrogenase. Earlier studies have linked its therapeutic effects to CYP2D6-generated metabolites. The aim of the current study was to investigate the differential generation of the CYP2D6 metabolites by racemic primaquine and its individual enantiomers.

Methods

Stable isotope ¹³C-labelled primaquine and its two enantiomers were incubated with recombinant cytochrome-P450 supersomes containing CYP2D6 under optimized conditions. Metabolite identification and time-point quantitative analysis were performed using LC-MS/MS. UHPLC retention time, twin peaks with a mass difference of 6, MS-MS fragmentation pattern, and relative peak area with respect to parent compound were used for phenotyping and quantitative analysis of metabolites.

Results

The rate of metabolism of (+)-(S)-primaquine was significantly higher (50% depletion of 20 μM in 120 min) compared to (−)-(R)-primaquine (30% depletion) when incubated with CYP2D6. The estimated V_max (μmol/min/mg) were 0.75, 0.98 and 0.42, with K_m (μM) of 24.2, 33.1 and 21.6 for (±)-primaquine, (+)-primaquine and (−)-primaquine, respectively. Three stable mono-hydroxylated metabolites, namely, 2-, 3- and 4-hydroxyprimaquine (2-OH-PQ, 3-OH-PQ, and 4-OH-PQ), were identified and quantified. 2-OH-PQ was preferentially formed from (+)-primaquine in a ratio of 4:1 compared to (−)-primaquine. The racemic (±)-primaquine showed a pattern similar to the (−)-primaquine; 2-OH-PQ accounted for about 15–17% of total CYP2D6-mediated conversion of (+)-primaquine. In contrast, 4-OH-PQ was preferentially formed with (−)-primaquine (5:1), accounting for 22% of the total (−)-primaquine conversion. 3-OH-PQ was generated from both enantiomers and racemate. 5-hydroxyprimaquine was unstable. Its orthoquinone degradation product (twice as abundant in (+)-primaquine compared to (−)-primaquine) was identified and accounted for 18–20% of the CYP2D6-mediated conversion of (+)-primaquine. Other minor metabolites included dihydroxyprimaquine species, two quinone-imine products of dihydroxylated primaquine, and a primaquine terminal alcohol with variable generation from the individual enantiomers.

Conclusion

The metabolism of primaquine by human CYP2D6 and the generation of its metabolites display enantio-selectivity regarding formation of hydroxylated product profiles. This may partly explain differential pharmacologic and toxicologic properties of primaquine enantiomers.

Genome-wide patterns of genetic polymorphism and signatures of selection in Plasmodium vivax

Plasmodium vivax is the most prevalent human malaria parasite outside of Africa. Yet, studies aimed to identify genes with signatures consistent with natural selection are rare. Here, we present a comparative analysis of the pattern of genetic variation of five sequenced isolates of P. vivax and its divergence with two closely related species, Plasmodium cynomolgi and Plasmodium knowlesi, using a set of orthologous genes. In contrast to Plasmodium falciparum, the parasite that causes the most lethal form of human malaria, we did not find significant constraints on the evolution of synonymous sites genome wide in P. vivax. The comparative analysis of polymorphism and divergence across loci allowed us to identify 87 genes with patterns consistent with positive selection, including genes involved in the “exportome” of P. vivax, which are potentially involved in evasion of the host immune system. Nevertheless, we have found a pattern of polymorphism genome wide that is consistent with a significant amount of constraint on the replacement changes and prevalent negative selection. Our analyses also show that silent polymorphism tends to be larger toward the ends of the chromosomes, where many genes involved in antigenicity are located, suggesting that natural selection acts not only by shaping the patterns of variation within the genes but it also affects genome organization.

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.

Hydroxylated derivatives of NPC1161: theoretical insights into their potential toxicity and the feasibility and regioselectivity of their formation

For antimalarial 8-aminoquinoline (8-AQ) drugs, the ionization potential (energy required to remove an electron) of their putative metabolites has been proposed to be correlated in part to their hemotoxicity potential. NPC1161 is a developmental candidate as an 8-AQ antimalarial drug. In this work, the ionization potentials (IPs) of the S-NPC1161 (NPC1161a) hydroxylated derivatives, which are possible metabolites derived from action of endogenous cytochrome P450 (CYP450) enzymes, were calculated at the B3LYP-SCRF(PCM)/6-311++G**//B3LYP/6-31G** level in water. The derivative hydroxylated at N1′ (8-amino) was found to have the smallest IP of ∼430 kJ/mol, predicting that it would be the most hemotoxic. The calculated IPs of the derivatives hydroxylated at the C2 and C7 positions were ∼475 and ∼478 kJ/mol, respectively, whereas the calculated IPs of those hydroxylated at all other possible positions were between 480 and 490 kJ/mol. The homolytic bond dissociation energies (HBDEs) of all C–H/N–H bonds in NPC1161a were also calculated. The smaller HBDEs of the C–H/N–H bonds on the 8-amino side chain suggest that these positions are more easily hydroxylated compared to other sites. Molecular orbital analysis implies that the N1′ position should be the most reactive center when NPC1161 approaches the heme in CYP450.

Evaluation of Antitrypanosomal Dihydroquinolines for Hepatotoxicity, Mutagenicity, and Methemoglobin Formation In Vitro

N1-Benzylated dihydroquinolin-6-ols and their corresponding esters display exceptional activity against African trypanosomes in vitro, and administration of members of this class of compounds to trypanosome-infected mice results in cures in a first-stage African trypanosomiasis model. Since a quinone imine intermediate has been implicated in the antiparasitic mechanism of action of these compounds, evaluation of the hepatotoxic, mutagenic, and methemoglobin-promoting effects of these agents was performed. 1-Benzyl-1,2-dihydro-2,2,4-trimethylquinolin-6-ol hydrochloride and 1-benzyl-1,2-dihydro-2,2,4-trimethylquinolin-6-yl acetate showed outstanding in vitro selectivity for Trypanosoma brucei compared to the HepG2, Hep3B, Huh7, and PLC5 hepatocyte cell lines. 1-Benzyl-1,2-dihydro-2,2,4-trimethylquinolin-6-ol hydrochloride and 1-(2-methoxybenzyl)-1,2-dihydro-2,2,4-trimethylquinolin-6-yl acetate were not mutagenic when screened in the Ames assay, with or without metabolic activation. The latter 2 compounds promoted time- and dose-dependent formation of methemoglobin when incubated in whole human blood, but such levels were below those typically required to produce symptoms of methemoglobinemia in humans. Although compounds capable of quinone imine formation require careful evaluation, these in vitro studies indicate that antitrypanosomal dihydroquinolines merit further study as drug candidates against the neglected tropical disease human African trypanosomiasis.

Methemoglobin generation by 8-aminoquinolines: effect of substitution at 5-position of primaquine

Currently, the only clinically approved antimalarial drug to treat relapsing malaria is primaquine (PQ), yet PQ administration can cause life-threatening hemolytic anemia in some patients. In our efforts to understand the connection between PQ and methemoglobin formation, the effect of 5-substituted primaquine derivatives on the basicity of hemoglobin-bound O2 was investigated using various computational methods, including quantum mechanics/molecular mechanics (QM/MM) calculations, molecular dynamics simulations and density functional theory calculations, to determine the geometries, relative energies, spin densities, proton affinities and ionization potentials of various PQ derivatives and PQ···hemoglobin complexes. We found that the protein environment and solvent do not change our previously proposed methemoglobin formation mechanism that 5-hydroxyprimaquine donates an electron to O2, facilitating its conversion to H2O2 and generating methemoglobin. Because of 5-hydroxyprimaquine's ability to lose an electron by this mechanism, we then used different substituents at primaquine's 5-position and found that an electron-withdrawing group (EWG) increases the ionization potential of the corresponding derivative. As a result, the EWG-substituted derivatives make the hemoglobin-bound O2 less basic, because of their weaker electron-donating ability. These derivatives hence are predicted to have a lower propensity to generate methemoglobin, which can inform future design of less hemotoxic antimalarial drugs. We also carried out experimental measurement of methemoglobin formation for some of the 5-substituted derivatives.

1,4-naphthoquinones and other NADPH-dependent glutathione reductase-catalyzed redox cyclers as antimalarial agents

The homodimeric flavoenzyme glutathione reductase catalyzes NADPH-dependent glutathione disulfide reduction. This reaction is important for keeping the redox homeostasis in human cells and in the human pathogen Plasmodium falciparum. Different types of NADPH-dependent disulfide reductase inhibitors were designed in various chemical series to evaluate the impact of each inhibition mode on the propagation of the parasites. Against malaria parasites in cultures the most potent and specific effects were observed for redox-active agents acting as subversive substrates for both glutathione reductases of the Plasmodium-infected red blood cells. In their oxidized form, these redox-active compounds are reduced by NADPH-dependent flavoenzyme-catalyzed reactions in the cytosol of infected erythrocytes. In their reduced forms, these compounds can reduce molecular oxygen to reactive oxygen species, or reduce oxidants like methemoglobin, the major nutrient of the parasite, to indigestible hemoglobin. Furthermore, studies on a fluorinated suicide-substrate of the human glutathione reductase indicate that the glutathione reductase-catalyzed bioactivation of 3-benzylnaphthoquinones to the corresponding reduced 3-benzoyl metabolites is essential for the observed antimalarial activity. In conclusion, the antimalarial lead naphthoquinones are suggested to perturb the major redox equilibria of the targeted cells. These effects result in developmental arrest of the parasite and contribute to the removal of the parasitized erythrocytes by macrophages.

Antiparasitic chemotherapy: from genomes to mechanisms

Owing to the absence of antiparasitic vaccines and the constant threat of drug resistance, the development of novel antiparasitic chemotherapies remains of major importance for disease control. A better understanding of drug transport (uptake and efflux), drug metabolism and the identification of drug targets, and mechanisms of drug resistance would facilitate the development of more effective therapies. Here, we focus on malaria and African trypanosomiasis. We review existing drugs and drug development, emphasizing high-throughput genomic and genetic approaches, which hold great promise for elucidating antiparasitic mechanisms. We describe the approaches and technologies that have been influential for each parasite and develop new ideas for future research directions, including mode-of-action studies for drug target deconvolution.

A primaquine-chloroquine hybrid with dual activity against Plasmodium liver and blood stages

We present a new class of hybrid molecules consisting of the established antiplasmodial drugs primaquine and chloroquine. No drug is known to date that acts comparably against all stages of Plasmodium in its life cycle. Starting from available precursors, we designed and synthesized a new-generation compound consisting of both primaquine and chloroquine components, with the intent to produce agents that exhibit bioactivity against different stages of the parasite's life cycle. In vitro, the hybrid molecule 3 displays activity against both asexual and sexual P. falciparum blood stages as well as P. berghei sporozoites and liver stages. In vivo, the hybrid elicits activity against P. berghei liver and blood stages. Our results successfully validate the concept of utilizing one compound to combine different modes of action that attack different Plasmodium stages in the mammalian host. It is our hope that the novel design of such compounds will outwit the pathogen in the spread of drug resistance. Based on the optimized synthetic pathway, the compound is accessible in a smooth and versatile way and open for potential further molecular modification.

The metabolism of primaquine to its active metabolite is dependent on CYP 2D6

BACKGROUND

The efficacy of the 8-aminoquinoline (8AQ) drug primaquine (PQ) has been historically linked to CYP-mediated metabolism. Although to date no clear evidence exists in the literature that unambiguously assigns the metabolic pathway or specific metabolites necessary for activity, recent literature suggests a role for CYP 2D6 in the generation of redox active metabolites.

METHODS

In the present study, the specific CYP 2D6 inhibitor paroxetine was used to assess its effects on the production of specific phenolic metabolites thought to be involved in PQ efficacy. Further, PQ causal prophylactic (developing liver stage) efficacy against Plasmodium berghei in CYP 2D knockout mice was assessed in comparison with a normal C57 background and with humanized CYP 2D6 mice to determine the direct effects of CYP 2D6 metabolism on PQ activity.

RESULTS

PQ exhibited no activity at 20 or 40 mg/kg in CYP 2D knockout mice, compared to 5/5 cures in normal mice at 20 mg/kg. The activity against developing liver stages was partially restored in humanized CYP 2D6 mice.

CONCLUSIONS

These results unambiguously demonstrate that metabolism of PQ by CYP 2D6 is essential for anti-malarial causal prophylaxis efficacy.

Profiling primaquine metabolites in primary human hepatocytes using UHPLC-QTOF-MS with 13C stable isotope labeling

Therapeutic efficiency and hemolytic toxicity of primaquine (PQ), the only drug available for radical cure of relapsing vivax malaria are believed to be mediated by its metabolites. However, identification of these metabolites has remained a major challenge apparently due to low quantities and their reactive nature. Drug candidates labeled with stable isotopes afford convenient tools for tracking drug-derived metabolites in complex matrices by liquid chromatography-tandem mass spectrometry (LC-MS-MS) and filtering for masses with twin peaks attributable to the label. This study was undertaken to identify metabolites of PQ from an in vitro incubation of a 1:1 w/w mixture of (13)C(6)-PQ/PQ with primary human hepatocytes. Acquity ultra-performance LC (UHPLC) was integrated with QTOF-MS to combine the efficiency of separation with high sensitivity, selectivity of detection and accurate mass determination. UHPLC retention time, twin mass peaks with difference of 6 (originating from (13)C(6)-PQ/PQ), and MS-MS fragmentation pattern were used for phenotyping. Besides carboxy-PQ (cPQ), formed by oxidative deamination of PQ to an aldehyde and subsequent oxidation, several other metabolites were identified: including PQ alcohol, predictably generated by oxidative deamination of PQ to an aldehyde and subsequent reduction, its acetate and the alcohol's glucuronide conjugate. Trace amounts of quinone-imine metabolites of PQ and cPQ were also detected which may be generated by hydroxylation of the PQ/cPQ quinoline ring at the 5-position and subsequent oxidation. These findings shed additional light on the human hepatic metabolism of PQ, and the method can be applied for identification of reactive PQ metabolites generated in vivo in preclinical and clinical studies.

CYP450 phenotyping and accurate mass identification of metabolites of the 8-aminoquinoline, anti-malarial drug primaquine

Background

The 8-aminoquinoline (8AQ) drug primaquine (PQ) is currently the only approved drug effective against the persistent liver stage of the hypnozoite forming strains Plasmodium vivax and Plasmodium ovale as well as Stage V gametocytes of Plasmodium falciparum. To date, several groups have investigated the toxicity observed in the 8AQ class, however, exact mechanisms and/or metabolic species responsible for PQ’s haemotoxic and anti-malarial properties are not fully understood.

Methods

In the present study, the metabolism of PQ was evaluated using in vitro recombinant metabolic enzymes from the cytochrome P450 (CYP) and mono-amine oxidase (MAO) families. Based on this information, metabolite identification experiments were performed using nominal and accurate mass measurements.

Results

Relative activity factor (RAF)-weighted intrinsic clearance values show the relative role of each enzyme to be MAO-A, 2C19, 3A4, and 2D6, with 76.1, 17.0, 5.2, and 1.7% contributions to PQ metabolism, respectively. CYP 2D6 was shown to produce at least six different oxidative metabolites along with demethylations, while MAO-A products derived from the PQ aldehyde, a pre-cursor to carboxy PQ. CYPs 2C19 and 3A4 produced only trace levels of hydroxylated species.

Conclusions

As a result of this work, CYP 2D6 and MAO-A have been implicated as the key enzymes associated with PQ metabolism, and metabolites previously identified as potentially playing a role in efficacy and haemolytic toxicity have been attributed to production via CYP 2D6 mediated pathways.

Understanding the mechanisms for metabolism-linked hemolytic toxicity of primaquine against glucose 6-phosphate dehydrogenase deficient human erythrocytes: evaluation of eryptotic pathway

Therapeutic utility of primaquine, an 8-aminoquinoline antimalarial drug, has been limited due to its hemolytic toxicity in population with glucose 6-phosphate dehydrogenase deficiency. Recent investigations at our lab have shown that the metabolites generated through cytochrome P(450)-dependent metabolic reactions are responsible for hemotoxic effects of primaquine, which could be monitored with accumulation of methemoglobin and increased oxidative stress. The molecular markers for succeeding cascade of events associated with early clearance of the erythrocytes from the circulation were evaluated for understanding the mechanism for hemolytic toxicity of primaquine. Primaquine alone though did not induce noticeable methemoglobin accumulation, but produced significant oxidative stress, which was higher in G6PD-deficient than in normal erythrocytes. Primaquine, presumably through redox active hemotoxic metabolites generated in situ in human liver microsomal metabolism-linked assay, induced a dose-dependent methemoglobin accumulation and oxidative stress, which were almost similar in normal and G6PD-deficient erythrocytes. Primaquine alone or in presence of pooled human liver microsomes neither produced significant effect on intraerythrocytic calcium levels nor affected the phosphatidyl serine asymmetry of the normal and G6PD-deficient human erythrocytes as monitored flowcytometrically with Annexin V binding assay. The studies suggest that eryptosis mechanisms are not involved in accelerated removal of erythrocytes due to hemolytic toxicity of primaquine.

Intoxicação experimental por Indigofera suffruticosa em caprinos e ovinos

Indigofera suffruticosa é uma planta invasora, ção. Em um caprino e um ovino foram realizados os testes que causa anemia hemolítica com hemoglobinúria em bo-de fragilidade osmótica, determinação de hemoglobina e vinos e, experimentalmente, anemia sem hemoglobinúria metemoglobina e pesquisa de corpúsculos de Heinz. Foi em cobaios. O objetivo deste trabalho foi determinar a comprovado que em caprinos e ovinos, I. suffruticosa cautoxicidade de I. suffruticosa para caprinos e ovinos. Par-sa anemia hemolítica sem hemoglobinúria com formação tes aéreas da planta foram administradas a seis caprinos de corpúsculos de Heinz. Os animais recuperaram-se da e quatro ovinos em doses diárias de 10, 20 e 40g por kg anemia, total ou parcialmente, mesmo com a continuidade de peso vivo, durante períodos de 2 a 24 dias. Foram ava-da administração da planta. Oito a 12 horas após a coleliados os parâmetros hematológicos (hematócrito, níveis ta observa-se pigmento azulado na urina. Sugere-se que o de hemoglobina e contagem de hemácias) e foi coletada pigmento seja anilina ou algum metabolito dessa substânurina para urinálise e observação de variações na coloracia e que a anilina seja o princípio ativo responsável pela hemólise causada por I. suffruticosa.

DFT study on the radical anions formed by primaquine and its derivatives

The electron affinities (EA) of the 8-aminoquinoline antimalarial drug primaquine and several of its metabolites were studied using the density functional theory method. We first considered six substituents at the 5-position, -CH(3), -OH, -OCH(3), -Ph, -OPh, and -CHO. We found that in the gas phase the adiabatic EAs are similar to that of the parent primaquine for the -CH(3), -OH, and -OCH(3) substituents. In contrast, the -Ph, -OPh, and -CHO substituents all markedly increase the adiabatic EA. However, only the -CHO substituted compound is predicted to form a stable covalently bound radical anion in the gas phase due to its significant positive vertical EA relative to that of the parent primaquine. In addition, when the 8-position is substituted by the N-hydroxyl group or a quinone-imine structure is formed, the electron capture ability is significantly increased. In aqueous solution, all these molecules have significantly larger adiabatic EAs than in the gas phase. In addition, all of the vertical EAs are positive in aqueous solution. The implications of these findings for contributing to our mechanistic understanding of the red cell toxicity of 8-aminoquinoline compounds are further discussed.

Methemoglobinemia caused by 8-aminoquinoline drugs: DFT calculations suggest an analogy to H4B's role in nitric oxide synthase

We suggest a possible mechanism of how 8-aminoquinolines (8-AQ's) cause hemotoxicity by oxidizing hemoglobin to methemoglobin. In our DFT calculations, we found that 5-hydroxyprimaquine is able to donate an electron to O(2) to facilitate its conversion to H(2)O(2). Meanwhile, Fe(II) is oxidized to Fe(III) and methemoglobin is formed. In this mechanism, the 8-AQ drug plays a similar role as that of H(4)B in nitric oxide synthase. Furthermore, our study offers an approach to inform the design of less toxic antimalarial drugs.

Antimalarial activity of novel 5-aryl-8-aminoquinoline derivatives

In an attempt to separate the antimalarial activity of tafenoquine (3) from its hemolytic side effects in glucose-6-phosphate dehydrogenase (G6PD) deficiency patients, a series of 5-aryl-8-aminoquinoline derivatives was prepared and assessed for antimalarial activities. The new compounds were found metabolically stable in human and mouse microsomal preparations, with t(1/2) > 60 min, and were equal to or more potent than primaquine (2) and 3 against Plasmodium falciparum cell growth. The new agents were more active against the chloroquine (CQ) resistant clone than to the CQ-sensitive clone. Analogues with electron donating groups showed better activity than those with electron withdrawing substituents. Compounds 4bc, 4bd, and 4be showed comparable therapeutic index (TI) to that of 2 and 3, with TI ranging from 5 to 8 based on IC(50) data. The new compounds showed no significant causal prophylactic activity in mice infected with Plasmodium berghei sporozoites, but are substantially less toxic than 2 and 3 in mouse tests.

Synthesis, antiprotozoal, antimicrobial, β-hematin inhibition, cytotoxicity and methemoglobin (MetHb) formation activities of bis(8-aminoquinolines)

In continuing our search of potent antimalarials based on 8-aminoquinoline structural framework, three series of novel bis(8-aminoquinolines) using convenient one to four steps synthetic procedures were synthesized. The bisquinolines were evaluated for in vitro antimalarial (Plasmodiumfalciparum), antileishmanial (Leishmaniadonovani), antimicrobial (a panel of pathogenic bacteria and fungi), cytotoxicity, β-hematin inhibitory and methemoglobin (MetHb) formation activities. Several compounds exhibited superior antimalarial activities compared to parent drug primaquine. Selected compounds (44, 61 and 79) when tested for in vivo blood-schizontocidal antimalarial activity (Plasmodiumberghei) displayed potent blood-schizontocial activities. The bisquinolines showed negligible MetHb formation (0.2-1.2%) underlining their potential in the treatment of glucose-6-phosphate dehydrogenase deficient patients. The bisquinoline analogues (36, 73 and 79) also exhibited promising in vitro antileishmanial activity, and antimicrobial activities (43, 44 and 76) against a panel of pathogenic bacteria and fungi. The results of this study provide evidence that bis(8-aminoquinolines), like their bis(4-aminoquinolines) and artemisinin dimers counterparts, are a promising class of antimalarial agents.

Targeting the hypnozoite reservoir of Plasmodium vivax: the hidden obstacle to malaria elimination

Plasmodium vivax is the major species of malaria parasite outside Africa. It is especially problematic in that the infection can relapse in the absence of mosquitoes by activation of dormant hypnozoites in the liver. Medicines that target the erythrocytic stages of Plasmodium falciparum are also active against P. vivax, except where these have been compromised by resistance. However, the only clinical therapy against relapse of vivax malaria is the 8-aminoquinoline, primaquine. This molecule has the drawback of causing haemolysis in genetically sensitive patients and requires 14 days of treatment. New, safer and more-easily administered drugs are urgently needed, and this is a crucial gap in the broader malaria-elimination agenda. New developments in cell biology are starting to open ways to the next generation of drugs against hypnozoites. This search is urgent, given the time needed to develop a new medication.

Cytochrome P(450)-dependent toxic effects of primaquine on human erythrocytes

Primaquine, an 8-aminoquinoline, is the drug of choice for radical cure of relapsing malaria. Use of primaquine is limited due to its hemotoxicity, particularly in populations with glucose-6-phosphate dehydrogenase deficiency [G6PD(-)]. Biotransformation appears to be central to the anti-infective and hematological toxicities of primaquine, but the mechanisms are still not well understood. Metabolic studies with primaquine have been hampered due to the reactive nature of potential hemotoxic metabolites. An in vitro metabolism-linked hemotoxicity assay has been developed. Co-incubation of the drug with normal or G6PD(-) erythrocytes, microsomes or recombinant cytochrome P(450) (CYP) isoforms has allowed in situ generation of potential hemotoxic metabolite(s), which interact with the erythrocytes to generate hemotoxicity. Methemoglobin formation, real-time generation of reactive oxygen intermediates (ROIs) and depletion of reactive thiols were monitored as multiple biochemical end points for hemotoxicity. Primaquine alone did not produce any hemotoxicity, while a robust increase was observed in methemoglobin formation and generation of ROIs by primaquine in the presence of human or mouse liver microsomes. Multiple CYP isoforms (CYP2E1, CYP2B6, CYP1A2, CYP2D6 and CYP3A4) variably contributed to the hemotoxicity of primaquine. This was further confirmed by significant inhibition of primaquine hemotoxicity by the selective CYP inhibitors, namely thiotepa (CYP2B6), fluoxetine (CYP2D6) and troleandomycin (CYP3A4). Primaquine caused similar methemoglobin formation in G6PD(-) and normal human erythrocytes. However, G6PD(-) erythrocytes suffered higher oxidative stress and depletion of thiols than normal erythrocytes due to primaquine toxicity. The results provide significant insights regarding CYP isoforms contributing to hemotoxicity and may be useful in controlling toxicity of primaquine to increase its therapeutic utility.