Population pharmacokinetics of primaquine and its metabolites in African males.

BACKGROUND: Primaquine (PQ) is the prototype 8-aminoquinoline drug, a class which targets gametocytes and hypnozoites. The World Health Organization (WHO) recommends adding a single low dose of primaquine to the standard artemisinin-based combination therapy (ACT) in order to block malaria transmission in regions with low malaria transmission. However, the haemolytic toxicity is a major adverse outcome of primaquine in glucose-6-phosphate dehydrogenase (G6PD)-deficient subjects. This study aimed to characterize the pharmacokinetic properties of primaquine and its major metabolites in G6PD-deficient subjects. METHODS: A single low-dose of primaquine (0.4-0.5 mg/kg) was administered in twenty-eight African males. Venous and capillary plasma were sampled up to 24 h after the drug administration. Haemoglobin levels were observed up to 28 days after drug administration. Only PQ, carboxy-primaquine (CPQ), and primaquine carbamoyl-glucuronide (PQCG) were present in plasma samples and measured using liquid chromatography mass spectrometry. Drug and metabolites' pharmacokinetic properties were investigated using nonlinear mixed-effects modelling. RESULTS: Population pharmacokinetic properties of PQ, CPQ, and PQCG can be described by one-compartment disposition kinetics with a transit-absorption model. Body weight was implemented as an allometric function on the clearance and volume parameters for all compounds. None of the covariates significantly affected the pharmacokinetic parameters. No significant correlations were detected between the exposures of the measured compounds and the change in haemoglobin or methaemoglobin levels. There was no significant haemoglobin drop in the G6PD-deficient patients after administration of a single low dose of PQ. CONCLUSIONS: A single low-dose of PQ was haematologically safe in this population of G6PD-normal and G6PD-deficient African males without malaria. Trial registration NCT02535767.

PHARMACOKINETICS OF PRIMAQUINE PHOSPHATE AFTER A SINGLE ORAL ADMINISTRATION TO AFRICAN PENGUINS (SPHENISCUS DEMERSUS).

Primaquine is an 8-aminoquinolone drug commonly used for the chemoprophylaxis and treatment of avian malarial infections in managed penguin populations worldwide. Little is known about its pharmacokinetic properties in avian species. The objective of this study was to describe the disposition of primaquine phosphate after a single oral dose in 15 healthy African penguins (Spheniscus demersus). A single tablet containing 26.3 mg of primaquine phosphate (equivalent to 15 mg primaquine base) was administered orally to each bird in a herring fish. Blood samples were collected prior to drug administration and at predetermined timepoints through 144 hr postadministration. Plasma was analyzed for drug concentration by high-performance liquid chromatography with ultraviolet detection. Mean maximum plasma concentration of primaquine phosphate was 277 ± 96 ng/ml at approximately 3.1 hr following oral administration. The mean disappearance half-life was 3.6 ± 1.6 hr. Plasma concentrations were below detectable limits in all but one penguin by 36 hr. A single oral administration of 26.3 mg of primaquine phosphate in African penguins resulted in a pharmacokinetic profile comparable to those attained in human studies. These results suggest that a dosing interval similar to human regimens may be of potential use in the prevention and treatment of avian malaria in penguins. Additional clinical studies are needed to determine the efficacy and safety of this regimen.

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.

Optimized Griess Reaction for UV-Vis and Naked-eye Determination of Anti-malarial Primaquine.

Primaquine (PMQ), an important anti-malarial drug, has been recommended by the World Health Organization (WHO) for the treatment of life-threatening infections caused by P. vivax and ovale. However, PMQ has unwanted adverse effects that lead to acute hemolysis in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency. There is a need to develop simple and reliable methods for PMQ determination with the purpose of dosage monitoring. In early 2019, we have reported an UV-Vis and naked-eye based approach for PMQ colorimetric quantification. The detection was based on a Griess-like reaction between PMQ and anilines, which can generate colored azo products. The detection limit for direct measurement of PMQ in synthetic urine is in the nanomolar range. Moreover, this method has shown great potential for PMQ quantification from human serum samples at clinically relevant concentrations. In this protocol, we will describe the technical details regarding the syntheses and characterization of colored azo products, the reagent preparation, and the procedures for PMQ determination.

CYP2D6 Polymorphisms and the Safety and Gametocytocidal Activity of Single-Dose Primaquine for Plasmodium falciparum.

Single-dose primaquine (PQ) clears mature gametocytes and reduces the transmission of Plasmodium falciparum after artemisinin combination therapy. Genetic variation in CYP2D6, the gene producing the drug-metabolizing enzyme cytochrome P450 2D6 (CYP2D6), influences plasma concentrations of PQ and its metabolites and is associated with PQ treatment failure in Plasmodium vivax malaria. Using blood and saliva samples of varying quantity and quality from 8 clinical trials across Africa (n = 1,076), we were able to genotype CYP2D6 for 774 samples (72%). We determined whether genetic variation in CYP2D6 has implications for PQ efficacy in individuals with gametocytes at the time of PQ administration (n = 554) and for safety in glucose-6-phosphate dehydrogenase (G6PD)-deficient individuals treated with PQ (n = 110). Individuals with a genetically inferred CYP2D6 poor/intermediate metabolizer status had a higher gametocyte prevalence on day 7 or 10 after PQ than those with an extensive/ultrarapid CYP2D6 metabolizer status (odds ratio [OR] = 1.79 [95% confidence interval {CI}, 1.10, 2.90]; P = 0.018). The mean minimum hemoglobin concentrations during follow-up for G6PD-deficient individuals were 11.8 g/dl for CYP2D6 extensive/ultrarapid metabolizers and 12.1 g/dl for CYP2D6 poor/intermediate metabolizers (P = 0. 803). CYP2D6 genetically inferred metabolizer status was also not associated with anemia following PQ treatment (P = 0.331). We conclude that CYP2D6 poor/intermediate metabolizer status may be associated with prolonged gametocyte carriage after treatment with single-low-dose PQ but not with treatment safety.

An unexpected Griess reaction on the important anti-malarial drug primaquine and its application for drug determination.

Primaquine (PMQ), a well-known anti-malarial drug, is of increasing importance as people moving toward global malaria eradication. PMQ has serious side effects that it often causes acute hemolytic toxicity in people with glucose-6-phosphate dehydrogenase (G6PD) deficiency. The development of simple and reliable approaches for quantitative dose monitoring is thus becoming important during malarial treatment with PMQ. Herein, an unexpected Griess reaction on PMQ was systematically studied. The reaction happened between substituted aniline and a primaquine molecule in the presence of nitrite. Both experimental measurements and theoretic calculation showed that UV-vis absorption of the azo products varied because of different electron contributing effects of substituents. Based on the optimized conditions, a novel colorimetric method has been developed for PMQ determination with excellent sensitivity and selectivity. The detection limits for PMQ in water and synthetic urine samples were down to nanomolar range. More importantly, this method has been successfully used to quantify PMQ from human serum samples within clinically relevant concentration ranges.

Population pharmacokinetics of primaquine and the effect of hepatic and renal dysfunction: An exploratory approach.

OBJECTIVES: We attempted to develop a population pharmacokinetic model for primaquine (PQ) and evaluate the effect of renal and hepatic dysfunction on PQ pharmacokinetics. MATERIALS AND METHODS: The data were collected from a prospective, nonrandomized clinical study in healthy volunteers and patients with mild-moderate hepatic dysfunction and renal dysfunction. Model development was conducted using NONMEM® software, and parameter estimation was conducted using first-order conditional estimation with interaction method. RESULTS: Final data included a total of 53 study participants (13 healthy individuals, 12 with mild hepatic dysfunction, 6 with moderate hepatic dysfunction, and 22 with renal dysfunction) with 458 concentrations records. Absorption rate constant (Ka) was constrained to be higher than elimination rate constant to avoid flip-flop situation. Mild hepatic dysfunction was a significant covariate on volume of distribution, and it is approximately three folds higher compared to other subjects. Fixed effects parameter estimates of the final model - absorption rate constant (Ka), volume of distribution (V), and clearance (CL) - were 0.95/h, 498 L, and 39 L/h, respectively. Between-subject variability estimates (% CV) on Ka, V, and CL were 77, 66, and 65, respectively. Residual error was modeled as combination error model with the parameter estimates for proportion error 12% CV and additive error (standard deviation) 1.5 ng/ml. CONCLUSION: Population pharmacokinetic modeling showed that the volume of distribution of PQ in subjects with moderate hepatic dysfunction increases approximately three folds resulting in a significantly lower plasma concentration.

Chloroquine-Primaquine Therapeutic Efficacy, Safety, and Plasma Levels in Patients with Uncomplicated Plasmodium vivax Malaria in a Colombian Pacific Region.

In Colombia, published studies for the treatment of uncomplicated Plasmodium vivax malaria with chloroquine-primaquine (CQ-PQ) are scarce. The aim of the study was to evaluate the therapeutic efficacy and safety profile of this combination. A clinical trial was performed in adults with uncomplicated P. vivax malaria using the 28-day World Health Organization validated protocol. Patients received supervised antimalarial treatment and the primary efficacy end point was the clinical and parasitological response. Safety was assessed through adverse events surveillance, and plasma levels of antimalarial drugs were measured. A total of 77 patients were included. Adequate clinical and parasitological response rate diagnosed by thick blood smear examination was achieved in 72 of 73 patients (98.6%) with a complete 28-day follow-up. There were two parasitological therapeutic failures (TFs) (2.9%) on day 28, established by polymerase chain reaction among 68 patients, one of them with a positive film. No adverse events were detected. After completing the antimalarial treatment, all patients reached adequate plasma concentrations of CQ and desethylchloroquine (DECQ), with medians of 302.9 and 104.0 ng/mL, respectively. Uncomplicated P. vivax malaria treatment with CQ-PQ standard treatment was effective and safe in the study population; TFs were not associated with low plasma levels of CQ and DECQ.

Levels of primaquine and carboxyprimaquine in patients with malaria vivax from the Brazilian Amazon basin.

In the last two years, a substantial increase in the number of malaria vivax cases has occurred in the Brazilian Amazon basin. The adequate exposure of hypnozoites to primaquine is a matter of interest as these dormant forms are responsible for the maintenance or even the increase of malaria burden in endemic areas. The aim of this study was to estimate the levels of primaquine and carboxyprimaquine in whole blood samples of patients with P. vivax treated with chloroquine and an abbreviated regimen of primaquine (0.5 mg/kg/d for 7 days), with adequate clinical and parasitological outcomes after 180 days of follow-up. A total of 40 male patients met the criteria for inclusion in the study. Primaquine and carboxyprimaquine were measured by high-performance liquid chromatography. The levels of primaquine in whole blood samples ranged from 40-238 ng/mL, 42-196 ng/mL and 42-150 ng/mL on days 1, 3 and 7. The levels of carboxyprimaquine in whole blood samples ranged from 87-234 ng/mL, 96-252 ng/mL and 74-448 ng/mL on days 1, 3 and 7. These data provide a reliable estimation of exposure of the infecting parasite to primaquine. Based on the regional pattern of relapse, the estimated blood levels of primaquine can be considered effective against hypnozoites of the local circulating strains of P. vivax.

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 Pharmacokinetics in Lactating Women and Breastfed Infant Exposures.

Background: Primaquine is the only drug providing radical cure of Plasmodium vivax malaria. It is not recommended for breastfeeding women as it causes hemolysis in glucose-6-phosphate dehydrogenase (G6PD)-deficient individuals, and breast milk excretion and thus infant exposure are not known. Methods: Healthy G6PD-normal breastfeeding women with previous P. vivax infection and their healthy G6PD-normal infants between 28 days and 2 years old were enrolled. Mothers took primaquine 0.5 mg/kg/day for 14 days. Primaquine and carboxyprimaquine concentrations were measured in maternal venous plasma, capillary plasma, and breast milk samples and infant capillary plasma samples taken on days 0, 3, 7, and 13. Results: In 20 mother-infant pairs, primaquine concentrations were below measurement thresholds in all but 1 infant capillary plasma sample (that contained primaquine 2.6 ng/mL), and carboxyprimaquine was likewise unmeasurable in the majority of infant samples (maximum value 25.8 ng/mL). The estimated primaquine dose received by infants, based on measured breast milk levels, was 2.98 µg/kg/day (ie, ~0.6% of a hypothetical infant daily dose of 0.5 mg/kg). There was no evidence of drug-related hemolysis in the infants. Maternal levels were comparable to levels in nonlactating patients, and adverse events in mothers were mild. Conclusions: The concentrations of primaquine in breast milk are very low and therefore very unlikely to cause adverse effects in the breastfeeding infant. Primaquine should not be withheld from mothers breastfeeding infants or young children. More information is needed in neonates. Clinical Trials Registration: NCT01780753.

A newly validated high-performance liquid chromatography method with diode array ultraviolet detection for analysis of the antimalarial drug primaquine in the blood plasma.

INTRODUCTION:: Primaquine (PQ) diphosphate is an 8-aminoquinoline antimalarial drug with unique therapeutic properties. It is the only drug that prevents relapses of Plasmodium vivax or Plasmodium ovale infections. In this study, a fast, sensitive, cost-effective, and robust method for the extraction and high-performance liquid chromatography with diode array ultraviolet detection (HPLC-DAD-UV ) analysis of PQ in the blood plasma was developed and validated. METHODS:: After plasma protein precipitation, PQ was obtained by liquid-liquid extraction and analyzed by HPLC-DAD-UV with a modified-silica cyanopropyl column (250mm × 4.6mm i.d. × 5µm) as the stationary phase and a mixture of acetonitrile and 10mM ammonium acetate buffer (pH = 3.80) (45:55) as the mobile phase. The flow rate was 1.0mL·min-1, the oven temperature was 50OC, and absorbance was measured at 264nm. The method was validated for linearity, intra-day and inter-day precision, accuracy, recovery, and robustness. The detection (LOD) and quantification (LOQ) limits were 1.0 and 3.5ng·mL-1, respectively. The method was used to analyze the plasma of female DBA-2 mice treated with 20mg.kg-1 (oral) PQ diphosphate. RESULTS:: By combining a simple, low-cost extraction procedure with a sensitive, precise, accurate, and robust method, it was possible to analyze PQ in small volumes of plasma. The new method presents lower LOD and LOQ limits and requires a shorter analysis time and smaller plasma volumes than those of previously reported HPLC methods with DAD-UV detection. CONCLUSIONS:: The new validated method is suitable for kinetic studies of PQ in small rodents, including mouse models for the study of malaria.

A newly validated high-performance liquid chromatography method with diode array ultraviolet detection for analysis of the antimalarial drug primaquine in the blood plasma

Abstract INTRODUCTION: Primaquine (PQ) diphosphate is an 8-aminoquinoline antimalarial drug with unique therapeutic properties. It is the only drug that prevents relapses of Plasmodium vivax or Plasmodium ovale infections. In this study, a fast, sensitive, cost-effective, and robust method for the extraction and high-performance liquid chromatography with diode array ultraviolet detection (HPLC-DAD-UV ) analysis of PQ in the blood plasma was developed and validated. METHODS: After plasma protein precipitation, PQ was obtained by liquid-liquid extraction and analyzed by HPLC-DAD-UV with a modified-silica cyanopropyl column (250mm × 4.6mm i.d. × 5μm) as the stationary phase and a mixture of acetonitrile and 10mM ammonium acetate buffer (pH = 3.80) (45:55) as the mobile phase. The flow rate was 1.0mL·min-1, the oven temperature was 50OC, and absorbance was measured at 264nm. The method was validated for linearity, intra-day and inter-day precision, accuracy, recovery, and robustness. The detection (LOD) and quantification (LOQ) limits were 1.0 and 3.5ng·mL-1, respectively. The method was used to analyze the plasma of female DBA-2 mice treated with 20mg.kg-1 (oral) PQ diphosphate. RESULTS: By combining a simple, low-cost extraction procedure with a sensitive, precise, accurate, and robust method, it was possible to analyze PQ in small volumes of plasma. The new method presents lower LOD and LOQ limits and requires a shorter analysis time and smaller plasma volumes than those of previously reported HPLC methods with DAD-UV detection. CONCLUSIONS: The new validated method is suitable for kinetic studies of PQ in small rodents, including mouse models for the study of malaria.

Primaquine in Plasma and Methemoglobinemia in Patients with Malaria Due to Plasmodium vivax in the Brazilian Amazon Basin.

AbstractPrimaquine is the only licensed drug available for the elimination of Plasmodium vivax hypnozoites. Methemoglobinemia is currently reported in the course of treatment. There is evidence that metabolites of primaquine formed by the cytochrome pathway are responsible for methemoglobin formation; a genetic polymorphism of cytochrome isoforms; and a potential influence of gender in the activities of these enzymes requiring the establishment of dose × response curves profiles in different population groups. Concentrations of primaquine in plasma and methemoglobin levels were investigated in 54 patients with malaria due to P. vivax during the course of the standard regimen of chloroquine with primaquine (0.25 mg/kg/day for 14 days). All study subjects lived in an endemic area of the Brazilian Amazon Basin. The blood samples were collected before initiation of treatment and 3 hours (range 2-4 hours) after the administration of antimalarial drugs on days 2, 7, and 14. Plasma primaquine concentrations were similar in both genders (males: range = 164-191 ng/mL, females: range = 193-212 ng/mL). Methemoglobin levels ranged from 3.3% to 5.9% in males and from 3.1% to 6.5% in females. There were no significant correlations between the plasma primaquine concentrations or total dose and methemoglobin levels, suggesting that unidentified metabolites rather than parent drug were likely responsible for changes in methemoglobin levels. There was no significant influence of gender on primaquine concentrations in plasma or methemoglobin levels.

Age, Weight, and CYP2D6 Genotype Are Major Determinants of Primaquine Pharmacokinetics in African Children.

Low-dose primaquine is recommended to prevent Plasmodium falciparum malaria transmission in areas threatened by artemisinin resistance and areas aiming for malaria elimination. Community treatment campaigns with artemisinin-based combination therapy in combination with the gametocytocidal primaquine dose target all age groups, but no studies thus far have assessed the pharmacokinetics of this gametocytocidal drug in African children. We recruited 40 children participating in a primaquine efficacy trial in Burkina Faso to study primaquine pharmacokinetics. These children received artemether-lumefantrine and either a 0.25- or a 0.40-mg/kg primaquine dose. Seven blood samples were collected from each participant for primaquine and carboxy-primaquine plasma levels determinations: one sample was collected before primaquine administration and six after primaquine administration according to partially overlapping sampling schedules. Physiological population pharmacokinetic modeling was used to assess the impact of weight, age, and CYP2D6 genotype on primaquine and carboxy-primaquine pharmacokinetics. Despite linear weight normalized dosing, the areas under the plasma concentration-time curves and the peak concentrations for both primaquine and carboxy-primaquine increased with age and body weight. Children who were CYP2D6 poor metabolizers had higher levels of the parent compound, indicating a lower primaquine CYP2D6-mediated metabolism. Our data indicate that primaquine and carboxy-primaquine pharmacokinetics are influenced by age, weight, and CYP2D6 genotype and suggest that dosing strategies may have to be reconsidered to maximize the transmission-blocking properties of primaquine. (This study has been registered at ClinicalTrials.gov under registration no. NCT01935882.).

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.

Pharmacokinetic interactions between primaquine and pyronaridine-artesunate in healthy adult Thai subjects.

Pyronaridine-artesunate is a newly introduced artemisinin-based combination treatment which may be deployed together with primaquine. A single-dose, randomized, three-sequence crossover study was conducted in healthy Thai volunteers to characterize potential pharmacokinetic interactions between these drugs. Seventeen healthy adults received a single oral dose of primaquine alone (30 mg base) and were then randomized to receive pyronaridine-artesunate alone (540-180 mg) or pyronaridine-artesunate plus primaquine in combination, with intervening washout periods between all treatments. The pharmacokinetic properties of primaquine, its metabolite carboxyprimaquine, artesunate, its metabolite dihydroartemisinin, and pyronaridine were assessed in 15 subjects using a noncompartmental approach followed by a bioequivalence evaluation. All drugs were well tolerated. The single oral dose of primaquine did not result in any clinically relevant pharmacokinetic alterations to pyronaridine, artesunate, or dihydroartemisinin exposures. There were significantly higher primaquine maximum plasma drug concentrations (geometric mean ratio, 30%; 90% confidence interval [CI], 17% to 46%) and total exposures (15%; 6.4% to 24%) during coadministration with pyronaridine-artesunate than when primaquine was given alone. Pyronaridine, like chloroquine and piperaquine, increases plasma primaquine concentrations. (This study has been registered at ClinicalTrials.gov under registration no. NCT01552330.).

Open-label crossover study of primaquine and dihydroartemisinin-piperaquine pharmacokinetics in healthy adult thai subjects.

Dihydroartemisinin-piperaquine is an artemisinin-based combination treatment (ACT) recommended by the WHO for uncomplicated Plasmodium falciparum malaria, and it is being used increasingly for resistant vivax malaria where combination with primaquine is required for radical cure. The WHO recently reinforced its recommendations to add a single dose of primaquine to ACTs to reduce P. falciparum transmission in low-transmission settings. The pharmacokinetics of primaquine and dihydroartemisinin-piperaquine were evaluated in 16 healthy Thai adult volunteers in a randomized crossover study. Volunteers were randomized to two groups of three sequential hospital admissions to receive 30 mg (base) primaquine, 3 tablets of dihydroartemisinin-piperaquine (120/960 mg), and the drugs together at the same doses. Blood sampling was performed over 3 days following primaquine and 36 days following dihydroartemisinin-piperaquine dosing. Pharmacokinetic assessment was done with a noncompartmental approach. The drugs were well tolerated. There were no statistically significant differences in dihydroartemisinin and piperaquine pharmacokinetics with or without primaquine. Dihydroartemisinin-piperaquine coadministration significantly increased plasma primaquine levels; geometric mean ratios (90% confidence interval [CI]) of primaquine combined versus primaquine alone for maximum concentration (Cmax), area under the concentration-time curve from 0 h to the end of the study (AUC0-last), and area under the concentration-time curve from 0 h to infinity (AUC0-∞) were 148% (117 to 187%), 129% (103 to 163%), and 128% (102 to 161%), respectively. This interaction is similar to that described recently with chloroquine and may result in an enhanced radical curative effect. (This study has been registered at ClinicalTrials.gov under registration no. NCT01525511.).

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.

Assessment of the prophylactic activity and pharmacokinetic profile of oral tafenoquine compared to primaquine for inhibition of liver stage malaria infections.

BACKGROUND: As anti-malarial drug resistance escalates, new safe and effective medications are necessary to prevent and treat malaria infections. The US Army is developing tafenoquine (TQ), an analogue of primaquine (PQ), which is expected to be more effective in preventing malaria in deployed military personnel. METHODS: To compare the prophylactic efficacy of TQ and PQ, a transgenic Plasmodium berghei parasite expressing the bioluminescent reporter protein luciferase was utilized to visualize and quantify parasite development in C57BL/6 albino mice treated with PQ and TQ in single or multiple regimens using a real-time in vivo imaging system (IVIS). As an additional endpoint, blood stage parasitaemia was monitored by flow cytometry. Comparative pharmacokinetic (PK) and liver distribution studies of oral and intravenous PQ and TQ were also performed. RESULTS: Mice treated orally with three doses of TQ at 5 mg/kg three doses of PQ at 25 mg/kg demonstrated no bioluminescence liver signal and no blood stage parasitaemia was observed suggesting both drugs showed 100% causal activity at the doses tested. Single dose oral treatment with 5 mg TQ or 25 mg of PQ, however, yielded different results as only TQ treatment resulted in causal prophylaxis in P. berghei sporozoite-infected mice. TQ is highly effective for causal prophylaxis in mice at a minimal curative single oral dose of 5 mg/kg, which is a five-fold improvement in potency versus PQ. PK studies of the two drugs administered orally to mice showed that the absolute bioavailability of oral TQ was 3.5-fold higher than PQ, and the AUC of oral TQ was 94-fold higher than oral PQ. The elimination half-life of oral TQ in mice was 28 times longer than PQ, and the liver tissue distribution of TQ revealed an AUC that was 188-fold higher than PQ. CONCLUSIONS: The increased drug exposure levels and longer exposure time of oral TQ in the plasma and livers of mice highlight the lead quality attributes that explain the much improved efficacy of TQ when compared to PQ.