5-Phenoxy Primaquine Analogs and the Tetraoxane Hybrid as Antimalarial Agents

The rapid emergence of drug resistance to the current antimalarial agents has led to the urgent need for the discovery of new and effective compounds. In this work, a series of 5-phenoxy primaquine analogs with 8-aminoquinoline core (7a–7h) was synthesized and investigated for their antimalarial activity against Plasmodium falciparum. Most analogs showed improved blood antimalarial activity compared to the original primaquine. To further explore a drug hybrid strategy, a conjugate compound between tetraoxane and the representative 5-phenoxy-primaquine analog 7a was synthesized. In our work, the hybrid compound 12 exhibited almost a 30-fold increase in the blood antimalarial activity (IC₅₀ = 0.38 ± 0.11 μM) compared to that of primaquine, with relatively low toxicity against mammalian cells (SI = 45.61). Furthermore, we found that these 5-phenoxy primaquine analogs and the hybrid exhibit significant heme polymerization inhibition, an activity similar to that of chloroquine, which could contribute to their improved antimalarial activity. The 5-phenoxy primaquine analogs and the tetraoxane hybrid could serve as promising candidates for the further development of antimalarial agents.

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.

Primaquine or other 8-aminoquinolines for reducing Plasmodium falciparum transmission

BACKGROUND

The 8-aminoquinoline (8AQ) drugs act on Plasmodium falciparum gametocytes, which transmit malaria from infected people to mosquitoes. In 2012, the World Health Organization (WHO) recommended a single dose of 0.25 mg/kg primaquine (PQ) be added to malaria treatment schedules in low-transmission areas or those with artemisinin resistance. This replaced the previous recommendation of 0.75 mg/kg, aiming to reduce haemolysis risk in people with glucose-6-phosphate dehydrogenase deficiency, common in people living in malarious areas. Whether this approach, and at this dose, is effective in reducing transmission is not clear.

OBJECTIVES

To assess the effects of single dose or short-course PQ (or an alternative 8AQ) alongside treatment for people with P. falciparum malaria.

SEARCH METHODS

We searched the Cochrane Infectious Diseases Group Specialized Register; the Cochrane Central Register of Controlled Trials (CENTRAL), published in the Cochrane Library; and the WHO International Clinical Trials Registry Platform (ICRTP) portal using 'malaria*', 'falciparum', 'primaquine', '8-aminoquinoline', and eight 8AQ drug names as search terms. We checked reference lists of included trials, and contacted researchers and organizations. Date of last search: 21 July 2017.

SELECTION CRITERIA

Randomized controlled trials (RCTs) or quasi-RCTs in children or adults, adding PQ (or alternative 8AQ) as a single dose or short course alongside treatment for P. falciparum malaria.

DATA COLLECTION AND ANALYSIS

Two authors screened abstracts, applied inclusion criteria, and extracted data. We sought evidence on transmission (community incidence), infectiousness (people infectious and mosquitoes infected), and potential infectiousness (gametocyte measures assessed by microscopy or polymerase chain reaction [PCR]). We grouped trials into artemisinin and non-artemisinin treatments, and stratified by PQ dose (low, 0.2 to 0.25 mg/kg; moderate, 0.4 to 0.5 mg/kg; high, 0.75 mg/kg). We used GRADE, and absolute effects of infectiousness using trial control groups.

MAIN RESULTS

We included 24 RCTs and one quasi-RCT, comprising 43 arms. Fourteen trials evaluated artemisinin treatments (23 arms), nine trials evaluated non-artemisinin treatments (13 arms), and two trials included both artemisinin and non-artemisinin arms (three and two arms, respectively). Two trial arms used bulaquine. Seven PQ arms used low dose (six with artemisinin), 11 arms used moderate dose (seven with artemisinin), and the remaining arms used high dose. Fifteen trials tested for G6PD status: 11 excluded participants with G6PD deficiency, one included only those with G6PD deficiency, and three included all, irrespective of status. The remaining 10 trials either did not test or did not report on testing.No cluster trials evaluating community effects on malaria transmission met the inclusion criteria.With artemisinin treatmentLow dose PQInfectiousness (participants infectious to mosquitoes) was reduced (day 3 or 4: RR 0.12, 95% CI 0.02 to 0.88, 3 trials, 105 participants; day 8: RR 0.34, 95% CI 0.07 to 1.58, 4 trials, 243 participants; low certainty evidence). This translates to a reduction in percentage of people infectious on day 3 or 4 from 14% to 2%, and, for day 8, from 4% to 1%; the waning infectiousness in the control group by day 8 making the absolute effect smaller by day 8. For gametocytes detected by PCR, there was little or no effect of PQ at day 3 or 4 (RR 1.02, 95% CI 0.87 to 1.21; 3 trials, 414 participants; moderate certainty evidence); with reduction at day 8 (RR 0.52, 95% CI 0.41 to 0.65; 4 trials, 532 participants; high certainty evidence). Severe haemolysis was infrequent, with or without PQ, in these groups with few G6PD-deficient individuals (RR 0.98, 95% CI 0.69 to 1.39; 4 trials, 752 participants, moderate certainty evidence).Moderate dose PQInfectiousness was reduced (day 3 or 4: RR 0.13, 95% CI 0.02 to 0.94; 3 trials, 109 participants; day 8 RR 0.33, 95% CI 0.07 to 1.57; 4 trials, 246 participants; low certainty evidence). Illustrative risk estimates for moderate dose were the same as low dose. The pattern and level of certainty of evidence with gametocytes detected by PCR was the same as low dose, and severe haemolysis was infrequent in both groups.High dose PQInfectiousness was reduced (day 4: RR 0.2, 95% CI 0.02 to 1.68, 1 trial, 101 participants; day 8: RR 0.18, 95% CI 0.02 to 1.41, 2 trials, 181 participants, low certainty evidence). The effects on gametocyte prevalence showed a similar pattern to moderate and low dose PQ. Trials did not systematically report evidence of haemolysis.With non-artemisinin treatmentTrials with non-artemisinin treatment have been conducted only for moderate and high dose PQ. With high dose, infectiousness appeared markedly reduced on day 5 (RR 0.09, 95% CI 0.01 to 0.62; 30 participants, very low certainty evidence), with similar reductions at day 8. For both moderate dose (two trials with 221 people) and high dose (two trials with 30 people), reduction in gametocytes (detected by microscopy) showed similar patterns as for artemisinin treatments, with little or no effect at day 4 or 5, and larger effects by day 8. No trials with non-artemisinin partner drugs systematically sought evidence of severe haemolysis.Two trials comparing bulaquine with PQ suggest bulaquine may have larger effects on gametocytes by microscopy on day 8 (RR 0.41, 95% CI 0.26 to 0.66; 2 trials, 112 participants).

AUTHORS' CONCLUSIONS

A single low dose of PQ (0.25 mg/kg) added to artemisinin-based combination therapy for malaria reduces infectiousness of people to mosquitoes at day 3-4 and day 8, and appears as effective as higher doses. The absolute effect is greater at day 3 or 4, and smaller at day 8, in part because of the lower infectiousness in the control group. There was no evidence of increased haemolysis at 0.25 mg/kg, but few G6PD-deficient individuals were included in the trials. The effect on infectiousness precedes the effect of PQ on gametocyte prevalence. We do not know whether single dose PQ could reduce malaria transmission at community level.

Design, Synthesis and Biological Evaluation of Novel Primaquine-Cinnamic Acid Conjugates of the Amide and Acylsemicarbazide Type

In this paper design and synthesis of a scaffold comprising primaquine (PQ) motif and cinnamic acid derivatives (CADs) bound directly (compounds 3a-k) or via a spacer (compounds 7a-k) are reported. In the first series of compounds, PQ and various CADs were connected by amide bonds and in the second series by acylsemicarbazide functional groups built from the PQ amino group, CONHNH spacer and the carbonyl group originating from the CADs. PQ-CAD amides 3a-k were prepared by a simple one-step condensation reaction of PQ with a series of CAD chlorides (method A) or benzotriazolides 2 (method B). The synthesis of acylsemicarbazides 7a-k included activation of PQ with benzotriazole, preparation of PQ-semicarbazide 6 and its condensation with CAD chlorides 4. All synthesized PQ-CAD conjugates were evaluated for their anticancer, antiviral and antioxidative activities. Almost all compounds from series 3 were selective towards the MCF-7 cell line and active at micromolar concentrations. The o-fluoro derivative 3h showed high activity against HeLa, MCF-7 and in particular against the SW 620 cell line, while acylsemicarbazide 7f with a benzodioxole ring and 7c, 7g and especially 7j with methoxy-, chloro- or trifluoromethyl-substituents in the para position showed high selectivity and high inhibitory activity against MCF-7 cell line at micromolar (7c, 7f, 7g) and nanomolar (7j) levels. Acylsemicarbazide derivatives with trifluoromethyl group(s) 7i, 7j and 7k showed specific activity against human coronavirus (229E) at concentrations which did not alter the normal cell morphology. The same compounds exerted the most potent reducing activity in the DPPH test, together with 7d and 7g, while methoxy (compounds 7c-e), benzodioxole (7f), p-Cl (7g) and m-CF₃ (7i) acylsemicarbazides and amide 3f presented the highest LP inhibition (83%-89%). The dimethoxy derivative 7d was the most potent LOX inhibitor (IC50 = 10 μΜ). The performed biological tests gave evidence of acylsemicarbazide functional group as superior binding group in PQ-CAD conjugates.

Evaluation of antimalarial activity and toxicity of a new primaquine prodrug

Plasmodium vivax is the most prevalent of the five species causing malaria in humans. The current available treatment for P. vivax malaria is limited and unsatisfactory due to at least two drawbacks: the undesirable side effects of primaquine (PQ) and drug resistance to chloroquine. Phenylalanine-alanine-PQ (Phe-Ala-PQ) is a PQ prodrug with a more favorable pharmacokinetic profile compared to PQ. The toxicity of this prodrug was evaluated in in vitro assays using a human hepatoma cell line (HepG2), a monkey kidney cell line (BGM), and human red blood cells deficient in the enzyme glucose-6-phosphate-dehydrogenase (G6PD). In addition, in vivo toxicity assays were performed with rats that received multiple doses of Phe-Ala-PQ to evaluate biochemical, hematological, and histopathological parameters. The activity was assessed by the inhibition of the sporogonic cycle using a chicken malaria parasite. Phe-Ala-PQ blocked malaria transmission in Aedes mosquitoes. When compared with PQ, it was less cytotoxic to BGM and HepG2 cells and caused less hemolysis of G6PD-deficient red blood cells at similar concentrations. The prodrug caused less alteration in the biochemical parameters than did PQ. Histopathological analysis of the liver and kidney did show differences between the control and Phe-Ala-PQ-treated groups, but they were not statistically significant. Taken together, the results highlight the prodrug as a novel lead compound candidate for the treatment of P. vivax malaria and as a blocker of malaria transmission.

Comparative analysis of in vitro rat liver metabolism of the antimalarial primaquine and a derived imidazoquine

The present study provides proof-of-concept regarding the expectedly high enzymatic stability of primaquinederived imidazolidin-4-ones, imidazoquines, formerly developed as alternatives to the parent antimalarial with potentially improved oral bioavailability [J. Med. Chem., 2009, 52, 7800-7807]. This study provides relevant experimental evidence on the remarkably low propensity of imidazoquines to undergo metabolic conversions mediated by rat liver enzymes. This, together with favourable key ADME parameters previously predicted for these compounds [Bioorg. Med. Chem. Lett. 2009, 19, 6914-6917], and proven lack of acute toxicity in mice, further reinforces the role of imidazoquines as reference leads for the development of novel primaquine surrogates. This is a particularly relevant issue in the present status of malaria chemotherapy worldwide, where primaquine remains the sole drug in clinical use able to block transmission between infected persons and the insect vector and to effectively act on liver-stage parasite forms, including hypnozoites.

Electrospray ionization mass spectrometry as a valuable tool in the characterization of novel primaquine peptidomimetic derivatives

Novel primaquine-derived antimalarials have been extensively characterized by electrospray ionization-ion trap mass spectrometry (ESI-MS). Experiments by in-source collision-induced dissociation (CID) in the nozzle- skimmer region (NSR) or by tandem-MS are shown to be most valuable tools for the physicochemical characterization of these 8-aminoquinolinic drugs that also bear the biologically relevant imidazolidin-4-one scaffold. It was possible to find parallelism between compound stability in the NSR and its reactivity towards hydrolysis at physiological pH and T. Moreover, tandem-MS fragmentation patterns were characteristic for each family, providing a means for structural distinction of isomers and allowing to find interesting correlations between the relative abundance of particular fragments and relevant structure-activity determinants, such as Charton steric parameter, v. In conclusion, this work provides solid grounds to establish ESI-MS as a key tool for the physicochemical characterization of biopharmaceuticals bearing the 8-aminoquinoline and/or the imidazolidin-4-one moieties.

Electrospray ionization-ion trap mass spectrometry study of PQAAPro and PQProAA mimetic derivatives of the antimalarial primaquine

Electrospray ionization-ion trap mass spectrometry (ESI-MS) of imidazolidin-4-one peptidomimetic derivatives of the antimalarial drug primaquine (PQ) is reported. These compounds contain the imidazolidin-4-one moiety either at the N- or the C-terminal of a dipeptide backbone, thus respectively mimicking PQ-Amino Acid-Proline (PQAAPro) and PQProAA derivatives of PQ. Both the peptidomimetics and precursors previously developed by us are promising drug candidates, as they were found to be active against rodent Plasmodium berghei malaria and Pneumocystis carinii pneumonia. Collision-induced dissociation (CID) and tandem-mass spectra (MS) of the title compounds, and fragmentation pathways thereof, led to the following findings: (1) CID patterns present some parallelism with the reactivity towards hydrolysis previously found for the same or related compounds; (2) a positional shift of the imidazolidin-4-one ring is reflected on both degree and pathways of fragmentation, which makes tandem-MS a key tool for differentiation of imidazolidin-4-one isomers; (3) the major MS/MS fragmentation of PQProAA mimetics involves release of a neutral diketopiperazine (DKP), in parallel to the "diketopiperazine pathway" described in tandem-MS studies of oligopeptides; (4) the relative abundance of a major fragment in tandem-MS spectra is inversely correlated with the size of the N-terminal AA in PQProAA mimetics. Overall, this work embodies an original and valuable contribution towards a deeper insight into the molecular properties of novel antimalarials, which can be viewed as representative of both the 8-aminoquinoline and, especially, the imidazolidin-4-one structural classes.

Protection of retinal cells from ischemia by a novel gap junction inhibitor

Retinal cells which become ischemic will pass apoptotic signal to adjacent cells, resulting in the spread of damage. This occurs through open gap junctions. A class of novel drugs, based on primaquine (PQ), was tested for binding to connexin 43 using simulated docking studies. A novel drug has been synthesized and tested for inhibition of gap junction activity using R28 neuro-retinal cells in culture. Four drugs were initially compared to mefloquine, a known gap junction inhibitor. The drug with optimal inhibitory activity, PQ1, was tested for inhibition and was found to inhibit dye transfer by 70% at 10 microM. Retinal ischemia was produced in R28 cells using cobalt chloride as a chemical agent. This resulted in activation of caspase-3 which was prevented by PQ1, the gap junction inhibitor. Results demonstrate that novel gap junction inhibitors may provide a means to prevent retinal damage during ischemia.

In Vitro and In Vivo pharmacokinetic studies of bulaquine (analogue of primaquine), a novel antirelapse antimalarial, in rat, rabbit and monkey--highlighting species similarities and differences

Bulaquine (BQ) is a potent antirelapse antimalarial developed by CDRI, India. Bulaquine was rapidly absorbed in rats and rabbits with no distinct absorption phase while in monkeys a variable irregular absorption profile was observed. BQ was extensively converted to primaquine (PQ) after oral administration and the conversion was maximum in rats and minimum in rabbits, which is possibly due to the species difference. Clearance was higher in rats (3.2 l/h/kg) than in rabbits and monkeys (1.2 l/h/kg) and it was found be negligibly excreted in rat urine and feces. The elimination half-life in rats and rabbits was comparable after both oral and i.v. administration ( approximately 1.2 h). In all three species, PQ was resident in the body for a period longer than BQ. PQ, being the major active metabolite of BQ, might be responsible for the extended therapeutic effect of BQ. The oral bioavailability of BQ was 3.12%, 5.3% and 12% in rats, rabbits and monkeys, respectively, which could be mainly due to the high instability of BQ at acidic pH as demonstrated from a simulated gastric fluid stability study. Protein binding in various species was in the range 50-65% while the partition coefficient between RBCs and plasma (K(rbc/pl)) was between 0.75 and 1, indicating significant RBC uptake.

Exploring biosynthetic diversity with trichodiene synthase

Trichodiene synthase is a terpenoid cyclase that catalyzes the cyclization of farnesyl diphosphate (FPP) to form the bicyclic sesquiterpene hydrocarbon trichodiene (89%), at least five sesquiterpene side products (11%), and inorganic pyrophosphate (PP(i)). Incubation of trichodiene synthase with 2-fluorofarnesyl diphosphate or 4-methylfarnesyl diphosphate similarly yields sesquiterpene mixtures despite the electronic effects or steric bulk introduced by substrate derivatization. The versatility of the enzyme is also demonstrated in the 2.85A resolution X-ray crystal structure of the complex with Mg(2+) (3)-PP(i) and the benzyl triethylammonium cation, which is a bulkier mimic of the bisabolyl carbocation intermediate in catalysis. Taken together, these findings show that the active site of trichodiene synthase is sufficiently flexible to accommodate bulkier and electronically-diverse substrates and intermediates, which could indicate additional potential for the biosynthetic utility of this terpenoid cyclase.

Synthesis and evaluation of naphthyridine compounds as antimalarial agents

Primaquine is the drug of choice for the radical cure of Plasmodium vivax malaria, but possesses serious side effects. In this study novel primaquine analogues were designed and synthesized. Lower toxicity was achieved by reducing or eliminating the tendency of forming chemically reactive and toxic intermediates and metabolites. In vitro and in vivo studies found that synthesized compounds were less toxic than the parent compound primaquine, while preserving the desired antimalarial activity. Some of these compounds possess a therapeutic index over 10 times superior to that of the commonly used antimalarial drug chloroquine. These compounds, as well as the underlying design rationale, may find usefulness in the discovery and development of new antimalarial drugs.

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

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

Safety and tolerability of elubaquine (bulaquine, CDRI 80/53) for treatment of Plasmidium vivax malaria in Thailand

We conducted a study to compare the safety and tolerability of anti-relapse drugs elubaquine and primaquine against Plasmodium vivax malaria. After standard therapy with chloroquine, 30 mg/kg given over 3 days, 141 patients with P. vivax infection were randomized to receive primaquine or elubaquine. The 2 treatment regimens were primaquine 30 mg once daily for 7 days (group A, n = 71), and elubaquine 25 mg once daily for 7 days (group B, n = 70). All patients cleared parasitemia within 7 days after chloroquine treatment. Among patients treated with primaquine, one patient relapsed on day 26; no relapse occurred with elubaquine treatement. Both drugs were well tolerated. Adverse effects occurred only in patients with G6PD deficiency who were treated with primaquine (group A, n = 4), whose mean hematocrit fell significantly on days 7, 8 and 9 (P = 0.015, 0.027, and 0.048, respectively). No significant change in hematocrit was observed in patients with G6PD deficiency who were treated with elubaquine (group B, n = 3) or in patients with normal G6PD. In conclusion, elubaquine, as anti-relapse therapy for P. vivax malaria, was as safe and well tolerated as primaquine and did not cause clinically significant hemolysis.

A randomized, parallel study of the safety and efficacy of 45 mg primaquine versus 75 mg bulaquine as gametocytocidal agents in adults with blood schizonticide-responsive uncomplicated falciparum malaria [ISCRTN50134587]

BACKGROUND

The WHO recommends that adults with uncomplicated P. falciparum successfully treated with a blood schizonticide receive a single dose of primaquine (PQ) 45 mg as a gametocytocidal agent. An earlier pilot study suggested that 75 mg of bulaquine (BQ), of which PQ is a major metabolite, may be a useful alternate to PQ.

METHODS

In a randomized, partial blind study, 90 hospitalized adults with Plasmodium falciparum malaria that was blood schizonticide-responsive and a gametocytemia of > 55/microl within 3 days of diagnosis were randomized to receive single doses of either PQ 45 mg or BQ 75 mg on day 4. We assessed gametocytemia on days 8, 15, 22 and 29 and gametocyte viability as determined by exflagellation (2 degrees end point) on day 8.

RESULTS

On day 8, 20/31 (65%) primaquine recipients versus 19/59 (32%) bulaquine recipients showed persistence of gametocytes (P = 0.002). At day 15 and beyond, all patients were gametocyte free. On day 8, 16/31 PQ and 7/59 BQ volunteers showed gametocyte viability (p = 0.000065).

CONCLUSION

BQ is a safe, useful alternate to PQ as a Plasmodium falciparum gametocytocidal agent and may clear gametocytemia faster than PQ.

Plasmodium cynomolgi: Gametocytocidal activity of the anti-malarial compound CDRI 80/53 (elubaquine) in rhesus monkeys

The gametocytocidal action of a new enamine analogue of primaquine, elubaquine (compound CDRI 80/53, bulaquine), has been evaluated against Plasmodium cynomolgi B in rhesus monkeys. Colony bred Anopheles stephensi mosquitoes were fed on gametocyte carrying rhesus monkeys prior to and at varying intervals after oral administration of a single dose of elubaquine at doses ranging between 0.63 and 5.00 mg/kg. Complete loss of oocyst development and mosquito infectivity was observed within 24 h after administering a single 1.25 mg/kg dose, while higher dose of 3.75 mg/kg inhibited oocyst development within 5 h, indicating gametocytocidal action of the compound. Elubaquine did not show any action against developing oocysts in the vector.

Primaquine-Induced Hemolytic Anemia: Role of Splenic Macrophages in the Fate of 5-Hydroxyprimaquine-Treated Rat Erythrocytes

Primaquine-induced hemolytic anemia is known to result from premature sequestration of damaged (but intact) erythrocytes by the spleen. We have shown previously that a phenolic metabolite, 5-hydroxyprimaquine (5-HPQ), is a direct-acting hemolytic agent in rats, suggesting that 5-HPQ is a mediator of the hemolytic response to primaquine. To investigate the fate of erythrocytes in vivo after in vitro exposure to 5-HPQ, rat (51)Cr-labeled erythrocytes were incubated with hemolytic concentrations of 5-HPQ and then readministered intravenously to rats. The time course of loss of radioactivity from blood and uptake into the spleen and liver was measured. In rats given 5-HPQ-treated erythrocytes, an increased rate of removal of radioactivity from the circulation was observed as compared with the vehicle control. The loss of blood radioactivity was accompanied by a corresponding increase in radioactivity appearing in the spleen but not in the liver. When rats were pretreated with clodronate-loaded liposomes to deplete splenic macrophages, there was a decreased rate of removal of radioactivity from the circulation and a markedly diminished uptake into the spleen. A role for phagocytic removal of 5-HPQ-treated red cells was confirmed in vitro using the J774A.1 macrophage cell line. Furthermore, depletion of red cell GSH with diethyl maleate significantly enhanced in vitro phagocytosis of 5-HPQ-treated red cells. The data indicate that splenic macrophages are responsible for removing 5-HPQ-treated red cells and support the postulate that this metabolite is a contributor to the hemolytic anemia induced after administration of the parent compound.

Primaquine-Induced Hemolytic Anemia: Role of Membrane Lipid Peroxidation and Cytoskeletal Protein Alterations in the Hemotoxicity of 5-Hydroxyprimaquine

Primaquine-induced hemolytic anemia is a toxic side effect that is due to premature splenic sequestration of intact erythrocytes. Previous studies have suggested that a phenolic metabolite, 5-hydroxyprimaquine (5-HPQ), mediates primaquine hemotoxicity by generating reactive oxygen species (ROS) within erythrocytes that overwhelm antioxidant defenses. However, the nature of the oxidative stress is not understood, and the molecular targets, whether protein and/or lipid, are unknown. To investigate the mechanism underlying the hemolytic activity of 5-HPQ, we have examined the effect of hemolytic concentrations of 5-HPQ on ROS formation within rat erythrocytes using the cellular ROS probe, 2',7'-dichlorodihydrofluoresein diacetate. In addition, we examined the effect of 5-HPQ on membrane lipids and cytoskeletal proteins. The data indicate that 5-HPQ causes a prolonged, concentration-dependent generation of ROS within erythrocytes. Interestingly, 5-HPQ-generated ROS was not associated with the onset of lipid peroxidation or an alteration in phosphatidylserine asymmetry. Instead, 5-HPQ induced oxidative injury to the erythrocyte cytoskeleton, as evidenced by changes in the normal electrophoretic pattern of membrane ghost proteins. Immunoblotting with an anti-hemoglobin antibody revealed that these changes were due primarily to the formation of disulfide-linked hemoglobin-skeletal protein adducts. The data suggest that cytoskeletal protein damage, rather than membrane lipid peroxidation or loss of phosphatidylserine asymmetry, underlies the process of removal of erythrocytes exposed to 5-HPQ.

Imidazolidin-4-one derivatives of primaquine as novel transmission-blocking antimalarials

Imidazolidin-4-one derivatives of primaquine were synthesized as potential double prodrugs of the parent drug. The title compounds inhibit the development of the sporogonic cycle of Plasmodium berghei, affecting the appearance of oocysts in the midguts of the mosquitoes. The imidazolidin-4-ones are very stable, both in human plasma and in pH 7.4 buffer, indicating that they are active per se. Thus, imidazolidin-4-ones derived from 8-aminoquinolines represent a new entry in antimalarial structure-activity relationships.

Enantioselective analysis of primaquine and its metabolite carboxyprimaquine by capillary electrophoresis

An enantioselective capillary electrophoresis method for the simultaneous determination of primaquine (PQ) and carboxyprimaquine (CPQ) in rat liver mitochondrial fraction, suitable for in vitro metabolism studies is presented. The drug and metabolite were extracted by liquid-liquid extraction using ethyl ether. The enantiomers were resolved in a fused-silica capillary, 50 microm inside diameter (ID) and 24 cm of effective length, using an electrolyte solution consisting of a 20 mmol/L sodium phosphate solution, pH 3.0, and 10% w/v maltodextrin. Hydrodynamic sample injection was used with a 10 s injection time at 50 mbar pressure. The applied voltage was 22 kV and the capillary temperature was controlled at 20 degrees C. Detection was carried out at 264 nm. Under these conditions, the enantiomeric fractions of the drug and of its metabolite were analyzed within 6 min. The extraction procedure was efficient in removing endogenous interferents and low values (<10%) for the coefficients of variation and deviation from theoretical values were demonstrated for both within-day and between-day assays. The method described allows the determination of PQ and CPQ enantiomers as low as 100 and 40 ng/mL, respectively. After validation, the method was used for an in vitro metabolism study of PQ. The results showed that the enantiomer (-)-PQ was preferentially metabolized to (-)-CPQ.

Pharmacokinetics of primaquine and carboxyprimaquine in korean patients with vivax malaria

Primaquine is used for relapses caused by vivax malaria hypnozoites. No studies on the pharmacokinetics of primaquine (PMQ) has been reported in Korean patients. In our study, thirty vivax malaria patients were given 15 mg primaquine daily for 14 days after 3 days of chloroquine treatment. Plasma samples were taken at intervals after each daily dose of PMQ for 3 days. Plasma concentrations of PMQ and carboxyprimaquine (CPMQ), the major metabolite of primaquine, were measured by HPLC. The PMQ concentrations reached a maximum of 0.28+/-0.18 microg/mL at 1.5 h after the first dose. The maximum concentration of CPMQ was 0.32+/-0.13 microg/mL at 4 h. Higher drug concentrations with repeated dosing were observed for CPMQ, but not for the parent drug, PMQ. The elimination half-life was 3.76+/-1.8 h and 15.7+/-12.2 h, for PMQ and CPMQ, respectively. Large variation in the plasma concentrations of both drugs was observed. Overall, PMQ is absorbed and metabolized rapidly after oral administration. It was noted that the mean peak plasma concentration of PMQ was significantly higher and that of CPMQ was lower in our patients compared to other studies. This suggests a potential difference of inter-ethnic groups, which warrants further investigations.

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

Primaquine is an important antimalarial agent because of its activity against exoerythrocytic forms of Plasmodium spp. Methemoglobinemia and hemolytic anemia, however, are dose-limiting side effects of primaquine therapy. These hemotoxic effects are believed to be mediated by metabolites, although the identity of the toxic specie(s) and the mechanism underlying hemotoxicity have remained unclear. Previous studies showed that an N-hydroxylated metabolite of primaquine, 6-methoxy-8-hydroxylaminoquinoline, was capable of mediating primaquine-induced hemotoxicity. The present studies were undertaken to investigate the hemolytic potential of 5-hydroxyprimaquine (5-HPQ), a phenolic metabolite that has been detected in experimental animals. 5-HPQ was synthesized, isolated by flash chromatography, and characterized by NMR spectroscopy and mass spectrometry. In vitro exposure of (51)Cr-labeled erythrocytes to 5-HPQ induced a concentration-dependent decrease in erythrocyte survival (TC(50) of ca. 40 microM) when the exposed cells were returned to the circulation of isologous rats. 5-HPQ also induced methemoglobin formation and depletion of glutathione (GSH) when incubated with suspensions of rat erythrocytes. Furthermore, when red cell GSH was depleted (>95%) by titration with diethyl maleate to mimic GSH instability in human glucose-6-phosphate dehydrogenase deficiency, a 5-fold enhancement of hemolytic activity was observed. These data indicate that 5-HPQ also has the requisite properties to contribute to the hemotoxicity of primaquine. The relative contribution of N-hydroxy versus phenolic metabolites to the overall hemotoxicity of primaquine remains to be assessed.

Discovery of a Bulky 2-tert-Butyl Group Containing Primaquine Analogue That Exhibits Potent Blood-Schizontocidal Antimalarial Activities and Complete Elimination of Methemoglobin Toxicity

To eliminate an unwarranted metabolic pathway of the quinoline ring, a set of two compounds, where C-2 position of the antimalarial drug primaquine is blocked by metabolically stable bulky alkyl group are synthesized. Compound 2 [R = C(CH(3))(3)] of the series has produced excellent antimalarial efficacy against P. berghei and highly virulent multidrug-resistant P. yoelii nigeriensis strain in vivo. Compound 2 was also evaluated for methemoglobin (MetHb) toxicity. This study describes the discovery of a highly potent blood-schizontocidal antimalarial analogue 2, completely devoid of MetHb toxicity.

HPLC and HPTLC assays for the antimalarial agents Chloroquine, Primaquine and Bulaquine

A combination Kit for antirelapse treatment of P. vivax malaria, consisting of Chloroquine phosphate tablets and Bulaquine capsules has been recently developed, and marketed under the trade name Aablaquine. Bulaquine is prepared from Primaquine. Several methods of analysis are reported for each drug separately as well two drugs in combination but no method for simultaneous estimation of these three drugs is known. Therefore, the present study was undertaken to develop a sensitive and reproducible high performance liquid chromatographic as well as high performance thin layer chromatographic assay method for the simultaneous estimation of Chloroquine, Primaquine and Bulaquine.

Chlorocoulometric determination of primaquine

A method for the coulometric determination of primaquine is presented. The coulometric titration is carried out in the anode compartment, in the supporting electrolyte consisting of 0.5 M sulfuric acid and 0.2 M sodium chloride solution, in the presence of methyl orange as indicator. The electrogenerated chlorine reacts with primaquine in a 2:1 molar stoichiometric ratio. Accurate and reproducible results have been obtained in the assay of small amounts of primaquine. One coulomb corresponds to 1.1797 mg of primaquine diphosphate.

Liquid chromatography-tandem mass spectrometric assay with a novel method of quantitation for the simultaneous determination of bulaquine and its metabolite, primaquine, in monkey plasma

A sensitive and selective liquid chromatography-tandem mass spectrometry method (LC-MS-MS) for the simultaneous estimation of bulaquine and primaquine has been developed and validated in monkey plasma. The mobile phase consisted of acetonitrile/ammonium acetate buffer (20 mM, pH 6) (50:50 v/v) at a flow-rate of 1 ml/min. The chromatographic separations were achieved on two spheri cyano columns (5 microm, 30 x 4.6 mm I.D.) connected in series. The quantitation was carried out using a Micromass LC-MS-MS with an electrospray source in the multiple reaction monitoring (MRM) mode. The analytes were quantified from the summed total ion value of their two most intense molecular transitions. This is another novel method leading to increased sensitivity and precision. A simple liquid-liquid extraction with 2 x 1.0 ml n-hexane/ethyl acetate/dimethyloctyl amine (90:10:0.05, v/v) was utilized. The method was validated in terms of recovery, linearity, accuracy and precision (within- and between-assay variation). The recoveries from spiked control samples were >or=90 and 50% for bulaquine and primaquine, respectively. Linearity in plasma was observed over a dynamic range of 1.56-400 and 3.91-1000 ng/ml for bulaquine and primaquine, respectively.

Analysis and pharmacokinetics of bulaquine and its major metabolite primaquine in rabbits using an LC-UV method--a pilot study

A precise and reproducible HPLC assay has been developed and validated for simultaneous determination of bulaquine (BQ) and its metabolite primaquine (PQ) in rabbit plasma. The method, applicable to 0.5 ml plasma, involves double extraction of samples with n-hexane: isopropanol (98:2, v/v) containing dimethyl octylamine (DMOA) (0.1%, v/ v). Separations were accomplished by reversed-phase liquid chromatography using a Spheri-5 cyano column with a low pressure gradient with mobile phase consisting of ammonium acetate buffer (50 mM, pH 6.0) and acetonitrile with DMOA. The method was sensitive with a limit of quantitation of 20 ng ml(-1) in rabbit plasma for both BQ and PQ and the recoveries were > 85 and > 45%, respectively. Excellent linear relationships (r > 0.99) were obtained between the measured and added concentration ratios of the plasma concentrations over a range of 20-1000 ng ml(-1) for both the analytes. Precision and accuracy were acceptable as indicated by relative standard deviations from 1.8 to 15.1%, bias values ranging from -14.2 to 15.7%. Moreover, BQ was stable in rabbit plasma for 15 days of storage at -60 degrees C and after being subjected to three freeze-thaw cycles. The method was applied to determine the levels and pharmacokinetics of BQ in rabbits following a single 2.5 mg kg(-1) oral and intravenous dose. The BQ levels declined and the PQ levels increased with time. The PQ/BQ ratio after oral dose at 1 and 1.5 h were higher than that after intravenous dose. In the pilot preclinical pharmacokinetic study after a single 2.5 mg kg(-1) oral dose, BQ levels were determined up to 6 h (post-prandial) and 8 h (fasting). The plasma concentration versus time data were best fitted to a two-compartment open model with first-order absorption and elimination processes without lag time. The AUC(0-infinity) and the elimination t1/2 in fasted rabbit was higher than that in post-prandial rabbit indicating the effect of food on BQ pharmacokinetics.

[Synthesis of primaquine analogues and their antimalarial activity in mice]

Ten 4-methyl-5-substituted-phenoxy-6-methoxy-8-[(1-ethyl-4-amino) butylamino] quinolines (10a-j) have been synthesized and evaluated preliminarily for both suppressive and causal prophylactic antimalarial activities. The results of preliminary screening test showed that three of these compounds exhibited significant activity against Plasmodium yoelii in mice, among which 10c was 4-8 times as effective as primaquine. Moreover, 10c was superior to primaquine in suppressive test against Plasmodium berghei K173 strain in mice. On the basis of ED50 and ED90, 10c was about 2 times as active as primaquine. Most of other compounds showed similar activity and the minority of them exhibited lower activity compared with primaquine.

Plasmodium vivax polymorphism in a clinical drug trial

Data from a double-blind randomized clinical drug trial were analyzed to find the comparative responses of two antirelapse drugs, bulaquine and primaquine, against different relapsing forms of Plasmodium vivax infection. A 1-year follow-up study strongly suggests that the duration of preerythrocytic development of P. vivax is a polymorphic characteristic, exhibited by two strains of hypnozoites responsible for early and late manifestations after primary infection. Short-term relapses were significantly higher in the first half year than long-term relapses, and the reverse was true in the second half year. Clinical drug response data showed that the hypnozoites characterized for short-term relapse were not susceptible to either of the antirelapse drugs in the currently administered dose, whereas hypnozoites characterized for long incubation were significantly susceptible.

Enhanced pneumocystis carinii activity of new primaquine analogues

New analogues of the venerable antimalarial drug primaquine have been synthesized and bioassayed in vivo against Pneumocystis carinii, a life-threatening infection common among immunosuppressed patients. Two of these new compounds are significantly more active than primaquine itself, and provide new information for future drug design and development in this area.

A simple and rapid evaluation of methemoglobin toxicity of 8-aminoquinolines and related compounds

Methemoglobin, a toxic ferric form of hemoglobin, is continuously formed in normal erythrocytes, but during abnormal situations in situ, the level is enhanced. 8-Amino-quinolines and related compounds are causative agents for methemoglobin formation. Employing oxyhemoglobin, methemoglobin toxicity was about six times higher with primaquine compared to CDRI Compound 80/53 at 10(-9) M concentration. Methemoglobin reductase activity was also completely inhibited by primaquine, whereas 24% inhibition was noted in the case of 80/53 at the same concentrations. Mastomys, a rodent animal model, was found to be equally good for comparative evaluation of methemoglobin toxicity. Further, with the use of primaquine transdermal tape on the Mastomys model, a rise in methemoglobin occurred with increase in time. In conclusion, the study presents simple, economical, less time-consuming methods for the evaluation of methemoglobin toxicity, in vitro and in vivo, without employing the conventional Beagle dog model.

Dipeptide derivatives of primaquine as transmission-blocking antimalarials: effect of aliphatic side-chain acylation on the gametocytocidal activity and on the formation of carboxyprimaquine in rat liver homogenates

PURPOSE

Dipeptide derivatives of primaquine (PQ) with reduced oxidative deamination to the inactive metabolite carboxyprimaquine were synthesized and evaluated as a novel class of transmission-blocking antimalarials. METHODS; Antimalarial activity was studied using a model consisting of mefloquine-resistant Plasmodium berghei ANKA 25R/10, Balb C mice, and Anopheles stephensi mosquitoes. Metabolic studies were performed with rat liver homogenates, and the incubates were analyzed by HPLC.

RESULTS

All dipeptide derivatives and glycyl-PQ completely inhibited the appearance of oocysts in the midguts of the mosquitoes at 15 mg/ kg, while N-acetylprimaquine was not active at this dose. However, none of the title compounds were able to block oocyst production at 3.75 mg/kg, in contrast with primaquine. Exception for sarc-gly-PQ, all remaining compounds prevented sporozoite formation in the salivary glands of mosquitoes at a dose of 3.75 mg/kg. Simultaneous hydrolysis to primaquine and gly-PQ ocurred with the following order of Vmax/Km: for primaquine formation. L-ala-gly-PQ > L-phe-gly-PQ > gly-gly-PQ; and for gly-PQ formation, L-phe-gly-PQ > L-ala-gly-PQ > gly-gly-PQ. In contrast, primaquine was not released from D-phe-gly-PQ, sarc-gly-PQ, and N-acetylprimaquine. Neither carboxyprimaquine nor 8-amino-6-methoxyquinoline were detected in any of the incubation mixtures.

CONCLUSIONS

The title compounds prevent the development of the sporogonic cycle of Plasmodium berghei. Gametocytocidal activity is independent of the rate and pathway of primaquine formation. Acylation of the aliphatic side-chain effectively prevents the formation of carboxyprimaquine, but the presence of a terminal amino group appears to be essential for the gametocytocidal activity.

A rapid and sensitive high performance liquid chromatographic assay of the new antimalarial compound 80/53 in serum with a novel sample clean-up method and its pharmacokinetics in rabbits

The compound 80/53 (AM) is a new antimalarial agent synthesized by this institute as a safer and less toxic analogue of primaquine. It was found to exhibit fluorescence in acetonitrile solution and this finding was exploited to develop a selective and sensitive high performance liquid chromatographic (HPLC) assay of the AM in rabbit serum. The sample clean-up was done in a single step by simultaneous protein precipitation and extraction with acetonitrile in the presence of sodium sulfate. The lower limit of quantitation of the method was 50 ng ml-1 using 100 microliters of serum sample. The method was fully validated from 50 to 1600 ng ml-1 concentration range with a recovery ranging from 70 to 75%. The within- and between-run variability was less than 10% and the drug in serum was stable over four freeze-thaw cycles and up to 24 h in injection solvent at 4 degrees C. The method was applied to determine the pharmacokinetic parameters of AM in 5 rabbits receiving a single bolus intravenous and peroral dose in a crossover study. The concentration-time data after a 5 mg kg-1 i.v. dose in rabbits was best fitted to the two compartment body model with first order absorption and elimination rate constants. The terminal half-life and MRT of AM were 95.3 +/- 43.5 and 104 +/- 10.6 min respectively. After administering a single 20 mg kg-1 oral dose, the serum levels of AM in all the rabbits declined below the quantitation limit by 90 min and it was not possible to fit the data by the compartmental approach. The MRT and AM after oral dose was 31.1+2-8.3 min. Application of the assay has also been extended to analyze the serum samples of rats, monkeys and humans.

Synthesis and in vitro evaluation of potential antichagasic dipeptide prodrugs of primaquine

American trypanosomiasis (Chagas' disease) is an endemic parasitic disease afflicting more than 20 million people in Latin America. Currently, therapy is unsatisfactory and only two drugs are available. Primaquine, an antimalarial drug, has trypanocidal activity. Dipeptide derivatives of primaquine, Phe-Arg-PQ, Lys-Arg-PQ, and Phe-Ala-PQ, were synthesized. The choice of the peptides was based on the primary specificity of cruzipain, the major cysteine proteinase from T cruzi. The prodrugs obtained were tested on the LLC-MK2 cell culture infected with trypomastigotes forms of T. cruzi. Phe-Arg-PQ, Lys-Arg-PQ, and Phe-Ala-PQ were active in all stages.

Primaquine analogues that are potent anti-Trypanosoma cruzi agents in a mouse model

Seventy-seven primaquine analogues were evaluated for their effectiveness in reducing the parasitaemias in female albino mice (aged 4-6 weeks) which had been infected with a Brazilian strain of Trypanosoma cruzi 15 days earlier. Of the analogues tested, 23 were more effective than the reference drug, nifurtimox, and one was > 14-fold as effective as the standard and almost four times as active as primaquine itself. Certain members of the series tested warrant further evaluation.

High-performance liquid chromatographic determination of primaquine and carboxyprimaquine concentrations in plasma and blood cells in Plasmodium vivax malaria cases following chronic dosage with primaquine

A reversed-phase HPLC method using acetonitrile-methanol-1 M perchloric acid-water (30:9:1:95, v/v) at a flow-rate of 1.5 ml/min on a mu-Bondapak C18 column with UV detection at 254 nm was developed for the separation of primaquine, its major metabolite carboxyprimaquine and other metabolites such as N-acetylprimaquine, 4-hydroxyprimaquine, 5-hydroxyprimaquine, 5-hydroxy-6-methoxyprimaquine, demethylprimaquine and 6-methoxyprimaquine, and also other antimalarials. The calibration graphs were linear in the range 0.025-100 micrograms/ml for primaquine and 4-1000 micrograms/ml for carboxyprimaquine. The within-day and day-to-day coefficients of variation averaged 3.65 and 6.95%, respectively, for primaquine and 3.0 and 7.52%, respectively for carboxyprimaquine in plasma. The extraction recoveries for primaquine and carboxyprimaquine were 89 and 83%, respectively. The mean carboxyprimaquine concentration was much higher in plasma and blood cells of Plasmodium vivax patients than that in plasma from healthy subjects. The carboxyprimaquine level was also higher in blood cells than plasma whereas the primaquine concentration was the same in both cases.

An improved pharmacodynamic model for formation of methemoglobin by antimalarial drugs

The widely used 8-aminoquinoline antimalarial group of compounds and the derivatives such as WR242511 that are being developed for possible prophylactic anticyanide applications have complex interactions with erythrocytes. Methemoglobin (MetHb) levels following the use of this drug predicted by earlier authors grossly deviated from the observed steady state levels under multiple-dose conditions. We propose a pharmacokinetic-pharmacodynamic model to characterize literature data for blood levels of MetHb generated after administration of WR242511. The model is based on an indirect mechanism involving WR242511 putative metabolite concentration, Cm on the formation of MetHb (rate constant, kr) and on depletion of reducing equivalents leading to accumulation of MetHb. Eventual depletion of MetHb is modeled as related to the disposition of both the drug metabolite and MetHb. The rate of change of MetHb concentration in the blood under the influence of a dose of WR242511 in dogs was governed by this relationship: d[MetHb]/d(t) = kr.Cm.[Hb]-kh.[MetHb], where kr is 2.9 x 10(-5) ml.ng-1.hr-1 and kh is 0.0418 hr-1. This model was validated with multiple-dose data. The model is simple and compatible with the physiological behavior of MetHb in vivo under single-dose and multiple-dose conditions of WR242511 administration.

Metabolism of a candidate 8-aminoquinoline antimalarial agent, WR 238605, by rat liver microsomes

The in vitro metabolism of the 8-aminoquinoline, 8-(4-amino-1- methylbutylamino-2,6-dimethoxy-4-methyl-5-(3-trifluromethyl- phenoxy)quinoline (WR 238605), by rat liver microsomes was studied. After incubation of WR 238605 with rat liver microsomes, the metabolites were isolated either by direct solvent extraction or by extraction in the presence of ethyl chloroformate. WR 238605 was extensively metabolized to aminophenolic compounds, which underwent air oxidation during the isolation process to a mixture of quinones and quinoneimines. Because of the instability of the metabolites toward air oxidation, most of them could only be isolated as the ethoxycarbonyl derivatives by in situ derivatization with ethyl chloroformate. The metabolism of WR 238605 involved the expected metabolic pathways, such as O-demethylation, N-dealkylation, N-oxidation, and oxidative deamination. In addition, C-hydroxylation involving the 8-aminoalkylamino side chain, which was previously unknown for 8-aminoquinoline analogs, was found to be an important metabolic pathway for WR 238605. Most of the metabolites retained the 5-(m-trifluoromethyl)phenoxy group of WR 238605. Direct and indirect supporting evidence for the structure of the metabolites of WR 238605 came from the concomitant study of the in vitro metabolism of six other compounds that are putative metabolites of WR 238605.

Side-chain hydroxylation in the metabolism of 8-aminoquinoline antiparasitic agents

Primaquine, 8-(4-amino-1-methylbutylamino)-6-methoxyquinoline, is an antimalarial 8-aminoquinoline derivative. Although it has been in use since 1952, its metabolism has not been clearly defined. This is due to the instability of the expected aminophenol metabolites and their amphoteric nature, which makes their isolation difficult. Recent studies on the metabolism of WR 238605, a new primaquine analog, has shown that these problems may be solved by extracting the metabolites in the presence of ethyl chloroformate. Subsequent identification of the ethoxycarbonyl derivatives of the metabolites has made it possible to define the in vitro metabolism of primaquine. The primary metabolic pathways of primaquine involved hydroxylation of the phenyl ring of the quinoline nucleus and C-hydroxylation of the 3'-position of the 8-aminoalkylamino side chain. Ring-hydroxylation of primaquine gives rise to 5-hydroxyprimaquine, which on demethylation produces 5-hydroxy-6-demethylprimaquine. Side-chain hydroxylation of primaquine gives rise to 3'-hydroxyprimaquine, which also undergoes O-demethylation to 3'-hydroxy-6-demethylprimaquine. 6-Demethylprimaquine, a putative metabolite of primaquine, also underwent metabolism involving 3'-hydroxylation of the side chain. WR 6026, 8-(6-diethylaminohexylamino)-6-methoxy-4-methylquinoline, is an antileishmanial 8-aminoquinoline derivative. The in vitro metabolism of WR 6026 also results in the formation of side chain-oxygenated metabolites. The present results, together with previous observations on the metabolism of WR 238605 and closely related primaquine analog, suggest that side-chain oxygenation is an important metabolic pathway of antiparasitic 8-aminoquinoline compounds in general.

Pharmacokinetics and kinetic-dynamic modeling of an 8-aminoquinoline candidate anticyanide and antimalarial drug (WR242511)

Malaria is a major cause of health problems in a large portion of the world. The 8-aminoquinoline compound, primaquine, is one of the only compounds useful for relapses of Plasmodium vivax and Plasmodium ovale malaria. Primaquine has several toxicities that include methemoglobinemia and hemolytic anemia. The induction of methemoglobinemia is a treatment for cyanide poisoning. We studied the pharmacokinetics and pharmacodynamics (percentage methemoglobin) for WR242511, an 8-aminoquinoline primaquine replacement and potential anticyanide compound. The drug's pharmacokinetics and pharmacodynamics are described for oral and intravenous dosing, and two kinetic-pharmacodynamic models are shown to describe the single dose data. A significant lag occurs between the onset of appearance of drug in the plasma and the onset of methemoglobinemia. Peak drug concentrations occurred within 4 hr for oral dosing, and peak effect (percentage methemoglobin) did not occur for 72-96 hrs for both the oral and intravenous routes. Elimination half-life for the drug was 30 +/- 14 hr. Two kinetic-dynamic models, one with an effect compartment relating drug concentration to effect and one with metabolite causing a first-order conversion of hemoglobin to methemoglobin, are compared as to their ability to predict multiple dose pharmacokinetics and pharmacodynamics. Both models were useful in predicting drug concentrations and methemoglobin levels for multiple-dose experiments.

Simultaneous determination of primaquine and carboxyprimaquine in plasma using high-performance liquid chromatography with electrochemical detection

A selective and sensitive high-performance liquid chromatographic method with electrochemical detection is described for the simultaneous quantitation of primaquine and carboxyprimaquine, its primary metabolite, in plasma. After addition of internal standard, plasma was deproteinized by addition of acetonitrile. Nitrogen-dried supernatants, resuspended in mobile phase were analyzed on a C8 reversed-phase column. Limits of detection for primaquine and carboxyprimaquine were 2 and 5 ng/ml with quantitation limits of 5 and 20 ng/ml, respectively. None of 47 tested antimicrobial agents interfered. In contrast to previously reported methods, the assay sensitivity and specificity are sufficient to permit quantitation of primaquine in plasma for pharmacokinetics following low dose (30 mg, base) oral administration of primaquine, typically used in the treatment of malaria and Pneumocystis carinii pneumonia.

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

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

[Effect of trifluoroacetoprimaquine on erythrocytic schizonts of rodent malaria]

Effect of trifluoroacetoprimaquine oxalate (M8506) and primaquine (PQ) on blood schizonts of Plasmodium berghei were determined using the method of 4-day suppressive test within extended observation period of 60 d. When mice infected with Plasmodium berghei ANKA strain were treated ig with M8506 or PQ at a same daily dose of 20 mg/kg for 4 d, the cure rates were 100% and 90%, respectively. The two drugs also showed prominent suppressive effects on chloroquine-resistant P. berghei NS line and pyronarine-resistant P. berghei RP line, but the parasitemia still remained positive or all of the mice treated recrudesced, indicating the existence of cross resistance between trifluoroacetoprimaquine and other erythrocytic schizonticides, including chloroquine and pyronaridine.

Effect of the antimalarial agents primaquine and (N'-3-acetyl-4-5-dihydro-2-furanyl)-N4-(6-methoxy-8-quinolinyl)1,4-pent ane-diamine on oxidative stress and antioxidant defences in mice

The effects of the newly developed antimalarial compound, CDRI 80/53 [(N'-3-acetyl-4-5-dihydro-2-furanyl)-N4-(6-methoxy-8-quinolinyl)1,4- pentane-diamine], and primaquine (PQ) on the antioxidant system of mice were determined at equi-effective antimalarial doses on enzyme systems responsible for protection against oxygen, i.e. hepatic superoxide dismutase and catalase. While PQ significantly inhibited these enzyme activities CDRI 80/53 did not. However, both compound 80/53 and PQ increased the level of superoxide anion and lipid peroxidation. It is concluded that compound 80/53 has less effect on antioxidant defence enzymes than PQ.

Effects of 8-aminoquinolines on the ultrastructural morphology of Pneumocystis carinii

Primaquine and other 8-aminoquinolines are effective against Pneumocystis carinii in culture and animal models but the way(s) in which they affect P. carinii are not known. This study used transmission electron microscopy to observe early effects of 8-aminoquinolines on P. carinii grown with human embryonic lung fibroblasts in microcarrier suspension culture. The 8-aminoquinolines evaluated were primaquine and Walter Reed Army Institute for Research (WR) compounds WR6026, WR238605 and WR242511. Samples of P. carinii were taken at 0, 3, 6, 12, 24 and 48 hours from culture flasks containing selected concentrations of the drugs. Time matched samples from a parallel culture without drug served as controls. All the 8-aminoquinolines produced similar morphologic alterations of the internal structure of P. carinii. Initially, dilatation of the nuclear envelopes and membranous arrays arising from the reticular system were observed. Later, more organisms displayed large arrays of smooth membranous material often presenting a concentric membranous pattern. Subsequently, cellular organization was lost resulting in necrosis. At concentrations tested WR242511 appeared to be the most effective, producing alterations in many trophozoites after 6 hours of exposure; WR6026 appeared to be the least effective with some organisms unaffected after 48 hours. The changes observed are consistent with damage to the reticular system of P. carinii, which might be caused by oxidation by the 8-aminoquinolines or their metabolites.

High-performance liquid chromatographic method for the determination of a candidate 8-aminoquinoline antimalarial drug (WR 242511) using oxidative electrochemical detection

WR 242511 (or I) is a new compound of the 8-aminoquinoline class designed to replace primaquine for the treatment of malaria. In order to perform preclinical and clinical testing, an assay was needed to determine drug levels in plasma samples. A simple and reliable reversed-phase high-performance liquid chromatographic (HPLC) method for the measurement of I in plasma using oxidative electrochemical detection is described. A 250-microliters plasma sample containing WR 256408 (or II) as internal standard was extracted with tert.-butyl methyl ether-2-propanol. A 25-microliters aliquot of the extractant was used for HPLC analysis. The mobile phase was 50:50 acetonitrile-sodium acetate (50 mM, pH 6) with 1 mM EDTA. Compounds I and II were separated within 10 min. The limit of detection for I was 10 ng/ml (plasma) with a recovery around 72%. The method was validated in a dog experiment where levels were followed for 48 h. The method is sensitive and robust and can be used for routine drug analysis during pharmacokinetic studies.

Simultaneous determination of a new antimalarial agent, CDRI compound 80/53, and its metabolite primaquine in serum by high-performance liquid chromatography

Compound 80/53 (I) is a new substance being developed as an antimalarial agent. It is unstable in acidic conditions where it is converted into primaquine. A high-performance liquid chromatographic assay for simultaneous determination in serum of I and primaquine has been developed. Conditions were optimized to minimize the conversion of I into primaquine. The method includes extraction of the unchanged compound and primaquine from serum samples with hexane-2-propanol (pH > 8). Separation was accomplished by reversed-phase chromatography on a C18 column with acetonitrile-tetrahydrofuran-phosphate buffer. The recoveries of I and primaquine were always greater than 70%. No interference was observed in extracts obtained from drug-free serum. The detector response was linear with concentrations of I and the metabolite in the ranges 25-400 and 10-180 ng/ml, respectively, and the within-day precision (coefficient of variation) remained less than 13.7% for I and 12.5% for primaquine. The method is suitable for the determination of concentration-time profiles of I and primaquine in human serum.

Interactions among primaquine, malaria infection and other antimalarials in Thai subjects

1. The pharmacokinetics of rac-primaquine (45 mg base) and its principal plasma metabolite, carboxyprimaquine have been investigated in healthy Thai adults prior to and following a single oral dose of mefloquine (10 mg kg-1). 2. Primaquine was rapidly absorbed, attaining peak plasma concentrations (median and range) of 167 (113-532) micrograms l-1 in 2 (1-4) h. Thereafter, concentrations declined rapidly with an apparent terminal half-life of 6.1 (1.7-16.1) h and an oral clearance (CLpo) of 33.1 (17.6-49.3) l h-1. Administration of mefloquine had no effect on the values of any of these parameters at the 5% level of significance [Cmax 229 (114-503) micrograms l-1; tmax 3 (2-4) h; t1/2,z 3.9 (1.7-13.5) h; CLpo 34.0 (21.7-49.0) l h-1]. 3. The carboxylic acid metabolite of primaquine achieved maximum concentrations (median and range) of 890 (553-3634) micrograms l-1 at 6 (3-16) h. Thereafter, plasma concentrations of carboxyprimaquine declined to 346 (99-918) micrograms l-1 at 24 h. AUC (0,24 h) was 12737 (6837-27388) micrograms l-1 h. Administration of mefloquine had no effect on the plasma concentrations of this metabolite [Cmax 1035 (174-3015) micrograms l-1; tmax 8 (2-24) h; AUC(0,24) 13471 (2132-17863) micrograms l-1 h]. 4. The effect of falciparum malaria and treatment with quinine (10 mg salt kg-1 p.o.) on the pharmacokinetics of primaquine (45 mg base p.o.) has been investigated in adult Thai patients during and after infection with falciparum malaria.(ABSTRACT TRUNCATED AT 250 WORDS)

High-performance liquid chromatographic determination of pamaquine, primaquine and carboxy primaquine in calf plasma using electrochemical detection

A high-performance liquid chromatographic method with electrochemical detection is described for quantification of pamaquine, primaquine and carboxy primaquine in calf plasma. After the proteins had been precipitated with acetonitrile, the drugs were separated on a 5-microns C18-modified polymer gel column with an isocratic mobile phase. The detection limit was 0.01 microgram/ml in plasma for all three compounds. The applicability of the method in pharmacokinetic studies was demonstrated by determining the plasma concentrations of the three substances in calves administered a single dose of pamaquine or primaquine.