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

An optimised age-based dosing regimen for single low-dose primaquine for blocking malaria transmission in Cambodia

BACKGROUND

In 2012, the World Health Organization recommended the addition of single low-dose primaquine (SLDPQ, 0.25 mg base/kg body weight) to artemisinin combination therapies to block the transmission of Plasmodium falciparum without testing for glucose-6-phosphate dehydrogenase deficiency. The targeted group was non-pregnant patients aged ≥ 1 year (later changed to ≥ 6 months) with acute uncomplicated falciparum malaria, primarily in countries with artemisinin-resistant P. falciparum (ARPf). No dosing regimen was suggested, leaving malaria control programmes and clinicians in limbo. Therefore, we designed a user-friendly, age-based SLDPQ regimen for Cambodia, the country most affected by ARPf.

METHODS

By reviewing primaquine's pharmacology, we defined a therapeutic dose range of 0.15-0.38 mg base/kg (9-22.5 mg in a 60-kg adult) for a therapeutic index of 2.5. Primaquine doses (1-20 mg) were tested using a modelled, anthropometric database of 28,138 Cambodian individuals (22,772 healthy, 4119 with malaria and 1247 with other infections); age distributions were: 0.5-4 years (20.0 %, n = 5640), 5-12 years (9.1 %, n = 2559), 13-17 years (9.1 %, n = 2550), and ≥ 18 years (61.8 %, n = 17,389). Optimal age-dosing groups were selected according to calculated mg base/kg doses and proportions of individuals receiving a therapeutic dose.

RESULTS

Four age-dosing bands were defined: (1) 0.5-4 years, (2) 5-9 years, (3) 10-14 years, and (4) ≥15 years to receive 2.5, 5, 7.5, and 15 mg of primaquine base, resulting in therapeutic doses in 97.4 % (5494/5640), 90.5 % (1511/1669), 97.7 % (1473/1508), and 95.7 % (18,489/19,321) of individuals, respectively. Corresponding median (1st-99th centiles) mg base/kg doses of primaquine were (1) 0.23 (0.15-0.38), (2) 0.29 (0.18-0.45), (3) 0.27 (0.15-0.39), and (4) 0.29 (0.20-0.42).

CONCLUSIONS

This age-based SLDPQ regimen could contribute substantially to malaria elimination and requires urgent evaluation in Cambodia and other countries with similar anthropometric characteristics. It guides primaquine manufacturers on suitable tablet strengths and doses for paediatric-friendly formulations. Development of similar age-based dosing recommendations for Africa is needed.

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

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

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

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

Enantioselective metabolism of primaquine by human CYP2D6

Background

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

Methods

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

Results

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

Conclusion

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

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

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