Primaquine derivatives: Modifications of the terminal amino group

Numerous modifications of the well-known antimalarial drug primaquine, both at the quinoline ring and at the primary amino group, have been reported, mostly to obtain antimalarial agents with improved bioavailability, reduced toxicity and/or prolonged activity. Modifications of the terminal amino group were made with the main idea to prevent the metabolic pathway leading to inactive and toxic carboxyprimaquine (follow-on strategy), but also to get compounds with different activity (repurposing strategy). The modifications undertaken until 2009 were included in a review published in the same year. The present review covers various classes of primaquine N-derivatives with diverse biological profiles, prepared in the last decade by our research group as well as the others. We have summarized the synthetic procedures applied for their preparation and discussed the main biological results. Several hits for the development of novel antiplasmodial, anticancer, antimycobacterial and antibiofilm agents were identified.

A review on diverse heterocyclic compounds as the privileged scaffolds in antimalarial drug discovery

The upward extend of malaria collectively with the emergence of resistance against predictable drugs has put enormous pressure on public health systems to introduce new malaria treatments. Heterocycles play an important role in the design and discovery of new malaria active compounds. Heterocyclic compounds have attracted significant attention for malaria treatment because of simplicity of parallelization and the examining power with regard to chemical space. Introduction of a variety of heterocyclic compounds have enabled to maintain the high levels of antimalarial potency observed for other more lipophilic analogues whilst improving the solubility and the oral bioavailability in pre-clinical species. In this review, we present an overview of recent literature to provide imminent into the applications of different heterocyclic scaffolds in fighting against malaria.

Synthesis and Biological Evaluation of 8-Quinolinamines and Their Amino Acid Conjugates as Broad-Spectrum Anti-infectives

In the search of therapeutic agents for emerging drug-resistant parasites, the synthesis of newer classes of 8-quinolinamines has emerged as a successful chemotherapeutic approach. We report synthesis of 8-quinolinamines bearing 5-alkoxy, 4-methyl, and 2-tert-butyl groups in the quinoline framework and their amino acid conjugates as broad-spectrum anti-infectives. 8-Quinolinamines exhibited potent in vitro antimalarial activity [IC50 = 20-4760 ng/mL (drug-sensitive Plasmodium falciparum D6 strain) and IC50 = 22-4760 ng/mL (drug-resistant P. falciparum W2 strain)]. The most promising analogues have cured all animals at 25 mg/kg/day against drug-sensitive Plasmodium berghei and at 50 mg/kg/day against multidrug-resistant Plasmodium yoelii nigeriensis infections in Swiss mice. The in vitro antileishmanial activities (IC50 = 0.84-5.0 μg/mL and IC90 = 1.95-7.0 μg/mL) comparable to standard drug pentamidine were exhibited by several of the synthesized 8-quinolinamines. At the same time, very promising antifungal activities (Candida albicans-IC50 = 4.93-19.38 μg/mL; Candida glabrata-IC50 = 3.96-19.22 μg/mL; Candida krusei-IC50 = 2.89-18.95 μg/mL; Cryptococcus neoformans-IC50 = 0.67-18.64 μg/mL; and Aspergillus fumigatus-IC50 = 6.0-19.32 μg/mL) and antibacterial activities (Staphylococcus aureus-IC50 = 1.33-18.9 μg/mL; methicillin-resistant S. aureus-IC50 = 1.38-15.34 μg/mL; and Mycobacterium intracellulare-IC50 = 3.12-20 μg/mL) were also observed. None of the 8-quinolinamines exhibited cytotoxicity and therefore are a promising structural class of compounds as antiparasitic and antimicrobials.

Metal-free synthesis of N-fused heterocyclic iodides via C-H functionalization mediated by tert-butylhydroperoxide

Direct, regioselective and metal-free synthesis of fused N-heterocyclic iodides is reported. This regioselective C-H functionalization is mediated by tert-butylhydroperoxide (TBHP), via dual activation of molecular iodine and a heterocyclic substrate, resulting in the in situ generation of electrophilic iodine species (I(+)), and free radical(s) (t)BuO˙ or (t)BuOO˙, driving the iodination reaction.

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.

Quinine conjugates and quinine analogues as potential antimalarial agents

Malaria is a tropical disease, prevalent in Southeast Asia and Africa, resulting in over half a million deaths annually; efforts to develop new antimalarial agents are therefore particularly important. Quinine continues to play a role in the fight against malaria, but quinoline derivatives are more widely used. Drugs based on the quinoline scaffold include chloroquine and primaquine, which are able to act against the blood and liver stages of the parasite's life cycle. The purpose of this review is to discuss reported biologically active compounds based on either the quinine or quinoline scaffold that may have enhanced antimalarial activity. The review emphasises hybrid molecules, and covers advances made in the last five years. The review is divided into three sections: modifications to the quinine scaffold, modifications to aminoquinolines and finally metal-containing antimalarial compounds.

Design and Synthesis of Novel Hybrid Molecules against Malaria

The effective treatment of malaria can be very complex: Plasmodium parasites develop in multiple stages within a complex life cycle between mosquitoes as vectors and vertebrates as hosts. For the full and effective elimination of parasites, an effective drug should be active against the earliest stages of the Plasmodium infection: liver stages (reduce the progress of the infection), blood stages (cure the clinical symptoms), and gametocytes (inhibit the transmission cycle). Towards this goal, here we report the design, the synthetic methodology, and the characterization of novel hybrid agents with combined activity against Plasmodium liver stages and blood stages and gametocytes. The divergent synthetic approach allows the access to differently linked primaquine-chloroquine hybrid templates in up to eight steps.

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

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

Effect of antimalarial drug primaquine and its derivatives on the ionization potential of hemoglobin: A QM/MM study

We used quantum mechanics/molecular mechanics calculations to test if antimalarial primaquine (PQ) and its derivatives aid the conversion of hemoglobin to methemoglobin by binding to hemoglobin and merely lowering hemoglobin's ionization potential (IP). Our results showed that PQ and its derivatives do not significantly lower the hemoglobin IP, disproving the hypothesis.