Imidazoquines as antimalarial and antipneumocystis agents

"Seasoning" antimalarial drugs' action: chloroquine bile salts as novel triple-stage antiplasmodial hits

Malaria is one of the "big three" global infectious diseases, having caused above two hundred million cases and over half a million deaths in 2020. The continuous demand for new treatment options prioritizes the cost-effective development of new chemical entities with multi-stage antiplasmodial activity, for higher efficacy and lower propensity to elicit drug-resistant parasite strains. Following up on our long-term research towards the rescue of classical antimalarial aminoquinolines like chloroquine and primaquine, we have developed new organic salts by acid-base pairing of those drugs with natural bile acids. These antimalarial drug-derived bile salts were screened in vitro against the hepatic, blood and gametocyte stages of Plasmodium parasites, unveiling chloroquine bile salts as unprecedented triple-stage antiplasmodial hits. These findings pave a new pathway for drug rescuing, even beyond anti-malarial and other anti-infective drugs.

Disclosure of cinnamic acid/4,9-diaminoacridine conjugates as multi-stage antiplasmodial hits

4,9-diaminoacridines with reported antiplasmodial activity were coupled to different trans-cinnamic acids, delivering a new series of conjugates inspired by the covalent bitherapy concept. The new compounds were more potent than primaquine against hepatic stages of Plasmodium berghei, although this was accompanied by cytotoxic effects on Huh-7 hepatocytes. Relevantly, the conjugates displayed nanomolar activities against blood stage P. falciparum parasites, with no evidence of hemolytic effects below 100 µM. Moreover, the new compounds were at least 25-fold more potent than primaquine against P. falciparum gametocytes. Thus, the new antiplasmodial hits disclosed herein emerge as valuable templates for the development of multi-stage antiplasmodial drug candidates.

Reaction of Picolinamides with Ketones Producing a New Type of Heterocyclic Salts with an Imidazolidin-4-One Ring

Reactions of picolinamides with 1,3-propanesultone in methanol followed by the treatment with ketones led to a series of previously unknown chemical transformations, yielding first pyridinium salts (2a-f), with a protonated endocyclic nitrogen atom, and then heterocyclic salts (3a-j) containing an imidazolidin-4-one ring. The structures of intermediate and final products were determined by IR and 1H, 13C NMR spectroscopy, and X-ray study. The effects of the ketone and alcohol structures on the product yield were studied by quantum-chemical calculations. The stability of salts 3a-j towards hydrolysis and alcoholysis makes them excellent candidates for the search for new types of biologically active compounds.

New 4-(N-cinnamoylbutyl)aminoacridines as potential multi-stage antiplasmodial leads

A novel family of 4-aminoacridine derivatives was obtained by linking this heteroaromatic core to different trans-cinnamic acids. The 4-(N-cinnamoylbutyl)aminoacridines obtained exhibited in vitro activity in the low- or sub-micromolar range against (i) hepatic stages of Plasmodium berghei, (ii) erythrocytic forms of Plasmodium falciparum, and (iii) early and mature gametocytes of Plasmodium falciparum. The most active compound, having a meta-fluorocinnamoyl group linked to the acridine core, was 20- and 120-fold more potent, respectively, against the hepatic and gametocyte stages of Plasmodium infection than the reference drug, primaquine. Moreover, no cytotoxicity towards mammalian and red blood cells at the concentrations tested was observed for any of the compounds under investigation. These novel conjugates represent promising leads for the development of new multi-target antiplasmodials.

The monoterpene 1,8-cineole prevents cerebral edema in a murine model of severe malaria

1,8-Cineole is a naturally occurring compound found in essential oils of different plants and has well-known anti-inflammatory and antimicrobial activities. In the present work, we aimed to investigate its potential antimalarial effect, using the following experimental models: (1) the erythrocytic cycle of Plasmodium falciparum; (2) an adhesion assay using brain microvascular endothelial cells; and (3) an experimental cerebral malaria animal model induced by Plasmodium berghei ANKA infection in susceptible mice. Using the erythrocytic cycle of Plasmodium falciparum, we characterized the schizonticidal effect of 1,8-cineole. This compound decreased parasitemia in a dose-dependent manner with a half maximal inhibitory concentration of 1045.53 ± 63.30 μM. The inhibitory effect of 972 μM 1,8-cineole was irreversible and independent of parasitemia. Moreover, 1,8-cineole reduced the progression of intracellular development of the parasite over 2 cycles, inducing important morphological changes. Ultrastructure analysis revealed a massive loss of integrity of endomembranes and hemozoin crystals in infected erythrocytes treated with 1,8-cineole. The monoterpene reduced the adhesion index of infected erythrocytes to brain microvascular endothelial cells by 60%. Using the experimental cerebral malaria model, treatment of infected mice for 6 consecutive days with 100 mg/kg/day 1,8-cineole reduced cerebral edema with a 50% reduction in parasitemia. Our data suggest a potential antimalarial effect of 1,8-cineole with an impact on the parasite erythrocytic cycle and severe disease.

Coupling the Antimalarial Cell Penetrating Peptide TP10 to Classical Antimalarial Drugs Primaquine and Chloroquine Produces Strongly Hemolytic Conjugates

Recently, we disclosed primaquine cell penetrating peptide conjugates that were more potent than parent primaquine against liver stage Plasmodium parasites and non-toxic to hepatocytes. The same strategy was now applied to the blood-stage antimalarial chloroquine, using a wide set of peptides, including TP10, a cell penetrating peptide with intrinsic antiplasmodial activity. Chloroquine-TP10 conjugates displaying higher antiplasmodial activity than the parent TP10 peptide were identified, at the cost of an increased hemolytic activity, which was further confirmed for their primaquine analogues. Fluorescence microscopy and flow cytometry suggest that these drug-peptide conjugates strongly bind, and likely destroy, erythrocyte membranes. Taken together, the results herein reported put forward that coupling antimalarial aminoquinolines to cell penetrating peptides delivers hemolytic conjugates. Hence, despite their widely reported advantages as carriers for many different types of cargo, from small drugs to biomacromolecules, cell penetrating peptides seem unsuitable for safe intracellular delivery of antimalarial aminoquinolines due to hemolysis issues. This highlights the relevance of paying attention to hemolytic effects of cell penetrating peptide-drug conjugates.

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.

One-step synthesis of N,N'-substituted 4-imidazolidinones by an isocyanide-based pseudo-five-multicomponent reaction

A pseudo-five-multicomponent reaction involving an isocyanide, a primary amine, two molecules of formaldehyde and water is reported, which gives N,N'-substituted 4-imidazolidinones when trifluoroethanol is used as the solvent. The reaction proceeds with good yields and with a wide variety of amines and isocyanides, providing an efficient new entry to these heterocycles. A preliminary study of the reaction mechanism suggests that trifluoroethanol, although acting as the solvent, is directly involved as a reagent in the reaction pathway.

Coupling the cell-penetrating peptides transportan and transportan 10 to primaquine enhances its activity against liver-stage malaria parasites

Novel primaquine-cell penetrating peptide conjugates were synthesised and tested in vitro against liver stage Plasmodium berghei parasites. Generally, the conjugates were more active than the parent peptides and, in some cases, than the parent drug. These are unprecedented findings that may open a new route towards antimalarial drug rescuing.

Chloroquine Analogues as Leads against Pneumocystis Lung Pathogens

The impact of Pneumocystis pneumonia (PcP) on morbidity and mortality remains substantial for immunocompromised individuals, including those afflicted by HIV infection, organ transplantation, cancer, autoimmune diseases, or subject to chemotherapy or corticosteroid-based therapies. Previous work from our group has shown that repurposing antimalarial compounds for PcP holds promise for treatment of this opportunistic infection. Following our previous discovery of chloroquine analogues with dual-stage antimalarial action both in vitro and in vivo, we now report the potent action of these compounds on Pneumocystis carinii in vitro Identification of chloroquine analogues as anti-PcP leads is an unprecedented finding.

Synthesis of Polycyclic Imidazolidinones via Amine Redox-Annulation

α-Ketoamides undergo redox-annulations with cyclic secondary amines, such as 1,2,3,4-tetrahydroisoquinoline, pyrrolidine, piperidine, and morpholine. Catalytic amounts of benzoic acid significantly accelerate these transformations. This approach provides polycyclic imidazolidinone derivatives in typically good yields.

A Quinacrine Analogue Selective Against Gastric Cancer Cells: Insight from Biochemical and Biophysical Studies

One of the earliest synthetic antimalarial drugs, quinacrine, was recently reported as interesting for the treatment of acute myeloid leukemia. Inspired by this and similar findings, we evaluated a set of quinacrine analogues against gastric (MKN-28), colon (Caco-2), and breast (MFC-7) cancer cell lines and one normal human fibroblast cell line (HFF-1). All the compounds, previously developed by us as dual-stage antimalarial leads, displayed antiproliferative activity, and one of the set stood out as selective toward the gastric cancer cell line, MKN-28. Interestingly, this compound was transported across an in vitro MKN-28 model cell line in low amounts, and approximately 80 % was trapped inside those cells. Nuclear targeting of the same compound and its interactions with calf thymus DNA were assessed through combined fluorescence microscopy, spectroscopy, and calorimetry studies, which provided evidence for the compound's ability to reach the nucleus and to interact with DNA.

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.

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.

Quinolin-4(1H)-imines are potent antiplasmodial drugs targeting the liver stage of malaria

We present a novel series of quinolin-4(1H)-imines as dual-stage antiplasmodials, several-fold more active than primaquine in vitro against Plasmodium berghei liver stage. Among those, compounds 5g and 5k presented low nanomolar IC50 values. The compounds are metabolically stable and modulate several drug targets. These results emphasize the value of quinolin-4(1H)-imines as a new chemotype and their suitable properties for further drug development.

In vitro efficiency of 9-(N-cinnamoylbutyl)aminoacridines against blood- and liver-stage malaria parasites

Novel 9-aminoacridine derivatives were synthesized by linking the heteroaromatic core to different cinnamic acids through an aminobutyl chain. The test compounds demonstrated mid-nanomolar in vitro activity against erythrocytic stages of the chloroquine-resistant W2 strain of the human malaria parasite Plasmodium falciparum. Two of the most active derivatives also showed in vitro activity against liver-stage Plasmodium berghei, with activity greater than that of the reference liver-stage antimalarial primaquine. The compounds were not toxic to human hepatoma cells at concentrations up to 5 μM. Hence, 9-(N-cinnamoylbutyl)aminoacridines are a new class of leads for prevention and treatment of malaria.

Peptidomimetic and organometallic derivatives of primaquine active against Leishmania infantum

The current treatment of visceral leishmaniasis is made difficult by the low efficacy, elevated costs, low bioavailability, and high toxicity of many of the available drugs. Primaquine, an antimalarial 8-aminoquinoline, displays activity against Leishmania spp., and several of its derivatives have been developed as potential antileishmanial drugs. However, primaquine exhibits low oral bioavailability due to oxidative deamination of its aliphatic chain. We previously developed peptidomimetic and organometallic derivatives of primaquine, with higher resistance to proteolytic degradation and oxidative deamination, which presented significant activity against primaquine-sensitive pathogens such as Plasmodium or Pneumocystis. In light of these relevant findings, we decided to evaluate these compounds against both the promastigote and intramacrophagic amastigote forms of Leishmania infantum, the agent of Mediterranean visceral leishmaniasis. We found that several of these compounds had significant activity against L. infantum. One of the peptidomimetic (3c) and one of the organometallic (7a) derivatives of primaquine were active against the clinically relevant intramacrophagic amastigote form of the parasite, causing >96% reductions in the number of amastigotes per 100 macrophages at 60 and 40 μM, respectively, while being less cytotoxic for host cells than the reference drugs sitamaquine and miltefosine. Hence, compounds 3c and 7a represent new entries toward the development of new antileishmanial leads.

Cinnamic acid/chloroquinoline conjugates as potent agents against chloroquine-resistant Plasmodium falciparum

Cinnamic acid derivatives containing a 4-amino-7-chloroquinoline scaffold (blue) and substituted cinnamoyl building blocks (green) linked through an alkylamine chain (red) were found to have potent (11-59 nM) in vitro activities against erythrocytic chloroquine- resistant Plasmodium falciparum.

Novel cinnamic acid/4-aminoquinoline conjugates bearing non-proteinogenic amino acids: towards the development of potential dual action antimalarials

A series of cinnamic acid/4-aminoquinoline conjugates conceived to link, through a proper retro-enantio dipeptide, a heterocyclic core known to prevent hemozoin formation, to a trans-cinnamic acid motif capable of inhibiting enzyme catalytic Cys residues, were synthesized as potential dual-action antimalarials. The effect of amino acid configuration and the absence of the dipeptide spacer were also assessed. The replacement of the D-amino acids by their natural L counterparts led to a decrease in both anti-plasmodial and falcipain-inhibitory activity, suggesting that the former are preferable. Molecules with such spacer were active against blood-stage Plasmodium falciparum, in vitro, and hemozoin formation, implying that the dipeptide has a key role in mediating these two activities. In turn, compounds without spacer were better falcipain-2 inhibitors, likely because these compounds are smaller and have their vinyl bonds in closer vicinity to the catalytic Cys, as suggested by molecular modeling calculations. These novel conjugates constitute promising leads for the development of new antiplasmodials targeted at blood-stage malaria parasites.

Novel potent metallocenes against liver stage malaria

Novel conjugates of the antimalarial drug primaquine (compound 1) with ferrocene, named primacenes, have been synthesized and screened for their activities against blood stage and liver stage malaria in vitro and host-vector transmission in vivo. Both transmission-blocking and blood-schizontocidal activities of the parent drug were conserved only in primacenes bearing a basic aliphatic amine group. Liver stage activity did not require this structural feature, and all metallocenes tested were comparable to or better than primaquine in this regard. Remarkably, the replacement of primaquine's aliphatic chain by hexylferrocene, as in compound 7, led to a ~45-fold-higher level activity against liver stage parasitemia than that of primaquine.

A toolbox to study liver stage malaria

The first obligatory phase of mammalian infection by Plasmodium parasites, the causative agents of malaria, occurs in the liver of the host. This stage of Plasmodium infection bears enormous potential for anti-malarial intervention. Recent technological progress has strongly contributed to overcoming some of the long-standing difficulties in experimentally assessing hepatic infection by Plasmodium. Here, we review appropriate infection models and infection assessment tools, and provide a comprehensive description of recent advances in experimental strategies to investigate the liver stage of malaria. These issues are discussed in the context of current challenges in the field to provide researchers with the technical tools that enable effective experimental approaches to study liver stage malaria.