Antimalarial activity of WR 243251, a Dihydroacridinedione

Role of Trifluoromethyl Substitution in Design of Antimalarial Quinolones: a Comprehensive Review

Malaria represents a significant health issue, and novel effective drugs are needed to address parasite resistance that has emerged to the current drug arsenal. The most popular antimalarial drugs are focused on the 7-chloro-4-aminoquinoline [e.g., chloroquine (CQ), amodiaquine (AQ), isoquine (IQ), and tebuquine (TBQ)], artemisinin, and atovaquone systems. Recently, endochin has been used as a platform to design a variety of novel potent and safe antimalarial agents named endochin-like quinolones (ELQs). Also, antimalarial quinolones have been constructed from other quinolones drugs such as ICI-56780 and floxacrine. Trifluoromethyl substitution has provided a significant increase in the antimalarial response of many of the designed ELQs against Plasmodium-resistant strains and for in vivo models. In particular, attachment of a substituted trifluoromethoxy (or trifluoromethyl in some cases) biaryl side chain at 2-, 3-, 4-, or 6-position of the quinolone core has shown to be crucially important to generate selective and potent novel ELQs. Furthermore, 6-chloro and 7-methoxy moieties on the quinolone core have been identified as essential pharmacophores when the trifluoromethoxy biaryl side chain is placed at 2- or 3-position of the quinolone core. Methyl or ethyl ester attached at 3-position is essential when the trifluoromethoxy aryl side chain is attached at 6- or 7-position of the quinolone core. Some promising ELQs are currently under clinical trials, representing an excellent platform for the design of new potent, selective, effective, and safe antimalarial drugs against emergent resistance malarial models.

Antiplasmodial and antimalarial activities of quinolone derivatives: An overview

Malaria remains one of the most deadly infectious diseases globally. Considering the growing spread of resistance, development of new and effective antimalarials remains an urgent priority. Quinolones, which are emerged as one of the most important class of antibiotics in the treatment of various bacterial infections, showed potential in vitro antiplasmodial and in vivo antimalarial activities, making them promising candidates for the chemoprophylaxis and treatment of malaria. This review presents the current progresses and applications of quinolone-based derivatives as potential antimalarials to pave the way for the development of new antimalarials.

Novel inhibitors of the Plasmodium falciparum electron transport chain

Due to an increased need for new antimalarial chemotherapies that show potency against Plasmodium falciparum, researchers are targeting new processes within the parasite in an effort to circumvent or delay the onset of drug resistance. One such promising area for antimalarial drug development has been the parasite mitochondrial electron transport chain (ETC). Efforts have been focused on targeting key processes along the parasite ETC specifically the dihydroorotate dehydrogenase (DHOD) enzyme, the cytochrome bc 1 enzyme and the NADH type II oxidoreductase (PfNDH2) pathway. This review summarizes the most recent efforts in antimalarial drug development reported in the literature and describes the evolution of these compounds.

Structural optimization of quinolon-4(1H)-imines as dual-stage antimalarials: toward increased potency and metabolic stability

Discovery of novel effective and safe antimalarials has been traditionally focused on targeting erythrocytic parasite stages that cause clinical symptoms. However, elimination of malaria parasites from the human population will be facilitated by intervention at different life-cycle stages of the parasite, including the obligatory developmental phase in the liver, which precedes the erythrocytic stage. We have previously reported that N-Mannich-based quinolon-4(1H)-imines are potent antiplasmodial agents but present several stability liabilities. We now report our efforts to optimize quinolon-4(1H)-imines as dual-stage antiplasmodial agents endowed with chemical and metabolic stability. We report compounds active against both the erythrocytic and exoerythrocytic forms of malaria parasites, such as the quinolon-4(1H)-imine 5p (IC50 values of 54 and 710 nM against the erythrocytic and exoerythrocytic forms), which constitute excellent starting points for further lead optimization as dual-stage antimalarials.

Lead optimization of 3-carboxyl-4(1H)-quinolones to deliver orally bioavailable antimalarials

Malaria is a protozoal parasitic disease that is widespread in tropical and subtropical regions of Africa, Asia, and the Americas and causes more than 800,000 deaths per year. The continuing emergence of multidrug-resistant Plasmodium falciparum drives the ongoing need for the development of new and effective antimalarial drugs. Our previous work has explored the preliminary structural optimization of 4(1H)-quinolone ester derivatives, a new series of antimalarials related to the endochins. Herein, we report the lead optimization of 4(1H)-quinolones with a focus on improving both antimalarial potency and bioavailability. These studies led to the development of orally efficacious antimalarials including quinolone analogue 20g, a promising candidate for further optimization.

Febrifugine analogue compounds: synthesis and antimalarial evaluation

Febrifugine is an alkaloid isolated from Dichroa febrifuga Lour as the active component against Plasmodium falciparum, but exhibits toxic side effects. In this study novel febrifugine analogues were designed and efficiently synthesized. New compounds underwent efficacy and toxicity evaluation. Some compounds are much less toxic than the natural product febrifugine and existing antimalarial drugs and are expected to possess wide therapeutic windows. In Aotus monkeys infected with the chloroquine resistant FVO strain of P. falciparum, one interesting compound possesses a 50% curative dose of 2mg/kg/day and a 100% curative dose of 8 mg/kg/day. These compounds, as well as the underlying design rationale, may find usefulness in the discovery and development of new antimalarial drugs.

Optimization of 1,2,3,4-tetrahydroacridin-9(10H)-ones as antimalarials utilizing structure-activity and structure-property relationships

Antimalarial activity of 1,2,3,4-tetrahydroacridin-9(10H)-ones (THAs) has been known since the 1940s and has garnered more attention with the development of the acridinedione floxacrine (1) in the 1970s and analogues thereof such as WR 243251 (2a) in the 1990s. These compounds failed just prior to clinical development because of suboptimal activity, poor solubility, and rapid induction of parasite resistance. Moreover, detailed structure-activity relationship (SAR) studies of the THA core scaffold were lacking and SPR studies were nonexistent. To improve upon initial findings, several series of 1,2,3,4-tetrahydroacridin-9(10H)-ones were synthesized and tested in a systematic fashion, examining each compound for antimalarial activity, solubility, and permeability. Furthermore, a select set of compounds was chosen for microsomal stability testing to identify physicochemical liabilities of the THA scaffold. Several potent compounds (EC(50) < 100 nM) were identified to be active against the clinically relevant isolates W2 and TM90-C2B while possessing good physicochemical properties and little to no cross-resistance.

Synthesis and structure-activity relationships of antimalarial 4-oxo-3-carboxyl quinolones

Malaria is endemic in tropical and subtropical regions of Africa, Asia, and the Americas. The increasing prevalence of multi-drug-resistant Plasmodium falciparum drives the ongoing need for the development of new antimalarial drugs. In this light, novel scaffolds to which the parasite has not been exposed are of particular interest. Recently, workers at the Swiss Tropical Institute discovered two novel 4-oxo-3-carboxyl quinolones active against the intra-erythrocytic stages of P. falciparum while carrying out rationally directed low-throughput screening of potential antimalarial agents as part of an effort directed by the World Health Organization. Here we report the design, synthesis, and preliminary pharmacologic characterization of a series of analogues of 4-oxo-3-carboxyl quinolones. These studies indicate that the series has good potential for preclinical development.

Evaluation and lead optimization of anti-malarial acridones

With 2-methoxy-6-chloroacridone as a lead compound, we synthesized and tested acridone derivatives to develop a better understanding of the anti-malarial structure-activity relationships. Over 30 acridone derivatives were synthesized. The most potent compounds contained extended alkyl chains terminated by trifluoromethyl groups and located at the 3-position of the tricyclic system. Acridones optimized in the length of the side chain and the nature of the terminal fluorinated moiety exhibited in vitro anti-malarial IC(50) values in the low nanomolar and picomolar range and were without cytotoxic effects on the proliferation and differentiation of human bone marrow progenitors or mitogen-activated murine lymphocytes at concentrations up to 100,000-fold higher. Based on a structural similarity to known anti-malarial agents it is proposed that the haloalkoxyacridones exert their anti-malarial effects through inhibition of the Plasmodium cytochrome bc(1) complex. Haloalkoxyacridones represent an extraordinarily potent novel class of chemical compounds with the potential for development as therapeutic agents to treat or prevent malaria in humans.

Severe anemia affects both splenectomized and non-splenectomized Plasmodium falciparum-infected Aotus infulatus monkeys

Severe anemia is the earliest and a frequently fatal complication of Plasmodium falciparum infection. Here we describe Aotus infulatus as a primate model suitable to study this malaria complication. Both non-splenectomized and splenectomized monkeys receiving different inocula of P. falciparum FVO strain presented large (> 50%) decreases in hematocrit values during infection. Non-splenectomized animals were able to control parasite growth (parasitemia did not exceed 4%), but they had to be treated because of severe anemia. Three of 4 splenectomized monkeys did not control parasitemia and were treated, but developed severe anemia after treatment when presenting a negative blood film. Destruction of parasitized red blood cells alone cannot account for the degree of anemia. Non-splenectomized monkeys repeatedly infected with homologous parasites became rapidly and progressively resistant to reinfection and to the development of severe anemia. The data presented here point to A. infulatus as a suitable model for studying the pathogenesis of severe malarial infection.

Plasmodium falciparum: the effects of atovaquone resistance on respiration

Atovaquone is an antimalarial agent that specifically inhibits the cytochrome bc(1) complex of the cytochrome pathway. High-level atovaquone resistance is associated with a point mutation in the cytochrome b gene. A pair of isogenic clinical isolates of Plasmodium falciparum derived from before and after the acquisition of atovaquone resistance was used to determine whether the change in the cytochrome b gene resulted in changes in respiration in response to atovaquone. Since P. falciparum appears to utilize a branched respiratory system comprising both the cytochrome and an alternative respiratory pathway, the proportion of each pathway utilized by the sensitive and resistant parasites was investigated. Atovaquone inhibited total parasite oxygen consumption by up to 66% in the sensitive isolate but only up to 28% in the resistant isolate. Both the atovaquone-sensitive and the atovaquone-resistant parasites were comparably sensitive to the alternative pathway inhibitor, salicylhydroxamic acid. Atovaquone appeared to partially inhibit the rate of oxygen consumed through the alternative pathway in only the atovaquone-sensitive isolate. Cross resistance was noted between atovaquone and a new antimalarial agent WR243251. However, the level of WR243251 resistance was very modest compared to the level of atovaquone resistance. WR243251 was shown to rapidly reduce the rate of parasite oxygen consumption by almost 80% in the atovaquone-sensitive isolate and by 57% in the atovaquone-resistant isolate. Drug interaction studies suggest that atovaquone and WR243251 may inhibit growth additively or with mild synergy. Together, these results suggest that while WR243251 may inhibit respiration, its target of action probably differs from that of atovaquone.