Antimetabolites of coenzyme Q. Their potential application as antimalarials

Prenylquinones in Human Parasitic Protozoa: Biosynthesis, Physiological Functions, and Potential as Chemotherapeutic Targets

Human parasitic protozoa cause a large number of diseases worldwide and, for some of these diseases, there are no effective treatments to date, and drug resistance has been observed. For these reasons, the discovery of new etiological treatments is necessary. In this sense, parasitic metabolic pathways that are absent in vertebrate hosts would be interesting research candidates for the identification of new drug targets. Most likely due to the protozoa variability, uncertain phylogenetic origin, endosymbiotic events, and evolutionary pressure for adaptation to adverse environments, a surprising variety of prenylquinones can be found within these organisms. These compounds are involved in essential metabolic reactions in organisms, for example, prevention of lipoperoxidation, participation in the mitochondrial respiratory chain or as enzymatic cofactors. This review will describe several prenylquinones that have been previously characterized in human pathogenic protozoa. Among all existing prenylquinones, this review is focused on ubiquinone, menaquinone, tocopherols, chlorobiumquinone, and thermoplasmaquinone. This review will also discuss the biosynthesis of prenylquinones, starting from the isoprenic side chains to the aromatic head group precursors. The isoprenic side chain biosynthesis maybe come from mevalonate or non-mevalonate pathways as well as leucine dependent pathways for isoprenoid biosynthesis. Finally, the isoprenic chains elongation and prenylquinone aromatic precursors origins from amino acid degradation or the shikimate pathway is reviewed. The phylogenetic distribution and what is known about the biological functions of these compounds among species will be described, as will the therapeutic strategies associated with prenylquinone metabolism in protozoan parasites.

Terpenoids with Special Pharmacological Significance: A Review

Terpenoids are a very prominent class of natural compounds produced in diverse genera of plants, fungi, algae and sponges. They gained significant pharmaceutical value since prehistoric times, due to their broad spectrum of medical applications. The fragrant leaves of Eucalyptus trees are a rich source of terpenoids. Therefore this review starts by summarizing the main terpenoid compounds present in Eucalyptus globulus, E. citriodora, E. radiata and E. resinifera and describing their biosynthetic pathways. Of the enormous number of pharmaceutically important terpenoids, this paper also reviews some well established and recently discovered examples and discusses their medical applications. In this context, the synthetic processes for (–)-menthol, (–)- cis-carveol, (+)-artemisinine, (+)-merrilactone A and (–)-sclareol are presented. The tricyclic sesquiterpene (–)-englerin A isolated from the stem bark of the Phyllanthus engleri plant ( Euphorbiaceae) is highly active against certain renal cancer cell lines. In addition, recent studies showed that englerin A is also a potent and selective activator of TRPC4 and TRPC5 calcium channels. These important findings were the motivation for several renowned research labs to achieve a total synthesis of (–)-englerin A. Two prominent examples – Christmann and Metz – are compared and discussed in detail.

1,4-naphthoquinone cations as antiplasmodial agents: hydroxy-, acyloxy-, and alkoxy-substituted analogues

Cations of hydroxy-substituted 1,4-naphthoquinones were synthesized and evaluated as antiplasmodial agents against Plasmodium falciparum. The atovaquone analogues were found to be inactive as antagonists of parasite growth, which was attributed to ionization of the acidic hydroxyl moiety. Upon modification to an alkoxy substituent, the antiplasmodial activity was restored in the sub-100 nM range. Optimal inhibitors were found to possess IC50 values of 17.4-49.5 nM against heteroresistant P. falciparum W2.

Phosphonium lipocations as antiparasitic agents

Phosphonium lipocations were synthesized and evaluated for inhibition of the development of Plasmodium falciparum and Trypanosoma cruzi, etiological agents of malaria and Chagas disease, respectively. Optimal phthalimides and 1,4-naphthoquinone-based lipocations were active in vitro at mid-high nM concentrations against P. falciparum and low μM concentrations against T. cruzi.

Stereochemistry of Products of Reactions between 3-diazo-naphthalene-1,2,4-trione and β-dicarbonyl Compounds. Structure of ethyl 2-[(3-hydroxy-1,4-dioxo-1,4-dihydro-naphthalen-2-yl)-hydrazono]-3-phenyl-3-oxo-propionate

3-Hydroxy-2-[(R1CO)(R2CO)]C=NNH-1,4-naphthoquinones, obtained from reactions of 3-diazonaphthalene-1,2,4-trione with β-diketones, R1C(O)CH2COR2, have been previously found to have high antibacterial activity. However, confirmation of the stereochemistry about the C=N bond could not be achieved by spectroscopic means for products having different R1and R2groups, thereby limiting the utility of the reaction. Full characterisation of the product isolated from reaction of 3-diazonaphthalene-1,2,4-trione with PhC(O)CH2CO2Et is now reported, from a single crystal X-ray structure determination: the product, 3-hydroxy-2-[(PhCO)(EtCO2)]C=NNH-1,4-naphthoquinone has a ( Z)-stereochemistry. The Z-isomer is obtained rather than the E form due to the preferred formation of the stronger intramolecular N–H—O hydrogen-bond with the ester carbonyl oxygen rather than a weaker one with the ketone oxygen. Weaker C–H—O hydrogen bonds link the molecules into columns. It is suggested that similar Z geometries will arise from other RC(O)CH2CO2Rireactants.

Malaria Chemotherapeutics Part I: History of Antimalarial Drug Development, Currently Used Therapeutics, and Drugs in Clinical Development

Since ancient times, humankind has had to struggle against the persistent onslaught of pathogenic microorganisms. Nowadays, malaria is still the most important infectious disease worldwide. Considerable success in gaining control over malaria was achieved in the 1950s and 60s through landscaping measures, vector control with the insecticide DDT, and the widespread administration of chloroquine, the most important antimalarial agent ever. In the late 1960s, the final victory over malaria was believed to be within reach. However, the parasites could not be eradicated because they developed resistance against the most widely used and affordable drugs of that time. Today, cases of malaria infections are on the rise and have reached record numbers. This review gives a short description of the malaria disease, briefly addresses the history of antimalarial drug development, and focuses on drugs currently available for malaria therapy. The present knowledge regarding their mode of action and the mechanisms of resistance are explained, as are the attempts made by numerous research groups to overcome the resistance problem within classes of existing drugs and in some novel classes. Finally, this review covers all classes of antimalarials for which at least one drug candidate is in clinical development. Antimalarial agents that are solely in early development stages will be addressed in a separate review.

Coenzyme Q homologs in parasitic protozoa as targets for chemotherapeutic attack

The central role of coenzyme Q (ubiquinone) in cellular energy metabolism is well established. Recent work has implicated this molecule in a wide range of other cellular functions, including roles in growth control, plasma membrane oxidase and as a cellular antioxidant. In this review, Jayne Ellis presents an overview of the current knowledge of this important cellular component in species of parasitic protozoa, discusses current therapies using its analogs and proposes its potential roles in these organisms.

Site of action of the antimalarial hydroxynaphthoquinone, 2-[trans-4-(4'-chlorophenyl) cyclohexyl]-3-hydroxy-1,4-naphthoquinone (566C80)

The site of action of the antimalarial compound 2-[trans-4-(4'-chlorophenyl) cyclohexyl]-3-hydroxy-1,4-naphthoquinone (566C80), would appear to be the mitochondrial respiratory chain. Studies reported herein have demonstrated 566C80 to be a potent and selective mitochondrial inhibitor with mitochondria isolated from Plasmodium falciparum and P. yoelii. Selective assay of individual respiratory chain complexes has shown the primary site of action of 566C80 to be the cytochrome bc1 complex (Complex III): supportive evidence from difference spectroscopy indicates the site of inhibition to lie between cytochromes b and c1 of this complex. Using [14C]566C80, evidence is presented which suggests that 566C80 may become irreversibly bound to a polypeptide with an approximate molecular mass of 11,500 Da.

Human spleen dihydroorotate dehydrogenase: properties and partial purification

Human spleen dihydroorotate dehydrogenase is associated with the mitochondrial membrane and is linked to the respiratory chain via ubiquinone. The enzyme activity was unaffected by pyridine nucleotides. The product of the reaction, orotate, was a potent inhibitor. However, a range of other naturally occurring pyrimidines or purines had no significant effect on the activity. No evidence for the involvement of a complexed metal ion or for an active sulfhydryl group was obtained. Purification of the enzyme was achieved by preparation of an acetone powder and extraction with Triton X-100, followed by preparative polyacrylamide gel electrophoresis. Activity was observed by the addition of the artificial electron acceptors, ubiquinone 50 or PMS. Purification resulted in alteration of the pH optimum and of other kinetic characteristics. Two molecular-weight species, of molecular weight 88,000 and 98,000, were consistently observed. The properties of the human spleen enzyme were similar in principle to those for the rat liver enzyme. Differences in the mode of linkage to the respiratory chain for the mitochondrially bound enzyme, and in the characteristics of the purified enzyme, were observed.

Novel anti-malarial hydroxynaphthoquinones with potent broad spectrum anti-protozoal activity

Novel hydroxynaphthoquinones are reported with outstanding efficacy against Plasmodium, Eimeria and Theileria species. Biochemical evidence is presented for the selective toxicity of these compounds being due to inhibition of parasite respiratory systems.

Potent and selective hydroxynaphthoquinone inhibitors of mitochondrial electron transport in Eimeria tenella (Apicomplexa: Coccidia)

Novel hydroxynaphthoquinones have been shown to be potent and selective inhibitors of mitochondrial electron transport in the protozoan Eimeria tenella, inhibiting at concentrations of 10(-10) to 10(-11)M. The primary site of electron transport inhibition has been localized to the ubiquinol-cytochrome c reductase span of the respiratory chain, whereas a secondary site of inhibition occurs in the NADH- and succinate-ubiquinone reductase complexes. Inhibition at the primary site is selective for the E. tenella enzyme; inhibition at the secondary sites is comparable in both E. tenella and chick (Gallus gallus) liver mitochondria. Hydroxynaphthoquinone inhibition of chick liver succinate-cyto-chrome c reductase was fully reversible by addition of the exogenous ubiquinone-2 analogue, 6-decyl-2,3-dimethoxy-5-methyl-1,4-benzoquinone; inhibition of the corresponding E. tenella enzyme was not reversed by this ubiquinone. E. tenella lines made resistant to the anticoccidial agents decoquinate or clopidol showed no cross-resistance to the hydroxynaphthoquinones, either at the level of electron transport or in vivo.