Leishmania donovani, Plasmodium berghei, Trypanosoma rhodesiense: antiprotozoal effects of some amidine types

Synthesis of N-substituted aryl amidines by strong base activation of amines

We describe an efficient method for the direct preparation of N-substituted aryl amidines from nitriles and primary amines. The protocol employs activation of amines by a strong base and provides greater access to a pharmaceutically relevant functional group. This synthetic approach tolerates deactivated nitriles, nitriles with competing substitution sites, and aryl amines.

CYP1A1 and CYP1B1-mediated biotransformation of the antitrypanosomal methamidoxime prodrug DB844 forms novel metabolites through intramolecular rearrangement

DB844 (CPD-594-12), N-methoxy-6-{5-[4-(N-methoxyamidino)phenyl]-furan-2-yl}-nicotinamidine, is an oral prodrug that has shown promising efficacy in both mouse and monkey models of second stage human African trypanosomiasis. However, gastrointestinal (GI) toxicity was observed with high doses in a vervet monkey safety study. In the current study, we compared the metabolism of DB844 by hepatic and extrahepatic cytochrome P450s to determine whether differences in metabolite formation underlie the observed GI toxicity. DB844 undergoes sequential O-demethylation and N-dehydroxylation in the liver to form the active compound DB820 (CPD-593-12). However, extrahepatic CYP1A1 and CYP1B1 produced two new metabolites, MX and MY. Accurate mass and collision-induced dissociation mass spectrometry analyses of the metabolites supported proposed structures of MX and MY. In addition, MY was confirmed with a synthetic standard and detection of nitric oxide (NO) release when DB844 was incubated with CYP1A1. Taken altogether, we propose that MX is formed by insertion of oxygen into the amidine CN to form an oxaziridine, which is followed by intramolecular rearrangement of the adjacent O-methyl group and subsequent release of NO. The resulting imine ester, MX, is further hydrolyzed to form MY. These findings may contribute to furthering the understanding of toxicities associated with benzamidoxime- and benzmethamidoxime-containing molecules.

Novel amidines and analogues as promising agents against intracellular parasites: a systematic review

Parasitic protozoa comprise diverse aetiological agents responsible for important diseases in humans and animals including sleeping sickness, Chagas disease, leishmaniasis, malaria, toxoplasmosis and others. They are major causes of mortality and morbidity in tropical and subtropical countries, and are also responsible for important economic losses. However, up to now, for most of these parasitic diseases, effective vaccines are lacking and the approved chemotherapeutic compounds present high toxicity, increasing resistance, limited efficacy and require long periods of treatment. Many of these parasitic illnesses predominantly affect low-income populations of developing countries for which new pharmaceutical alternatives are urgently needed. Thus, very low research funding is available. Amidine-containing compounds such as pentamidine are DNA minor groove binders with a broad spectrum of activities against human and veterinary pathogens. Due to their promising microbicidal activity but their rather poor bioavailability and high toxicity, many analogues and derivatives, including pro-drugs, have been synthesized and screened in vitro and in vivo in order to improve their selectivity and pharmacological properties. This review summarizes the knowledge on amidines and analogues with respect to their synthesis, pharmacological profile, mechanistic and biological effects upon a range of intracellular protozoan parasites. The bulk of these data may contribute to the future design and structure optimization of new aromatic dicationic compounds as novel antiparasitic drug candidates.

Recent initiatives and strategies to developing new drugs for tropical parasitic diseases

BACKGROUND

Despite the toll of tropical parasitic diseases on human life in the developing world, present therapies still rely on drugs developed decades ago. In many cases, the clinical usefulness of these compounds is limited due to poor efficacy, toxicity and the constant attrition of drug resistance. The absence of a profit incentive regarding diseases afflicting the very poor has resulted in a lack of investment by the pharmaceutical industry in new chemotherapies.

OBJECTIVE

Given this background, this review addresses what alternative economic and scientific strategies have been implemented to procure novel drugs.

METHODS

The latest chemical, genetic and screening technologies to discover and develop drugs for tropical parasitic diseases are reviewed. In many cases these strategies are being implemented within the framework of public-private partnerships established to sustain dynamic drug development portfolios. Examples of public-private partnerships and their portfolios are discussed. Further, the contribution of dedicated academic screening centres to target discovery and preclinical prosecution of new small molecules is also highlighted. In every case, the latest scientific literature is cited, but also relevant press releases and website information to indicate the present vitality in the field.

CONCLUSION

The tools, institutions and consortia are now in place and evolving to deliver new pharmaceuticals. Short-term results have already been realised in the clinic, mainly through the provision of new formulations of existing drugs. Long-term and consistent investment will be required, however, to identify, develop and clinically validate new chemical entities.

In situ potentiometric method to evaluate bacterial outer membrane-permeabilizing ability of drugs: example using antiprotozoal diamidines

We introduced a new assay system, combining tyrocidine A and a K(+)-selective electrode, to evaluate the bacterial outer membrane-permeabilizing ability of drugs. Tyrocidine A, in the presence of an outer membrane permeabilizer, increased the permeability to K(+) of the cytoplasmic membrane of Escherichia coli, because this antibiotic could markedly increase the permeability of phospholipid layers constituting the cytoplasmic membrane, while it acted weakly on the outer membrane. Hence, the novel function of agents increasing the permeability of the outer membrane could be examined directly by monitoring the tyrocidine A-induced leakage of K(+) from the bacterial cytoplasm using a K(+)-selective electrode. We found that antiprotozoal diamidines, such as diminazene, pentamidine, and 4',6-diamidino-2-phenylindole (DAPI), can increase the permeability of the bacterial outer membrane and appropriate lipophilicity is important for diamidines to permeabilize the outer membrane.

Synthesis and antiprotozoal activity of cationic 1,4-diphenyl-1H-1,2,3-triazoles

Novel dicationic triazoles 1-60 were synthesized by the Pinner method from the corresponding dinitriles, prepared via the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC). The type and the placement of cationic moieties as well as the nature of aromatic substituents influenced in vitro antiprotozoal activities of compounds 1-60 against Trypanosoma brucei rhodesiense, Plasmodium falciparum, and Leishmania donovani and their cytotoxicity for mammalian cells. Eight congeners displayed antitrypanosomal IC(50) values below 10 nM. Thirty-nine dications were more potent against P. falciparum than pentamidine (IC(50) = 58 nM), and eight analogues were more active than artemisinin (IC(50) = 6 nM). Diimidazoline 60 exhibited antiplasmodial IC(50) value of 0.6 nM. Seven congeners administered at 4 x 5 mg/kg by the intraperitoneal route cured at least three out of four animals in the acute mouse model of African trypanosomiasis. At 4 x 1 mg/kg, diamidine 46 displayed better antitrypanosomal efficacy than melarsoprol, curing all infected mice.

Structure-activity study of pentamidine analogues as antiprotozoal agents

Diamidine 1 (pentamidine) and 65 analogues (2-66) have been tested for in vitro antiprotozoal activities against Trypanosoma brucei rhodesiense, Plasmodium falciparum, and Leishmania donovani, and for cytotoxicity against mammalian cells. Dications 32, 64, and 66 exhibited antitrypanosomal potencies equal or greater than melarsoprol (IC(50) = 4 nM). Nine congeners (2-4, 12, 27, 30, and 64-66) were more active against P. falciparum than artemisinin (IC(50) = 6 nM). Eight compounds (12, 32, 33, 44, 59, 62, 64, and 66) exhibited equal or better antileishmanial activities than 1 (IC(50) = 1.8 microM). Several congeners were more active than 1 in vivo, curing at least 2/4 infected animals in the acute mouse model of trypanosomiasis. The diimidazoline 66 was the most promising compound in the series, showing excellent in vitro activities and high selectivities against T. b. rhodesiense, P. falciparum, and L. donovani combined with high antitrypanosomal efficacy in vivo.

Synthesis, DNA affinity, and antiprotozoal activity of linear dications: Terphenyl diamidines and analogues

Diamidines 10a-g and 18a,b were obtained from dinitriles 9a-g and 15a,b by treatment with lithium trimethylsilylamide or upon hydrogenation of bis-O-acetoxyamidoximes. Dinitriles 9a-g were prepared via Suzuki reactions between arylboronic acids and arylnitriles. Potential prodrugs 12a-f and 17 were prepared via methylation of the diamidoximes 11a-f and 16a. Significant DNA affinities for rigid-rod molecules were observed. Compounds 10a, 10b, 10d, 18a, and 18b show IC50 values of 5 nM or less against Trypanosoma brucei rhodesiense (T. b. r.) and 10a, 10b, 10e, 18a, and 18b gave similar ones against Plasmodium falciparum (P.f.). The dications, 10a, 10d, 10f, and 10g are more active than furamidine in vivo. The prodrugs are only moderately effective on oral administration. Mouse liver microsome bioconversion of the methamidoxime prodrugs is significantly reduced from that of pafuramidine and suggests that the in vivo efficacy of these prodrugs is, in part, due to poor bioconversion.

CYP4F enzymes are the major enzymes in human liver microsomes that catalyze the O-demethylation of the antiparasitic prodrug DB289 [2,5-bis(4-amidinophenyl)furan-bis-O-methylamidoxime]

DB289 [2,5-bis(4-amidinophenyl)furan-bis-O-methylamidoxime] is biotransformed to the potent antiparasitic diamidine DB75 [2,5-bis(4-amidinophenyl) furan] by sequential oxidative O-demethylation and reductive N-dehydroxylation reactions. Previous work demonstrated that the N-dehydroxylation reactions are catalyzed by cytochrome b5/NADH-cytochrome b5 reductase. Enzymes responsible for catalyzing the DB289 O-demethylation pathway have not been identified. We report an in vitro metabolism study to characterize enzymes in human liver microsomes (HLMs) that catalyze the initial O-demethylation of DB289 (M1 formation). Potent inhibition by 1-aminobenzotriazole confirmed that M1 formation is catalyzed by P450 enzymes. M1 formation by HLMs was NADPH-dependent, with a Km and Vmax of 0.5 microM and 3.8 nmol/min/mg protein, respectively. Initial screening showed that recombinant CYP1A1, CYP1A2, and CYP1B1 were efficient catalysts of M1 formation. However, none of these three enzymes was responsible for M1 formation by HLMs. Further screening showed that recombinant CYP2J2, CYP4F2, and CYP4F3B could also catalyze M1 formation. An antibody against CYP4F2, which inhibited both CYP4F2 and CYP4F3B, inhibited 91% of M1 formation by HLMs. Two inhibitors of P450-mediated arachidonic acid metabolism, HET0016 (N-hydroxy-N'-(4-n-butyl-2-methylphenyl)formamidine) and 17-octadecynoic acid, effectively inhibited M1 formation by HLMs. Inhibition studies with ebastine and antibodies against CYP2J2 suggested that CYP2J2 was not involved in M1 formation by HLMs. Additionally, ketoconazole preferentially inhibited CYP4F2, but not CYP4F3B, and partially inhibited M1 formation by HLMs. We conclude that CYP4F enzymes (e.g., CYP4F2, CYP4F3B) are the major enzymes responsible for M1 formation by HLMs. These findings indicate that, in human liver, members of the CYP4F subfamily biotransform not only endogenous compounds but also xenobiotics.

Plasmodium vivax: In vitro susceptibility of blood stages to synthetic trioxolane compounds and the diamidine DB75

Plasmodium vivax is an important human pathogen causing malaria in more temperate climates of the world. Similar to Plasmodium falciparum, the causative agent for malaria tropica, drug resistance is beginning to emerge for this parasite species and this hampers adequate treatment of infection. We have used a short-term ex vivo drug assay to monitor activity of OZ277 (RBx-11160), a fully synthetic anti-malarial peroxide, and the diamidine DB75 against P. vivax. For both compounds as well as the anti-malarial reference compounds artesunate, artemether, and chloroquine, the in vitro IC(50) values were determined in one-cycle hypoxanthine incorporation assays. Results from such assays were found to be very similar compared to IC(50) values obtained from one-cycle P. falciparum hypoxanthine assays. We demonstrate the anti-parasite activity of OZ277 and the reference compounds to be faster than that of DB75. These data warrant clinical testing of OZ277 against P. vivax malaria and support recent data on clinical activity against P. vivax for DB75.

In vitro metabolism of an orally active O-methyl amidoxime prodrug for the treatment of CNS trypanosomiasis

A new aza-analogue of furamidine, 6-[5-(4-amidinophenyl)-furan-2-yl]nicotinamidine (DB820), has potent in vitro antitrypanosomal activity; however, it suffers from poor oral activity because of its positively charged amidine groups. The dimethoxyamidine prodrug of DB820, N-methoxy-6-{5-[4-(N-methoxyamidino)phenyl]-furan-2-yl}-nicotinamidine (DB844), has potent oral activity in mouse models of both early-stage and CNS African trypanosomiasis. Metabolism of DB844 in human liver microsomes (HLM) was investigated using liquid chromatography-mass spectrometry (LC-MS/MS). The metabolism of DB844 in HLM was NADPH-dependent and resulted in the production of eight metabolites over a 90?min incubation. O-Demethylation and N-dehydroxylation reactions resulted in the metabolic conversion of DB844 to its active DB820 metabolite. Chromatographic conditions used for LC-MS analysis allowed for the separation and identification of all metabolites including positional isomers. Demethylation of either the phenyl or pyridine side of DB844 (DB844 m/z 366.2) resulted in the production of two metabolites (M1A, M1B), each with a molecular ion of m/z of 352.3 and MS(2) fragments of 288.1, 305.2, 321.2 and 335.2. However, the intensities of the MS(2) fragments were different among the two isomeric metabolites, and comparison to an authentic standard allowed for the structural determination of each metabolite. The isomeric metabolites M2A and M2B, resulting from amidoxime reductions of M1A and M1B, were also chromatographically separated and had distinguishable MS(2) profiles that allowed for their structural assignments when compared to an authentic standard. The di-amidoxime product resulting from O-demethylation of either side of DB844 was also identified as an abundant metabolite during microsomal incubations. The active antitrypanosomal metabolite, DB820, was the last metabolite to be formed and thus provides evidence that DB844 may effectively be metabolized to its active metabolite in vivo.

The activity of diguanidino and 'reversed' diamidino 2,5-diarylfurans versus Trypanosoma cruzi and Leishmania donovani

The in vitro activity of 20 dicationic molecules containing either diguanidino or reversed amidine cationic groups were evaluated versus Trypanosoma cruzi and Leishmania donovani. The most active compounds were in the reversed amidine series and six exhibited IC(50) values of less than 1 micro mol versus T. cruzi and five gave similar values versus L. donovani.

Enhanced permeability of the antimicrobial agent 2,5-bis(4-amidinophenyl)furan across Caco-2 cell monolayers via its methylamidoidme prodrug

PURPOSE

DB75 [2.5-bis(4-amidinophenyl)furan] is a promising antimicrobial agent although it has poor oral potency. In contrast, its novel prodrug, 2,5-bis(4-amidinophenyl)furan-bis-O-methylamidoxime (DB289). has excellent oral potency. The mechanisms of transport of DB289 and DB75 across intestinal epithelium have been investigated in these studies to understand differences in their oral potency.

METHODS

Caco-2 cell monolayers were used as an in vitro model to examine the mechanisms of transport of DB289 and DB75. Samples collected from the transport studies were quantified using high-performance liquid chromatography with ultraviolet and fluorescence detection.

RESULTS

A low permeability coefficient (3.8 x 10(-7) cm/s for transport in apical [AP] to basolateral [BL] direction) and high sensitivity to extracellular Ca2+ suggest that AP to BL transport of DB75 across Caco-2 cell monolayers occurs predominantly via a paracellular route. DB289 has an 85-fold higher transport rate (322.0 x 10(-7) cm/s for transport in the AP to BL direction) across Caco-2 monolayers than that of DB75. This, with its insensitivity to extracellular Ca2+ indicates that AP to BL transport of DB289 across Caco-2 cell monolayers occurs predominantly via a transcellular route.

CONCLUSIONS

DB75 is transported across Caco-2 cell monolayers predominantly via paracellular pathways, whereas the prodrug DB289 is transported via transcellular pathways. This could account for the much higher oral activity of DB289 over DB75.

Characterizing the fragmentation of 2,5-bis (4-amidinophenyl)furan-bis-O-methylamidoxime and selected metabolites using ion trap mass spectrometry

A novel prodrug [2,5-bis(4-amidinophenyl)furan-bis-O-methylamidoxime (DB289)] of the promising antimicrobial agent, 2,5-bis(4-amidinophenyl)furan (DB75), has excellent oral activity. It is currently undergoing phase II clinical evaluation as an orally administered drug candidate against African trypanosomiasis and Pneumocystis carinii pneumonia. The sequential product ion (MS(n)) fragmentations of DB289 and selected metabolites were characterized using ion trap mass spectrometry with electrospray ionization. An unusual homolytic bond cleavage, formation of an odd-electron ion from an even-electron ion with the loss of a radical, was commonly seen in the fragmentation patterns of DB289 and its metabolites. Both O-ethyl and N-methyl homologues of DB289 were utilized to confirm this fragmentation pathway. The labile hydrogen atoms in DB289 are readily exchanged with deuterium atoms in the solvent containing deuterium oxide (D2O) instead of water. The mass shift patterns displayed in the product ion spectra of DB289 in D2O proved useful in verifying the fragmentation pathway. Octadeuterated DB289 and DB75 (d-labeling on the diphenyl rings) showed unequivocally that the diphenylfuran moiety is not involved in the fragmentation. The fragmentation pathways uncovered in this work will facilitate structural characterization of all the metabolites produced in the metabolic activation of DB289.

Pentamidine accumulates in rat liver lysosomes and inhibits phospholipid degradation

The subcellular distribution and the effects of pentamidine on the ultrastructure of the rat liver were studied. Rats were given single or repeated daily intraperitoneal injections of 10, 25 or 50 mg pentamidine isethionate/kg b. wt. for 1, 4, 6, 9 or 16 days. The livers were removed for ultrastructural and biochemical analyses on the day after termination of each series of injections and in addition 7 and 35 days after the 16th injection. Electron microscopy of liver tissues showed that the general cellular architecture of the hepatocytes was preserved. The subcellular organelles were normal, except for the secondary lysosomes, which were severely altered and laden with multilamellar, myelin structures (myelin bodies) that gradually increased with dose and time course following repeated injections. These altered lysosomes were enriched in phospholipids. The alteration of the lysosomes persisted for up to 5 weeks after cessation of administration. Pentamidine was highly enriched in the lysosomal fraction (30-50 times more than in the liver homogenate). It was calculated that the lysosomal pentamidine accounted for practically all pentamidine distributed to the liver. The demonstrated accumulation of pentamidine in the lysosomes may explain the known large volume of distribution of this drug and may be one mechanism for organ toxicity.

Destruction of intracellular Trypanosoma cruzi after treatment of infected macrophages with cationic electron carriers

Intracellular forms of Trypanosoma cruzi were destroyed in infected mouse macrophage cultures treated for 24 h with micromolar concentrations of phenazine methosulfate, brilliant cresyl blue, or crystal violet. Parasites were not killed in macrophages which were only treated with the dyes before infection, indicating that the drugs did not induce a protracted state of macrophage trypanocidal activity.