Double prodrugs of a fosmidomycin surrogate as antimalarial and antitubercular agents

A series of eleven double prodrug derivatives of a fosmidomycin surrogate were synthesized and investigated for their ability to inhibit in vitro growth of P. falciparum and M. tuberculosis. A pivaloyloxymethyl (POM) phosphonate prodrug modification was combined with various prodrug derivatisations of the hydroxamate moiety. The majority of compounds showed activity comparable with or inferior to fosmidomycin against P. falciparum. N-benzyl substituted carbamate prodrug 6f was the most active antimalarial analog with an IC₅₀ value of 0.64?µM. Contrary to fosmidomycin and parent POM-prodrug 5, 2-nitrofuran and 2-nitrothiophene prodrugs 6i and 6j displayed promising antitubercular activities.     

Phosphonodiamidate prodrugs of N-alkoxy analogs of a fosmidomycin surrogate as antimalarial and antitubercular agents

A series of N-alkoxy analogs of a l-leucine ethyl ester phosphonodiamidate prodrug of a fosmidomycin surrogate were synthesized and investigated for their ability to inhibit in vitro growth of P. falciparum and M. tuberculosis. These compounds originate by merging a previously reported successful phosphonate derivatisation with favorable modifications of the hydroxamate moiety. None of the synthesized compounds showed enhanced activity against either P. falciparum or M. tuberculosis in comparison with the parent free hydroxamate analog.     

Synthesis of beta- and gamma-oxa isosteres of fosmidomycin and FR900098 as antimalarial candidates

To expand the structure-activity relationships of fosmidomycin and FR900098, two potent antimalarials interfering with the MEP-pathway, we decided to replace a methylene group in beta-position of the phosphonate moiety of these leads by an oxygen atom. beta-oxa-FR900098 (11) proved equally active as the parent compound. When applied to 4-[hydroxyl(methyl)amino]-4-oxobutyl phosphonic acid, featuring a hydroxamate instead of the retrohydroxamate moiety, a beta-oxa modification yielded a derivative (13) with superior activity against the Plasmodium falciparum 3D7 strain than fosmidomycin, while a gamma-oxa modification resulted in less active derivatives. A bis(pivaloyloxymethyl)ester of phosphonate 13 proved twice as active in inhibiting cultured parasites as a similar prodrug of FR900098.     

Evaluation of amidoxime derivatives as prodrug candidates of potent bis-cationic antimalarials

Plasmodium falciparum is responsible for most of the cases of malaria and its resistance to established antimalarial drugs is a major issue. Thus, new chemotherapies are needed to fight the emerging multi-drug resistance of P. falciparum malaria, like choline analogues targeting plasmodial phospholipidic metabolism. Here we describe the synthesis of amidoxime derivatives as prodrug candidates of reverse-benzamidines and hybrid compounds able to mimic choline, as well as the design of a new series of asymmetrical bis-cationic compounds. Bioconversion studies were conducted on amidoximes in asymmetrical series and showed that amidoxime prodrug strategy could be applied on C-alkylamidine moieties, like benzamidines and that N-substituents did not alter the bioconversion of amidoximes. The antimalarial activity of the three series of compounds was evaluated in vitro against P. falciparum and in vivo against P. vinckei petteri in mice.     

Pharmacokinetic studies of naproxen amides of some amino acid esters with promising colorectal cancer chemopreventive activity

Naproxen (nap) is belonging to Non-steriodal anti-inflammatory drugs (NSAIDs) group of drugs that characterized by their free carboxylic group. The therapeutic activity of nap is usually accompanied by GI untoward side effects. Recently synthesized naproxen amides of some amino acid esters prodrugs to mask the free carboxylic group were reported. Those prodrugs showed a promising colorectal cancer chemopreventive activity. The current study aims to investigate the fate and hydrolysis of the prodrugs kinetically in different pH conditions, simulated gastric and intestinal fluids with pHs of 1.2, 5.5 and 7.4 in vitro at 37 °C. The effect of enzymes on the hydrolysis of prodrugs was also studied through incubation of these prodrugs at 37 °C in human plasma and rat liver homogenates. The pharmacokinetic parameters of selected prodrugs and the liberated nap were studied after oral and intraperitoneal administration in male wistar rats. The results showed the hydrolysis of naproxen amides of amino acid esters to nap through two steps first by degradation of the ester moiety to form the amide of nap with amino acid and the second was through the degradation of the amide link to liberate nap. The two reactions were followed and studied kinetically where K₁ and K₂ (rate constants of degradation) is reported. The hydrolysis of prodrugs was faster in liver homogenates than in plasma. The relative bioavailability of the liberated nap in vivo was higher in case of prodrug containing ethyl glycinate moiety than that occupied l-valine ethyl ester moiety. Each of nap. prodrugs containing ethyl glycinate and l-valine ethyl ester moieties appears promising in liberating nap, decreasing direct GI side effect and consequently their colorectal cancer chemopreventive activity.     

Synthesis of alpha-substituted fosmidomycin analogues as highly potent Plasmodium falciparum growth inhibitors

In view of the promising antimalarial activity of fosmidomycin or its N-acetyl homologue FR900098, the objective of this work was to investigate the influence of aromatic substituents in the alpha-position of the phosphonate moiety. The envisaged analogues were prepared using a linear route involving a 3-aryl-3-phosphoryl propanal intermediate. The activities of all compounds were evaluated on Eschericia coli 1-deoxy-d-xylulose 5-phosphate reductoisomerase and against two Plasmodium falciparum strains. Compared with fosmidomycin, several analogues displayed enhanced activity towards the P. falciparum strains. Compound 1e with a 3,4-dichlorophenyl substitution in the alpha-position of fosmidomycin emerged as the most potent analogue of this series. It is approximately three times more potent in inhibiting the growth of P. falciparum than FR900098, the most potent representative of this class reported so far.     

Design,synthesis, and biological evaluation of pyrrolobenzodiazepine-containing hypoxia-activated prodrugs

The ability of various pyrrolobenzodiazepine(PBD)-containing cytotoxic compounds to function as hypoxia-activated prodrugs was assessed. These molecules incorporated a 1-methyl-2-nitro-1H-imidazole hypoxia-activated trigger (present in the clinically evaluated compound TH-302) in a manner that masked a reactive imine moiety required for cytotoxic activity. Incubation of the prodrugs with cytochrome P450-reductase under normoxic and hypoxic conditions revealed that some, but not all, were efficient substrates for the enzyme. In these experiments, prodrugs derived from PBD-monomers underwent rapid conversion to the parent cytotoxic compounds under low-oxygen conditions while related PBD-dimers did not. The ability of a given prodrug to function as an efficient cytochrome P450-reductase substrate correlated with the ratio of cytotoxic potencies measured for the compound against NCI460 cells under normoxic and hypoxic conditions.     

Double-drug development against antioxidant enzymes from Plasmodium falciparum

Abstract

New drugs against malaria are urgently and continuously needed. Plasmodium parasites are exposed to higher fluxes of reactive oxygen species and need high activities of intracellular antioxidant systems. A most important antioxidative system consists of (di)thiols which are recycled by disulfide reductases (DR), namely both glutathione reductases (GR) of the malarial parasite Plasmodium falciparum and man, and the thioredoxin reductase (TrxR) of P. falciparum. The aim of our interdisciplinary research is to substantiate DR inhibitors as antimalarial agents. Such compounds are active per se but, in addition, they can reverse thiol-based resistance against other drugs in parasites. Reversal of drug resistance by DR inhibitors is currently investigated for the commonly used antimalarial drug chloroquine (CQ). Our recent strategy is based on the synthesis of inhibitors of the glutathione reductases from parasite and host erythrocyte. With the expectation of a synergistic or additive effect, double-headed prodrugs were designed to be directed against two different and essential functions of the malarial parasite P. falciparum, namely glutathione regeneration and heme detoxification. The prodrugs were prepared by linking bioreversibly a GR inhibitor to a 4-aminoquinoline moiety which is known to concentrate in the acidic food vacuole of parasites. Drug-enzyme interaction was correlated with antiparasitic action in vitro on strains resistant towards CQ and in vivo in Plasmodium berghei-infected mice as well as absence of cytotoxicity towards human cells. Because TrxR of P. falciparum was recently shown to be responsible for the residual glutathione disulfide-reducing capacity observed after GR inhibition in P. falciparum, future development of antimalarial drug-candidates that act by perturbing the redox equilibrium of parasites is based on the design of new double-drugs based on TrxR inhibitors as potential antimalarial drug candidates.     

Diaryl ester prodrugs of FR900098 with improved in vivo antimalarial activity

The fosmidomycin derivative FR900098 represents an inhibitor of the 1-deoxy-D-xylulose 5-phosphate (DOXP) reductoisomerase with potent antimalarial activity. Prodrugs of FR900098 with increased activity after oral administration were obtained by chemical modification of the phosphonate moiety to yield phosphodiaryl esters. One diaryl ester prodrug demonstrated efficacy in mice infected with the rodent malaria parasite Plasmodium vinckei comparable to i.p. drug administration.     

Synthesis of 3-(3-aryl-pyrrolidin-1-yl)-5-aryl-1,2,4-triazines that have antibacterial activity and also inhibit inorganic pyrophosphatase

Inorganic pyrophosphatases are potential targets for the development of novel antibacterial agents. A pyrophosphatase-coupled high-throughput screening assay intended to detect o-succinyl benzoic acid coenzyme A (OSB CoA) synthetase inhibitors led to the unexpected discovery of a new series of novel inorganic pyrophosphatase inhibitors. Lead optimization studies resulted in a series of 3-(3-aryl-pyrrolidin-1-yl)-5-aryl-1,2,4-triazine derivatives that were prepared by an efficient synthetic pathway. One of the tetracyclic triazine analogues 22h displayed promising antibiotic activity against a wide variety of drug-resistant Staphylococcus aureus strains, as well as activity versus Mycobacterium tuberculosis and Bacillus anthracis, at a concentration that was not cytotoxic to mammalian cells.     

Optimization of spirocyclic proline tryptophan hydroxylase-1 inhibitors

As a follow-up to the discovery of our spirocyclic proline-based TPH1 inhibitor lead, we describe the optimization of this scaffold. Through a combination of X-ray co-crystal structure guided design and an in vivo screen, new substitutions in the lipophilic region of the inhibitors were identified. This effort led to new TPH1 inhibitors with in vivo efficacy when dosed as their corresponding ethyl ester prodrugs. In particular, 15b (KAR5585), the prodrug of the potent TPH1 inhibitor 15a (KAR5417), showed robust reduction of intestinal serotonin (5-HT) levels in mice. Furthermore, oral administration of 15b generated high and sustained systemic exposure of the active parent 15a in rats and dogs. KAR5585 was selected for further pharmacological evaluation in disease models associated with a dysfunctional peripheral 5-HT system.     

Potential application of thymidylate kinase in nucleoside analogue activation in Plasmodium falciparum

Plasmodium falciparum thymidylate kinase (PfTMPK) shows a broad range of substrate tolerance when compared to the corresponding human enzyme. Besides 2′-deoxythymidine monophosphate (dTMP), PfTMPK can phosphorylate 3′-azido-2′,3′-dideoxythymidine monophosphate (AZTMP) very efficiently. In contrast, human thymidylate kinase (hTMPK) is 200 times less active towards AZTMP. We were interested to see if we could use PfTMPK to activate 3′-azido-2′,3′-deoxythymidine (AZT) derivatives as a strategy to treat malaria. P. falciparum lacks a pyrimidine nucleoside kinase which usually activates nucleoside and nucleoside analogues to the corresponding monophosphates. Therefore, several prodrug analogues of AZT and related nucleoside monophosphates were prepared and analysed for antiparasitic activity. The prodrugs showed an increase in activity over the parent nucleoside analogues, which showed no inhibition of parasite growth at the concentration tested. The evidence here reported provides a strategy that could be exploited for further anti-malarial design.     

N4-Acyl derivatives as lipophilic prodrugs of cidofovir and its 5-azacytosine analogue, (S)-HPMP-5-azaC: Chemistry and antiviral activity

Even number fatty acid residues—docosanoyl (behenoyl) and stearoyl were selected for introduction to the N⁴-position of (S)-1-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine) (HPMPC, cidofovir), and its 5-azacytosine counterpart, (S)-1-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine) (HPMP-5-azaC) with the aim to prepare a new type of lipophilic prodrugs. The study on the influence of these modifications to the stability and biological activity of both antivirals was performed. Different reactivity of both systems towards acylation reactions was also found: the 4-NH₂ group of cidofovir was more reactive compared to that of HPMP-5-azaC. In 5-azacytosine derivatives, we found mostly a destabilizing effect of the N⁴-acylation but this could be compensated by a positive influence of the esterification of the phosphonate group. Chemical stability of the 5-azacytosine moiety in the HPMP series is increasing in the following order: HPMP-5-azaC < cyclic HPMP-5-azaC < HPMP-5-azaC esters. From the view of prodrug development, the best chemical stability was observed in case of the double prodrug 7: the N⁴-behenoyl derivative of the hexadecyloxyethyl ester of cyclic HPMP-5-azaC. The free phosphonic acid (N⁴-behenoyl-HPMPC) appeared to be a more potent and selective inhibitor of herpesvirus replication than the parent HPMPC derivative.     

Synthesis and potent antiprotozoal activity of mono/di amidino 2-anilinobenzimidazoles versus Plasmodium falciparum and Trypanosoma brucei rhodesiense

A series of mono and dicationic new 2-anilinobenzimidazole carboxamidines were prepared in a four step process starting from 4-amino-3-nitrobenzonitrile and corresponding o-phenylenediamines. Their antiparasitic activity against Plasmodium falciparum (P. falciparum) and Trypanosoma brucei rhodesiense (T.b. rhodesiense) were evaluated in vitro. Some of the dicationic compounds (10,12,14) showed equal or very close activity against T.b. rhodesiense with melarsoprol and also showed promising activity against P. falciparum as compared to chloroquine. Among the monocationic derivatives compound 21 exhibited best inhibitory activity against P. falciparum.     

Antiprotozoal activity of bicyclic diamines with a N-methylpiperazinyl group at the bridgehead atom

ω-Aminoacyl and -alkyl derivatives of 4-(4-methylpiperazin-1-yl)bicyclo[2.2.2]octan-2-amines and of 5-(4-methylpiperazin-1-yl)-2-azabicyclo[3.2.2]nonanes were prepared and their activities were examined in vitro against the multiresistant K₁ strain of Plasmodium falciparum and against Trypanosoma brucei rhodesiense (STIB 900). Some of the newly synthesized compounds showed very promising antiprotozoal activity and selectivity. A few of the alkylamino-2-azabicyclo[3.2.2]nonanes exhibited high antiplasmodial activity, whereas a single bicyclo[2.2.2]octane derivative was the most potent antitrypanosomal compound. The results of the newly synthesized compounds were compared with the activities of already synthesized compounds and of drugs in use. Structure–activity relationships were discussed.     

Chemoenzymatic Synthesis, Inhibition Studies, and X-ray Crystallographic Analysis of the Phosphono Analog of UDP-Galp as an Inhibitor and Mechanistic Probe for UDP-Galactopyranose Mutase

UDP (uridine diphosphate) galactopyranose mutase (UGM) is involved in the cell wall biosynthesis of many pathogenic microorganisms. UGM catalyzes the reversible conversion of UDP-α-d-galactopyranose into UDP-α-d-galactofuranose, with the latter being the precursor of galactofuranose (Galf) residues in cell walls. Glycoconjugates of Galf are essential components in the cell wall of various pathogenic bacteria, including Mycobacterium tuberculosis, the causative agent of tuberculosis. The absence of Galf in humans and its bacterial requirement make UGM a potential target for developing novel antibacterial agents. In this article, we report the synthesis, inhibitory activity, and X-ray crystallographic studies of UDP-phosphono-galactopyranose, a nonhydrolyzable C-glycosidic phosphonate. This is the first report on the synthesis of a phosphonate analog of UDP-α-d-galactopyranose by a chemoenzymatic phosphoryl coupling method. The phosphonate was evaluated against three bacterial UGMs and showed only moderate inhibition. We determined the crystal structure of the phosphonate analog bound to Deinococcus radiodurans UGM at 2.6 Å resolution. The phosphonate analog is bound in a novel conformation not observed in UGM-substrate complex structures or in other enzyme-sugar nucleotide phosphonate complexes. This complex structure provides a structural basis for the observed micromolar inhibition towards UGM. Steric clashes, loss of electrostatic stabilization between an active-site arginine (Arg305) and the phosphonate analog, and a 180° flip of the hexose moiety account for the differences in the binding orientations of the isosteric phosphonate analog and the physiological substrate. This provides new insight into the ability of a sugar-nucleotide-binding enzyme to orient a substrate analog in an unexpected geometry and should be taken into consideration in designing such enzyme inhibitors.     

Synthesis and in vitro evaluation of S-acyl-3-thiopropyl prodrugs of Foscarnet

A new enzyme-labile group called S-acyl-3-thiopropyl group (SATP) has been synthesized from allylic esters of phosphonate. After demonstration of the enzyme-labile character of the SATP in cellular extracts, it has been introduced onto the phosphonate moiety of PFA (Foscarnet) to obtain potential lipophilic prodrugs. To ponder the lipophilicity of the triesters of PFA, esters of monomethylether of polyethyleneglycols and of thioglycerol were introduced on the PFA carboxylate moiety. The SATP groups were introduced in an attempt to deliver PFA after bioactivation inside the cells. The PFA prodrugs were evaluated in vitro for their activity against human immunodeficiency viruses (HIV-1 and HIV-2).     

The N-Reductive System Composed of Mitochondrial Amidoxime Reducing Component (mARC), Cytochrome b5 (CYB5B) and Cytochrome b5 Reductase (CYB5R) Is Regulated by Fasting and High Fat Diet in Mice

The mitochondrial amidoxime reducing component mARC is the fourth mammalian molybdenum enzyme. The protein is capable of reducing N-oxygenated structures, but requires cytochrome b5 and cytochrome b5 reductase for electron transfer to catalyze such reactions. It is well accepted that the enzyme is involved in N-reductive drug metabolism such as the activation of amidoxime prodrugs. However, the endogenous function of the protein is not fully understood. Among other functions, an involvement in lipogenesis is discussed. To study the potential involvement of the protein in energy metabolism, we tested whether the mARC protein and its partners are regulated due to fasting and high fat diet in mice. We used qRT-PCR for expression studies, Western Blot analysis to study protein levels and an N-reductive biotransformation assay to gain activity data. Indeed all proteins of the N-reductive system are regulated by fasting and its activity decreases. To study the potential impact of these changes on prodrug activation in vivo, another mice experiment was conducted. Model compound benzamidoxime was injected to mice that underwent fasting and the resulting metabolite of the N-reductive reaction, benzamidine, was determined. Albeit altered in vitro activity, no changes in the metabolite concentration in vivo were detectable and we can dispel concerns that fasting alters prodrug activation in animal models. With respect to high fat diet, changes in the mARC proteins occur that result in increased N-reductive activity. With this study we provide further evidence that the endogenous function of the mARC protein is linked with lipid metabolism.     

Auxiliary in vitro and in vivo biological evaluation of hydrogen peroxide sensitive prodrugs of methotrexate and aminopterin for the treatment of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease that causes severe joints damage and other extra-articular alterations. Despite the efficacy of low-dose methotrexate (LD-MTX) in RA treatment, adverse effects are the predominant reasons for discontinuation of therapy. As a therapeutic targeting strategy, the presence of increased concentrations of reactive oxygen species (ROS) in the inflammatory environment can serve as the stimulus for prodrug activation in site-selective drug delivery systems. Our group has previously reported novel ROS sensitive prodrugs (1–3) of MTX and aminopterin (AMT) for site-selective delivery to inflammatory tissue associated with RA, with the aim of reducing side effects in RA therapy. Herein, we investigate the effect and toxicity of the same prodrugs in a rat CIA (collagen-induced arthritis) model of RA. We find that prodrug 1, an arylboronic acid ROS-sensitive MTX-prodrug, displays similar in vivo efficacy as MTX at an equimolar dose, while avoiding adverse effects known to restrict MTX treatment. To further characterize prodrug 1 and its ROS mediated activation, we synthesized compound 4, a negative control lacking the boronic acid moiety. We then investigated the effect of molecules on cell proliferation and cytotoxicity in the presence of the ROS scavenger pyruvate, as well as their stability in buffer and cell media, demonstrating a direct correlation between ROS concentration and the prodrug activity. Moreover, the in vitro ADME properties were investigated, including permeability, rat plasma and microsomal stability.     

Glycerol: An unexpected major metabolite of energy metabolism by the human malaria parasite

Background

Malaria is the third most prevalent cause of infectious disease in the world. Resistance of the parasite to classical drugs makes the discovery of new and effective drugs more urgent. The oxidized derivative of hydroxy- cis terpenone (OHCT) is a synthetic molecule that is not toxic to cultured human liver cells at concentrations as high as 60 μM and inhibits activity of cytochrome P450s that metabolize many drugs.

Methods

OHCT activity against chloroquine-sensitive and -resistant strains of Plasmodium falciparum, and a P. falciparum clone that is partially resistant to artemisinin was assayed in vitro.

Results

OHCT at nanomolar concentrations was effective against all intraerythrocytic stages of P. falciparum and exhibited activity in vitro against both chloroquine-sensitive and -resistant strains of P. falciparum as well as a P. falciparum clone that is partially resistant to artemisinin. Moreover, OHCT exhibited potent activity against gametocytes, the form that is transmitted by mosquitoes and essential for the spread of malaria.

Conclusion

OHCT displays strong growth inhibitory activity against all stages of P. falciparum and no evidence of toxicity to human cells in culture. It is easily synthesized and has the potential for inhibiting metabolism of drugs used in combination therapies.