Effects of isoxazolyl-naphthoquinoneimines on growth and oxygen radical production in Trypanosoma cruzi and Crithidia fasciculata

Naphthoquinones and Derivatives for Chemotherapy: Perspectives and Limitations of their Anti-trypanosomatids Activities

Chagas disease, Sleeping sickness and Leishmaniasis, caused by trypanosomatids Trypanosoma cruzi, Trypanosoma brucei and Leishmania spp., respectively, are considered neglected tropical diseases, and they especially affect impoverished populations in the developing world. The available chemotherapies are very limited, and a search for alternatives is still necessary. In folk medicine, natural naphthoquinones have been employed for the treatment of a great variety of illnesses, including parasitic infections. This review is focused on the anti-trypanosomatid activity and mechanistic analysis of naphthoquinones and derivatives. Among all the series of derivatives tested in vitro, naphthoquinone-derived 1,2,3-triazoles were very active on T. cruzi infective forms in blood bank conditions, as well as in amastigotes of Leishmania spp. naphthoquinones containing a CF₃ on a phenyl amine ring inhibited T. brucei proliferation in the nanomolar range, and naphthopterocarpanquinones stood out for their activity on a range of Leishmania species. Some of these compounds showed a promising selectivity index (SI) (30 to 1900), supporting further analysis in animal models. Indeed, high toxicity to the host and inactivation by blood components are crucial obstacles to be overcome to use naphthoquinones and/or their derivatives for chemotherapy. Multidisciplinary initiatives embracing medicinal chemistry, bioinformatics, biochemistry, and molecular and cellular biology need to be encouraged to allow the optimization of these compounds. Large scale automated tests are pivotal for the efficiency of the screening step, and subsequent evaluation of both the mechanism of action in vitro and pharmacokinetics in vivo is essential for the development of a novel, specific and safe derivative, minimizing adverse effects.

The trypanocidal activity of naphthoquinones: a review

Naphthoquinones are compounds present in several families of higher plants. Their molecular structures confer redox properties, and they are involved in multiple biological oxidative processes. In folk medicine, especially among Indian populations, plants containing naphthoquinones have been employed for the treatment of various diseases. The biological redox cycle of quinones can be initiated by one electron reduction leading to the formation of semiquinones, unstable intermediates that react rapidly with molecular oxygen, generating free radicals. Alternatively, the reduction by two electrons, mediated by DT-diphorase, leads to the formation of hydroquinone. Lapachol, alpha-lapachone and beta-lapachone, which are isolated from the heartwood of trees of the Bignoniaceae family, are examples of bioactive naphthoquinones. In this review, we will discuss studies investigating the activity of these natural products and their derivatives in the context of the search for alternative drugs for Chagas disease, caused by Trypanosoma cruzi, a neglected illness that is endemic in Latin America.

Synthesis, antiprotozoal and cytotoxic activities of new N-(3,4-dimethyl-5-isoxazolyl)-1,2-naphthoquinone-4-amino derivatives

Three derivatives of N-(3,4-dimethyl-5-isoxazolyl)-1,2-naphthoquinone-4-amino (1), a compound which exhibits significant activity against Trypanosoma cruzi and Plasmodium falciparum but with cytotoxicity toward murine L-6 cells, were synthesized with the aim of ameliorating its cytotoxicity. The in vitro antiprotozoal and cytotoxic activities of the synthesized compounds were evaluated against T. cruzi, Trypanosoma brucei rhodesiense, P. falciparum and murine L-6 cells. The hydroxymethyl (2) and the oxime (3) derivatives were active against T. cruzi, with IC50 values in a range comparable to those of 1 (IC50: 0.65 microg/ml) and benznidazole (IC50: 0.56 microg/ml) while the carboxymethyloxime (4) was inactive. Compounds 2 and 3 were cytotoxic toward L-6 cells, with IC50 values identical to that of 1 (IC50: 0.50 microg/ml). The results did not support the suggestion that 2 and 3 may be used as prodrugs of 1.

Synthesis and biological evaluation of pyrazolylnaphthoquinones as new potential antiprotozoal and cytotoxic agents

The importance of American trypanosomiasis (Chagas' disease) in human pathology is widely known. The prognosis of this disease is poor and the choice of effective medicines limited, thus study of new drugs is absolutely necessary. In this work, the activities of three new pyrazolylnaphthoquinones, heterocyclic naphthoquinones bearing 3-aminopyrazole rings, were evaluated on Trypanosoma cruzi, the etiological agent of Chagas' disease. These activities were compared with those of three 5-aminoisoxazole analogues. In addition, since these compounds belong to a family of antiprotozoal and cytotoxic/antitumor agents, the activities of all six against Plasmodium falciparum, Trypanosoma brucei rhodesiense, and murine L-6 cells were also investigated. In the biological tests, five of the compounds showed significant in vitro trypanocidal activities against T. cruzi, with activities similar to that of benznidazole. Two of the 5-aminoisoxazole analogues also showed good activities, in one case highly selective, against the K1 and NF54 strains of P. falciparum (IC(50)<0.12 microg mL(-1)). Three of the compounds were cytotoxic to murine L-6 cells (IC(50)=0.21-0.50 microg mL(-1)). The results suggested that the three pyrazolylnaphthoquinones and one of the 5-aminoisoxazole analogues could be starting points for lead optimization programs against T. cruzi and P. falciparum, respectively.

Effect of hydroxypropyl-beta-cyclodextrin on the solubility of an antibacterial isoxazolyl-naphthoquinone

The complexation of 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1,4-naphthoquinone-4-imine (I) with a highly soluble cyclodextrin, hydroxypropyl-beta-cyclodextrin (HP-beta-CD) was studied in aqueous media by solubility methods. I is an antibacterial and trypanocidal agent that is undergoing preclinical testing. Unfortunately, I exhibits low water solubility, and it is therefore difficult to prepare the solutions for biological tests. I inclusion took place with 1:1 stoichiometry. The stability constants of the I complexes calculated from the slope and the intercept of the phase solubility diagrams are larger in the less ionized form, whereas greater overall solubility is obtained in basic media.

Mechanism of antibacterial and degradation behavior of a chlorinated isoxazolylnaphthoquinone

The chemical stability of 3-chloro-2-hydroxy-(3, 4-dimethyl-5-isoxazolyl)-1,4-naphthoquinon-4-imine (ClQ(1)), a new potential antimicrobial agent was analyzed at different pH values by first-derivative spectroscopy. The degradation of ClQ(1) followed a pseudo-first-order kinetics in aqueous media at different pH values. The interaction of antibiotics with respiratory chain of Staphylococcus aureus generates superoxide anion, an oxygen radical capable of producing damage to the bacteria. The performed assays have demonstrated that ClQ(1) presents higher activity and toxic oxidant generation at pH 5.0 than at pH 7.5. In addition, the antibacterial activity of other halogenated isoxazolylnaphthoquinones was also studied in different collection and clinical strains which presented the following decreasing activity, ClQ(1) > BrQ(1) > DClQ(1) whereas DBrQ(1) did not show inhibition properties. The antibacterial and stability properties evidenced by ClQ(1) are so important that must be taken into account when new alternative treatments against beta-lactamase-positive S. aureus strains are investigated.

Synthesis of New Isoxazolylnaphthoquinones as Potential Trypanocidal and Antibacterial Agents

The synthesis of three new isoxazolylnaphthoquinones with an ethyl group on the isoxazole ring is reported.

Redox cycling of o-naphthoquinones in trypanosomatids. Superoxide and hydrogen peroxide production

beta-Lapachone and structurally related lipophilic o-naphthoquinones, namely, CG 8-935, CG 9-442, CG 10-248 and mansonones A, C, E, and F, were investigated for redox cycling, production of reactive oxygen species, and cytotoxicity in the trypanosomatids Crithidia fasciculata and Leptomonas seymouri. Structural analysis of the assayed quinones indicated that a tricyclic structure, including a naphthalene ring, a 1,2b or 1,8bc pyran ring, and two ortho-carbonyl groups were required for quinone activities. The contribution of oxygen radical production to quinone cytotoxicity was supported by: (a) spectroscopic observation of quinone redox cycling; (b) production of the semiquinone radical; (c) H2O2 and O2- production; (d) the effect of beta-lapachone on thiol pools in C. fasciculata; (e) the effect of quinones on cell respiration; (f) superoxide dismutase inactivation after incubation of C. fasciculata with CG 8-935; and (g) the effect of quinones on cell growth.

Antibacterial effect of 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1, 4-naphthoquinone-4-imine on Staphylococcus aureus

The mechanism by which a new naphthoquinone derivative, the 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1, 4-naphthoquinone-4-imine (INQI-E) has antibacterial effect against Staphylococcus aureus was studied. The interaction of INQI-E with the bacteria was followed by absorption spectroscopy at 323 and 490 nm. The absorption band of INQI-E at 490 nm undergoes a hypochromic shift with a decrease of intensity. This effect was found to be reversible by oxygenation during the first hours of incubation. The participation of an oxidation-reduction process related to the respiratory chain was demonstrated by oxygen consumption. An increase in O2 uptake and inhibition of S. aureus growth was observed. Experiments with three inhibitors of the respiratory chain demonstrated that the pathway induced by INQI-E was antimycin-resistant and KCN- and salicylhydroxamic acid (SHAM)-sensitive, which suggests that INQI-E is capable of diverting the normal electron flow to an alternate superoxide-producing route. On the other hand, experiments with Tiron, a specific scavenger of superoxide, hindered the effect of INQI-E against S. aureus, indicating that the inhibitory growth effect of this quinone-imine is mainly due to the production of the cytotoxic superoxide radical.

Mutagenic activity of isoxazolylnaphthoquinoneimines assayed by micronucleus bone marrow test

Studies were undertaken to evaluate the ability of various quinoneimines to induce micronuclei in bone marrow cells as a measure of their genotoxicity. Accordingly, 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1,4-naphthoquinone-4-imine (I), its 2-acetyl derivative (II) and 2-[(5-methyl-3-isoxazolyl)amino]-N-(5-methyl-3-isoxazolyl)-1 ,4- naphthoquinone-4-imine (III), as well as two of their precursors, 2-hydroxynaphthoquinone (NQ-2-OH) and 3,4-dimethyl-5-aminoisoxazole (DMAI) were given by intraperitoneal injection at 5, 50, 100 and 200 mg/Kg doses to S.J.L. Swiss mice with 24 h sampling time. Compounds I and II displayed highly significant differences at 50, 100 and 200 mg/kg doses (p < 0.01) and their mutagenic dose response curves correlated closely with an inverted U-shaped form whose interpretation is still the subject of controversy. NQ-2-OH only produced a significant increase in micronucleus frequency at 50 mg/kg, whereas no mutagenic activity was found for compound III and DMAI at the doses assayed. At 50 mg/kg the order of relative mutagenic potencies was I > II > NQ-2-OH. Mechanisms advanced to explain loss of drug activity at high doses include capture saturation, enzymatic induction during metabolism and participation of an independent defense system.

Antibiotic activity of isoxazolylnaphthoquinone imines on mice infected with Staphylococcus aureus

The antibiotic activity of new synthetic isoxazolylnaphthoquinone imines was studied. Pseudomonas aeruginosa ATCC 27853 and Escherichia coli ATCC 25922 were resistant to the four compounds studied (MIC > 128 micrograms ml-1), but Staphylococcus aureus ATCC 25923, ATCC 29213 and 30 clinical isolates of Staph. aureus were inhibited by 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1,4-naphthoquinone-4-imine (I). This compound diminished bloodstream infection of mice injected i.m. with Staph. aureus; septicaemia decayed significantly when I was applied at the beginning of the infection while when I was given 3 d after bacterial challenge, a significant protection was afforded. Bactericidal activity in serum increased during the 5 h after I was administered i.p. The acetyl derivative of I had a high MIC but when inoculated orally in mice decreased the Staph. aureus counts in circulation. This protection occurred only when the schedule of administration started close to the bacterial challenge. Antibiotic activity in vivo may be associated with in vitro effects.

Stability determination of 3-bromo-2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)- 1,4-naphthoquinon-4-imine in ethanol by first-derivative spectrophotometry

The degradation kinetics of a new potential tripanocidal and antibacterial agent, 3-bromo-2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1,4- naphthoquinon-4-imine (2), in 95% ethanol, was investigated between 35 and 50 degrees C under room-light and light-protected conditions. The decomposition product was isolated and identified as 2-hydroxy-N-(3,4-dimethyl- 5-isoxazolyl)-1,4-naphthoquinon-4-imine (1). A simple, rapid, and stability-indicating method for the determination of 2 in the presence of 1 using "zero crossing" first-derivative spectrophotometry is reported. The validity of this method was proved using synthetic mixtures of the intact drug with its decomposition product and by statistical analysis of the calibration data. Pseudo-first-order constants for the degradation reaction of 2, obtained from linear plots of the residual concentration logarithms vs time, the calculated activation parameters Ea, delta H not equal to, and delta S not equal to were similar under room-light and light-protected conditions. The in vitro antibacterial activity of 2 was also evaluated.

Synthesis and some physiochemical properties of 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1,4-naphthoquinone 4-imine derivatives

Some derivatives of 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)- 1,4-naphthoquinone 4-imine (3), a poorly soluble drug, were synthesized in an attempt to improve their physicochemical properties. The new compounds were characterized by spectroscopic methods including an iterative NMR method (the LAOCOON III program). The physicochemical properties such as solubility, relative lipophilicity (RM), and partition coefficients (Leo-Hansch fragmental system) were determined. Some derivatives were more lipophilic than 3 and one was water soluble. In vitro antibacterial activity was also reported for some derivatives.

The challenge of Chagas' disease chemotherapy: an update of drugs assayed against Trypanosoma cruzi

The chemotherapy of Chagas' disease remains an unsolved problem, and the search for alternative drugs continues. Only two nitroheterocyclic drugs are in clinical use at the present time, and these have severely restricted applicability for chronic patients, as well as being highly toxic. This review covers drugs tested in the last 12 years. A large number of different compounds have been assayed in a variety of ways, most commonly in terms of their capacity to inhibit epimastigote proliferation. Allopurinol has emerged for the treatment of chronic cases. However, only with greater knowledge of the biochemistry of the parasite and in particular of its peculiarities, will it be possible to shift the emphasis of drug research away from random screenings onto a more rational footing. This is exemplified by recent studies carried out using purine derivatives and trypanothione reductase inhibitors.