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Infections with protozoan parasites are a major cause of disease and mortality in many tropical countries of the world. Diseases caused by species of the genera Trypanosoma (Human African Trypanosomiasis and Chagas Disease) and Leishmania (various forms of Leishmaniasis) are among the seventeen "Neglected Tropical Diseases" (NTDs) defined as such by WHO due to the neglect of financial investment into research and development of new drugs by a large part of pharmaceutical industry and neglect of public awareness in high income countries. Another major tropical protozoan disease is malaria (caused by various Plasmodium species), which -although not mentioned currently by the WHO as a neglected disease- still represents a major problem, especially to people living under poor circumstances in tropical countries. Malaria causes by far the highest number of deaths of all protozoan infections and is often (as in this review) included in the NTDs. The mentioned diseases threaten many millions of lives world-wide and they are mostly associated with poor socioeconomic and hygienic environment. Existing therapies suffer from various shortcomings, namely, a high degree of toxicity and unwanted effects, lack of availability and/or problematic application under the life conditions of affected populations. Development of new, safe and affordable drugs is therefore an urgent need. Nature has provided an innumerable number of drugs for the treatment of many serious diseases. Among the natural sources for new bioactive chemicals, plants are still predominant. Their secondary metabolism yields an immeasurable wealth of chemical structures which has been and will continue to be a source of new drugs, directly in their native form and after optimization by synthetic medicinal chemistry. The current review, published in two parts, attempts to give an overview on the potential of such plant-derived natural products as antiprotozoal leads and/or drugs in the fight against NTDs.

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Three series of 4-anilino-1H-pyrazolo[3,4-b]pyridine-5-carboxylic esters were synthesized as part of a program to study potential anti-Leishmania drugs. These compounds were obtained by a condensation reaction of 4-chloro-1H-pyrazolo[3,4-b]pyridine with several aniline derivatives. Some of them were also obtained by an alternative pathway involving a Mannich-type reaction. The hydrophobic parameter, log P, was determined by shake-flask methodology, and using the Hansch-Fujita addictive hydrophobic fragmental constants. These compounds were tested against promastigote forms of Leishmania amazonensis. The very promising results showed the 3'-diethylaminomethyl-substituted compounds as the most active [IC50 = 0.39 (21) and 0.12 microM (22)]. Molecular modeling, using semiempirical AM1 method, predicted the most active compounds through the low-energy conformers superimposition on amodiaquine structure. QSAR equations, derived from the IC50 values against L. amazonensis, showed the hydrophobic (log P) and Sterimol steric (L and B2) parameters as most significant contributions on biological activity.

Bioassay-guided fractionation of the bark extract of Annona foetida afforded a new antileishmanial pyrimidine-beta-carboline alkaloid, N-hydroxyannomontine (1), together with the previously reported annomontine (2), O-methylmoschatoline (3), and liriodenine (4). The structure of compound 1 was established on the basis of extensive 1D and 2D NMR and MS analyses. This is the third reported pyrimidine-beta-carboline-type alkaloid and is particularly important for Annona genus chemotaxonomy. In addition, all compounds exhibit in vitro antileishmanial activity against promastigote forms of Leishmania braziliensis. Compounds 2 and 4 showed better activity than compounds 1 and 3 against L. braziliensis. Compound 2 was not active against L. guyanensis.

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Graphical abstractClones isolated from a single benznidazole-resistant Trypanosoma cruzi population contain a stop-codon-generating mutation in the nitroreductase gene TcNTR. Clonal variation in resistance suggests that additional mechanisms must also operate.

Recebido em 2/1/08; aceito em 6/8/08; publicado na web em 15/12/08The essential oil from leaves of Annona foetida obtained by hydrodistillation was analyzed by GC/FID and GC/MS. The results showed that the major constituents were bicyclogermacrene (35.12%), (E)-caryophyllene (14.19%) and α-copaene (8.19%). The antimicrobial and antileishmanial activities were investigated. The oil showed potent antimicrobial activity against Candida albicans and Rhodococcus equi. The oil also showed significant antileishmanial activity, giving the best results against Leishmania guyanensis. A preliminary cytotoxicity assay for this oil was carried out on hamster and mice (Balb/c) peritoneal macrophages. The results obtained were similar to pentamidine and considered not to be cytotoxic to macrophages.