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 by the WHO. Furthermore, malaria (caused by various Plasmodium species) can be considered a neglected disease in certain countries and with regard to availability and affordability of the antimalarials. Living organisms, especially plants, provide an innumerable number of molecules with potential for the treatment of many serious diseases. The current review 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. In part I, a general description of the diseases, the current state of therapy and need for new therapeuticals, assay methods and strategies applied in the search for new plant derived natural products against these diseases and an overview on natural products of terpenoid origin with antiprotozoal potential were given. The present part II compiles the current knowledge on natural products with antiprotozoal activity that are derived from the shikimate pathway (lignans, coumarins, caffeic acid derivatives), quinones of various structural classes, compounds formed via the polyketide pathways (flavonoids and related compounds, chromenes and related benzopyrans and benzofurans, xanthones, acetogenins from Annonaceae and polyacetylenes) as well as the diverse classes of alkaloids. In total, both parts compile the literature on almost 900 different plant-derived natural products and their activity data, taken from over 800 references. These data, as the result of enormous efforts of numerous research groups world-wide, illustrate that plant secondary metabolites represent an immensely rich source of chemical diversity with an extremely high potential to yield a wealth of lead structures towards new therapies for NTDs. Only a small percentage, however, of the roughly 200,000 plant species on earth have been studied chemically and only a small percentage of these plants or their constituents has been investigated for antiprotozoal activity. The repository of plant-derived natural products hence deserves to be investigated even more intensely than it has been up to present.

Leishmaniasis is a neglected infectious disease caused by kinetoplastid protozoans. An urgent need for novel chemotherapeutics exists. The current approaches to discover new antileishmanial compounds present many drawbacks, including high-cost and time-consuming bioassays. Thus, advances in leishmaniasis treatment are limited, and the development of screening assays is hindered. The combination of multidisciplinary approaches using standardised methods and synchronous projects could be an alternative to develop novel drugs for leishmaniasis treatment. In this review, we discuss the current status of leishmaniasis occurrence and treatment. In addition, we address the advantages and limitations of in vitro leishmaniasis bioassays and discuss the findings of drug discovery research using natural products. Finally, we comprehensively review the marine natural products that are active against Leishmania spp., including their natural sources and bioactivity profile.

Objectives: We devised liposome-entrapped antimony with the negatively charged lipid phosphatidylserine—liposome-entrapped antimony (Sb-LP)—in order to improve their targeting to infected macrophages through the interaction with scavenger receptors (SRs). Methods: SR production was indirectly evaluated by its mRNA synthesis in infected and uninfected peritoneal macrophages using RT–PCR. The interaction and cytotoxicity of Sb-LP with SRs and their metabolism were determined by incubation with macrophages in the presence of cytochalasin B, chloroquine or different competitive ligands, with determination of the 50% inhibitory concentration (IC₅₀) in vitro in infected macrophages. The intracellular trafficking of Sb-LP was evaluated by confocal microscopy using trapped fluorescent dyes. Results: Our results showed an up-regulation of macrophage SR mRNA during the initial steps of Leishmania (L.) chagasi infection. By competitive ligand assays, we demonstrated the preferential uptake of Sb-LP by macrophage SRs. Sb-LP was 16-fold more effective (IC₅₀=14.11 μM) than the free drug (IC₅₀=225.9 μM) against L. (L.) chagasi-infected macrophages. The binding and uptake of Sb-LP in macrophages were shown to be energy-dependent and were reduced in the presence of cytochalasin B, showing the dependency of the cell microfilament system. Confocal analysis using trapped fluorescent dyes showed fluorescence of parasites or in their close proximity, compatible with the localized delivery of the liposomes. Conclusions: The uptake of Sb-LP was reduced in infected macrophages, despite their effectiveness and targeting ability, suggesting a low metabolic rate in infected macrophages that could be overcome by the higher efficiency of the liposomal formulation. These in vitro results suggest that liposomes could improve the therapeutic index of old drugs, such as pentavalent antimony, via targeted delivery to Leishmania-infected cells.