Novel compounds to combat trypanosomatid infections: a medicinal chemical perspective

Introduction: The current therapeutic arsenal against the kinetoplastids Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp. is clearly inadequate and underscores the urgent need to develop new effective, safe and cost-effective drugs.

Areas covered: Accordingly, this review of patented products and processes using anti-kinetoplastid agents provides insight into the identification of novel or more refined drugs. In this review, we describe products developed in recent years for the treatment of human African trypanosomiasis, American trypanosomiasis and leishmaniasis from a medicinal chemical perspective.

Expert opinion: Applications so far have looked only superficially for candidate anti-trypanosomatid drugs and are deficient in the final stages of drug development studies, i.e., tolerance/safety, selectivity, drug-resistance, scaling-up, pharmacokinetic and pharmacodynamic assays. The ultimate goal for production of agents with anti-HAT activity has been the development of dicationic agents with parasite DNA-binding activity. Another goal for control of human African trypanosomiasis as well as for Chagas disease and leishmaniasis is the development of protease inhibitors. It should also be noted that several recent studies describing promising targets and compounds have not yet been patented. An effort should be made by foundations, international health organizations and pharmaceutical corporations to support analysis and development of promising new chemotherapeutic agents for controlling the trypanosomiasis and leishmaniasis.     

Natural products from plants as drug candidates and lead compounds against leishmaniasis and trypanosomiasis

Millions of people in the developing world are affected by diseases caused by the kinetoplastid parasites: the leishmaniases, African trypanosomiasis, and Chagas disease. In many cases the drugs employed for treatment are toxic, marginally effective, given by injection, and/or compromised by the development of resistance. Since safe, effective, and affordable chemotherapeutic agents for leishmaniasis and trypanosomiasis are clearly needed, the identification of new antikinetoplastid drug candidates should be an urgent priority. Numerous plant-derived natural products from different structural classes have been investigated as antileishmanial and antitrypanosomal candidates, including various alkaloids, terpenoids, flavonoids, and quinonoids. This review outlines the antikinetoplastid activities of plant-derived natural products reported in the literature and also provides an overview of mechanistic studies that have been conducted with these compounds. Given the activities of these agents and their diverse range of effects on parasite biology, natural products are a potentially rich source of drug candidates and leads against leishmaniasis and trypanosomiasis.     

Therapeutic potential of phosphodiesterase inhibitors in parasitic diseases

Protozoan parasites of the order kinetoplastida are the causative agents of three of the world's most important neglected human diseases: African trypanosomiasis, American trypanosomiasis, and leishmaniasis. Current therapies are limited, with some treatments having serious and sometimes lethal side effects. The growing number of cases that are refractory to treatment is also of concern. With few new drugs in development, there is an unmet medical need for new, more effective, and safer medications. Recent studies employing genetic and pharmacological techniques have begun to shed light on the role of the cyclic nucleotide phosphodiesterases in the life cycle of these pathogens and suggest that these important regulators of cyclic nucleotide signaling may be promising new targets for the treatment of parasitic diseases.     

Discovery of trypanocidal compounds by whole cell HTS of Trypanosoma brucei

Chemotherapy against human African trypanosomiasis relies on four drugs that cause frequent and occasionally severe side-effects. Because human African trypanosomiasis is a disease of poor people in Africa, the traditional market-driven pathways to drug development are not available. One potentially rapid and cost-effective approach to identifying and developing new trypanocidal drugs would be high throughput-screening of existing drugs already approved for other uses, as well as clinical candidates in late development. We have developed an ATP-bioluminescence assay that could be used to rapidly and efficiently screen compound libraries against trypanosomes in a high throughput-screening format to validate this notion. We screened a collection of 2160 FDA-approved drugs, bioactive compounds and natural products to identify hits that were cytotoxic to cultured Trypanosoma brucei at a concentration of 1 mum or less. This meant that any hit identified would be effective at a concentration readily achievable by standard drug dosing in humans. From the screen, 35 hits from seven different drug categories were identified. These included the two approved trypanocidal drugs, suramin and pentamidine, several other drugs suspected but never validated as trypanocidal, and 17 novel trypanocidal drugs.     

Carlina oxide--a natural polyacetylene from Carlina acaulis (Asteraceae) with potent antitrypanosomal and antimicrobial properties

Carlina acaulis (Asteraceae) has a long history of medicinal use in Europe due to its antimicrobial properties. The strong activity of Carlina oxide, themain compound of the essential oil of C. acaulis against two MRSA strains, Streptococcus pyogenes, Pseudomonas aeruginosa, Candida albicans, and C. glabratawas confirmed. A strong and selective activity against Trypanosoma brucei brucei with an IC?? of 1.0 μg/mL and a SI of 446 compared to human HeLa cells was recorded. The selective toxicity of Carlina oxidemakes it a promising lead compound for the development of drugs to treat African trypanosomiasis and multiresistant gram-positive bacteria.     

The proteasome as a potential target for chemotherapy of African trypanosomiasis

Trypanosomes are parasitic protozoans that afflict both humans and livestock in sub‐Saharan Africa. The current chemotherapy of African trypanosomiasis relies on a few drugs, most of which were developed decades ago. Combinations of toxic side effects and poor efficacy of current drugs demand an urgent need for the development of novel and effective therapeutic agents. The proteasome is a multisubunit proteinase complex that plays a critical role in intracellular protein degradation. Despite being essential to all eukaryotic cells, there are significant differences between the trypanosomal and mammalian proteasomes that makes this enzyme complex a promising target for anti‐trypanosomal drug development. In this review article, the structural properties, enzymatic activities, and inhibitor sensitivities of the proteasomes of Trypanosoma brucei and mammalian cells are compared. In addition, the trypanocidal activities of the different classes of proteasome inhibitors are summarized. Drug Dev Res 68:205–212, 2007. © 2007 Wiley‐Liss, Inc.     

The therapeutic potential of inhibitors of the trypanothione cycle

There is an urgent need for new drugs in the treatment of human African trypanosomiasis, Chagas’ disease and leishmaniasis. This article provides an overview of current drugs, formulations and their deficiencies. Targets for the design of new drugs and the rational provided for targeting enzymes of the trypanothione cycle are described. Biochemical aspects of the cycle and the currently investigated target trypanothione reductase are discussed as are the several classes of inhibitors and their in vitro potencies. Evidence is provided for considering the tryparedoxins as a new target for antiprotozoal chemotherapy and a summary of glutathione-based inhibitors with significant in vitro activity is reported.     

The therapeutic potential of inhibitors of the trypanothione cycle

There is an urgent need for new drugs in the treatment of human African trypanosomiasis, Chagas' disease and leishmaniasis. This article provides an overview of current drugs, formulations and their deficiencies. Targets for the design of new drugs and the rational provided for targeting enzymes of the trypanothione cycle are described. Biochemical aspects of the cycle and the currently investigated target trypanothione reductase are discussed as are the several classes of inhibitors and their in vitro potencies. Evidence is provided for considering the tryparedoxins as a new target for antiprotozoal chemotherapy and a summary of glutathione-based inhibitors with significant in vitro activity is reported.     

Small molecules and antibodies for the treatment of psoriasis: a patent review (2010–2015)

Introduction: Psoriasis is a chronic condition whose therapeutic armamentarium is increasingly being discussed, particularly when compared to past decades. The use of biologic agents has profoundly changed the history of this disease, as well as the management of psoriatic patients. Due to the enormous interest in psoriasis, as demonstrated within the scientific community and pharmaceuticals, new therapeutic targets have been identified and novel patented therapeutics are being tested.

Areas covered: This review sought to give an overview of small molecules and antibodies patented in the last five years for the treatment of psoriasis. Therapeutic agents either in the early or advanced phase of development have been described, primarily based on a systematic search using the PubMed Medline database.

Expert opinion: Though the recent introduction of new antipsoriatic agents has facilitated the management of long-term psoriasis, there is still a strong desire for alternative therapeutic options. Indeed, there remain unmet needs regarding safety and efficacy of psoriasis treatment that should be addressed. In this context, recently patented drugs may prove valid, interesting, and promising within the therapeutic paradigm.     

The synthesis of parasitic cysteine protease and trypanothione reductase inhibitors

The presence of parasitic cysteine proteases and trypanothione reductase in the parasitic protozoa of the genus Trypanosoma and Leishmania has made these enzymes attractive targets for the development of antitrypanosomal and antileishmanial agents. Furthermore, the presence of cysteine proteases in Plasmodium falciparum has presented additional opportunities for the development of chemical scaffolds that could potentially be utilized against all of the aforementioned parasites. While previous reviews on parasitic cysteine proteases and trypanothione reductase covered various aspects, none emphasized the chemistry behind the synthesis of described inhibitors. This review focuses on recent developments in the synthesis of low-molecular weight inhibitors of these enzymes with a bearing on the human diseases of leishmaniasis, malaria and trypanosomiasis. Only those inhibitors whose synthesis has been described in the open literature during the period 1993-mid 2002 have been highlighted. The review thus excludes what may be in the patent literature. Inhibitors synthesized using combinatorial and/or parallel synthesis chemistry as well as polymer-assisted synthesis methodologies have been deliberately omitted from this review because they are a subject of a separate and focused review.     

Diamidines as antitrypanosomal, antileishmanial and antimalarial agents

Diamidine-containing compounds have a long history of use in the treatment of African trypanosomiasis and leishmaniasis. The discovery that diamidine prodrugs possess in vivo antimicrobial activity when administered orally has led to a renewed interest in this class of compounds for the treatment of parasitic infections. In this review, the selectivity of diamidines against trypanosomes, Leishmania and Plasmodium is rationalized through mechanism-of-action studies. An overview of the antiprotozoal activities of newer diamidines and diamidine prodrugs is also presented, along with a summary of the progress made toward the clinical development of new diamidines for use against these parasitic diseases.     

Neglected disease - african sleeping sickness: recent synthetic and modeling advances

Human African Trypanosomiasis (HAT) also called sleeping sickness is caused by subspecies of the parasitic hemoflagellate Trypanosoma brucei that mostly occurs in sub-Saharan Africa. The current chemotherapy of the human trypanosomiases relies on only six drugs, five of which have been developed more than 30 years ago, have undesirable toxic side effects and most of them show drug-resistance. Though development of new anti-trypanosomal drugs seems to be a priority area research in this area has lagged far behind. The given review mainly focus upon the recent synthetic and computer based approaches made by various research groups for the development of newer anti-trypanosomal analogues which may have improved efficacy and oral bioavailability than the present ones. The given paper also attempts to investigate the relationship between the various physiochemical parameters and anti-trypanosomal activity that may be helpful in development of potent anti-trypanosomal agents against sleeping sickness.     

Sterol 14alpha-demethylase (CYP51) as a therapeutic target for human trypanosomiasis and leishmaniasis

Pathogenic protozoa threaten lives of several hundred million people throughout the world and are responsible for large numbers of deaths globally. The parasites are transmitted to humans by insect vectors, more than a hundred of infected mammalian species forming reservoir. With human migrations, HIV-coinfections, and blood bank contamination the diseases are now spreading beyond the endemic tropical countries, being found in all parts of the world including the USA, Canada and Europe. In spite of the widely appreciated magnitude of this health problem, current treatment for sleeping sickness (Trypanosoma brucei), Chagas disease (Trypanosoma cruzi) and leishmaniasis (Leishmania spp.) remains unsatisfactory. The drugs are decades old, their efficacy and safety profiles are unacceptable. This review describes sterol 14α-demethylase, an essential enzyme in sterol biosynthesis in eukaryotes and clinical target for antifungal azoles, as a promising target for antiprotozoan chemotherapy. While several antifungal azoles have been proven active against Trypanosomatidae and are under consideration as antiprotozoan agents, crystal structures of sterol 14α-demethylases from three protozoan pathogens, Trypanosoma brucei, Trypanosoma cruzi and Leishmania infantum provide the basis for the development of new, highly potent and pathogen-specific drugs with rationally optimized pharmacological properties.     

High-throughput analysis of an RNAi library identifies novel kinase targets in Trypanosoma brucei

New drugs are needed to treat human African trypanosomiasis because the currently approved treatments are toxic or limited in efficacy. One strategy for developing new drugs involves discovering novel genes whose products can be targeted for modulation by small‐molecule chemotherapeutic agents. The Trypanosoma brucei genome contains many genes with the potential to become such targets. Kinases represent one group of genes that regulate many important cell functions and can be modulated by small molecules, thus representing a promising group of enzymes to screen for potential therapeutic targets. RNAi screens could help identify the most promising kinase targets, but the lack of suitable assays represents a barrier for optimizing the use of this technology in T. brucei. Here, we describe an RNAi screen of a small RNAi library targeting 30 members of the T. brucei kinome utilizing a luciferase‐based assay. This screen both validated the luciferase‐based assay as a suitable method for conducting RNAi screens in T. brucei and also identified two kinases (CRK12 and ERK8) that are essential for normal proliferation by the parasite.     

The kinetoplastid chemotherapy revisited: current drugs, recent advances and future perspectives

Leishmaniasis, African sleeping sickness and Chagas disease, caused by the kinetoplastid parasites Leishmania spp, Trypanosoma brucei and Trypanosoma cruzi, respectively, are among the most important parasitic diseases, affecting millions of people and considered to be within the most relevant group of neglected tropical diseases. The main alternative to control such parasitosis is chemotherapy. Nevertheless, the current chemotherapeutic treatments are far from being satisfactory. This review outlines the current understanding of different drugs against leishmaniasis, African sleeping sickness and Chagas disease, their mechanism of action and resistance. Recent approaches in the area of anti-leishmanial and trypanocidal therapies are also enumerated, new modulators from the mode of action, development of new formulations of old drugs, therapeutic switching and "in silico" drug design.     

Antitrypanosomal naphthylisoquinoline alkaloids and related compounds

In view of the need to develop new drugs against human African trypanosomiasis, a series of naturally occurring naphthylisoquinoline alkaloids, axially chiral acetogenic products derived from tropical plants, have been investigated for their activity against Trypanosoma brucei brucei TC 221. Likewise compounds corresponding to the two molecular portions, the naphthalene and the isoquinoline parts were tested, as well as molecules related to the central biaryl core of the alkaloids. Among all compounds tested, the natural, genuine alkaloids themselves, in particular dioncophylline B with its biaryl system and a moderate number of free hydroxy functions, showed the highest activities. Our results demonstrate that naphthylisoquinoline alkaloids constitute an interesting novel class of antitrypanosomal compounds worth further optimization.     

Characterizing the bi-subunit type IB DNA topoisomerase of Leishmania parasites; a novel scenario for drug intervention in trypanosomatids

African and South American trypanosomes and leishmanias are unicellular protozoan parasites, forming part of the order Kinetoplastida. These ancient eukaryotes are causative agents of some of the most devastating neglected Tropical Diseases called trypanosomiasis and leishmaniasis. Despite the efforts to develop effective vaccines, immunoprophylaxis is not even a method of prevention of these diseases at present. Current antiprotozoal chemotherapy is often expensive, has side or toxic effects and it does not provide economic profits to the Pharmaceuticals, which have scant enthusiasm in R + D investments in this field. The surprising finding of unusual bi-subunit type IB DNA-topoisomerase in kinetoplastids adds a new promising drug target to antiprotozoal chemotherapy. The remarkable differences between trypanosomal and leishmanial DNA-topoisomerase IB with respect to the one in the mammalian hosts, have provided a new lead in the study of structural determinants that can be effectively targeted. This review provides an update on recent progress in research in kinetoplastid's topoisomerase IB as potential chemotherapeutic target against this group of parasitic diseases.     

Ultrastructural alterations in organelles of parasitic protozoa induced by different classes of metabolic inhibitors

Parasitic protozoa such as Leishmania, Trypanosoma, Plasmodium, Toxoplasma gondii, Giardia and Trichomonas are able to cause several diseases affecting millions of people around the world with dramatic consequences to the socio-economic life of the affected countries. Diseases like malaria, leishmaniasis and trypanosomiasis have been classified by the World Health Organization as neglected diseases, because they have been almost completely forgotten by the governments as well as the pharmaceutical companies. The specific chemotherapy currently employed for the treatment of these diseases has serious limitations due to lack of efficacy, toxic side effects, growth of drug-resistance and high costs. Thus, it is urgent to develop new chemotherapeutic agents that are more effective, safe and accessible. In this context, several works have been focused on understanding the effect of different drug-treatments on these parasitic protozoa. Organelles and structures such as mitochondrion, kinetoplast, apicoplast, glycosome, acidocalcisome, hydrogenosome, plasma membrane and the cytoskeleton have been studied using different approaches to identify new targets for the development of new chemotherapeutic agents that are required. Some studies on alterations in the fine structure, as assayed using electron microscopy, have indicated the nature of lesions induced by several drugs, allowing deductions on possible modes of action. Here, we briefly review the available data of the effects of several drugs on the ultrastructure of parasitic protozoa and show how electron microscopy can contribute to elucidate the different mechanisms of these anti-parasitic drugs.     

Evaluation of azasterols as anti-parasitics

In this article, the design and synthesis of some novel azasterols is described, followed by their evaluation against Trypanosoma brucei rhodesiense, T. cruzi, Leishmania donovani, and Plasmodium falciparum, the causative agents of human African trypanosomiasis, Chagas disease, leishmaniasis, and malaria, respectively. Some of the compounds showed anti-parasitic activity. In particular, a number of compounds appeared to very potently inhibit the growth of the blood stream form T. b. rhodesiense, with one compound giving an IC50 value of 12 nM. Clear structure activity relationships could be discerned. These compounds represent important leads for further optimization. Azasterols have previously been shown to inhibit sterol biosynthesis in T. cruzi and L. donovani by the inhibition of the enzyme sterol 24-methyltransferase. However, in this case, none of the compounds showed inhibition of the enzyme. Therefore, these compounds have an unknown mode of action.     

Parasitic diseases: Liposomes and polymeric nanoparticles versus lipid nanoparticles

Parasitic diseases such as malaria, leishmaniasis, and trypanosomiasis represent a significant global burden and pose a great challenge to drug discovery and delivery scientists due to their intracellular nature and disseminated locations. Moreover, poor rate of discovery in the anti-parasitic segment seen in last few decades has necessitated effective management of existing drugs by modulating their delivery. The review focuses on the biological and biopharmaceutical issues to be considered in the design of delivery strategy for treating parasitic infections such as malaria, leishmaniasis, and trypanosomiasis. Also, it describes the role of the colloidal carriers liposomes, polymeric nanoparticles, lipid nanoparticles including lipid drug conjugate (LDC) nanoparticles in optimizing the delivery of anti-malarial, anti-leishmanial and anti-trypanosomial agents. Furthermore, the review emphasizes especially the potential of solid lipid nanoparticles (SLN) in the treatment of parasitic infections with the help of recent reports and our own experience.