This review discusses the challenges of chemotherapy for human African trypanosomiasis (HAT). The few drugs registered for use against the disease are unsatisfactory for a number of reasons. HAT has two stages. In stage 1 the parasites proliferate in the haemolymphatic system. In stage 2 they invade the central nervous system and brain provoking progressive neurological dysfunction leading to symptoms that include the disrupted sleep wake patterns that give HAT its more common name of sleeping sickness. Targeting drugs to the central nervous system offers many challenges. However, it is the cost of drug development for diseases like HAT, that afflict exclusively people of the world's poorest populations, that has been the principal barrier to new drug development and has led to them becoming neglected. Here we review drugs currently registered for HAT, and also discuss the few compounds progressing through clinical trials. Finally we report on new initiatives that might allow progress to be made in developing new and satisfactory drugs for this terrible disease.

Designed, synthetic heterocyclic diamidines have excellent activity against eukaryotic parasites that cause diseases such as sleeping sickness and leishmania and adversely affect millions of people each year. The most active compounds bind specifically and strongly in the DNA minor groove at AT sequences. The compounds enter parasite cells rapidly and appear first in the kinetoplast that contains the mitochondrial DNA of the parasite. With time the compounds are also generally seen in the cell nucleus but are not significantly observed in the cytoplasm. The kinetoplast decays over time and disappears from the mitochondria of treated cells. At this point the compounds begin to be observed in other regions of the cell, such as the acidocalcisomes. The cells typically die in 24-48 hours after treatment. Active compounds appear to selectively target extended AT sequences and induce changes in kinetoplast DNA minicircles that cause a synergistic destruction of the catenated kinetoplast DNA network and cell death.

Eighteen substituted 2,5-bis(4-guanylphenyl)furans and related analogues, including "masked" amidines in which the guanyl function is incorporated into a heterocyclic ring, have been synthesized and their antimalarial and antitrypanosomal activity has been evaluated. None of the compounds exhibited high orders of antimalarial activity; however, 11 were very active against Trypanosoma rhodesiense in mice. Six compounds, including 2,5-bis(4-guanylphenyl)furan (4) and its 3-chloro (32), 3,4-dichloro (31), 3-methyl (25), 3,4-dimethyl (20), and 3-chloro-4-methyl (38) derivatives, produced cures in mice at submilligram dosage levels; the 3,4-dimethyl (20) analogue exhibited a prolonged curative effect providing protection for 30 days after a single dose against a challenge by T. rhodesiense. These six compounds are somewhat more active in this screen than stilbamidine, hydroxystilbamidine, and pentamidine. The "masked" amidines generally exhibited lower antitrypanosomal activity than their true guanyl counterparts. Compound 4 was synthesized from 1,4-di-p-bromophenyl-1,4-butanedione by cyclodehydrative furanization to 2,5-bis(4-bromophenyl)furan (2) which was allowed to react with Cu2(CN)2 to produce the corresponding bis-nitrile 3. The latter compound was ultimately converted by way of an imidate ester into 4. Similarly, the 3- and/or 4-substituted derivatives of 2 were employed to prepare the other members of the series.

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African sleeping sickness is a fatal parasitic disease, and all drugs currently in use for treatment have strong liabilities. It is essential to find new, effective, and less toxic drugs, ideally with oral application, to control the disease. In this study, the aromatic diamidine DB75 (furamidine) and two aza analogs, DB820 and DB829 (CPD-0801), as well as their methoxyamidine prodrugs and amidoxime metabolites, were evaluated against African trypanosomes. The active parent diamidines showed similar in vitro profiles against different Trypanosoma brucei strains, melarsoprol-and pentamidine-resistant lines, and a P2 transporter knockout strain (AT1KO), with DB75 as the most trypanocidal molecule. In the T. b. rhodesiense strain STIB900 acute mouse model, the aza analogs DB820 and DB829 demonstrated activities superior to that of DB75. The aza prodrugs DB844 and DB868, as well as two metabolites of DB844, were orally more potent in the T. b. brucei strain GVR35 mouse central nervous system (CNS) model than DB289 (pafuramidine maleate). Unexpectedly, the parent diamidine DB829 showed high activity in the mouse CNS model by the intraperitoneal route. In conclusion, DB868 with oral and DB829 with parenteral application are potential candidates for further development of a second-stage African sleeping sickness drug.