Pentamidine in the prevention and treatment of trypanosomiasis

Drug resistance in animal trypanosomiases: Epidemiology, mechanisms and control strategies

Animal trypanosomiasis (AT) is a complex of veterinary diseases known under various names such as nagana, surra, dourine and mal de caderas, depending on the country, the infecting trypanosome species and the host. AT is caused by parasites of the genus Trypanosoma, and the main species infecting domesticated animals are T. brucei brucei, T. b. rhodesiense, T. congolense, T. simiae, T. vivax, T. evansi and T. equiperdum. AT transmission, again depending on species, is through tsetse flies or common Stomoxys and tabanid flies or through copulation. Therefore, the geographical spread of all forms of AT together is not restricted to the habitat of a single vector like the tsetse fly and currently includes almost all of Africa, and most of South America and Asia. The disease is a threat to millions of companion and farm animals in these regions, creating a financial burden in the billions of dollars to developing economies as well as serious impacts on livestock rearing and food production. Despite the scale of these impacts, control of AT is neglected and under-resourced, with diagnosis and treatments being woefully inadequate and not improving for decades. As a result, neither the incidence of the disease, nor the effectiveness of treatment is documented in most endemic countries, although it is clear that there are serious issues of resistance to the few old drugs that are available. In this review we particularly look at the drugs, their application to the various forms of AT, and their mechanisms of action and resistance. We also discuss the spread of veterinary trypanocide resistance and its drivers, and highlight current and future strategies to combat it.

Fexinidazole for Human African Trypanosomiasis, the Fruit of a Successful Public-Private Partnership

After 100 years of chemotherapy with impractical and toxic drugs, an oral cure for human African trypanosomiasis (HAT) is available: Fexinidazole. In this case, we review the history of drug discovery for HAT with special emphasis on the discovery, pre-clinical development, and operational challenges of the clinical trials of fexinidazole. The screening of the Drugs for Neglected Diseases initiative (DNDi) HAT-library by the Swiss TPH had singled out fexinidazole, originally developed by Hoechst (now Sanofi), as the most promising of a series of over 800 nitroimidazoles and related molecules. In cell culture, fexinidazole has an IC50 of around 1 µM against Trypanosoma brucei and is more than 100-fold less toxic to mammalian cells. In the mouse model, fexinidazole cures both the first, haemolymphatic, and the second, meningoencephalitic stage of the infection, the latter at 100 mg/kg twice daily for 5 days. In patients, the clinical trials managed by DNDi and supported by Swiss TPH mainly conducted in the Democratic Republic of the Congo demonstrated that oral fexinidazole is safe and effective for use against first- and early second-stage sleeping sickness. Based on the positive opinion issued by the European Medicines Agency in 2018, the WHO has released new interim guidelines for the treatment of HAT including fexinidazole as the new therapy for first-stage and non-severe second-stage sleeping sickness caused by Trypanosoma brucei gambiense (gHAT). This greatly facilitates the diagnosis and treatment algorithm for gHAT, increasing the attainable coverage and paving the way towards the envisaged goal of zero transmission by 2030.

Optimal kinetic exposures for classic and candidate antitrypanosomals

OBJECTIVES

Efficacy is determined not only by size, but also by shape, of drug exposure. Here the critical importance of the temporal pattern of drug concentrations (pharmacokinetic profile) is examined for antitrypanosomals in vitro.

METHODS

An in vitro hollow-fibre cartridge system was used to study contrasting drug profiles with four clinically used agents and two experimental candidates against the deadly parasite Trypanosoma brucei. Artificial kinetics were employed intentionally to favour either high peak concentration or sustained duration of drug.

RESULTS

Changing the shape of drug exposure significantly impacted drug efficacy. Suramin, melarsoprol and pentamidine were concentration-driven and therefore more efficacious when applied as short-lived high peaks. In contrast, difluoromethylornithine (DFMO) was time-driven, and therefore maximally effective as a constant infusion. Kinetic preference was robust over a wide range of drug exposures. Promising clinical candidates SCYX-7158 (acoziborole) and fexinidazole (parent and sulfone) were concentration-driven, suggesting optimal clinical regimens would involve relatively high but intermittent dosing.

CONCLUSIONS

Antitrypanosomals have an intrinsic pharmacokinetic driver for optimal efficacy, with important implications for clinical management and future candidate development.

The Drugs of Sleeping Sickness: Their Mechanisms of Action and Resistance, and a Brief History

With the incidence of sleeping sickness in decline and genuine progress being made towards the WHO goal of eliminating sleeping sickness as a major public health concern, this is a good moment to evaluate the drugs that 'got the job done': their development, their limitations and the resistance that the parasites developed against them. This retrospective looks back on the remarkable story of chemotherapy against trypanosomiasis, a story that goes back to the very origins and conception of chemotherapy in the first years of the 20 century and is still not finished today.

Pentamidine uptake and resistance in pathogenic protozoa: past, present and future

Diamidines, and pentamidine in particular, have a long history as valuable chemotherapeutic agents against infectious disease. Their selectivity is due mostly to selective accumulation by the pathogen, rather than the host cell; and acquired resistance is frequently the result of changes in transmembrane transport of the drug. Here, recent progress in elucidating the mechanisms of diamidine transport in three important protozoan pathogens, Trypanosoma brucei, Leishmania and Plasmodium falciparum, is reviewed, and the implications for drug resistance are discussed.

Differential sensitivity of Trypanosoma brucei rhodesiense isolates to in vitro lysis by arsenicals

Clinical isolates of Trypanosoma brucei rhodesiense, which were resistant to arsenical drugs in murine infections, were examined for resistance in vitro. A rapid lysis assay was developed which was able to predict in vivo sensitivity to melarsoprol (Mel B, Arsobal) and melarsen oxide. The assay was based on the finding that long slender bloodforms of drug-sensitive isolates would lyse in the presence of arsenicals upon incubation in heat-inactivated fetal bovine serum. On the basis of plots of decrease in the absorbance of trypanosome suspensions vs time of incubation with drug, L50 values, reflecting the drug concentration necessary for lysis of 50% of the cells within 30 min. were calculated for five strains. These values ranged from less than 30 microM for arsenical-sensitive strains to greater than 75 microM in proven arsenic refractory isolates. Calcium was essential for lysis, and the presence of the Ca2+ chelator EGTA (10 mM) in serum delayed lysis of sensitive strains. Ca2+ channel antagonists (Verapamil, Diltiazem), however, did not enhance lysis of refractory isolates when used at 20 to 30 microM. Intracellular concentrations of reduced trypanothione, the apparent target of arsenicals, were similar for all isolates, approximately 1.02 +/- 0.28 nmol/10(8) cells, as detected by monobromobimane derivitization and HPLC analysis. Uptake of melarsen oxide was found to be reduced in arsenical refractory strains. Uptake was judged by reduction of free reduced trypanothione as a result of formation of the trypanothione-arsenic complex Mel T. Little change was found in arsenical-resistant strains, but sensitive strains had 50 to 70% reductions in trypanothione levels after incubation with a low (1 microM) level of melarsen oxide.

Arsenic and old taxa: subspeciation and drug sensitivity in Trypanosoma brucei

Resistance to the trypanocidal drugs atoxyl and tryparsamide was traditionally considered to be a diagnostic feature of rhodesian sleeping sickness and, consequently, of Trypanosoma rhodesiense. In examining the tryparsamide sensitivity of 13 isoenzymically defined stocks of the subgenus Trypanozoon, typical West African stocks showed no greater drug sensitivity than did those of East and Central Africa. The greatest resistance to tryparsamide was shown by two stocks isolated in the Ivory Coast. There was no evidence of strain differences in drug sensitivity to melarsoprol (Mel B) among 26 tested populations; none the less, differential melarsoprol sensitivity was evident in clones from a single mixed population. By contrast, isoenzymically defined West African stocks appeared to be less sensitive to pentamidine and diminazene aceturate (Berenil) than were typical East African stocks. Drug sensitivity was measured in a novel in vivo test designed to minimize the influence of host-parasite interactions, in particular trypanosome penetration of drug-inaccessible sites and host-antibody induced remission of parasitaemia. Drug effect was expressed as the DS0.1, the dose required to suppress parasitaemia to 0.1% of that in untreated control mice.