Drug resistance in African trypanosomiasis: the melarsoprol and pentamidine story
2013; Elsevier BV; Volume: 29; Issue: 3 Linguagem: Inglês
10.1016/j.pt.2012.12.005
ISSN1471-5007
AutoresNicola Baker, Harry P. de Koning, Pascal Mäser, David Horn,
Tópico(s)Synthesis and Biological Evaluation
ResumoMelarsoprol and pentamidine represent the two main classes of drugs, the arsenicals and diamidines, historically used to treat the diseases caused by African trypanosomes: sleeping sickness in humans and Nagana in livestock. Cross-resistance to these drugs was first observed over 60 years ago and remains the only example of cross-resistance among sleeping sickness therapies. A Trypanosoma brucei adenosine transporter is well known for its role in the uptake of both drugs. More recently, aquaglyceroporin 2 (AQP2) loss of function was linked to melarsoprol–pentamidine cross-resistance. AQP2, a channel that appears to facilitate drug accumulation, may also be linked to clinical cases of resistance. Here, we review these findings and consider some new questions as well as future prospects for tackling the devastating diseases caused by these parasites. Melarsoprol and pentamidine represent the two main classes of drugs, the arsenicals and diamidines, historically used to treat the diseases caused by African trypanosomes: sleeping sickness in humans and Nagana in livestock. Cross-resistance to these drugs was first observed over 60 years ago and remains the only example of cross-resistance among sleeping sickness therapies. A Trypanosoma brucei adenosine transporter is well known for its role in the uptake of both drugs. More recently, aquaglyceroporin 2 (AQP2) loss of function was linked to melarsoprol–pentamidine cross-resistance. AQP2, a channel that appears to facilitate drug accumulation, may also be linked to clinical cases of resistance. Here, we review these findings and consider some new questions as well as future prospects for tackling the devastating diseases caused by these parasites. ‘Cellular therapy is a consequence of cellular nutrition, for only those compounds can affect the cell that are actually eaten by it.’ – Paul Ehrlich, 1907 [1Ehrlich P. Chemotherapeutische Trypanosomen-Studien.Berl. Klin. Wochenschr. 1907; 9: 233-236Google Scholar]. African trypanosomes are parasitic protists that circulate in the bloodstream and tissue fluids of their mammalian hosts. Transmitted by tsetse flies, they cause important human and animal diseases. Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense cause human African trypanosomiasis (HAT), also known as sleeping sickness, which is typically fatal without chemotherapy, whereas the closely related, but human-serum sensitive, T. b. brucei, Trypanosoma congolense, and Trypanosoma vivax cause Nagana, an important veterinary disease. HAT affects 8.7 million km2 of Sub-Saharan Africa, areas where the climate and environment are suitable for the tsetse fly [2Simarro P.P. et al.Eliminating human African trypanosomiasis: where do we stand and what comes next?.PLoS Med. 2008; 5: e55Crossref PubMed Scopus (342) Google Scholar]. T. b. gambiense is endemic in many areas of West and Central Africa and is currently responsible for the vast majority (>90%) of HAT cases. For early-stage HAT cases in West Africa, pentamidine, an aromatic diamidine, is the drug of choice. Diagnosis is often late, however [3Wastling S.L. Welburn S.C. Diagnosis of human sleeping sickness: sense and sensitivity.Trends Parasitol. 2011; 27: 394-402Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar], revealing advanced infection with trypanosomes in the central nervous system (CNS). In these cases, eflornithine (in combination with nifurtimox) is the safest therapy [4Priotto G. et al.Nifurtimox–eflornithine combination therapy for second-stage African Trypanosoma brucei gambiense trypanosomiasis: a multicentre, randomised, phase III, non-inferiority trial.Lancet. 2009; 374: 56-64Abstract Full Text Full Text PDF PubMed Scopus (408) Google Scholar], and availability of these drugs has increased in recent years [5Simarro P.P. et al.Update on field use of the available drugs for the chemotherapy of human African trypanosomiasis.Parasitology. 2012; 139: 842-846Crossref PubMed Scopus (102) Google Scholar]; nevertheless, the highly toxic, melaminophenyl arsenical melarsoprol is still used. This is explained by the lack of efficacy of eflornithine against T. b. rhodesiense [6Iten M. et al.Alterations in ornithine decarboxylase characteristics account for tolerance of Trypanosoma brucei rhodesiense to D,L-α-difluoromethylornithine.Antimicrob. Agents Chemother. 1997; 41: 1922-1925PubMed Google Scholar] and the high cost and difficulty of administration for use against T. b. gambiense [5Simarro P.P. et al.Update on field use of the available drugs for the chemotherapy of human African trypanosomiasis.Parasitology. 2012; 139: 842-846Crossref PubMed Scopus (102) Google Scholar]. Thus, melarsoprol, a drug that causes an often fatal reactive encephalopathy in approximately 10% of patients [7Kuepfer I. et al.Safety and efficacy of the 10-day melarsoprol schedule for the treatment of second stage rhodesiense sleeping sickness.PLoS Negl. Trop. Dis. 2012; 6: e1695Crossref PubMed Scopus (40) Google Scholar], is currently the only drug active against both advanced T. b. rhodesiense and T. b. gambiense infections. Melarsoprol and pentamidine are also the most potent drugs used to treat HAT, both displaying low nanomolar 50%-effective growth-inhibitory concentrations (EC50). There have been three major epidemics of sleeping sickness recorded since the late 19th century. Tsetse control, the systematic screening for patients in at-risk populations followed by chemotherapy, and the introduction of nifurtimox–eflornithine combination therapy (NECT), have all contributed to the recent successful reduction in cases [8Nimmo C. Time to put out the lights on sleeping sickness?.Travel Med. Infect. Dis. 2010; 8: 263-268Abstract Full Text Full Text PDF PubMed Scopus (6) Google Scholar]. However, the WHO recently warned against neglect and complacency if further epidemics are to be avoided [9World Health Organization Human African trypanosomiasis: better hope for disease management.Neglected Tropical Diseases. WHO, 2009Google Scholar]. With no vaccine available and limited therapeutic alternatives, the emergence of drug resistance is a major threat in this regard [10Fairlamb A.H. Chemotherapy of human African trypanosomiasis: current and future prospects.Trends Parasitol. 2003; 19: 488-494Abstract Full Text Full Text PDF PubMed Scopus (285) Google Scholar], especially because loss of a single, non-essential transporter can result in eflornithine resistance [11Vincent I.M. et al.A molecular mechanism for eflornithine resistance in African trypanosomes.PLoS Pathog. 2010; 6: e1001204Crossref PubMed Scopus (142) Google Scholar]. In fact, the high cost and logistical burden of NECT might render this particular treatment unsustainable [5Simarro P.P. et al.Update on field use of the available drugs for the chemotherapy of human African trypanosomiasis.Parasitology. 2012; 139: 842-846Crossref PubMed Scopus (102) Google Scholar]. Therapies based on arsenicals and diamidines have been prominent in efforts to tackle HAT for over 100 years, and selected important developments during this time are summarised in Figure 1 and detailed below. The first organic arsenical, aminophenyl arsonic acid, euphemistically named atoxyl, was introduced as a treatment for HAT in the early 1900s [12Thomas H.W. Some experiments in the treatment of trypanosomiasis.Br. Med. J. 1905; 1: 1140-1143Crossref PubMed Scopus (14) Google Scholar]. This drug was partly replaced by its less toxic N-substituted derivative tryparsamide (arsonophenylglycineamide) [13Jacobs W.A. Heidelberger M. Chemotherapy of trypanosome and spirochete infections: chemical series I. N-Phenylglycineamide-p-arsonic acid.J. Exp. Med. 1919; 30: 411-415Crossref PubMed Scopus (5) Google Scholar] in the early 1920s, although it was not effective against T. b. rhodesiense or arsenic-resistant T. b. gambiense [14Williamson J. Review of chemotherapeutic and chemoprophylactic agents.in: Mulligan H.W. The African Trypanosomiases. George Allen and Unwin, 1970: 125-221Google Scholar]. 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Evolution of dyskinetoplastic trypanosomes: how, and how often?.Trends Parasitol. 2010; 26: 557-558Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar]. In the case of pentamidine, toxicity by other means is certainly likely because the drug accumulates in trypanosomes to millimolar concentrations [36Damper D. Patton C.L. Pentamidine transport and sensitivity in brucei-group trypanosomes.J. Protozool. 1976; 23: 349-356Crossref PubMed Scopus (78) Google Scholar]. Indeed, cells apparently lacking kinetoplast DNA remain sensitive to pentamidine [36Damper D. Patton C.L. Pentamidine transport and sensitivity in brucei-group trypanosomes.J. Protozool. 1976; 23: 349-356Crossref PubMed Scopus (78) Google Scholar], possibly due to disruption of mitochondrial membrane potential [37Lanteri C.A. et al.The mitochondrion is a site of trypanocidal action of the aromatic diamidine DB75 in bloodstream forms of Trypanosoma brucei.Antimicrob. 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By testing different classes of compounds, Ehrlich was also able to deduce that cross-resistance was often restricted to one class, and did not extend to an unrelated class, postulating that changes in specific ‘chemioreceptors’ conferred resistance [42Ehrlich P. Address in Pathology – On Chemiotherapy. Delivered before the Seventeenth International Congress of Medicine.Br. Med. J. 1913; 2: 353-359Crossref PubMed Scopus (115) Google Scholar]. These remarkable insights hold true today, but the identity of many of the molecules potentially involved in resistance have been revealed only recently. Trypanosomes rely on uptake of essential nutrients from the host and, therefore, bear a number of transporters at the cell surface and in the flagellar pocket, an invagination of the pellicular membrane that is inaccessible to host innate immune effectors and the exclusive site for endocytosis, exocytosis, and specific receptors [43Field M.C. Carrington M. The trypanosome flagellar pocket.Nat. 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The prime candidate was the high-affinity pentamidine transporter (HAPT1), an activity recorded in bloodstream-form T. brucei using low nanomolar concentrations of [3H]-pentamidine [66de Koning H.P. Uptake of pentamidine in Trypanosoma brucei brucei is mediated by three distinct transporters: implications for cross-resistance with arsenicals.Mol. Pharmacol. 2001; 59: 586-592Crossref PubMed Scopus (131) Google Scholar]. Melarsoprol also appears to be a substrate for HAPT1 [62Matovu E. et al.Mechanisms of arsenical and diamidine uptake and resistance in Trypanosoma brucei.Eukaryot. Cell. 2003; 2: 1003-1008Crossref PubMed Scopus (187) Google Scholar], and selection for increased resistance to pentamidine or melarsoprol in Tbat1 null cells led to specific loss of HAPT1 activity [64Bridges D.J. et al.Loss of the high-affinity pentamidine transporter is responsible for high levels of cross-resistance between arsenical and diamidine drugs in African trypanosomes.Mol. Pharmacol. 2007; 71: 1098-1108Crossref PubMed Scopus (108) Google Scholar]. Apart from resistance through loss of drug uptake, the other common resistance mechanism associated with changes in net drug accumulation is the energy-dependent extrusion of the compound, prodrug, or active metabolite by ATP-binding cassette (ABC) transporters, which include P-glycoprotein and multidrug resistance-associated transporters (MRPs). These transporters perform many biological functions besides the exclusion of xenobiotics from cells [68Sauvage V. et al.The role of ATP-binding cassette (ABC) proteins in protozoan parasites.Mol. Biochem. Parasitol. 2009; 167: 81-94Crossref PubMed Scopus (71) Google Scholar], but have frequently been associated with antibiotic resistance in bacteria and fungi, and with treatment failure in cancer [69Borst P. Elferink R.O. Mammalian ABC transporters in health and disease.Annu. Rev. Biochem. 2002; 71: 537-592Crossref PubMed Scopus (1361) Google Scholar, 70Nikaido H. Multidrug resistance in bacteria.Annu. Rev. Biochem. 2009; 78: 119-146Crossref PubMed Scopus (1126) Google Scholar]. Their involvement in drug resistance in protists, including Plasmodium and Leishmania spp., is also well documented [71Klokouzas A. et al.ABC transporters and drug resistance in parasitic protozoa.Int. J. Antimicrob. Agents. 2003; 22: 301-317Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar]. In T. brucei, overexpression of the ABC transporter, MRPA, resulted in increased efflux of the melarsoprol–trypanothione adduct Mel T [72Shahi S.K. et al.Overexpression of the putative thiol conjugate transporter TbMRPA causes melarsoprol resistance in Trypanosoma brucei.Mol. Microbiol. 2002; 43: 1129-1138Crossref PubMed Scopus (81) Google Scholar]. However, MRPA overexpression was insufficient to cause melarsoprol-resistance i
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