Trypanosomatid Diseases (Molecular Routes to Drug Discovery) || New Developments in the Treatment of Late-Stage Human African Trypanosomiasis

27New Developments in the Treatment of Late-Stage HumanAfrican TrypanosomiasisCyrus J. Bacchi, Robert T. Jacobs, and Nigel Yarlett�

AbstractHuman African trypanosomiasis (African sleeping sickness) is a disease caused byvarious subspecies of the parasite Trypanosoma brucei and spread through an insectvector, the tsetse fly, in sub-Saharan Africa. Although only relatively small numbersof patients (estimated by the World Health Organization to be approximately 10 000per annum) are diagnosed each year, all victims of this disease will progress to asecond-stage central nervous system disease that is 100% fatal if untreated. Currenttreatment options for HAT are limited to old, toxic, and ineffective drugs that aredifficult to administer in the disease endemic area, particularly for the second-stagedisease. Consequently, there is an urgent need for the discovery and development ofnew agents to treat Stage 2 HAT. This chapter reviews some recent efforts in thisarea, in particular the discovery and development of a new class of novel boron-containing compounds, the benzoxaboroles.

Introduction

African trypanosomiasis, human African trypanosomiasis (HAT), and animaltrypanosomiasis (Ngana) go back to reports at the beginning of recorded Africanhistory. European explorers prior to the nineteenth century describe a disease whichcould only have been HAT. Later, explorers such as Livingstone, Speke, Stanley,Burton, Bruce, and others began noticing dramatic loss of livestock to a wastingdisease that disrupted their intended destination (e.g., the head waters of the Nile,Lake Victoria). Soon human disease, resembling the animal wasting disease, beganplaguing members of these expeditions, with usually fatal results. Very often portersand other expeditionmembers would come down with fever, chills, and an extendedwasting disease, always after being bitten by a large fly with the hatchet-shapedvenations in its wings – the tsetse fly [1].Epidemics of huge proportions were frequent in preclinical Africa with reports of

entire villages being wiped out. By 1900, the disease had been attributed to amicroscopic blood flagellate, Trypanosoma brucei, with different subspecies proposed

� Corresponding Author

Trypanosomatid Diseases: Molecular Routes to Drug Discovery, First edition. Edited by T. Jäger, O. Koch, and L. Flohé.# 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.

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as agents for the human diseases in West Africa ( T. b. gambiense ), East Africa (T. b .rhodesiense ), and domestic and wild animals (T. b., rhodesiense and T. b. brucei). Theveterinary trypanosomes prevented large-scale cattle ranching and remain a con-stant menace for farmers who own livestock [1].

Life Cycle of T. b. brucei

The agent of HAT, the trypanosome, is transmitted by the tsetse fly, and has a cyclicallife cycle (Figure 27.1) in which it changes morphologically and biochemicallybetween vector insect and human (or veterinary) hosts. After a blood meal from aninfected host, bloodstream trypanosomes migrate to the stomach of the fly, undergotransformation (procyclic trypomastigotes), and migrate to the salivary glandswhere they undergo another transformation (epimastigotes). In the salivary glandsof the tsetse they become infectious in the saliva. When the fly bites, these forms(metacyclic trypomastigotes) are transmitted, multiplying at the bite site, and thenprogressively invading the bloodstream and lymphatics of the new host. The cycle inthe fly takes 25–50 days depending on environmental conditions. The fly injects upto 40 000 metacyclic forms when it feeds.

Figure 27.1 Life cycle of T. brucei, causative agent of human African trypanosomiasis.(Reproduced from: http://dpd.cdc.gov/dpdx/HTML/ImageLibrary/TrypanosomiasisAfrican_il.htm).

516j 27 New Developments in the Treatment of Late-Stage Human African Trypanosomiasis

HAT is endemic over 10 million square kilometers of sub-Saharan Africa,threatening 60 million people [2] and all livestock. The human disease is alwaysfatal if untreated. In humans, two types of human disease are recognized, WestAfrican sleeping sickness (T. b. gambiense) and East African disease (T. b. rhodesiense).Of the two diseases, East African HAT is the more rapid developing disease, lastingless than 6 months from initiation of symptoms to death. In West African diseaseprogression is slower with death coming in 1–5 years. Progression of the disease isidentical in both cases, with an abscess (chancre) forming at the site of the bite, thentrypanosomes migrating to the lymph nodes and eventually to the bloodstream. Theorganisms then migrate to internal organs and then to the central nervous system(CNS) and brain. Symptoms include fever, convulsions, and later behavioralchanges, blindness, coma, and death.The entire trypanosome surface is covered by a coat – the variant surface

glycopeptide (VSG). During infection of mammalian hosts, the VSG genes rapidlyswitch on and off expression, resulting in variation of the surface coat, whichconstantly evades the host immune system. There are about 900 genes for VSGs,which prevent practical development of a vaccine [2]. Aside from vector (tsetse fly)control, drug treatment is the only way to control the disease.Development of chemotherapy has been limited, with few drugs available and

only one new drug available clinically since 1947. Early treatment centered oninorganic arsenic administered as H3AsO4. After extreme toxicity of this compoundwas recognized, the scene was ready for a synthetic chemical approach. Paul Ehrlichbegan synthesizing organic arsenicals in 1905. His early efforts resulted in “Salvar-san 606”, the “Magic Bullet” which killed trypanosomes in vitro and was later usedsuccessfully against syphilis. His work also involved compounds being developed bythe German dye industry, which later led to the synthesis of suramin, now used forearly-stage disease [1].

Current Chemotherapy

Melarsoprol

In the mid-1940s Ernst Freidheim, working at the Rockefeller University inNew York, began chemical synthesis of organic arsenicals based on the earlywork of Ehrlich. In 1947 he synthesized melarsoprol, a melanophenyl-basedarsenical (Figure 27.2). Freidheim traveled to Africa and successfully treated patientswith late-stage West African disease using melarsoprol [1,3].Until recently, melarsoprol was the only available agent for late-stage CNS disease;

however, there are serious considerations concerning its use. Melarsoprol causes apost-treatment reactive encephalopathy (PTRE) in about 20% of all patients. About5% of all patients receiving melarsoprol die from complications of PTRE. Melarso-prol is not water soluble and is soluble only in propylene glycol. Intravenousadministration of melarsoprol is painful, and vein thrombophlebitis and atrophyis common [4]. Dose regimens are usually once a day for 10 days, followed by 7 days

Current Chemotherapy j517

rest and an additional 10-day treatment [5]. Recently, failure rates up to 30% havebeen reported at several clinical locations [6], and in the highly endemic foci ofSouthern Sudan, Democratic Republic of Congo, Uganda, and Angola [7].

Suramin

This “colorless dye” is used only for early-stage HAT. Suramin (Figure 27.2), firstsynthesized aspart of theGermandye industry,wasbasedon theworkofEhrlichusingthe naphthalene dyes Trypan red and blue. It was first synthesized by Jacobs andHeidelbergat theRockefellerUniversity in1919.Suraminhasbeenusedforearly-stageT.b. gambienseandT.b. rhodesiense infectionssince1940. It is ahighly chargedmoleculeandbindstobloodproteins, resultingina longhalf-lifeandprophylacticactivity.Since itdoes not penetrate the blood–brainbarrier (BBB) it is not usedwhere late-stage diseaseis suspect [8]. It is given as a series of five intravenous infusions every 3–7 days. Side-effects include neuropathy, rash, fatigue, and renal insufficiency. Resistance tosuramin is rare in human disease. The highly charged nature of suramin promotesbinding to serum proteins which extends its half-life but prevent BBB penetration [8].

Pentamidine

This diamidine (Figure 27.2) arose from work on synthalin, an agent used to controlblood sugar levels in the 1930s [9]. Pentamidine has been in use since the early1940s, and is used primarily for early-stage T. b. gambiense and given as intra-muscular injections over 7–10 days [8]. Common side-effects (10% of patients)include reduction of serum glucose, hypotension, and pain at the injection site.Owing to its extensive use, long serum half-life, and multiple uptake transporters inthe trypanosome, resistance to pentamidine is not common [7]. It is currently usedto treat early-stage HAT infections, primarily T. b. gambiense. As it is a highly chargedmolecule, it does not penetrate the BBB and is not used for late-stage disease [8].

O

HN

NH

NH

OO

NHHN

O

NH

O

SO3NaSO3Na

SO3NaNaO3S

SO3NaNaO3S

Suramin

H2N

NH

O O

NH2

NHPentamidine

N

N

N

HN

AsS

S OH

NH2H2N

Melarsoprol

H2N F

F

HO2CNH2

Eflornithine

OO2NN N

SO2

Nifurtimox

Figure 27.2 Current chemotherapeutic agents for HAT.


There is no solid evidence for clinical resistance with most failures attributed toincorrect dosing or the presence of low level CNS disease. Uptake of pentamidinehas been attributed to three adenine/adenosine-type transporters [10,11] and it isunlikely that resistance due to uptake would arise.

Eflornithine (Ornidyl, Difluoromethylornithine)

Eflornithine (Figure 27.2), an inhibitor of ornithine decarboxylase was developed asan anti-tumor agent that failed in extensive clinical trials. This agent was found tocure early-stage acute T. b. brucei laboratory infection [12], and later found to cure alate-stage T. b. brucei model infection both alone and in combination with otheragents [13,14]. In 1990 eflornithine was licensed for clinical use for late-stage T. b.gambiense infection, the first new anti-trypanosomal agent in over 40 years [7].Although it has no serious side-effects, it must be given in four intravenous doses(total 400mg), 6 h apart for 2 weeks. This is a serious undertaking in rural clinicsresulting in frequent bloodstream infections. Recently, dosing has been reduced totwo intravenous doses per day at 12 h intervals for 1 week [6]. In general, eflornithinehas been around 90% curative in late-stage T. b. gambiense infections [15]. It is notused in T. b. rhodesiense infections. Eflornithine is expensive and its synthesis isdifficult – a major drawback to widespread use.

Nifurtimox–Eflornithine Combination Therapy

Recent clinical observations with eflornithine and nifurtimox, a nitroimidazole(Figure 27.2) used in single-drug compassionate therapy for relapsed diseases [5],led to the development of an eflornithine-nifurtimox combination regimen [16].This consists of eflornithine 400mg/kg/day, intravenously every 12 h for 1 weekaccompanied by nifurtimox 15mg/kg/day orally every 8 h for 10 days. This regimenis 96% curative with low adverse effects and is now the standard clinical regimen.Particularly important in the nifurtimox–eflornithine combination therapy (NECT)regimen is the reduction of eflornithine infusions from four per day for 2 weeks totwo per day for 1 week [16].

Need for New Chemotherapy

As evident in the preceding discussion, the major problems with current chemo-therapy for HAT are drug resistance, toxicity, and inability to penetrate the BBB.Consequently, continued research is needed for new agents that can be given orallyin a short-term regimen, and are both non-toxic and effective against Stage 2 CNSdisease. This need has been recognized, and there has been a resurgence in researchin neglected tropical diseases, including HAT, over the past decade [8]. Numerousreasons for this renewed interest are at play, including the establishment of private–public partnerships such as the Drugs for Neglected Diseases initiative (DNDi) and

Need for New Chemotherapy j519

increased philanthropic support from foundations such as the Bill and MelindaGates Foundation.

Recent Approaches to New Trypanocidal Agents

Nitroimidazoles

Nitroimidazoles (Figure 27.3) are an important class of anti-bacterial and anti-protozoal agents. They include the anti-trichomonad metronidazole, the relatedtinidazole, the anti-trypanosomal megazole, and the anti-Chagas agent benznida-zole. Although metronidazole (trichomoniasis) and benznidazole (Chagas disease)have been in use for many years, these compounds are perceived to be mutagenicbecause of the nitroaromatic group. More extensive examination of the mechanismof action of nitroheterocycles has suggested that concerns regardingmutagenicity ofthis class may be less than initially suspected and hence a renewed interest in thisclass has emerged [17,18].

Fexinidazole

This 5-nitroimidazole (Figure 27.3) is a member of a class of compounds known tohave anti-trypanosomal activity [19,20], but was abandoned because of mutagenicityconcerns [21]. Following a comprehensive review and in vitro assessment ofhundreds of nitroheterocycles by the DNDi, recent studies with fexinidazolehave demonstrated in vivo activity in murine models of both acute and CNSinfections [22]. Following extensive characterization of fexinidazole’s pharmaco-kinetics, coupled with no evidence for toxicity or mutagenicity, this compound wasprogressed to phase I clinical trials in 2010 [22].

NNHO

O2N

metronidazole

NNS

O2N

O

O

tinidazole

N

NN N

SO2N

NH2

megazol

N N

O2NNH

O

benznidazole

N

N NO2OS

fexinidazole

NN

O2NOCF3

1-aryl-4-nitro-1H-imidazoles

Figure 27.3 Nitroheterocycles for HAT.


Aryl-4-nitroimidazoles

In part based on the successful preclinical characterization of fexinidazole, addi-tional nitroimidazoles have been considered as potential candidates for optimiza-tion. For example, recent developments in chemistry have allowed synthesis of aseries of 1-aryl-4-nitro-1H-imidazoles (Figure 27.3), several of which have beenfound to be curative in murine models of acute and CNS infections of T. b .rhodesiense and T. b. brucei [23]. Dosing was orally twice daily for 4 or 5 days. Thesecompounds were not substrates of mammalian nitro-reductases and not genotoxicin mammalian cells. This series needs further investigation to progress to clinicaltrials as orally administered trypanocides.

Diamidines

Given the clinical utility of pentamidine for treatment of Stage 1 HAT, exploration ofthe diamidine class has been an active area of research for the Consortium forParasitic Drug Development (http://www.unc.edu/�jonessk/). Initially focusedon exploration of conformationally constrained analogs of pentamidine as potentialanti-malarials, the trypanocidal activity of the 2,5-bis(4-guanylphenyl)furan (struc-ture 2, Figure 27.4) was noted [24,25]. These early diamidine analogs were morepotent than pentamidine in animal models, but were active only when dosedparenterally. A key advance in the diamidine class occurred when it was discoveredthat carbamate analogs (structure 3, Figure 27.4) could serve as orally bioavailableprodrugs, initially in anti-microbial models [26]. The prodrug strategy was extendedto include amidoximes, resulting in the discovery of DB289 (structure 4, Figure 27.4)as an orally available diamidine analog efficacious inmouse [27–29] andmonkey [30]models of Stage 1 HAT. DB289 was not curative in Stage 2 models of HAT [31],presumably due to poor transport across the BBB. DB289 progressed to clinical trialsfor Stage 1 HAT, where it was found to be effective following a 10-day course oftreatment [32]. Unfortunately, further development of DB289 was suspended due toliver and kidney toxicities [32]. More recent research efforts on the diamidine classhave identified aza analogs of DB289 (structure 5, Figure 27.4) as having signifi-cantly improved efficacy in a murine Stage 2 HAT model [33].

OHN

H2N

NH

NH22

ON

H2N

N

NH23

H3CO

O

OCH3

O

ON

H2N

N

NH24

H3CO OCH3O

NNN

H2N

N

NH25

H3CO OCH3

Figure 27.4 Diamidines for HAT.

Recent Approaches to New Trypanocidal Agents j521

Benzoxaboroles

Very recently, a novel series of boron-containing drug candidates, the benzoxaboroles(Figure 27.5), have emerged as a potential treatment for both Stage 1 and Stage 2sleeping sickness. Initially explored by Anacor Pharmaceuticals as anti-fungal, anti-bacterial, and anti-inflammatory agents [34–36], the benzoxaboroles were found tohave anti-parasitic efficacy through screening againstT. brucei at the Sandler Center ofDrug Discovery, UCSF [37]. From approximately 400 benzoxaboroles screened in thisearly work, two classes of compounds, the 6-carboxamides (represented by AN3520,Figure 27.5) and the 6-sulfoxides (represented by AN2920, Figure 27.5), were deter-mined to have interesting activity andpotential for optimization [38]. In addition to thein vitro screening conducted by the UCSF group, evaluation of AN2920 in a mousemodel of Stage 1 HATdemonstrated that this compound was able to cure mice of theparasitic infection, albeit at a reasonably high dose of 50mg/kg (i.p.� 5 days).Encouraged by these results, Anacor approached the DNDi to further explore andgranted the DNDi a license to develop this series for HAT, leishmaniasis, and Chagasdisease. The DNDi had already engaged the biotechnology company Scynexis, incollaboration with the Haskins Laboratories at Pace University, to conduct a leadoptimization drug discovery program for HAT and the benzoxaboroles were intro-duced into the program in early 2008. Following confirmation of the activity of the leadbenzoxaboroles in a high throughput in vitro assay at Scynexis [39], AN2920 andAN3520 and several close analogs were progressed to a range of in vitro ADME(absorption, distribution, metabolism, and elimination) and physicochemical prop-erty assays. The profile exhibited by these compounds in these assays suggested thatthe benzoxaboroles were indeed an attractive series for lead optimization.Described in detail elsewhere [40,41], it was determined that instability to

metabolizing enzymes was one of the primary limitations of the early leadcompounds. This liability was confirmed in an in vivo pharmacokinetic study, inwhich relatively rapid disappearance of compounds from the plasma of mice was

OBOH

SO

ClO

BOHH

N

OCF3

OBOHH

N

OCF3

F

AN2920

OBOHH

N

O

6, R = OCH3

7, R = FAN3520

R

OBOHH

N

OCF3

F

SCYX-6759 SCYX-7158

OBOHH

N

OCF3

F

8

Figure 27.5 Benzoxaboroles for HAT.


observed after either intraperitoneal or oral dosing. In this study, both AN2920 andAN3520 were found to provide exposure sufficient to suggest that they shouldbe active in a mouse model of Stage 1 HAT. This relationship between pharma-cokinetics and pharmacodynamics was confirmed in the Stage 1 mouse model(Table 27.1), where both compounds were found to be active when dosed i.p. at20mg/kg (b.i.d.� 4 days) [37,41]. When evaluated in the same model following oraladministration at 20mg/kg (b.i.d.� 4 days), AN2920 was found to be inactive,whereas AN3520 was found to be active. Lower doses of AN3520 were found to bevariably active by either route of administration. Analogs of AN3520 were also activefollowing intraperitoneal administration, but exhibited variable efficacy when dosedorally. For example, the 4-methoxybenzamide (structure 6, Figure 27.5) was inactivefollowing a 20mg/kg (b.i.d.� 4 days) oral dose (Table 27.1). In contrast, the4-fluorobenzamide (structure 7, Figure 27.5) exhibited full efficacy at 20mg/kgand partial efficacy at 10mg/kg (Table 27.1). This structure–activity relationship trendwas broadly reflectiveofmetabolic stability, as6was less stable in several in vitro assays,and the in vivo pharmacokinetics of 6 inmice suggested this compound was removedfrom the bloodstream much more rapidly than either AN3520 or 7 (Wring,unpublished data). Consequently, the focus of the lead optimization program wasto improve the metabolic stability of the benzoxaborole 6-carboxamide series.Of several approaches taken, one of the most straightforward and effective was to

simply add a fluorine atom to the 4-position of benzamide region of AN3520,

Table 27.1 Efficacy of benzoxaboroles in a murine Stage 1 HAT model [13,42].

Compound Dose(mg/kg)

Route andfrequency

Duration(days)

Relapses Numbercured

AN2920 20 i.p., b.i.d. 4 — 3/320 p.o., b.i.d. 4 1-d10, 1-d11 1/3

AN3520 20 i.p., b.i.d. 4 — 3/320 p.o., b.i.d. 4 — 3/310 p.o., b.i.d. 4 1-d11 2/3

6 20 p.o., b.i.d. 4 3-d10 0/37 20 p.o., b.i.d. 4 — 3/3

10 p.o., b.i.d. 4 1-d7 2/3SCYX-6759

10 p.o., b.i.d. 4 — 5/5

10 p.o., q.d. 4 — 5/55 p.o., q.d. 4 — 5/52.5 p.o., q.d. 4 — 5/51.25 p.o., q.d. 4 4-d4, 1-d5 0/5

SCYX-7158

10 p.o., q.d. 4 — 5/5

5 p.o., q.d. 4 — 5/52.5 p.o., q.d. 4 1-d5, 1-d7, 3-

d100/5

i.p., intraperitoneally; p.o., per os; b.i.d., twice daily; q.d., once daily.


resulting in SCYX-6759 (Figure 27.5). This compound was also found to be verypotent in the T. b. brucei in vitro assay, and had good physicochemical and ADMEproperties [40]. Furthermore, when dosed orally to mice at doses as low as6.25mg/kg, plasma levels of SCYX-6759 were found to remain at concentrationsat or above the in vitro IC50 for at least 12 h, suggesting that this compound wouldexhibit improved efficacy in the in vivomouse model. This was confirmed, as SCYX-6759 was fully efficacious when dosed orally at 2.5mg/kg (b.i.d.� 4 days). In thepharmacokinetics experiment, brain levels of SCYX-6759 were also measured; thisdata suggested that concentrations of compound in the brain were maintained at orabove the in vitro IC50 for at least 12 h following a dose of 50mg/kg (Figure 27.6).Consequently, SCYX-6759 was progressed to a mouse model of Stage 2 HAT [40],where it was found to be curative following a dosing regimen of 50mg/kg (b.i.d., i.p.� 14 days). While encouraged by this result, it was clear that further improve-ments in either efficacy and/or brain exposure were required for the benzoxaborolesto be viable as once-daily oral treatments for Stage 2 HAT, which was the TargetProduct Profile defined by the DNDi at the outset of the project.As described above, the in vitro ADME profiles of SCYX-6759 and related

compounds were very good, and it was difficult to identify reasons for the apparentdisconnect between plasma and brain exposure. For example, permeability acrossthe BBB, as predicted by an in vitro assay using anMDCK-MDRI cell monolayer, wasuniformly high for all of the benzoxaborole 6-carboxamides examined, with noevidence of efflux liability by P-glycoprotein [43]. It was hypothesized that othertransporters could be at least in part responsible for efflux of the weakly acidicbenzoxaboroles from the brain. Therefore, modifications to SCYX-6759 werepursued in an effort to electronically and/or sterically limit access to the acidic

10.00

mL)

0.10

1.00

Con

cent

ratio

n (µ

g/m

0.010 5 10 15 20 25

Time (hr)

Figure 27.6 Pharmacokinetic profiles for SCYX-6759 (red symbols) and SCYX-7158 (bluesymbols) in plasma (solid lines) and brain (dashed lines) of mice.


B-OH moiety. Of several approaches pursued, it was found that introduction ofsubstituents at C(3) of the benzoxaborole nucleus were particularly successful, albeitwith some limitations [41]. More specifically, introduction of a single methyl group,such as in structure 8 (Figure 27.5), afforded compounds that were more cytotoxic tomammalian cells relative to the C(3)-unsubstituted analogs. From a pharmaco-kinetic perspective, these analogs did not exhibit the improvements required interms of brain exposure. More sterically demanding C(3) substituents were found tosignificantly erode in vitro potency versus T. brucei. In contrast, the 3,3-dimethylanalogs exemplified by SCYX-7158 (Figure 27.5) were found to not exhibit mam-malian cell cytotoxicity, and maintained sufficient in vitro potency to progress toin vivo efficacy and pharmacokinetic studies.When evaluated in both the Stage 1 and Stage 2 HAT models, SCYX-7158

exhibited impressive efficacy [44]. In particular, it was observed that completecure of mice infected with the TREU667 strain in the Stage 2 model could beachieved following a 25mg/kg (q.d.� 7 days) oral dosing paradigm. Partial cure(80%) was observed following a 12.5mg/kg (q.d.� 7 days) oral dose schedule. In vivopharmacokinetic characterization of SCYX-7158 was consistent with these observa-tions, as it was observed that brain concentrations of SCYX-7158 were maintained attherapeutically relevant levels for close to 24 h after a 25mg/kg dose and that a12.5mg/kg dose provided brain levels slightly below the in vitro IC50 and/orminimum inhibitory concentration. The relationship between brain concentrationand observed efficacy in the Stage 2 HATmodel has formed the basis for develop-ment of a pharmacokinetics/pharmacodynamics model (Wring, personal commu-nication). SCYX-7158 was thoroughly evaluated in the mouse Stage 2 HATmodel asshown in Table 27.2.Interestingly, while most studies were performed using a standardized 7-day

dosing protocol, it was found that cures could be obtained with SCYX-7158 with aslittle as 3 days of dosing at 50mg/kg. Reduction of the dosing period from 7 days (to5 or 3 days) at 25mg/kg resulted in partial efficacy. Coupled with the extensivepharmacokinetic data generated for SCYX-7158 allowed one to conclude that activityin the mouse model of Stage 2 HAT was both time- and concentration-dependent.

Table 27.2 Efficacy of SCYX-7158 in Stage 2 HAT model [13,45].

Dose (mg/kg) Route/frequency Duration (days) Relapses Number cured

6.25 p.o., q.d. 7 6-d34, 1-d41, 3-d48 0/1012.5 p.o., q.d. 7 1-d34, 1-d48 8/1025 p.o., q.d. 7 10/1025 p.o., q.d. 5 1-d35, 1-d42, 1-d63 7/1025 p.o., q.d. 3 2-d35, 1-d42 6/950 p.o., q.d. 7 32/3250 p.o., q.d. 5 14/1450 p.o., q.d. 3 14/1450 p.o. 1 1-d22, 4-d28 0/5

p.o., per os; q.d., once daily.


Based on these properties, SCYX-7158 was selected for progression to preclinicaldevelopment activities [44]. When evaluated in a broad array (greater than 100) ofin vitro receptor binding, enzyme inhibition, and ion channel inhibition assays,SCYX-7158 showed no significant activity (defined as greater than 50% inhibition at10 mM) against any of these mammalian targets. In addition, the compound wasfound to be functionally inactive against the hERG ion channel [46] and was notmutagenic in an in vitro bacterial reverse mutation assay [47].SCYX-7158 has also been evaluated in a number of in vivo safety pharmacology

and toxicology studies in rodents and dogs, including single-dose, 7- and 28-daymultiple-dose toxicology studies, rat CNS safety pharmacology, dog cardiovascularsafety pharmacology, rat respiratory safety pharmacology, and rat gastrointestinalmotility. In these studies, no significant safety or toxicity issues were identi fied. Inthe toxicology studies, pharmacokinetics were also further evaluated, and it wasobserved that plasma exposure to SCYX-7158 was well in excess of that required forefficacy in the mouse models, allowing adequate safety margins to be predictedbased on these studies. Based on these efforts, SCYX-7158 was progressed to phase Iclinical trials following regulatory approval from the European Medicines Associa-tion (http://clinicaltrials.gov/show/NCT01533961).

Conclusion

Based on the above compilation of results, it is evident that oxaboroles hold muchpromise as effective human trypanocides. The results of ongoing clinical trials arekeenly awaited and we maintain hope for the emergence of an effective, non-toxic,orally available agent for HAT. This research has shown the rapid progression of adrug lead to clinical candidate through mining of compound data, collaboration ofpharmaceutical resources, academic research, and effective private–public partner-ship and philanthropic foundation support. There is, however, another issue closelylinked to HAT: the continuing poverty and malnutrition found in the tsetse belt.Cure of HAT is only part of the answer. The rest of the solution lies in the emergenceof a veterinary trypanocide that will allow effective farming and cattle ranching intsetse-infected areas. Only then will the “curse of flies” be lifted from Africa [1].

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Trypanosomatid Diseases (Molecular Routes to Drug Discovery) || New Developments in the Treatment of Late-Stage Human African Trypanosomiasis