New Therapeutics for Chagas Disease: Charting a Course to Drug Approval

Abstract

Little progress has been made since the 1960s and 1970s to widen the therapeutic arsenal against Trypanosoma cruzi, the causative pathogen of Chagas disease, which remains a frustrating and perplexing infectious disease. This chapter focuses on the strategic and operational challenges in the clinical drug development of a novel antitrypanosomal agent for Chagas disease. The various elements that contribute to a robust assessment of treatment effect including dose selection, choice of patient population, trial methodology, endpoint measures, and regulatory perspectives are discussed. The learnings herein should serve as resource to help researchers and other stakeholders optimize their clinical development plans and speed delivery of new medicines to patients with Chagas disease.

1. Introduction

Chagas disease is a largely vector-borne infection caused by Trypanosoma cruzi, a kinetoplastid protozoan parasite endemic to Latin America. It affects an estimated 6–7 million people globally, is associated with 1.2 million cases of cardiomyopathy, and causes 10–14,000 deaths annually [1, 2]. While most infections are transmitted by triatomine insects, infection can be acquired through oral routes, congenital infection, and organ transplantation. For many decades, Chagas disease was mainly seen in rural populations of Central and South America, but in recent years the number of cases diagnosed in urban endemic areas and outside Latin America has increased due to economic, cultural, and migratory patterns [3, 4]. Consequently, a drug development plan for Chagas disease must consider the international regulatory perspectives and different national healthcare systems to enable faster patient access to diagnosis and treatment.

Beyond the direct clinical impact of the disease on patients, the substantial economic impact of Chagas disease on society has been estimated by Lee et al. [5]. The annual societal cost (i.e., healthcare costs plus productivity loss) of Chagas disease globally was projected at more than over 800,000 life years lost to disability (DALYs) with a financial burden exceeding USD $600 million. When projected future costs (including cardiomyopathy and heart failure) are taken into account, the economic burden of this disease reaches a staggering USD $7.19 billion per year. In contrast, global R&D funding for Chagas disease was reported as only USD $37.12 million in 2019 (representing less than 1% of R&D funding for neglected diseases in developing countries), of which 35% came from private industry [6].

Against this economic background, the substantial resources needed to develop and approve a novel drug should be noted. Novel drug development is complex and multidisciplinary with a predicted success rate to bring a new drug to approval of less than 12% [7]. Estimated fully capitalized costs range from USD $800 million to USD $2.2 billion per compound approved. Clinical trial phases of drug development account for the majority of these investments and further costs are incurred post-approval for activities such as patient registries, follow-on studies, and drug safety monitoring.

We will discuss the various strategic and operational challenges facing physicians, scientists, and clinical researchers designing drug development programs for new antitrypanosomal agents for Chagas disease. Highlighting these issues allows researchers to explore solutions that generate sound scientific evidence for cost-effective and timely drug development programs.

2. Current drug therapy

The two mainstays of drug therapy in adult and children with Chagas disease are benznidazole (BZN) and nifurtimox (NFX). These two nitroheterocyclic prodrugs have been in clinical use for over four decades, and their background, efficacy, and safety are well documented [8, 9, 10, 11, 12]. Both reduce parasitemia and cause seroreversion but with less effect as the infection becomes more established. Both drugs are genotoxic, clastogenic, and in animals, are carcinogenic. The main inconveniences associated with their use are treatment duration of several weeks, avoidance of alcohol, and the associated clinical side effects. Adverse effects commonly seen with benznidazole are hypersensitivity reactions, skin rashes, gastrointestinal intolerance, peripheral neuropathy, and bone marrow suppression [8, 9, 10, 11, 12, 13, 14]. Nifurtimox is less well tolerated and associated with more nervous system toxicity (anorexia, sleepiness) and gastrointestinal intolerance [10].

While two drugs differ in safety profile, there is little evidence to favor efficacy of one over the other although preliminary results of one small randomized study appeared to show a slight trend in favor of BZN [15]. Results from two ongoing randomized studies (NCT02369978, NCT03981523) comparing multiple BZN and NFX regimens in adults with chronic indeterminate disease may help clarify this situation.

A simplified summary of the essential strengths and weaknesses of these two pillars of Chagas disease treatment from a drug developers’ perspective is give in Table 1.

	Benznidazole (BZN)	Nifurtimox (NFX)
Mechanism of action	Prodrug converted by trypanosome specific nitro reductase (TcNTR-1)to highly reactive toxic intermediates forming adducts with DNA, proteins, and small molecules
Drug resistance	Inducible in vitro; multifactorial mechanisms including TcNTR mutation Intrinsic drug susceptibility highly variable across parasite subtypes
Formulation & regimen	2.5 and 100 mg oral tablets Weight-based dosing, tablet slurry for children; Typically 2–3 x day x 60 days	30 and 120 mg oral tablets Weight-based dosing, tablet slurry for children; Typically 3x day x 60 days
PK Characteristics	Cmax ≈ 2–3 hours post dose elimination half-life ≈13 hours Wide tissue distribution and transplacental passage Metabolic pathways unknown	Cmax ≈ 4 hours post dose elimination half-life ≈3 hours Wide tissue distribution and transplacental passage Metabolism via nitroreductases
Efficacy	Both drugs achieve >70% parasitological cure rate in acute phase Limited efficacy in children and adults with chronic indeterminate disease (<10–35% cure as assessed by seroreversion)
Safety	Hypersensitivity reactions (inc. dermatitis in 30% of patients) Bone marrow depression Peripheral neuropathy Mutagenic and clastogenic Embryofetal toxicity risk	Anorexia/weight loss Nausea/vomiting Psychic alterations Excitability or sleepiness Mutagenic and clastogenic Embryofetal toxicity risk
Drug interactions	Alcohol, disulfiram	Alcohol
Tolerability	BZN is often preferred to NFX due to a better safety profile Tolerability better in children and acute disease Early therapy discontinuation due to side effects is common (within 19d) Discontinuation of treatment in 12–24% patients

Table 1.

Simplified summary of essential product characteristics of benznidazole and nifurtimox.

Data summarized from multiple sources and approved product labels [14, 15].

While a 60d treatment regimen is approved for both drugs, shorter BZN regimens show encouraging data of improved tolerability without loss of short-term efficacy [16, 17]. The results of three ongoing trials are awaited with interest; the MULTIBENZ three-arm randomized phase II trial (NCT03191162) compares both low dose for 60d and high dose for 15 d against standard 60d treatment; the nonrandomized NuestroBen phase III study (NCT 04897516) compares 2 weeks to 8 weeks of BZN and uses historical controls; and the phase III BETTY trial (NCT03672487) compares 30d of low-dose BZN to 60d standard-dose BZN in women of reproductive age.

The Pan American Health Organization (PAHO) has issued guidance on the treatment and management of Chagas disease with these drugs based on critical review of available scientific evidence [18]. These comprehensive guidelines consider the both drugs to be effective in reducing short-term parasitemia but evidence is much less convincing for improving long-term clinical outcomes. Hence, there is still much room for improvement in efficacy and safety for different patient populations. Salient points of interest in these guidelines from a drug development perspective are summarized in Table 2.

Use of antitrypanosomal therapy	Evidence-based level of certainty		Conclusion
	Antiparasitic and serological effect	Impact on clinical outcomes
Acute Chagas (children and adults) and Congenital Chagas	Moderate; Parasitemia clearance (75–90%) Seroreversion (50–60%)	Low; Symptomatic benefit No evidence of impact on long-term outcomes	Benefit > Risk; Strong Recommendation
Adults with; chronic Chagas; and no organ damage	Moderate; Clearance of short-term parasitemia (RR 1.44) Long term seroreversion (OR 3.32)	Low; Could reduce development of heart disease (OR 0.38) Impact on mortality unknown	Benefit > Risk; Conditional Recommendation
Adults with; chronic Chagas’ and specific organ damage	High; Parasitemia clearance (RR 1.98)	Low; No impact on progression or death	Risk > Benefit; Not recommended
Seropositive girls and women of childbearing age	Moderate; Decreased likelihood of vertical transmission (OR 0.07)		Benefit > Risk; Strong Recommendation
Benznidazole; vs.; Nifurtimox	Moderate for both	Low for both	Benefit > Risk; No evidence to favor efficacy of one or other but side effect profiles differ

Table 2.

Current treatment recommendations for antitrypanosomal therapy.

^*Adapted from [18]: Pan American Health Organization. Guidelines for the diagnosis and treatment of Chagas’ disease. Washington, D.C.: PAHO; 2019.

OR = Odds ratio; and RR = Relative risk.

The preclinical and development pipeline of experimental antitrypanosomal drug candidates is covered by several reviews [12, 19, 20, 21]. Only a few new drugs have reached phase II trials, notably the azoles class represented by posaconazole and fosravuconazole (E1224). Unfortunately, both failed to produce sustained parasite suppression compared with BZN [17, 22, 23, 24] and have not progressed further. Fexinidazole, a 5-nitroimidazole approved for treating Human African Trypanosomiasis (HAT), has been tested in two phase II trials of Chagas disease (NCT 02498782, NCT 03587766). Following poor tolerability in the first study [25], a second study with modified dose regimens was conducted but results have not been published. Other novel drug classes (oxaboroles, nitroimadazoles, proteosome inhibitors) being developed for other trypanosomal diseases such as leishmaniasis and HAT may also have potential to treat Chagas disease [26].

3. The clinical development a new therapeutic agent

A rational starting point of a new drug development program is a thorough understanding of the “patient journey,” i.e., the pathway of patient experiences from contracting infection to their end outcome which includes access to the healthcare systems, diagnosis, treatment, and follow-up. Inevitably, the perspectives of multiple stakeholders (e.g., patients, caregivers, regulators, payers, policymakers) in the Chagas disease ecosystem must be integrated into program design.

Drug developers aspire to design new medicines to match a preconceived set of criteria called a “target product profile” or “TPP.” This is a statement of the desired characteristics of the drug candidate and serves multiple purposes. It is a framework for generating scientific and clinical evidence to support a final product label; it guides program decision-making and facilitates dialog with regulatory authorities. The TPP may be modified during the development process based on the generated scientific data or a shift in the external environment. Characteristics of an “ideal” drug for a Chagas disease therapeutic are given in Table 3, and examples of TPPs have been given by Rao et al. [19] and the Drugs for Neglected Diseases initiative (DNDi) [27].

The remainder of this chapter will focus on the clinical development stage of a new drug and assumes that discovery and preclinical development stages have been completed. Readers wishing to have deeper insights into design of preclinical programs for Chagas drug candidates are referred to the comprehensive reviews by Romanha [28] and Kratz [29].

3.1 Target indications and choice of study populations

As patient needs lie at the core of any new drug development program, a critical strategic decision is the choice of the patient population and the projected clinical benefit. This drives a more specific TPP, determines the associated development risks, and ultimately determines the approved drug label.

The unique and complex nature of Chagas disease, characterized by varying parasitological and clinical effects at different stages of disease, has substantial implications for clinical trial study populations and the indication chosen for approval. A schema showing the possible disease intervention points and associated challenges is given in Figure 1.

Figure 1.

Development challenges vary by disease stage. (Adapted from Lidani et al. The Complement System: A Prey of Trypanosoma cruzi. Front. Microbiol., 20 April 2017).

We will focus our principal discussion on issues associated with development in a general population of acute and chronic Chagas disease including children and adults. A pediatric development plan would be expected by most regulatory authorities. From a drug development perspective, neonates, pregnant women, and immunocompromised patients are considered special study populations and will not be further discussed in any detail. They would seldom be chosen as the first indication as the benefit-risk profile of the drug has not been fully established, but they may be included later when the appropriate supportive data are available.

3.1.1 Acute Chagas’ disease (ACD)

It stands to reason that early intervention in Chagas disease with an effective and safe drug, without the mutagenicity liabilities or drug interactions associated with BZN or NFX, would be beneficial. Symptomatic acute Chagas disease (ACD) is a potentially favorable setting for testing a new drug for both early efficacy and to seek an approved indication. Patients with acute disease typically have significant parasitemia and many exhibit severe clinical manifestations [30], which could serve as the basis for measurement of a treatment effect. Endpoints measures are further discussed in section 3.4.

Since BZN and NFX response rates in ACD are already high, clinical trials intended for a regulatory submission must be powered to show superiority or noninferiority and require a large sample size. In early phase II studies of a new drug, using historical BZN control data is feasible, but better is the inclusion of a concurrent BZN calibration arm to gauge assay sensitivity. A concurrent placebo control would be unacceptable in a trial of monotherapy with a new agent and rescue medication (BZN or NFX) should be offered.

The main operational challenge in ACD is the enrollment of patients for clinical trials. The number of documented ACD cases in endemic countries is low [31, 32], and even in larger countries such as Brazil, where acute Chagas disease due to oral ingestion of contaminated foodstuffs has become more common, the reported incidence is less than 0.15 cases per 100,000 of the population [33]. Prospective studies in ACD are uncommon with most published studies being case series, longitudinal cohort, or observational studies [34, 35, 36].

A modest size early phase II trial in ACD may be feasible if focused on a region of high endemicity, and a few trial sites but restricting eligibility to adults may slow recruitment rates. Interpretation of study result may also be challenging in that geographical variability in measured drug effect varies for many reasons [37, 38], and results from a small study in one region may not be reproducible in another region nor representative of a broader study population.

3.1.2 Chronic indeterminate disease (CID)

Chronic indeterminate disease (CID) is an alternative study population with both high medical need and greater prevalence than ACD. The number of studies published in chronic disease far exceed those in the acute setting and accounted for almost 80% of 109 studies in a recent review of trials in Chagas disease [36]. Patients with CID are usually asymptomatic, physically active, and have no or minimal evidence of functionally significant organ damage. As evidence for the benefit of using antiparasitic therapy in CID is not compelling, a case can be made for designing trials with concurrent placebo controls, and the CID population is becoming increasingly used for early placebo-controlled studies of experimental drugs.

Including this population in a development program is a key opportunity to prevent progression to end-organ damage and have a major public health impact. Parasite persistence is considered to play a key role in developing clinical sequelae including cardiomyopathy [39], but the pathogenesis of organ damage is not fully understood [40, 41]. Choosing cardiac-related events as study endpoints in this patient population has significant logistic challenges as event rates are low (1–2% per year), and multiyear follow-up is needed [42, 43, 44]. Even with 13 years of follow-up in patients at low risk of progression, the large randomized placebo-controlled TRAENA study of BZN (Clinicaltrials.gov-NCT02386358) failed to detect any benefit on cardiovascular outcomes despite clear evidence of an antiparasitic effect [45].

The main study methodology challenges in studying CID are choice of appropriate measures of treatment effect (parasitological, laboratory, and clinical endpoints); variability in response associated with geographical, parasitological, and immune factors; and the extended follow-up needed to establish a relationship between short-term response and long-term outcomes.

3.1.3 Chronic determinate disease (CDD)

In patients with chronic determinate disease (CDD), an antiparasitic drug might not be expected to improve established organ dysfunction, but intervention might still stop further tissue damage, worsening of cardiomyopathy and frequency of adverse cardiovascular outcomes. To address this hypothesis, a large, prospective, multicenter, controlled trial was conducted across five endemic countries. The BENEFIT study randomized over 2800 patients with Chagas cardiomyopathy to benznidazole or placebo [46]. The primary endpoint was a composite of cardiovascular mortality and morbidity events. The mean follow-up was over 5 years at the time of reporting with excellent patient retention rates. Over a 7-year period, some 60% of patients were PCR positive as baseline. and importantly these assays were standardized across reference laboratories. There was no significant difference in the clinical outcomes with primary endpoint events seen in just under 30% of patients in both arms. In PCR-positive patients, BZN causes seroreversion in two-thirds of patients compared with one-third of placebo patients. No correlation between seroreversion and clinical outcomes was seen. Additionally, the PCR seroreversion rates were much higher in Brazil than in the other countries.

While BZN therapy in CDD significantly reduces parasite load, this has not translated into significant clinical benefit. For this reason, the PAHO guidelines do not recommend antitrypanosomal therapy in this population. This trial result, however, may only hold true for benznidazole (or the same drug class) and may not predict for a new drug class with a novel mechanism of action or combination approaches.

Specific drug development challenges associated with the different populations in Chagas disease are summarized in Table 4.

Active across all T. cruzi strains with a low level of resistance induction Active on replicative, non-replicative, and dormant forms Sustained clearance of circulating and deep tissue parasitemia Convenient route of administration with age-appropriate formulations Low level of Drug-Drug interactions (e.g., HIV/Post TX reactivation, cardiac disease) Shorter duration of treatment than BZN/NFX Clinical efficacy in all endemic regions Clinical efficacy in immunocompromised patients Affordable and adequate product stability in endemic regions Superior to benznidazole/ nifurtimox in across all disease stages Positive treatment effect on short-term endpoints (Clinical, Serology, PCR) Prevention or stabilization of progression to complications (e.g. cardiomyopathy) Better safety and tolerability than BZN/NFX in adults, children, neonates, females of childbearing potential Safe for use in pregnancy Safe in patients with cardiac complications (myocarditis, cardiomyopathy)

Table 3.

Characteristics of an “ideal” antitrypanosomal drug for Chagas disease.

Population	Medical need	Considerations
Congenital Chagas (neonates)	High antiparasitic activity better than equal to SoC; Prevent death, acute complications, and cardiomyopathy; Non-genotoxic	BZN already very effective; Convincing evidence of safety and long-term; efficacy in older children; Placebo control unacceptable; Clean juvenile toxicology; Age-appropriate formulation
Acute disease (Adults and Children)	High antiparasitic activity better than equal to SoC; Short dosing regimen; Non-genotoxic; No arrhythmogenic or QT liability; Low DDI potential	SoC highly effective for majority of patients; Difficult to find/recruit patients; Trials must include rescue therapy; Placebo control unacceptable; Consider regions with low BZN susceptibility; Clean juvenile toxicology; Age-appropriate formulation
Children (early) Chronic Indeterminate	Short dosing regimen; Prevent cardiomyopathy; Less side effects and better tolerability; No-genotoxic molecule	Evidence of safety and prospect of benefit established in adults; Placebo control unacceptable; BZN relatively well tolerated in children; Clean juvenile toxicology; Low cardiac event rates need long follow up and large sample size; Age-appropriate formulation
Adults chronic indeterminate	Long-term seroreversion; Less side effects and better tolerability; Prevent cardiomyopathy; Non-genotoxic molecule	High hurdle due to long-term B-cell memory; BZN 30d better tolerated than 60d; Low cardiac event rates need long follow-up and large sample size; Placebo control possible
Adults chronic determinate	Damaged end organ rescue/support treatments	Would a new MoA affect clinical outcomes?; Consider for a combination approach with antiparasitic or antiparasitic + host response modulator
Transplant reactivation/ immunosuppression-HIV	Effective parasite reduction as good as BZN but better tolerated; Low DDI liability	Patients uncommon; Patient polypharmacy but well supervised; Studies complex
Females of Childbearing Potential (FCBP); Pregnancy	As for adults above; Non genotoxic; Safe in pregnancy	Include FCBP in acute and chronic studies with contraception; Clean reproductive toxicology; Requires extensive preclinical and clinical safety information
Patients with early stage I/II cardiomyopathy	No arrhythmogenic or QT liability; Low DDI potential	Consider enriched population study or stratify for subpopulation in Ph III; Stratify for other risk CV factors; Large study with long follow-up needed

Table 4.

Drug development challenges associated with different Chagas disease populations.

Once the primary indication has been chosen, a cohesive set of clinical trials must be designed to generate the scientific and medical evidence to support drug registration. The foundation of the development plan will be significantly shaped by the drugs’ pharmacology and its anticipated effect in humans.

3.2 Pharmacology and dose determination

Determining the correct dosing schedule and dosage form for a new drug is driven by basic principles of drug pharmacology. For a drug to be effective, enough biologically active substance must traverse the body’s natural physiological barriers and metabolic pathways to reach the target site of action to cause the desired effect, without undesirable “off target” effects. Establishing effective and safe therapeutic doses in infectious diseases has been well served historically by pharmacokinetic and pharmacodynamic (PK/PD) modeling. This process integrates in vitro, in vivo, and human data to predict clinical efficacy and provide a rational basis for clinical regimens and trial design. The PK/PD drivers of efficacy are well recognized for many classes of antibacterials [47] and for some antiparasitics [48].

A specific drug exposure response model is assembled from preclinical data for each new compound class and validated during clinical development. All models have limitations and in the case of Chagas disease, they may be limited by the complex parasite-host relationship including differential tissue distribution, mixed infections, variable drug susceptibility, and parasite dormancy [49, 50]. Exploration of drug effects in different parasite stages, especially the intracellular amastigote form and clinical isolates from endemic regions, is recommended to support selection of a clinically effective dose. In addition, essential elements of a development program will be assessing the potential for inducing drug resistance and monitoring for resistance emergence as a cause of therapeutic failure during clinical trials. The effect on host immune response (serology) in animal models and under immunocompromised conditions provides further useful information to support clinical dosing decisions.

Sustainable duration of response is a critical success factor and the shortest possible duration of drug exposure required to obtain cure, while limiting emergence of resistance is desirable. To achieve these goals, special attention must be paid to the therapeutic index (TI), which is the quantitative relationship between drug exposure causing the desired therapeutic effect and that at which toxic develops. Animal models and PK/PD modeling simulations can provide guidance. Monitoring for recurrence in clinical trials is indispensable, and molecular testing may distinguish between recurrence and new infection in hyperendemic areas. Pharmacokinetic sampling during phase II and III clinical trial is required to support dose-regimen selection, explain variability of response, and help interpret reasons of treatment failure.

3.3 Design of clinical trial programs

3.3.1 Trial methodology considerations

Despite the long history of clinical research in Chagas disease, the PAHO guidelines emphasize the paucity of high-quality clinical evidence. Clinical trials must be rigorously conducted, tightly controlled scientific experiments in carefully selected patient populations yet sufficiently representative and pragmatic to make the results relevant to real-world clinical practice. This balancing act is not easy to achieve and may be a contributing factor to inconsistency of trial results in Chagas disease. Trial heterogeneity is been a major confounding factor in achieving a consistent and robust conclusion on treatment effects and is evident from several systematic reviews.

A comprehensive analysis of the clinical trials landscape in Chagas disease conducted by Maguire et al. [36] analyzed 109 interventional trials of antitrypanosomal agents conducted from 1997 to 2019. One-fifth of the trials were conducted in nonendemic regions. Just over three quarters of the 23,000 patients in these studies were in active treatment arms and almost one-quarter had no therapy or placebo. Study sample sizes covered a wide range from 6 to 3703 patients with median value of 53 patients, and most studies were conducted in patients with chronic disease. Benznidazole (mainly monotherapy) accounted for 85% of active treatments and NFX for a mere 5.6%, with the remainder being azoles (2%) or unspecified drugs. These authors highlight the enormous heterogeneity of treatment regimens, study designs, and diagnostic methods and advocate a common pooled data platform to facilitate further research.

Studies addressing the impact of antitrypanosomal therapy on progression to cardiac disease were evaluated by Villar et al. [51] in their systematic review of 13 studies. Ten of these 13 studies had BZN arms, 5 included NFX, 4 allopurinol, one itraconazole, and 7 included placebo controls. Significant heterogeneity in study design, response to treatment, and patient outcomes were noted. Overall, no significant treatment effect on cardiac progression was found.

Chadalawada et al. [52] analyzed 32 trials to assess the impact of antitrypanosomal therapy on developing chronic cardiomyopathy in patients with both acute (9 studies) and chronic indeterminate (23 trials) disease. All but three of these trials were prospective cohort studies with notably heterogeneous designs. These authors found a much higher rate of cardiac progression in patients with acute disease (4.6% annually) than those with chronic disease (1.9% per annum). This increase did not appear to be influenced by the route of parasite transmission. Studies conducted in Brazil also had higher rates of cardiomyopathy than other countries. In contrast to Villar et al. this study observed lower rates of cardiomyopathy in trials with antitrypanosomal therapy compared with those without.

Patient availability may be an underlying contributing factor to the diversity of clinical trials noted above and some populations such as acute disease and pediatric patients are challenging to recruit. Identifying patients soon after infection is ideal, but unfortunately, most Trypanosoma cruzi infections are asymptomatic and access to diagnostics in rural areas of endemic countries is poor. While national-level screening programs may exist in antenatal/perinatal settings, blood banks, and some hospital settings such as organ transplantation, we are unaware of coordinated systematic population screening across endemic regions, which could support multinational trial participation. Most patients are diagnosed in the stages of chronic indeterminate disease (CID) or having developed chronic determinate disease (CDD) with overt clinical end organ damage. Clinical researchers should pay special attention to optimizing population screening to efficiently use study resources, time, and costs. Widening the availability and use of immunochromatographic rapid diagnostic tests [53] in endemic regions may be a useful and pragmatic approach to improved study subject screening. Furthermore, the increasing trend toward decentralizing studies and use of remote technologies [54] may facilitate participation of patients in more remote regions, especially for screening and follow-up visits.

3.3.2 Phase I trials

Phase I first-in-human (FIH) studies are typically done in healthy adult volunteers. The dosing regimen, the initial human starting dose, and dose escalation scheme will be based on preclinical efficacy and adequate safety margin considerations. The usual aim is to establish either a maximum tolerated dose or a minimum biologically active dose. A comprehensive outline of the framework and general considerations for the design of FIH studies is provided by Shen et al. [55]. Although initial testing of novel drug candidates has also been conducted in seropositive patients with HIV, HBV, and HCV, this has rarely been done in Chagas disease. As of December 2021, several phase I studies in patients with Chagas disease for formulation bioequivalence, drug interactions, or drug repurposing were listed in the WHO International Clinical Trials Registry Platform (ICTRP) [56] and ClinicalTrials.gov database [57], but no FIH studies were found. Given the need to establish new and robust pharmacodynamic markers, opportunities to conduct FIH studies in populations of chronic indeterminate disease may be considered.

3.3.3 Phase II trials

The first test of therapeutic efficacy where both antimicrobial and clinical effects can be measured is traditionally a phase II study (also termed a “Proof -of- Concept” or PoC) in patients with well-defined disease characteristics. The dosing regimen chosen in these studies is derived from the preclinical animal data, PK/PD modeling, and results of phase I studies. A dose close to the maximum tolerated dose seen in phase I studies may be chosen with provision for modification if significant tolerability or safety issues occur. The initial PoC study population is usually adults because only limited amount of human safety information is available. Pediatric patients are typically included later after safety has been documented in juvenile preclinical toxicology studies and in adult clinical trials.

Sample sizes in PoC studies are usually limited though efficient designs with multiple parallel arms for regimen comparison and combination therapy have become more common.

Two pairs of published phase II trials that merit further discussion illustrate some of the methodological issues for PoC and dose finding. Features common across these four studies are a multicenter, multiarm, randomized, parallel group design; enrollment of RT-PCR-positive patients with chronic (mainly indeterminate) disease; use of serial short- and long-term biomarker measurements; and PK sampling for drug exposure.

The CHAGAZOL trial [22] was an open-label, multicenter randomized trial of 79 adults conducted in a Spain comparing high and low doses of posaconazole (POZ) to 60d of standard BZN. The subject screen failure rate for study entry was 57%, and most patients (> 90%) originated from Bolivia. Sixty-five percent of subjects had chronic indeterminate disease (CID), and the remainder had some evidence of end-organ damage, primarily cardiac (22%). Serial RT-PCR measures showed a high PCR reversion in all arms during the treatment period, but by 10 months post treatment, more than 80% of POZ-treated patients relapsed versus 38% of BZN patients. The transient effectiveness of POZ in this study prompted exploration in a combination study with BZN. The STOP-CHAGAS trial [23] was a multinational study of 120 adults with CID. This partially blinded study randomized patients across four arms comprising POZ alone, BZN plus placebo, BZN plus POZ, and placebo. The higher screen failure rate of 70% may reflect the narrower patient population and broader site participation. The primary outcome measure was RT-PCR negativity at 180d. High response rates (>90%) were seen in the three active arms during treatment but was not sustained for POZ monotherapy with only 13% response at d180 compared with >80% for the BZN arms. Notably, the treatment discontinuation rate is 32% almost halving the per protocol population in the BZN arms.

The second pair of randomized studies are the trials of fosravuconazole (E1224). The single-blind, five-arm trial from the E1224 study group [24] allocated 231 adults with CID to low, high, or short doses of E1224 or to BZN or placebo. This study was conducted in two sites in Bolivia and had screen failure rate of 59% and the antiparasitic effect was assessed through serial RT-PCR and serology. The study was powered to show superiority over placebo for a primary endpoint of sustained PCR response at 12 months after treatment start. The high initial response observed for both E1224 and BZN during treatment was only sustained in BZN-treated group. The follow-on BENDITA trial [17] was a seven-arm, double-blind, double dummy study in 210 adults with CID conducted in three regions of Bolivia. Of the 518 patients screened, 210 were eligible and randomized to one of three BZN treatment arms (2,4, or 8 weeks), one of two BZN plus E1224 combination arms, or to placebo. The primary efficacy endpoint was sustained parasite clearance at 6 months based on RT-PCR. This trial showed that 2 weeks of BZN therapy was almost as good as standard therapy in achieving sustained response with far better tolerability. Whether a 2-week regimen can translate into longer-term benefit remains to be seen. The addition of E1224 to BZN did not improve efficacy but increased the incidence severe adverse events. The low rates of protocol violation and treatment discontinuation in these two trials attest to the quality of the design and execution of the study investigators.

These well-designed and well-executed phase II designs using RT-PCR in patients with CID provided a robust evaluation of drug effect and assist dose selection. This same design approach has also been used in the studies of fexinidazole (NCT02498782, NCT03587766).

3.3.4 Phase III trials

The enormous heterogeneity in published studies conducted in Chagas discussed above emphasizes the need to have robust and consistent approaches to clinical testing. Typically, data from one or more adequately conducted randomized trial would be expected by regulatory authorities to support license approval. The clinical research gold standard for generating evidence of efficacy and safety is a prospective (ideally double blind) randomized clinical trial against the current standard-of-care (SoC) or placebo. In the majority of Chagas’ settings, either benznidazole or nifurtimox would be the active comparative control in registration enabling clinical studies. In certain circumstances, a historical or concurrent placebo may be acceptable. The evolution of potentially shorter BZN regimens from four ongoing trials (NCT03191162, NCT03981523, NCT04897516, NCT03672487) may significantly impact both a new drug TPP and the design of future comparative trials.

Beyond the fundamental requirements for minimizing bias and variability through randomization, blinding, and stratification, the numerous possibilities for comparative phase III and adaptive study designs will not be covered here as they must be tailored to answer the precise clinical research question of interest. The logistic challenges around designing and executing such trials to meet current regulatory standards are significant and a major investment in research time, finances, and resources. It behooves drug developers to have compelling phase II data before deciding to embark on such studies and to engage in early discussions with regulatory authorities.

A clinically meaningful endpoint (or an acceptable surrogate marker) should have been chosen and agreed with Health Regulatory Authorities; a dose adequately determined; and a minimum follow-up period of 12 months (or more) planned. Ideally, a trial should be prospective, multicenter, multiregion randomized, double-blind (double dummy), and powered at a minimum for noninferiority against the SoC (BZN/NFX) and superiority against placebo (where used). Factors known to influence variability in response such as age and geographical distribution required careful balancing. Preplanned interim analyses with early stopping rules may be useful to manage risk, and an independent data safety monitoring board is strongly recommended. Adjudication committees for some endpoints may be added as needed. Centralized laboratory assessment for serology and biomarkers is recommended.

The operational challenges of mounting large phase III interventional studies in Chagas disease include effective community engagement; cooperation with local centers of expertise; engagement of government health screening and awareness programs; rapid diagnostics and facilitated access to medical facilities capable of conducting regulatory standard studies. Rigorous trial execution and patient retention are needed to minimize protocol deviations that could compromise the noninferiority designs. Post-approval requirements may include further clinical trials or extended follow-up (3+ years) of study patients.

Where more than one drug candidate exists, prioritization decisions must be made as the resource requirements to run several simultaneous phase III programs may well be prohibitive. Clinical trial designs (master protocols) to improve efficiency of testing of multiple drugs of the same therapeutic class are well accepted by Health Authorities and have been used in oncology and more recently COVID-19 settings [58, 59]. An alternative strategic option would be a combination approach with two new drugs (antiparasitic or antiparasitic plus host response modulator) in a single phase II/III program.

3.3.5 Alternative development strategies

Discussion of drug development for Chagas disease would not be complete without considering the many outstanding strategic questions concerning the ultimate goal of halting disease progression.

After four decades of use, is it realistic to expect any more gains from BZN (or NFX) alone or is the problem simply related to poor tissue targeting [60]? Is chronic indeterminate disease already too late a setting to influence the course of the disease for any drug and if so, should efforts be redirected to early detection and acute disease where the risk of cardiomyopathy is higher [52]? Finally, would combinations of antiparasitic drugs with or without anti-inflammatory/immune-modifying drugs in both ACD and CID be more effective than monotherapy? Combination antimicrobial therapy is well established in some infectious diseases (e.g., HIV, HCV, Tuberculosis) but relatively uncommon in Chagas disease. A phase I/II trial (ICTRP REBEC-RBR-5n4htp) studying the combination of BZN and disulfiram antitrypanosomal agents is currently underway in Brazil [61]. While immunomodulatory approaches to Chagas cardiomyopathy have met with some success in preclinical models [41], there are few human trials of antiparasitic and immunomodulatory combinations. Simvastatin combined with BZN was shown to reduce cardiac inflammation in a murine model despite parasite persistence [62], and the ongoing ATOCHA trial (NCT04984616) is evaluating the effects of atorvastatin combined with BZN or NFX with chronic indeterminate disease. In a murine model, attenuated cardiac dysfunction and tissue parasite clearance were seen with the combination of fenofibrate (a peroxisome proliferator-activated receptor ligand) and BZN [63], but no human study of this combination has been published. The poor progress with monotherapy in many settings warrants preclinical and clinical exploration of combination approaches despite the potential for increased costs, more side effects, drug interactions, and increased risk of medication nonadherence.

3.4 Endpoint considerations in clinical trials

The scientific soundness and clinical relevance of an interventional trial are heavily dependent of choice of a robust, reproducible, and validated measurement of the desired treatment effect. Some issues related to selection of trial endpoints have been discussed briefly in the context of patient population.

3.4.1 Parasitological, serological, and clinical endpoints

Sustained decrease in parasitemia and resolution of clinical effects would be appropriate clinical trial endpoints for efficacy in patients with acute Chagas disease. Well-established microscopy-based methods exist to directly observe detection of circulating parasites; however, these methods lack the exquisite sensitivity and quantitative advantages of PCR for monitoring treatment effect [64]. Use of standardized PCR and serology assays using central laboratories is advisable during phase II and III trials to ensure consistency and comparability. Detailed time course profiles of quantitative changes in parasitemia from presentation to resolution (with and without treatment) are not available in the literature and would need to be generated de novo.

While a parasitological response alone may be sufficient to determine drug activity from a mechanistic viewpoint, clinically meaningful trial endpoints that reflect how a patient feels, functions, or survives are essential. Elegant and detailed descriptions of the clinical course of acute disease can be found in the literature [34, 35, 65, 66] but lack the granularity required to develop a robust clinical endpoint for regulatory purposes. If needed, additional data that enhance understanding of clinical parameters in acute disease may be sought through patient chart review (preferably from recent datasets) or by prospective collection, in parallel to, or as part of a Proof-of-Concept trial.

As, patients with CID are asymptomatic and parasitemia is often absent or of low-grade intermittent nature, most phase II studies have employed PCR testing and serological tests as to quantitate drug effects. Long-term seroreversion of T. cruzi specific IgG, measured by serial assays, is the mostly widely accepted evidence of cure and reflects both parasite and host response [67, 68]. PCR is considered a sensitive tool to assess peripheral parasite load for diagnosis and relapse [69] and is useful as a short-term endpoint for antiparasitic drug effect and dose finding in clinical trials [70]. Nonconventional serological tests with antibodies against specific Chagas’ antigens [71] may show earlier time course changes and some (e.g., F29- ELISA, AT-ELISA) have been included in regulatory submissions for benznidazole and nifurtimox [72, 73, 74].

Some specific challenges associated with choosing cardiac related endpoints have been discussed above in Section 3.1.2 and 3.1.3 will not be repeated. Trials using cardiovascular event endpoints must also be designed to adjust for the other competing risk factors that may confound interpretation of the results [75]. Based on available evidence, the probability to detect meaningful changes in clinical cardiac parameters in studies with BZN monotherapy is extremely low. Finding new biomarkers or imaging tools that may correlate with clinical outcomes or predict for risk of later end-organ damage would significantly accelerate the development of new drugs.

The efficiency of clinical trials in Chagas disease could also be improved if study populations could be enriched by patients most likely to develop progression. Currently, no such early predictive markers of long-term seroreversion or cure exist although many markers of disease progression are under exploration. The ideal characteristic for biomarkers along with a summary of numerous parasitic, molecular, cellular, and host biomarkers studied to date has been reviewed by Pinazo et al. [67].

3.4.2 Quality-of-life (QoL) measures

Most drug studies in Chagas disease have reported adverse events or clinical effects as patient-related outcomes [36]. Quality-of-life (QoL) research using different validated tools has mainly been studied in patients with cardiac disease [76, 77]. In CID patients, QoL measures observed during treatment may be a worthwhile consideration when comparing tolerability or convenience of two active treatments. Regulatory authorities have an active interest in how patient feels and functions, and the United States Food and Drug administration (US FDA) held a patient workshop for Chagas disease that highlighted emotional, financial, and other pragmatic concerns associated with the disease [78]. Where it is appropriate to measure QoL during new therapeutic interventions, carefully constructed patient measures are useful supplements to conventional safety and efficacy parameters.

4. Regulatory health authority perspectives

While the epicenter of the burden of Chagas disease continues to be Latin America, migration and other factors have made the disease a global health concern. As such an international regulatory approach to drug approval is required. Early engagement with regulatory authorities in endemic regions of Central and South America is essential when designing a development program to address Chagas disease. Indeed, there is a compelling case for a single pan-Latin American approach to the evaluation and approval to accelerate patient access to new anti-T. cruzi therapeutics.

Outside Latin America, the US FDA has played an important role in stimulating research and development for neglected tropical diseases through several mechanisms, most notably the award of transferable priority review vouchers to eligible products and the use of expedited approval programs [79, 80]. While available in Latin America for decades, benznidazole and nifurtimox were only approved in the United States (for the treatment of children with Chagas disease) in 2017 and 2020, respectively. These drugs were assigned US FDA Orphan Drug status, and both were granted an accelerated approval based upon randomized clinical trials in children with primarily indeterminate disease using serology as the primary endpoint. Post-approval requirements (PMR) included phase IV studies to confirm activity and complete missing data gaps. As these drugs may serve as the SoC comparator in pivotal trials of a new therapy, review of the FDA assessment reports [72, 73, 74] provides useful insights into how a rigorous health authority viewed the challenges of developing a drug to treat Chagas disease.

Similar to the US FDA, the European Medicines Agency (EMA) can also grant Orphan Drug designation [81]. Furthermore, the EMA can support faster approval in certain non-European lower-income countries by providing a thorough scientific assessment and the facilitating WHO prequalification under their EU-4Mall (formerly called “Article 58”) program [82]. Medicines reviewed under this provision benefit from scientific advice, EMA’s PRIME (PRIority MEdicines) scheme [83], and accelerated review [84].

The authors did not find any regulatory assessment documents for indications of congenital and acute Chagas disease in the public domain. A summary of regulatory considerations linked with a clinical development program for an antitrypanosomal drug for use in chronic indeterminate Chagas disease is given in Table 5.

Serology indicates total body parasite burden and sustained seroreversion of ELISA/IIF/IHA at 3 or more years remain the standard for cure Serology is acceptable as a surrogate endpoint in chronic indeterminate disease Clearance of parasitemia alone probably insufficient for efficacy assessment PCR assays are an acceptable marker of failure or relapse Nonconventional serologic assays or PCR is acceptable as supportive evidence if multiple concordances shown with conventional tests Historical control data from placebo/untreated patients may be acceptable Efficacy demonstrated across multiple endemic regions is desirable Prospective randomized trials are essential because of response heterogeneity

Table 5.

Considerations for clinical programs to support drug approval in chronic indeterminate Chagas disease.

5. Conclusions

The development of a new antiparasitic agent against T. cruzi to improve the lives of patients with Chagas disease is an urgent priority and one of the most challenging in infectious diseases. Despite the discovery of the disease over a century ago, substantial knowledge gaps persist in disease biology and the pathophysiology of disease progression. Clinical trials face a wide range of challenges that include disease and response heterogeneity and operational challenges associated with early diagnosis, patient access, intervention, and prolonged follow-up. The clinical impact of the benznidazole and nifurtimox in chronic disease settings has been disappointing, but decades of research provide valuable insights that can be applied to new drug development. This discourse highlights the Chagas disease associated pitfalls to be navigated and risks that need to be managed. Ultimately, achieving improved patient outcomes requires a multidisciplinary approach encompassing novel compound classes, rapid diagnostics, early progression biomarkers, rigorous drug candidate selection, and efficient, focused clinical development plans. The advent of new digital technologies and trial methodologies has significant potential to improve clinical trial efficiency and patient access. The WHO, PAHO, Mundo Sano, the Oswaldo Cruz Foundation (FIOCRUZ), Chagas’ Coalition, the Barcelona Institute for Global Health (ISGlobal), DNDi, and various private sector companies are among the many international institutions dedicated to conquering this disease. The long and complex path to the approval of new drugs will best be served by broad, long-term collaborative engagement, knowledge sharing, and partnerships between stakeholders in private and public sectors.

Acknowledgments

The authors would like to acknowledge insights contributed by our colleagues Jonathan Spector, Caroline Demacq, Monica Quijano, and Claudio Gimpelewicz.

Conflict of interest

The authors were employees of Novartis at the time of writing. All views are expressed by the authors to support further dialog between researchers in Chagas Disease. The opinions are not intended to represent the position of the Novartis company nor are promotional in nature.

List of abbreviations

ACD

Acute Chagas’ disease

BZN

Benznidazole

CDD

Chronic determinate disease

CID

Chronic indeterminate disease

DALY

Disease-adjusted life year

DDI

Drug-drug interaction

DNDi

Drugs for neglected disease initiative

DTU

Discrete typing units

ECG

Electrocardiogram

ELISA

Enzyme-linked immunosorbent assay

EMA

Europe Medicines Agency

FCBP

Females of childbearing potential

FDA

Food and Drugs Administration

FIH

First-in-human

HAT

Human African Trypanosomiasis

HIV

Human immunodeficiency virus

HBV

Hepatitis B virus

ICTRP

International Clinical Trial Registry Platform

IHA

Indirect hemagglutination assay

IIF

Indirect immunofluorescence assay

MoA

Mechanism of action

NFX

Nifurtimox

PAHO

Pan American Health Organization

PCR

Polymerase chain reaction

POZ

Posaconazole

PMR

Post-marketing requirement

QOL

Quality of life

R&D

Research and development

SoC

Standard of care

TcNTR

Trypanosome cruzi mitochondrial nitroreductase

TI

Therapeutic index

TPP

Target product profile

TX

Transplantation