Dogs as a Model for Chemotherapy of Chagas Disease and Leishmaniasis

Animal models for exploring Chagas disease pathogenesis and supporting drug discovery

SUMMARYInfections with the parasitic protozoan Trypanosoma cruzi cause Chagas disease, which results in serious cardiac and/or digestive pathology in 30%-40% of individuals. However, symptomatic disease can take decades to become apparent, and there is a broad spectrum of possible outcomes. The complex and long-term nature of this infection places a major constraint on the scope for experimental studies in humans. Accordingly, predictive animal models have been a mainstay of Chagas disease research. The resulting data have made major contributions to our understanding of parasite biology, immune responses, and disease pathogenesis and have provided a platform that informs and facilitates the global drug discovery effort. Here, we provide an overview of available animal models and illustrate how they have had a key impact across the field.

New drug discovery strategies for the treatment of benznidazole-resistance in Trypanosoma cruzi, the causative agent of Chagas disease

INTRODUCTION

Benznidazole, the drug of choice for treating Chagas Disease (CD), has significant limitations, such as poor cure efficacy, mainly in the chronic phase of CD, association with side effects, and parasite resistance. Understanding parasite resistance to benznidazole is crucial for developing new drugs to treat CD.

AREAS COVERED

Here, the authors review the current understanding of the molecular basis of benznidazole resistance. Furthermore, they discuss the state-of-the-art methods and critical outcomes employed to evaluate the efficacy of potential drugs against T. cruzi, aiming to select better compounds likely to succeed in the clinic. Finally, the authors describe the different strategies employed to overcome resistance to benznidazole and find effective new treatments for CD.

EXPERT OPINION

Resistance to benznidazole is a complex phenomenon that occurs naturally among T. cruzi strains. The combination of compounds that inhibit different metabolic pathways of the parasite is an important strategy for developing a new chemotherapeutic protocol.

Global Distribution of Canine Visceral Leishmaniasis and the Role of the Dog in the Epidemiology of the Disease

Visceral leishmaniasis is a disease caused by protozoa of the species Leishmania (Leishmania) infantum (syn = Leishmania chagasi) and Leishmania (Leishmania) donovani, which are transmitted by hematophagous insects of the genera Lutzomyia and Phlebotomus. The domestic dog (Canis familiaris) is considered the main urban reservoir of the parasite due to the high parasite load on its skin, serving as a source of infection for sandfly vectors and, consequently, perpetuating the disease in the urban environment. Some factors are considered important in the perpetuation and spread of canine visceral leishmaniasis (CVL) in urban areas, such as stray dogs, with their errant behavior, and houses that have backyards with trees, shade, and organic materials, creating an attractive environment for sandfly vectors. CVL is found in approximately 50 countries, with the number of infected dogs reaching millions. However, due to the difficulty of controlling and diagnosing the disease, the number of infected animals could be even greater. In the four continents endemic for CVL, there are reports of disease expansion in endemic countries such as Brazil, Italy, Morocco, and Tunisia, as well as in areas where CVL is not endemic, for example, Uruguay. Socio-environmental factors, such as migration, drought, deforestation, and global warming, have been pointed out as reasons for the expansion into areas where it had been absent. Thus, the objective of this review is to address (i) the distribution of CVL in endemic areas, (ii) the role of the dog in the visceral leishmaniasis epidemiology and the factors that influence dog infection and the spread of the disease, and (iii) the challenges faced in the control of CVL.

Positive clinical outcome using a modified dosing regimen of benznidazole in dogs at high risk for infection or acutely infected with Trypanosoma cruzi

Trypanosoma cruzi infection in dogs can cause heart failure and sudden death with few treatment options available. A litter of 4 dogs living in a T cruzi endemic area were randomized to prophylaxis and nonprophylaxis groups as part of a study evaluating a modified benznidazole dosing regimen administered twice weekly to prevent T cruzi infection during a vector transmission season. The 2 dogs that received prophylaxis remained healthy without T cruzi infection or cardiac disease for >2 years. One dog that did not receive prophylaxis died unexpectedly with acute T cruzi-induced pancarditis, and the second dog tested positive for T cruzi and developed complex arrhythmias with markedly increased cardiac troponin I and improved with a higher benznidazole treatment dose. Although the small sample size precludes definitive conclusions, we describe the potential clinical benefit of prophylactic and early treatment with modified benznidazole dosing regimens for dogs with T cruzi infection.

Neglected Zoonotic Diseases: Advances in the Development of Cell-Penetrating and Antimicrobial Peptides against Leishmaniosis and Chagas Disease

In 2020, the WHO established the road map for neglected tropical diseases 2021-2030, which aims to control and eradicate 20 diseases, including leishmaniosis and Chagas disease. In addition, since 2015, the WHO has been developing a Global Action Plan on Antimicrobial Resistance. In this context, the achievement of innovative strategies as an alternative to replace conventional therapies is a first-order socio-sanitary priority, especially regarding endemic zoonoses in poor regions, such as those caused by Trypanosoma cruzi and Leishmania spp. infections. In this scenario, it is worth highlighting a group of natural peptide molecules (AMPs and CPPs) that are promising strategies for improving therapeutic efficacy against these neglected zoonoses, as they avoid the development of toxicity and resistance of conventional treatments. This review presents the novelties of these peptide molecules and their ability to cross a whole system of cell membranes as well as stimulate host immune defenses or even serve as vectors of molecules. The efforts of the biotechnological sector will make it possible to overcome the limitations of antimicrobial peptides through encapsulation and functionalization methods to obtain approval for these treatments to be used in clinical programs for the eradication of leishmaniosis and Chagas disease.

State-of-the-Art in the Drug Discovery Pathway for Chagas Disease: A Framework for Drug Development and Target Validation

Chagas disease is the most important protozoan infection in the Americas, and constitutes a significant public health concern throughout the world. Development of new medications against its etiologic agent, Trypanosoma cruzi, has been traditionally slow and difficult, lagging in comparison with diseases caused by other kinetoplastid parasites. Among the factors that explain this are the incompletely understood mechanisms of pathogenesis of T. cruzi infection and its complex set of interactions with the host in the chronic stage of the disease. These demand the performance of a variety of in vitro and in vivo assays as part of any drug development effort. In this review, we discuss recent breakthroughs in the understanding of the parasite's life cycle and their implications in the search for new chemotherapeutics. For this, we present a framework to guide drug discovery efforts against Chagas disease, considering state-of-the-art preclinical models and recently developed tools for the identification and validation of molecular targets.

Cardiac Magnetic Resonance Imaging Detects Myocardial Abnormalities in Naturally Infected Dogs with Chronic Asymptomatic Chagas Disease

Trypanosoma cruzi infection causes inflammation and fibrosis, resulting in cardiac damage in dogs. The objectives of this study were to describe cardiac magnetic resonance imaging (CMR) in naturally infected dogs with chronic Chagas disease and the frequency of abnormalities for CMR and cardiac diagnostic tests. Ten asymptomatic, client-owned dogs seropositive for T. cruzi were prospectively enrolled in an observational study evaluating echocardiography, ECG (standard and ambulatory), cardiac troponin I (cTnI), and CMR. Standard ECG measurements (3/10) and cTnI concentration (1/10) outside the reference range were uncommon. Ambulatory ECG abnormalities were documented more frequently (6/10 dogs) than with standard ECG and included ventricular arrhythmias (4), supraventricular premature beats (3), second-degree atrioventricular block (2), and sinus arrest (1). Echocardiographic abnormalities were documented in 6/10 dogs including mildly increased left ventricular internal dimension in diastole (1) and decreased right ventricular (RV) systolic function based on reductions in tricuspid annular plane systolic excursion (3) and RV S' (4). Abnormalities were detected with CMR in 7/10 dogs including delayed myocardial enhancement in 5 of which 2 also had increased extracellular volume, abnormal wall motion in 5, and loss of apical compact myocardium in 1. In conclusion, CMR abnormalities were common, and the results of this study suggest CMR can provide useful information in dogs with T. cruzi infection and may support naturally infected dogs for future clinical investigation as an animal model for Chagas disease.

Benznidazole, itraconazole, and their combination for the treatment of chronic experimental Chagas disease in dogs

Treatment for Chagas disease has limited efficacy in the chronic phase. We evaluated benznidazole (BZ) and itraconazole (ITZ) individually and in association in dogs 16 months after infection with a BZ-resistant Trypanosoma cruzi strain. Four study groups (20 animals) were evaluated and treated for 60 days with BZ, ITZ, or BZ + ITZ, and maintained in parallel to control group infected and not treated (INT). All dogs were evaluated in the first, sixth, 12th, 18th and 24th months of study. Polymerase chain reaction (PCR) was negative in 2 of 3 animals in the BZ + ITZ group, 2 of 5 in the BZ group, and 4 of 5 in the ITZ group. Hemoculture performed in the 24th month was negative in all groups. Enzyme-linked immunoassay remained reactive in all treated animals. Echocardiography differentiated treated animals from control animals. Quantitative PCR analysis of cardiac tissue was negative in the BZ + ITZ and BZ groups, positive in 2 of 5 dogs in the ITZ group and in 2 of 3 dogs in the control group, but negative in colon tissue in all groups. Inflammation was significantly reduced in the right atrium and left ventricle of dogs treated with BZ + ITZ and BZ compared with those receiving ITZ alone. Fibrosis was absent in most dogs treated with BZ + ITZ, mild in those treated with BZ or ITZ alone, and intense in the control group. Parasitological and histopathological evaluations showed that BZ + ITZ treatment improved or stabilized the clinical condition of the dogs.