BNZ Pharmacology
Benznidazole is an oral, broad spectrum nitroimidazole antimicrobial that has activity against bacteria and several parasites. It has demonstrated efficacy against in vitro T. cruzi strains in several in vivo animal models 52–56. According to the Biopharmaceutical Classification System (BCS)57,58 BNZ belongs to Class IV drugs (reduced solubility and permeability); it is a liposoluble drug with very low solubility in water, and a weak base at physiological pH range. BNZ solubility in distilled water or simulated gastric and enteric fluids is reported between 0.2 mg/ml and 0.4 mg/ml. According to this, BZN is classified as a low-permeability drug with a log P of 1.6458.
BZN is considered a prodrug, requiring activation by parasite nitroreductase enzymes that reduce BZN, initiating a cascade of reactions leading to the formation of highly reactive drug metabolites27. The main parasite enzyme involved in BZN activation is believed to be a type 1 nitroreductase. The resulting BZN metabolites, such as dialdehyde glyoxal, bind to parasite macromolecules disrupting T. cruzi metabolism and other vital functions, and leading to parasite cell death27. However, using a metabolomics approach to asses BZN mechanism of action, Trochine et al. proposed that the covalent binding of BZN with low molecular weight thiols as well as with protein thiols is a primary cause of the drug’s toxicity against T. cruzi, instead of glyoxal generation as formerly stated59. This suggests that BZN acts in a complex manner and there are still some remaining uncertainties about its mechanisms of action: metabolomic studies are a promising frontier in this research area.
T cruzi resistance to BZN is not well described in literature. Some in-vitro studies have reported that some parasite strains have a ‘natural’ in-vitro resistance to BZN associated with overexpression of ABCG transporter60, but this evidence has been questioned, as in-vitro results do not correlate with therapeutic outcomes in humans61. Other studies with in vitro data suggest that susceptibility of different T.cruzi strains to BZN fluctuates, but the 50% inhibitory concentration (IC50) values remain ≤19.5 μg/mL (75 μM) and can vary 10-fold within the same assay. Activity against different forms of the parasite (epimastigotes, trypomastigotes, or amastigotes) also appears to vary within a relatively small range62,63, and it should be considered that many studies are performed on the epimastigote stage, which is easier to culture but not the human stage of the parasite. Additionally, time-kill studies indicate that BZN trypanocidal effect is both time and concentration dependent 62–64. Using multipleT.cruzi strains and a high-throughput screening platform, a rapid trypanocidal effect was demonstrated with 100% parasite clearance against multiple divergent T. cruzi  genotypes, a rate superior to that for ergosterol biosynthesis inhibitors63.
After oral administration, BZN is quickly absorbed from the human intestine (Ka = 1.14/h), with a plasmatic peak within 2–4 hours after drug intake8,65. The impact of food on absorption has not been systematically investigated. Some evidence points to first step elimination by hepatic biotransformation and entero-hepatic recirculation, possibly with some degree of enteric metabolism as well, but little research has been conducted in this area. Absolute bioavailability in humans has never been formally estimated due to the absence of an intravenous formulation apt for human use, though a mean relative oral bioavailability of 91.7% in three healthy adults when comparing liquid to solid oral formulations was reported66. Steady-state plasma concentrations are reached within 3 days of initiation of a twice-daily dosing regimen8,66. BZN distributes widely into tissues, including the central nervous system (CNS)67,68, with higher volume of distribution in children compared to adults8. The drug reaches CNS concentrations close to 70% of those observed in plasma, which has allowed successful treatment of Chagas CNS infections (e.g. meningoencephalitis) in immunosuppressed patients69–72. Plasma protein binding of BZN is approximately 50% and is thus not expected to lead to significant interactions with other drugs 54.
Clearance of BZN is mainly by biotransformation (>80%)68,73, believed to take place mostly in the liver, probably by members of the cytochrome P450 (CYP) family and/or tissue nitro-reductases. However, few studies to date have explored the details of the metabolic pathways responsible for BZN elimination. Approximately 6–20% of the drug can be found unchanged in urine, with differences depending on age of the patient (e.g., children seem to eliminate more unchanged drug in urine compared to adults); and the rest of the drug has been observed as reduced and conjugated.74
Mean BZN half-life is 13 hours in adults66 and significantly shorter in children (3 to 6 hours for 2 to 7 year-old patients and 9 to 10 hours in children 7 to 12 years) as observed in two prospective clinical trials8. This difference in clearance and half-life between different age groups implies average steady-state concentrations of BZN lower in children than in adults. Interestingly, this difference does not seem to affect the efficacy of BNZ since in a prospective clinical trial, all treated children showed good response to treatment despite lower plasma concentrations of the drug8. When comparing the data obtained in this study with previously reported adult results66, a progressive decrease in the clearance rate of BZN with increasing age was observed (i.e. the older the patient, the slower the drug was eliminated). The specific mechanisms for drug elimination in children and adults remain undiscovered. Research in the area is actively testing different hypotheses such as slower drug metabolism in adults and impaired drug absorption in younger children. BNZ pharmacokinetics and treatment response in teenagers and young adults have never been studied, so the assumption that it would be in between children and adults is so far unsupported by actual evidence.
The most commonly used BNZ dosing regimen, reported in the majority of the evidence published to date (see table 1 ) uses doses ranging from 5 to 8 mg/kg/day orally, in two daily doses for 30 to 60 days. BZN can also be administered in three daily doses, with a clear tendency in international guides for recommending 5 rather than 8 mg/kg/day and twice daily rather than thrice.48
Duration of treatment in children and adults is currently under review and some expert guides are already recommending shorter treatments8,75, supported by the fact that treatment in children is proven to be effective despite differences in PK with adults leading to lower concentrations and shorter half-lives, without detectable drop in effectiveness and with less adverse reactions8,75. A few trials enrolling children who received 30 days of treatment have showed good results42,76, and recent evidence points towards possible efficacy of lower BNZ doses or less frequent dosing for adults and teenagers too65,77,78. Lower treatment duration has also proven to be effective in adults in preliminary results of an unpublished clinical trial (NCT03378661) that showed 89.3% of therapeutic response (measured as a negative PCR) after a 4 week treatment compared to 82.8% after 8 week-treatment. There are currently other ongoing trials addressing BZN dairy doses and duration as well such as BETTY trial79 and MULTIBENZ (NCT03191162), that may change treatment regimen in the next few years, assuming that sustained long term responses are demonstrated.
The most commonly observed adverse drug reactions (ADRs) associated with BZN use includes rash and pruritus (usually after 7 to 12 days of treatment), headache, myalgia, and gastrointestinal discomfort (in the first days of the treatment). Drug-associated hepatitis, leucopenia, peripheral neuropathy, and severe drug hypersensitivity (Stevens-Johnson syndrome and other reactions with systemic symptoms) are less frequent. The median proportion of severe side effects is 2.7%45 and trough BZN serum concentrations did not appear to be related to the appearance of serious ADRs in a small study in adults80, evidence for a concentration-adverse event relationship has been observed in pediatric studies8,34. A recent prospective study in 99 participants reported some previously unreported ADRs; ten subjects presented psychiatric symptoms (anxiety, panic attacks, emotional lability and persecutory delusions), four patients reported sexual alterations (erectile dysfunction or delay in menstrual cycle with no alternative explanations) and one patient had a bronchospasm. The results of this study were in other aspects similar to previously published literature about BZN adverse reactions.81The safety profile of BZN in children is well described in the literature; data are consistent and do not suggest any signals of clinical concern8,34.
The incidence of ADRs between children and adults has not been compared directly (i.e. in a study enrolling both age groups), but ADRs seem rare and almost universally mild in younger children, and appear to increase gradually after 7 years of age in both frequency and severity. It is very infrequent to observe ADRs in newborns and children under 1 year old, and rates of treatment discontinuation due to ADRs are significantly low in children34,47,76,82–85 while these ranges between 11 and 45% in adult studies16,45,82,86.
The underlying biological mechanisms for the observed ADRs have not been studied in depth, but the immune system seems to play an important role, particularly in the case of cutaneous rashes and hypersensitivity reactions. This assumption is based on the timing for the moderate cutaneous reactions (7–12 days after onset of treatment) that mimics the time course of similar reactions associated to other unrelated medications known to cause rash (e.g. lamotrigine), and the observation of rare severe adverse reactions such as Stevens-Johnson syndrome and drug reactions with eosinophilia and systemic symptoms83,84. A common immunological trigger for these reactions and possibly a pharmacogenetic predisposition could explain these similarities, but studies of potential pharmacogenomic markers are lacking. Some authors had formerly proposed to associate BZN with thioctic acid in order to prevent ADRs, based on this compound to increase hepatic elimination of BZN, but this has proven not to be effective when evaluated in-vivo.87
BZN has never been formally studied during pregnancy, but it is not recommended for pregnant women due to the lack of safety data; there is insufficient information about reproductive safety of this drug, other than the fact that there have been no reports of malformations or any other pregnancy complications. However, it should be considered that it is likely that an unknown number of women were exposed to BZN in the first trimester by accident and the lack of reports on safety data might be a good sign so far. Also, there are some reports of treatment during late-stage pregnancy in emergency situations that did not result in any complications for the baby and may have saved the mother’s life69. The main recommendation therefore remains to avoid BZN during the first trimester of pregnancy and throughout pregnancy whenever possible until further information becomes available, though in case of an emergency or a life-threatening situation caused by CD, we recommend not delaying treatment because of an unproven teratogenic risk69.
BNZ has been classically contraindicated during lactation, but recent prospective studies and pharmacokinetic evaluations suggest that the risk of exposure to BZN from breastmilk for a breastfed baby is negligible, and lactation should not be considered a contraindication for CD treatment in those circumstances when treatment cannot be postponed. 29