Pharmacokinetics and pharmacodynamics utilizing unbound target tissue exposure as part of a disposition-based rationale for lead optimization of benzoxaboroles in the treatment of Stage 2 Human African Trypanosomiasis

Practical Synthesis of 6-Amino-1-hydroxy-2,1-benzoxaborolane: A Key Intermediate of DNDI-6148

Visceral leishmaniasis (VL), a parasitic, poverty-linked, neglected disease, is endemic across multiple regions of the world and fatal if untreated. There is an urgent need for a better and more affordable treatment for VL. DNDI-6148 is a promising drug candidate being evaluated for the treatment of VL; however, the current process for producing the key intermediate of DNDI-6148, 6-amino-1-hydroxy-2,1-benzoxaborolane, is expensive and difficult to scale up. Herein, we describe two practical approaches to synthesizing 6-amino-1-hydroxy-2,1-benzoxaborolane from inexpensive and readily available raw materials. Starting with 4-tolunitrile, the first approach is a five-step sequence involving a Hofmann rearrangement, resulting in an overall yield of 40%. The second approach utilizes 2-methyl-5-nitroaniline as the starting material and features borylation of aniline and continuous flow hydrogenation as the key steps, with an overall yield of 46%. Both routes bypass the nitration of 1-hydroxy-2,1-benzoxaborolane, which is challenging and expensive to scale. In particular, the second approach is more practical and scalable because of the mild operating conditions and facile isolation process.

Drug resistance in animal trypanosomiases: Epidemiology, mechanisms and control strategies

Animal trypanosomiasis (AT) is a complex of veterinary diseases known under various names such as nagana, surra, dourine and mal de caderas, depending on the country, the infecting trypanosome species and the host. AT is caused by parasites of the genus Trypanosoma, and the main species infecting domesticated animals are T. brucei brucei, T. b. rhodesiense, T. congolense, T. simiae, T. vivax, T. evansi and T. equiperdum. AT transmission, again depending on species, is through tsetse flies or common Stomoxys and tabanid flies or through copulation. Therefore, the geographical spread of all forms of AT together is not restricted to the habitat of a single vector like the tsetse fly and currently includes almost all of Africa, and most of South America and Asia. The disease is a threat to millions of companion and farm animals in these regions, creating a financial burden in the billions of dollars to developing economies as well as serious impacts on livestock rearing and food production. Despite the scale of these impacts, control of AT is neglected and under-resourced, with diagnosis and treatments being woefully inadequate and not improving for decades. As a result, neither the incidence of the disease, nor the effectiveness of treatment is documented in most endemic countries, although it is clear that there are serious issues of resistance to the few old drugs that are available. In this review we particularly look at the drugs, their application to the various forms of AT, and their mechanisms of action and resistance. We also discuss the spread of veterinary trypanocide resistance and its drivers, and highlight current and future strategies to combat it.

The pipeline for drugs for control and elimination of neglected tropical diseases: 1. Anti-infective drugs for regulatory registration

The World Health Organization 'Ending the neglect to attain the Sustainable Development Goals: A road map for neglected tropical diseases 2021-2030' outlines the targets for control and elimination of neglected tropical diseases (NTDs). New drugs are needed to achieve some of them. We are providing an overview of the pipeline for new anti-infective drugs for regulatory registration and steps to effective use for NTD control and elimination. Considering drugs approved for an NTD by at least one stringent regulatory authority: fexinidazole, included in WHO guidelines for Trypanosoma brucei gambiense African trypanosomiasis, is in development for Chagas disease. Moxidectin, registered in 2018 for treatment of individuals ≥ 12 years old with onchocerciasis, is undergoing studies to extend the indication to 4-11-year-old children and obtain additional data to inform WHO and endemic countries' decisions on moxidectin inclusion in guidelines and policies. Moxidectin is also being evaluated for other NTDs. Considering drugs in at least Phase 2 clinical development, a submission is being prepared for registration of acoziborole as an oral treatment for first and second stage T.b. gambiense African trypanosomiasis. Bedaquiline, registered for tuberculosis, is being evaluated for multibacillary leprosy. Phase 2 studies of emodepside and flubentylosin in O. volvulus-infected individuals are ongoing; studies for Trichuris trichuria and hookworm are planned. A trial of fosravuconazole in Madurella mycetomatis-infected patients is ongoing. JNJ-64281802 is undergoing Phase 2 trials for reducing dengue viral load. Studies are ongoing or planned to evaluate oxantel pamoate for onchocerciasis and soil-transmitted helminths, including Trichuris, and oxfendazole for onchocerciasis, Fasciola hepatica, Taenia solium cysticercosis, Echinococcus granulosus and soil-transmitted helminths, including Trichuris. Additional steps from first registration to effective use for NTD control and elimination include country registrations, possibly additional studies to inform WHO guidelines and country policies, and implementation research to address barriers to effective use of new drugs. Relative to the number of people suffering from NTDs, the pipeline is small. Close collaboration and exchange of experience among all stakeholders developing drugs for NTDs may increase the probability that the current pipeline will translate into new drugs effectively implemented in affected countries.

Determination of the Optimal Single Dose Treatment for Acoziborole, a Novel Drug for the Treatment of Human African Trypanosomiasis: First-in-Human Study

BACKGROUND AND OBJECTIVES

Acoziborole is a novel boron-containing candidate developed as an oral drug for the treatment of human African trypanosomiasis (HAT). Results from preclinical studies allowed progression to Phase 1 trials. We aimed to determine the best dose regimen for all stages of HAT.

METHODS

Acoziborole was assessed in 128 healthy adult males of sub-Saharan African origin living in France. The study included a single oral administration of a 20- to 1200-mg dose in a randomised double-blind study in cohorts of 8 (6 active, 2 placebo) to assess safety, tolerability, and pharmacokinetics. In three additional open cohorts of 6 participants, the effect of activated charcoal was evaluated, bioequivalence of capsules versus tablets was assessed, and safety in the 960-mg tablet cohorts was monitored.

RESULTS

Acoziborole was well tolerated at all doses tested; no dose-related adverse events were observed. The drug appeared rapidly in plasma (at 1 h), reached t_max between 24 and 72 h, and remained stable for up to 96 h, after which a slow decrease was quantifiable until 14 weeks after dosing. Charcoal had little impact on the enterohepatic recirculation effect, except for the 20-mg dose. Bioequivalence between capsule and tablet formulations was demonstrated. The therapeutic single dose for administration under fasted conditions was fixed to 960 mg. The maximum administered dose was 1200 mg.

CONCLUSIONS

This study showed that acoziborole could be safely assessed in patients as a potential single-dose oral cure for both stages of gambiense HAT.

TRIAL REGISTRATION

The study was registered with ClinicalTrials.gov: NCT01533961.

Hypothesis-generating proteome perturbation to identify NEU-4438 and acoziborole modes of action in the African Trypanosome

NEU-4438 is a lead for the development of drugs against Trypanosoma brucei, which causes human African trypanosomiasis. Optimized with phenotypic screening, targets of NEU-4438 are unknown. Herein, we present a cell perturbome workflow that compares NEU-4438's molecular modes of action to those of SCYX-7158 (acoziborole). Following a 6 h perturbation of trypanosomes, NEU-4438 and acoziborole reduced steady-state amounts of 68 and 92 unique proteins, respectively. After analysis of proteomes, hypotheses formulated for modes of action were tested: Acoziborole and NEU-4438 have different modes of action. Whereas NEU-4438 prevented DNA biosynthesis and basal body maturation, acoziborole destabilized CPSF3 and other proteins, inhibited polypeptide translation, and reduced endocytosis of haptoglobin-hemoglobin. These data point to CPSF3-independent modes of action for acoziborole. In case of polypharmacology, the cell-perturbome workflow elucidates modes of action because it is target-agnostic. Finally, the workflow can be used in any cell that is amenable to proteomic and molecular biology experiments.

Synthesis of Benzoxaboroles by ortho-Oxalkylation of Arylboronic Acids with Aldehydes/Ketones in the Presence of Brønsted Acids

Herein we describe a simple and efficient synthesis of benzoxaboroles from arylboronic acids and aldehydes or ketones in the presence of a Brønsted acid. This method greatly simplifies the starting materials and reduces the number of reaction steps. The reaction can also be accomplished with acetals and ketals. The reaction has a wide substrate scope and high practicability.

Optimal kinetic exposures for classic and candidate antitrypanosomals

OBJECTIVES

Efficacy is determined not only by size, but also by shape, of drug exposure. Here the critical importance of the temporal pattern of drug concentrations (pharmacokinetic profile) is examined for antitrypanosomals in vitro.

METHODS

An in vitro hollow-fibre cartridge system was used to study contrasting drug profiles with four clinically used agents and two experimental candidates against the deadly parasite Trypanosoma brucei. Artificial kinetics were employed intentionally to favour either high peak concentration or sustained duration of drug.

RESULTS

Changing the shape of drug exposure significantly impacted drug efficacy. Suramin, melarsoprol and pentamidine were concentration-driven and therefore more efficacious when applied as short-lived high peaks. In contrast, difluoromethylornithine (DFMO) was time-driven, and therefore maximally effective as a constant infusion. Kinetic preference was robust over a wide range of drug exposures. Promising clinical candidates SCYX-7158 (acoziborole) and fexinidazole (parent and sulfone) were concentration-driven, suggesting optimal clinical regimens would involve relatively high but intermittent dosing.

CONCLUSIONS

Antitrypanosomals have an intrinsic pharmacokinetic driver for optimal efficacy, with important implications for clinical management and future candidate development.

New Drugs for Human African Trypanosomiasis: A Twenty First Century Success Story

The twentieth century ended with human African trypanosomiasis (HAT) epidemics raging across many parts of Africa. Resistance to existing drugs was emerging, and many programs aiming to contain the disease had ground to a halt, given previous success against HAT and the competing priorities associated with other medical crises ravaging the continent. A series of dedicated interventions and the introduction of innovative routes to develop drugs, involving Product Development Partnerships, has led to a dramatic turnaround in the fight against HAT caused by Trypanosoma brucei gambiense. The World Health Organization have been able to optimize the use of existing tools to monitor and intervene in the disease. A promising new oral medication for stage 1 HAT, pafuramidine maleate, ultimately failed due to unforeseen toxicity issues. However, the clinical trials for this compound demonstrated the possibility of conducting such trials in the resource-poor settings of rural Africa. The Drugs for Neglected Disease initiative (DNDi), founded in 2003, has developed the first all oral therapy for both stage 1 and stage 2 HAT in fexinidazole. DNDi has also brought forward another oral therapy, acoziborole, potentially capable of curing both stage 1 and stage 2 disease in a single dosing. In this review article, we describe the remarkable successes in combating HAT through the twenty first century, bringing the prospect of the elimination of this disease into sight.

Identification of a potent benzoxaborole drug candidate for treating cryptosporidiosis

Cryptosporidiosis is a leading cause of life-threatening diarrhea in young children and causes chronic diarrhea in AIDS patients, but the only approved treatment is ineffective in malnourished children and immunocompromised people. We here use a drug repositioning strategy and identify a promising anticryptosporidial drug candidate. Screening a library of benzoxaboroles comprised of analogs to four antiprotozoal chemical scaffolds under pre-clinical development for neglected tropical diseases for Cryptosporidium growth inhibitors identifies the 6-carboxamide benzoxaborole AN7973. AN7973 blocks intracellular parasite development, appears to be parasiticidal, and potently inhibits the two Cryptosporidium species most relevant to human health, C. parvum and C. hominis. It is efficacious in murine models of both acute and established infection, and in a neonatal dairy calf model of cryptosporidiosis. AN7973 also possesses favorable safety, stability, and PK parameters, and therefore, is an exciting drug candidate for treating cryptosporidiosis.

Half- and mixed-sandwich metallacarboranes for potential applications in medicine

Today, medicinal chemistry is still clearly dominated by organic chemistry, and commercially available boron-based drugs are rare. In contrast to hydrocarbons, boranes prefer the formation of polyhedral clusters via delocalized 3c2e bonds, such as polyhedral dicarba-closo-dodecaborane(12) (closo-C2B10H12). These clusters have remarkable biological stability, and the three isomers, 1,2- (ortho), 1,7- (meta), and 1,12-dicarba-closo-dodecaborane(12) (para), have attracted much interest due to their unique structural features. Furthermore, anionic nido clusters ([7,8-C2B9H11]2−), derived from the neutral icosahedral closo cluster 1,2-dicarba-closo-dodecaborane(12) by deboronation followed by deprotonation are suitable ligands for transition metals and offer the possibility to form metallacarboranes, for example via coordination through the upper pentagonal face of the cluster. The isolobal analogy between the cyclopentadienyl(–1) ligand (Cp−) and [C2B9H11]2− clusters (dicarbollide anion, Cb2−) is the motivation in using Cb2− as ligand for coordination to a metal center to design compounds for various applications. This review focuses on potential applications of half- and mixed-sandwich-type transition metal complexes in medicine.

Discovery of boron-containing compounds as Aβ aggregation inhibitors and antioxidants for the treatment of Alzheimer's disease

A novel series of boron-containing compounds were designed, synthesized and evaluated as multi-target-directed ligands against Alzheimer's disease. The biological activity results demonstrated that these compounds possessed a significant ability to inhibit self-induced Aβ aggregation (20.5-82.8%, 20 μM) and to act as potential antioxidants (oxygen radical absorbance capacity assay using fluorescein (ORAC-FL) values of 2.70-5.87). In particular, compound 17h is a potential lead compound for AD therapy (IC₅₀ = 3.41 μM for self-induced Aβ aggregation; ORAC-FL value = 4.55). Compound 17h also functions as a metal chelator. These results indicated that boron-containing compounds could be new structural scaffolds for the treatment of AD.

Model System Identifies Kinetic Driver of Hsp90 Inhibitor Activity against African Trypanosomes and Plasmodium falciparum

Hsp90 inhibitors, well studied in the laboratory and clinic for antitumor indications, have promising activity against protozoan pathogens, including Trypanosoma brucei which causes African sleeping sickness, and the malaria parasite, Plasmodium falciparum To progress these experimental drugs toward clinical use, we adapted an in vitro dynamic hollow-fiber system and deployed artificial pharmacokinetics to discover the driver of their activity: either concentration or time. The activities of compounds from three major classes of Hsp90 inhibitors in development were evaluated against trypanosomes. In all circumstances, the activities of the tested Hsp90 inhibitors were concentration driven. By optimally deploying the drug to match its kinetic driver, the efficacy of a given dose was improved up to 5-fold, and maximal efficacy was achieved with a significantly lower drug exposure. The superiority of concentration-driven regimens was evident in vitro over several logs of drug exposure and was predictive of efficacy in a mouse model of African trypanosomiasis. In studies with P. falciparum, antimalarial activity was similarly concentration driven. This experimental strategy offers an expedient and versatile translational tool to assess the impact of pharmacokinetics on antiprotozoal activity. Knowing kinetic governance early in drug development provides an additional metric for judging lead compounds and allows the incisive design of animal efficacy studies.

Aromatic metamorphosis: conversion of an aromatic skeleton into a different ring system

Aromatic skeletons are generally regarded as being uncleavable because of their aromatic stabilization energy. Compared to exocyclic functionalizations of aromatic compounds, much less attention has been paid to substitutions of endocyclic atoms in aromatic cores through partial disassembly of the cyclic skeletons and subsequent ring reconstruction. In this Feature Article, we describe our endeavours to establish "aromatic metamorphosis", where general aromatic compounds such as dibenzothiophenes, dibenzofurans, and benzofurans are transformed into different ring systems using a multi-step strategy or ideally in one step.

In vitro and in vivo pharmacodynamics of three novel antileishmanial lead series

Objectives

Three new chemical series (bicyclic nitroimidazoles, aminopyrazoles and oxaboroles) were selected by Drugs for Neglected Diseases initiative as potential new drug leads for leishmaniasis. Pharmacodynamics studies included both in vitro and in vivo efficacy, cross-resistance profiling against the current antileishmanial reference drugs and evaluation of their cidal activity potential.

Methods

Efficacy against the reference laboratory strains of Leishmania infantum (MHOM/MA(BE)/67/ITMAP263) and L. donovani (MHOM/ET/67/L82) was evaluated in vitro on intracellular amastigotes and in vivo in the early curative hamster model. Cidal activity was assessed over a period of 15 days in an in vitro ‘time-to-kill’ assay. Cross-resistance was assessed in vitro on a panel of L. infantum strains with different degrees of resistance to either antimony, miltefosine or paromomycin.

Results

All lead compounds showed potent and selective in vitro activity against the Leishmania strains tested and no cross-resistance could be demonstrated against any of the current antileishmanial drugs. Cidal activity was obtained in vitro for all series within 15 days of exposure with some differences noted between L. donovani and L. infantum. When evaluated in vivo, all lead compounds showed high efficacy and no adverse effects were observed.

Conclusions

The new lead series were shown to have cidal pharmacodynamic activity. The absence of cross-resistance with any of the current antileishmanial drugs opens possibilities for combination treatment to reduce the likelihood of treatment failures and drug resistance.

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Good efficacy was evaluated for all series in vitro and in vivo.

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No cross-resistance towards current anti-leishmanial drugs was observed.

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Cidal activity was obtained in vitro for all series within 15 days of exposure.

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Some differences were observed between L. infantum and L. donovani.

Influence of fluorine substituents on the properties of phenylboronic compounds

Rapid development of research on the chemistry of boronic acids is connected with their applications in organic synthesis, analytical chemistry, materials’ chemistry, biology and medicine. In many applications Lewis acidity of boron atoms plays an important role. Special group of arylboronic acids are fluoro-substituted compounds, in which the electron withdrawing character of fluorine atoms influences their properties. The present paper deals with fluoro-substituted boronic acids and their derivatives: esters, benzoxaboroles and boroxines. Properties of these compounds, i.e. acidity, hydrolytic stability, structures in crystals and in solution as well as spectroscopic properties are discussed. In the next part examples of important applications are given.

Antileishmanial and antitrypanosomal drug identification

Although the treatments for human African trypanosomiasis (HAT), leishmaniasis and Chagas disease (CD) still rely on drugs developed several decades ago, there has been significant progress in the identification, development and use of novel drugs and formulations. Notably, there are now two drugs in clinical trial for HAT, fexinidazole and acoziborole; the liposomal amphotericin B formulation AmBisome has become an essential tool for both treatment and control of visceral leishmaniasis; and antifungal triazoles, posoconazole and ravuconazole, together with fexinidazole, have reached clinical trials for CD. Several other novel and diverse candidates are moving through the pipeline; sustained funding for their clinical development will now be the key to bring new safe, oral, shorter-course treatments to the clinic.

Assessment of a pretomanid analogue library for African trypanosomiasis: Hit-to-lead studies on 6-substituted 2-nitro-6,7-dihydro-5H-imidazo[2,1-b][1,3]thiazine 8-oxides

A 900 compound nitroimidazole-based library derived from our pretomanid backup program with TB Alliance was screened for utility against human African trypanosomiasis (HAT) by the Drugs for Neglected Diseases initiative. Potent hits included 2-nitro-6,7-dihydro-5H-imidazo[2,1-b][1,3]thiazine 8-oxides, which surprisingly displayed good metabolic stability and excellent cell permeability. Following comprehensive mouse pharmacokinetic assessments on four hits and determination of the most active chiral form, a thiazine oxide counterpart of pretomanid (24) was identified as the best lead. With once daily oral dosing, this compound delivered complete cures in an acute infection mouse model of HAT and increased survival times in a stage 2 model, implying the need for more prolonged CNS exposure. In preliminary SAR findings, antitrypanosomal activity was reduced by removal of the benzylic methylene but enhanced through a phenylpyridine-based side chain, providing important direction for future studies.

Leishmania and other intracellular pathogens: selectivity, drug distribution and PK-PD

New drugs and treatments for diseases caused by intracellular pathogens, such as leishmaniasis and the Leishmania species, have proved to be some of the most difficult to discover and develop. The focus of discovery research has been on the identification of potent and selective compounds that inhibit target enzymes (or other essential molecules) or are active against the causative pathogen in phenotypic in vitro assays. Although these discovery paradigms remain an essential part of the early stages of the drug R & D pathway, over the past two decades additional emphasis has been given to the challenges needed to ensure that the potential anti-infective drugs distribute to infected tissues, reach the target pathogen within the host cell and exert the appropriate pharmacodynamic effect at these sites. This review will focus on how these challenges are being met in relation to Leishmania and the leishmaniases with lessons learned from drug R & D for other intracellular pathogens.

Challenges and Opportunities for the Application of Boron Clusters in Drug Design

There are two branches in boron medicinal chemistry: the first focuses on single boron atom compounds, and the second utilizes boron clusters. Boron clusters and their heteroatom counterparts belong to the family of cage compounds. A subset of this extensive class of compounds includes dicarbadodecaboranes, which have the general formula C2B10H12, and their metal biscarboranyl complexes, metallacarboranes, with the formula [M(C2B10H12)2(-2)]. The unique properties of boron clusters have resulted in their utilization in applications such as in pharmacophores, as scaffolds in molecular construction, and as modulators of bioactive compounds. This Perspective presents an overview of the properties of boron clusters that are pertinent for drug discovery, recent applications in the design of various classes of drugs, and the potential use of boron clusters in the construction of new pharmaceuticals.

Genomic and Proteomic Studies on the Mode of Action of Oxaboroles against the African Trypanosome

SCYX-7158, an oxaborole, is currently in Phase I clinical trials for the treatment of human African trypanosomiasis. Here we investigate possible modes of action against Trypanosoma brucei using orthogonal chemo-proteomic and genomic approaches. SILAC-based proteomic studies using an oxaborole analogue immobilised onto a resin was used either in competition with a soluble oxaborole or an immobilised inactive control to identify thirteen proteins common to both strategies. Cell-cycle analysis of cells incubated with sub-lethal concentrations of an oxaborole identified a subtle but significant accumulation of G2 and >G2 cells. Given the possibility of compromised DNA fidelity, we investigated long-term exposure of T. brucei to oxaboroles by generating resistant cell lines in vitro. Resistance proved more difficult to generate than for drugs currently used in the field, and in one of our three cell lines was unstable. Whole-genome sequencing of the resistant cell lines revealed single nucleotide polymorphisms in 66 genes and several large-scale genomic aberrations. The absence of a simple consistent mechanism among resistant cell lines and the diverse list of binding partners from the proteomic studies suggest a degree of polypharmacology that should reduce the risk of resistance to this compound class emerging in the field. The combined genetic and chemical biology approaches have provided lists of candidates to be investigated for more detailed information on the mode of action of this promising new drug class.

The mode of action of a new class of boron-containing chemicals (the oxaboroles), currently under development for the treatment of human African trypanosomiasis, is unknown. Here we identify a number of potential candidate proteins that could be involved either in the mode of action of these compounds or in the mechanism of resistance. This information could prove critical in protecting the compounds against resistance emerging in the field as well as opening up new avenues for drug discovery.

Emerging paradigms in anti-infective drug design

The need for new drugs to treat microbial infections is pressing. The great progress made in the middle part of the twentieth Century was followed by a period of relative inactivity as the medical needs relating to infectious disease in the wealthier nations receded. Growing realisation that anti-infectives are needed in many parts of the world, to treat neglected diseases as well as to combat the burgeoning risk of resistance to existing drugs, has galvanised a new wave of research into anti-microbial drugs. The transfer of knowledge from the Pharmaceutical industry relating to the importance of understanding how to target drugs successfully within the body, and improved understanding of how pathogens interact with their hosts, is driving a series of new paradigms in anti-infective drug design. Here we provide an overview of those processes as an introduction to a series of articles from experts in this area that emerged from a meeting entitled “Emerging Paradigms in Anti-Infective Drug Design” held in London on the 17th and 18th September 2012. The symposium was organised jointly by British Society for Parasitology (BSP) and the Biological & Medicinal Chemistry sector of the Royal Society of Chemistry (RSC) and held at the London School of Hygiene & Tropical Medicine. The symposium set out to cover all aspects of the identification of new therapeutic modalities for the treatment of neglected and tropical diseases. We aimed to bring together leading scientists from all the disciplines working in this field and cover the pharmacology, medicinal chemistry and drug delivery of potential new medicines. Sessions were held on: “Target diseases and targets for drugs”, “Target based medicinal chemistry”, “Bioavailability and chemistry”, “Targeting intracellular microbes”, “Alternative approaches and models”, and “New anti-infectives – how do we get there?”

This symposium was organised by Simon Croft (LSHTM) and Mike Barrett (University of Glasgow) for the BSP, and David Alker (David Alker Associates) and Andrew Stachulski (University of Liverpool) for the Biological & Medicinal Chemistry sector of the RSC.