Brain-Penetrant Triazolopyrimidine and Phenylpyrimidine Microtubule Stabilizers as Potential Leads to Treat Human African Trypanosomiasis

Tractable Quinolone Hydrazides Exhibiting Sub-Micromolar and Broad Spectrum Antitrypanosomal Activities

Nagana and Human African Trypanosomiasis (HAT), caused by (sub)species of Trypanosoma, are diseases that impede human and animal health, and economic growth in Africa. The few drugs available have drawbacks including suboptimal efficacy, adverse effects, drug resistance, and difficult routes of administration. New drugs are needed. A series of 20 novel quinolone compounds with affordable synthetic routes was made and evaluated in vitro against Trypanosoma brucei and HEK293 cells. Of the 20 compounds, 12 had sub-micromolar potencies against the parasite (EC50 values=0.051-0.57 μM), and most were non-toxic to HEK293 cells (CC50 values>5 μM). Two of the most potent compounds presented sub-micromolar activities against other trypanosome (sub)species (T. cruzi and T. b. rhodesiense). Although aqueous solubility is poor, both compounds possess good logD values (2-3), and either robust or poor microsomal stability profiles. These varying attributes will be addressed in future reports.

Design, Synthesis, and Evaluation of An Anti-trypanosomal [1,2,4]Triazolo[1,5-a]pyrimidine Probe for Photoaffinity Labeling Studies

Studies have shown that depending on the substitution pattern, microtubule (MT)-targeting 1,2,4-triazolo[1,5-a]pyrimidines (TPDs) can produce different cellular responses in mammalian cells that may be due to these compounds interacting with distinct binding sites within the MT structure. Selected TPDs are also potently bioactive against the causative agent of human African trypanosomiasis, Trypanosoma brucei, both in vitro and in vivo. So far, however, there has been no direct evidence of tubulin engagement by these TPDs in T. brucei. Therefore, to enable further investigation of anti-trypanosomal TPDs, a TPD derivative amenable to photoaffinity labeling (PAL) was designed, synthesized, and evaluated in PAL experiments using HEK293 cells and T. brucei. The data arising confirmed specific labeling of T. brucei tubulin. In addition, proteomic data revealed differences in the labeling profiles of tubulin between HEK293 and T. brucei, suggesting structural differences between the TPD binding site(s) in mammalian and trypanosomal tubulin.

Tubulin inhibitors. Selected scaffolds and main trends in the design of novel anticancer and antiparasitic agents

Design of tubulin inhibitors as anticancer drugs dynamically developed over the past 20 years. The modern arsenal of potential tubulin-targeting anticancer agents is represented by small molecules, monoclonal antibodies, and antibody-drug conjugates. Moreover, targeting tubulin has been a successful strategy in the development of antiparasitic drugs. In the present review, an overall picture of the research and development of potential tubulin-targeting agents using small molecules between 2018 and 2023 is provided. The data about some most often used and prospective chemotypes of small molecules (privileged heterocycles, moieties of natural molecules) and synthetic methodologies (analogue-based, fragment-based drug design, molecular hybridization) applied for the design of novel agents with an impact on the tubulin system are summarized. The design and prospects of multi-target agents with an impact on the tubulin system were also highlighted. Reported in the review data contribute to the "structure-activity" profile of tubulin-targeting small molecules as anticancer and antiparasitic agents and will be useful for the application by medicinal chemists in further exploration, design, improvement, and optimization of this class of molecules.

Structure-Activity Relationships, Tolerability and Efficacy of Microtubule-Active 1,2,4-Triazolo[1,5-a]pyrimidines as Potential Candidates to Treat Human African Trypanosomiasis

Tubulin and microtubules (MTs) are potential protein targets to treat parasitic infections and our previous studies have shown that the triazolopyrimidine (TPD) class of MT-active compounds hold promise as antitrypanosomal agents. MT-targeting TPDs include structurally related but functionally diverse congeners that interact with mammalian tubulin at either one or two distinct interfacial binding sites; namely, the seventh and vinca sites, which are found within or between α,β-tubulin heterodimers, respectively. Evaluation of the activity of 123 TPD congeners against cultured Trypanosoma brucei enabled a robust quantitative structure-activity relationship (QSAR) model and the prioritization of two congeners for in vivo pharmacokinetics (PK), tolerability and efficacy studies. Treatment of T. brucei-infected mice with tolerable doses of TPDs significantly decreased blood parasitemia within 24 h. Further, two once-weekly doses at 10 mg/kg of a candidate TPD significantly extended the survival of infected mice relative to infected animals treated with vehicle. Further optimization of dosing and/or the dosing schedule of these CNS-active TPDs may provide alternative treatments for human African trypanosomiasis.

Structure-Activity Relationships, Tolerability and Efficacy of Microtubule-Active 1,2,4-Triazolo[1,5- a ]pyrimidines as Potential Candidates to Treat Human African Trypanosomiasis

Tubulin and microtubules (MTs) are potential protein targets to treat parasitic infections and our previous studies have shown that the triazolopyrimidine (TPD) class of MT- active compounds hold promise as antitrypanosomal agents. MT-targeting TPDs include structurally related but functionally diverse congeners that interact with mammalian tubulin at either one or two distinct interfacial binding sites; namely, the seventh and vinca sites, which are found within or between α,β-tubulin heterodimers, respectively. Evaluation of the activity of 123 TPD congeners against cultured Trypanosoma brucei enabled a robust quantitative structure-activity relationship (QSAR) model and the prioritization of two congeners for in vivo pharmacokinetics (PK), tolerability and efficacy studies. Treatment of T. brucei -infected mice with tolerable doses of TPDs 3 and 4 significantly decreased blood parasitemia within 24 h. Further, two once-weekly doses of 4 at 10 mg/kg significantly extended the survival of infected mice relative to infected animals treated with vehicle. Further optimization of dosing and/or the dosing schedule of these CNS-active TPDs may provide alternative treatments for human African trypanosomiasis.

Structure-activity relationship of dibenzylideneacetone analogs against the neglected disease pathogen, Trypanosoma brucei

Trypanosoma brucei is a protozoan parasite that causes Human African Trypanosomiasis (HAT), a neglected tropical disease (NTD) that is endemic in 36 countries in sub-Saharan Africa. Only a handful drugs are available for treatment, and these have limitations, including toxicity and drug resistance. Using the natural product, curcumin, as a starting point, several curcuminoids and related analogs were evaluated against bloodstream forms of T. b. brucei. A particular subset of dibenzylideneacetone (DBA) compounds exhibited potent in vitro antitrypanosomal activity with sub-micromolar EC₅₀ values. A structure-activity relationship study including 26 DBA analogs was initiated, and several compounds exhibited EC₅₀ values as low as 200 nM. Cytotoxicity counter screens in HEK293 cells identified several compounds having selectivity indices above 10. These data suggest that DBAs offer starting points for a new small molecule therapy of HAT.

Synthetic approaches to the 2015-2018 new agrochemicals

In this review, the synthesis of 33 agrochemicals that received an international standardization organization (ISO) name between January 2015 and December 2018 is described. The aim is to showcase the broad range and scope of reactions, reagents and intermediates used to discover and produce the latest active ingredients addressing the crop protection industry's needs.

Congeners Derived from Microtubule-Active Phenylpyrimidines Produce a Potent and Long-Lasting Paralysis of Schistosoma mansoni In Vitro

Schistosomiasis is a parasitic disease that affects approximately 200 million people in developing countries. Current treatment relies on just one partially effective drug, and new drugs are needed. Tubulin and microtubules (MTs) are essential constituents of the cytoskeleton in all eukaryotic cells and considered potential drug targets to treat parasitic infections. The α- and β-tubulin of Schistosoma mansoni have ∼96% and ∼91% sequence identity to their respective human tubulins, suggesting that compounds which bind mammalian tubulin may interfere with MT-mediated functions in the parasite. To explore the potential of different classes of tubulin-binding molecules as antischistosomal leads, we completed a series of in vitro whole-organism screens of a target-based compound library against S. mansoni adults and somules (postinfective larvae), and identified multiple biologically active compounds, among which phenylpyrimidines were the most promising. Further structure-activity relationship studies of these hits identified a series of thiophen-2-yl-pyrimidine congeners, which induce a potent and long-lasting paralysis of the parasite. Moreover, compared to the originating compounds, which showed cytotoxicity values in the low nanomolar range, these new derivatives were 1-4 orders of magnitude less cytotoxic and exhibited weak or undetectable activity against mammalian MTs in a cell-based assay of MT stabilization. Given their selective antischistosomal activity and relatively simple drug-like structures, these molecules hold promise as candidates for the development of new treatments for schistosomiasis.

In-vitro Anti-trypanosomal and Cytotoxicity Evaluation of 3-methyl-3,4-dihydroquinazolin-2(1H)-one Derivatives

Sleeping sickness, caused by trypanosomes, is a debilitating, neglected tropical disease wherein current treatments suffer from several drawbacks such as toxicity, low activity, and poor pharmacokinetic properties, and hence the need for alternative treatment is apparent. To this effect, we screened in vitro a library of 2-quinazolinone derivatives for antitrypanosomal activity against T.b. brucei and cytotoxicity against HeLa cells. Seven compounds having no overt cytotoxicity against HeLa cells exhibited antitrypanosomal activity in the range of 0.093-45 µM were identified. The activity data suggests that the antitrypanosomal activity of this compound class is amenable to substituents at N1 and C6 positions. Compound 14: having a molecular weight of 238Da, ClogP value of 1 and a total polar surface area of 49 was identified as the most active, exhibiting an IC₅₀ value of 0.093 µM Graphical Abstract.

The high-resolution X-ray structure of vinca-domain inhibitors of microtubules provides a rational approach for drug design

Tubulin vinca-domain ligands can inhibit microtubule polymerization, causing cell death in mitosis, and their potential against multiple cancer types has been demonstrated. However, due to drug resistance and toxicities, development of novel vinca-domain ligands is still needed. In this study, we determined the high-resolution crystal structures of vinorelbine, YXD, and Phomopsin A in complex with tubulin at 2.5 Å. Additionally, we recapitulated all previously published high-resolution crystal structures of the vinca binding site to reveal critical residues and the molecular mechanism of vinca-domain ligands interacting with tubulin. Furthermore, we designed putatively novel triazolopyrimidine derivatives by introducing secondary amine groups to establish salt-bridge and H-bond interactions with Asp179^β1 and Asn329^α2 . Our studies provided the structural basis for designing novel tubulin vinca-domain ligands.

The discovery of novel antitrypanosomal 4-phenyl-6-(pyridin-3-yl)pyrimidines

Human African trypanosomiasis, or sleeping sickness, is a neglected tropical disease caused by Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense which seriously affects human health in Africa. Current therapies present limitations in their application, parasite resistance, or require further clinical investigation for wider use. Our work herein describes the design and syntheses of novel antitrypanosomal 4-phenyl-6-(pyridin-3-yl)pyrimidines, with compound 13, the 4-(2-methoxyphenyl)-6-(pyridine-3-yl)pyrimidin-2-amine demonstrating an IC₅₀ value of 0.38 μM and a promising off-target ADME-Tox profile in vitro. In silico molecular target investigations showed rhodesain to be a putative candidate, supported by STD and WaterLOGSY NMR experiments, however, in vitro evaluation of compound 13 against rhodesain exhibited low experimental inhibition. Therefore, our reported library of drug-like pyrimidines present promising scaffolds for further antikinetoplastid drug development for both phenotypic and target-based drug discovery.

Identification of anisomycin, prodigiosin and obatoclax as compounds with broad-spectrum anti-parasitic activity

Parasitic infections are a major source of human suffering, mortality, and economic loss, but drug development for these diseases has been stymied by the significant expense involved in bringing a drug though clinical trials and to market. Identification of single compounds active against multiple parasitic pathogens could improve the economic incentives for drug development as well as simplifying treatment regimens. We recently performed a screen of repurposed compounds against the protozoan parasite Entamoeba histolytica, causative agent of amebic dysentery, and identified four compounds (anisomycin, prodigiosin, obatoclax and nithiamide) with low micromolar potency and drug-like properties. Here, we extend our investigation of these drugs. We assayed the speed of killing of E. histolytica trophozoites and found that all four have more rapid action than the current drug of choice, metronidazole. We further established a multi-institute collaboration to determine whether these compounds may have efficacy against other parasites and opportunistic pathogens. We found that anisomycin, prodigiosin and obatoclax all have broad-spectrum antiparasitic activity in vitro, including activity against schistosomes, T. brucei, and apicomplexan parasites. In several cases, the drugs were found to have significant improvements over existing drugs. For instance, both obatoclax and prodigiosin were more efficacious at inhibiting the juvenile form of Schistosoma than the current standard of care, praziquantel. Additionally, low micromolar potencies were observed against pathogenic free-living amebae (Naegleria fowleri, Balamuthia mandrillaris and Acanthamoeba castellanii), which cause CNS infection and for which there are currently no reliable treatments. These results, combined with the previous human use of three of these drugs (obatoclax, anisomycin and nithiamide), support the idea that these compounds could serve as the basis for the development of broad-spectrum anti-parasitic drugs.

Anti-Trypanosomal and Antimalarial Properties of Tetralone Derivatives and Structurally Related Benzocycloalkanones

Background and objectives: Sleeping sickness and malaria alike are insect-borne protozoan diseases that share overlapping endemic areas in sub-Saharan Africa. The causative agent for malaria has developed resistance against all currently deployed anti-malarial agents. In the case of sleeping sickness, the currently deployed therapeutic options are limited in efficacy and activity spectra, and there are very few drug candidates in the development pipeline. Thus, there is a need to search for new drug molecules with a novel mode of actions. Materials and Methods: In the current study, an in vitro screening of a library of tetralone derivatives and related benzocycloalkanones was effected against T. b. brucei and P. falciparum. Results: Several hits with low micromolar activity (0.4–8 µM) against T. b. brucei were identified. Conclusions: The identified hits have a low molecular weight (<280 Da), a low total polar surface area (<50 Ų), and a defined structure activity relationship, which all make them potential starting points for further hit optimization studies.

1,2,4-Triazolo[1,5-a]pyrimidines in drug design

The 1,2,4-triazolo[1,5-a]pyrimidine (TP) heterocycle, in spite of its relatively simple structure, has proved to be remarkably versatile as evidenced by its use in many different applications reported over the years in different areas of drug design. For example, as the ring system of TPs is isoelectronic with that of purines, this heterocycle has been proposed as a possible surrogate of the purine ring. However, depending on the choice of substituents, the TP ring has also been described as a potentially viable bio-isostere of the carboxylic acid functional group and of the N-acetyl fragment of ε-N-acetylated lysine. In addition, the metal-chelating properties of the TP ring have also been exploited to generate candidate treatments for cancer and parasitic diseases. In the present review article, we discuss recent applications of the TP scaffold in medicinal chemistry, and provide an overview of its properties and methods of synthesis.