Novel hydrazone compounds with broad-spectrum antiplasmodial activity and synergistic interactions with antimalarial drugs.

The development of novel antiplasmodial compounds with broad-spectrum activity against different stages of Plasmodium parasites is crucial to prevent malaria disease and parasite transmission. This study evaluated the antiplasmodial activity of seven novel hydrazone compounds (referred to as CB compounds: CB-27, CB-41, CB-50, CB-53, CB-58, CB-59, and CB-61) against multiple stages of Plasmodium parasites. All CB compounds inhibited blood stage proliferation of drug-resistant or sensitive strains of Plasmodium falciparum in the low micromolar to nanomolar range. Interestingly, CB-41 exhibited prophylactic activity against hypnozoites and liver schizonts in Plasmodium cynomolgi, a primate model for Plasmodium vivax. Four CB compounds (CB-27, CB-41, CB-53, and CB-61) inhibited P. falciparum oocyst formation in mosquitoes, and five CB compounds (CB-27, CB-41, CB-53, CB-58, and CB-61) hindered the in vitro development of Plasmodium berghei ookinetes. The CB compounds did not inhibit the activation of P. berghei female and male gametocytes in vitro. Isobologram assays demonstrated synergistic interactions between CB-61 and the FDA-approved antimalarial drugs, clindamycin and halofantrine. Testing of six CB compounds showed no inhibition of Plasmodium glutathione S-transferase as a putative target and no cytotoxicity in HepG2 liver cells. CB compounds are promising candidates for further development as antimalarial drugs against multidrug-resistant parasites, which could also prevent malaria transmission.

Lead Optimization of Second-Generation Acridones as Broad-Spectrum Antimalarials.

The global impact of malaria remains staggering despite extensive efforts to eradicate the disease. With increasing drug resistance and the absence of a clinically available vaccine, there is an urgent need for novel, affordable, and safe drugs for prevention and treatment of malaria. Previously, we described a novel antimalarial acridone chemotype that is potent against both blood-stage and liver-stage malaria parasites. Here, we describe an optimization process that has produced a second-generation acridone series with significant improvements in efficacy, metabolic stability, pharmacokinetics, and safety profiles. These findings highlight the therapeutic potential of dual-stage targeting acridones as novel drug candidates for further preclinical development.

Scaffold and Parasite Hopping: Discovery of New Protozoal Proliferation Inhibitors.

Utilizing a target repurposing and parasite-hopping approach, we tested a previously reported library of compounds that were active against Trypanosoma brucei, plus 31 new compounds, against a variety of protozoan parasites including Trypanosoma cruzi, Leishmania major, Leishmania donovani, and Plasmodium falciparum. This led to the discovery of several compounds with submicromolar activities and improved physicochemical properties that are early leads toward the development of chemotherapeutic agents against kinetoplastid diseases and malaria.

Discovery and Structural Optimization of Acridones as Broad-Spectrum Antimalarials.

Malaria remains one of the deadliest diseases in the world today. Novel chemoprophylactic and chemotherapeutic antimalarials are needed to support the renewed eradication agenda. We have discovered a novel antimalarial acridone chemotype with dual-stage activity against both liver-stage and blood-stage malaria. Several lead compounds generated from structural optimization of a large library of novel acridones exhibit efficacy in the following systems: (1) picomolar inhibition of in vitro Plasmodium falciparum blood-stage growth against multidrug-resistant parasites; (2) curative efficacy after oral administration in an erythrocytic Plasmodium yoelii murine malaria model; (3) prevention of in vitro Plasmodium berghei sporozoite-induced development in human hepatocytes; and (4) protection of in vivo P. berghei sporozoite-induced infection in mice. This study offers the first account of liver-stage antimalarial activity in an acridone chemotype. Details of the design, chemistry, structure-activity relationships, safety, metabolic/pharmacokinetic studies, and mechanistic investigation are presented herein.

Anilinoquinoline based inhibitors of trypanosomatid proliferation.

We recently reported the medicinal chemistry re-optimization of a series of compounds derived from the human tyrosine kinase inhibitor, lapatinib, for activity against Plasmodium falciparum. From this same library of compounds, we now report potent compounds against Trypanosoma brucei brucei (which causes human African trypanosomiasis), T. cruzi (the pathogen that causes Chagas disease), and Leishmania spp. (which cause leishmaniasis). In addition, sub-micromolar compounds were identified that inhibit proliferation of the parasites that cause African animal trypanosomiasis, T. congolense and T. vivax. We have found that this set of compounds display acceptable physicochemical properties and represent progress towards identification of lead compounds to combat several neglected tropical diseases.

Series of Alkynyl-Substituted Thienopyrimidines as Inhibitors of Protozoan Parasite Proliferation.

Discovery of new chemotherapeutic lead agents can be accelerated by optimizing chemotypes proven to be effective in other diseases to act against parasites. One such medicinal chemistry campaign has focused on optimizing the anilinoquinazoline drug lapatinib (1) and the alkynyl thieno[3,2-d]pyrimidine hit GW837016X (NEU-391, 3) into leads for antitrypanosome drugs. We now report the structure-activity relationship studies of 3 and its analogs against Trypanosoma brucei, which causes human African trypanosomiasis (HAT). The series was also tested against Trypanosoma cruzi, Leishmania major, and Plasmodium falciparum. In each case, potent antiparasitic hits with acceptable toxicity margins over mammalian HepG2 and NIH3T3 cell lines were identified. In a mouse model of HAT, 3 extended life of treated mice by 50%, compared to untreated controls. At the cellular level, 3 inhibited mitosis and cytokinesis in T. brucei. Thus, the alkynylthieno[3,2-d]pyrimidine chemotype is an advanced hit worthy of further optimization as a potential chemotherapeutic agent for HAT.

Optimization of Physicochemical Properties for 4-Anilinoquinoline Inhibitors of Plasmodium falciparum Proliferation.

We recently reported the medicinal chemistry reoptimization of a known human tyrosine kinase inhibitor, lapatinib, against a variety of parasites responsible for numerous tropical diseases, including human African trypanosomiasis ( Trypanosoma brucei), Chagas disease ( T. cruzi), Leishmaniasis ( Leishmania spp.), and malaria ( Plasmodium falciparum). Herein, we report our continuing efforts to optimize this series against P. falciparum. Through the design of a library of compounds focused on reducing the lipophilicity and molecular weight, followed by an SAR exploration, we have identified NEU-1953 (40). This compound is a potent inhibitor of P. falciparum with an improved ADME profile over the previously reported compound, NEU-961 (3).

Optimization of physicochemical properties for 4-anilinoquinazoline inhibitors of trypanosome proliferation.

Human African trypanosomiasis (HAT) is a deadly disease in need of new chemotherapeutics that can cross into the central nervous system. We previously reported the discovery of 2 (NEU-617), a small molecule with activity against T. brucei bloodstream proliferation. Further optimization of 2 to improve the physicochemical properties (LogP, LLE, [1], and MPO score) [2] have led us to twelve sub-micromolar compounds, most importantly the headgroup variants 9i and 9j, and the linker variant 18. Although these 3 compounds had reduced potency compared to 2, they all had improved LogP, LLE and MPO scores. Cross-screening these analogs against other protozoan parasites uncovered 9o with potent activity towards T. brucei, T. cruzi and L. major, while four others compounds (17, 18, 21, 26) showed activity towards P. falciparum D6. This reinforces the effectiveness of lead repurposing for the discovery of new protozoan disease therapeutics.

Antiparasitic Lead Discovery: Toward Optimization of a Chemotype with Activity Against Multiple Protozoan Parasites.

Human African trypanosomiasis (HAT), Chagas disease, and leishmaniasis present a significant burden across the developing world. Existing therapeutics for these protozoal neglected tropical diseases suffer from severe side effects and toxicity. Previously, NEU-1045 (3) was identified as a promising lead with cross-pathogen activity, though it possessed poor physicochemical properties. We have designed a library of analogues with improved calculated physicochemical properties built on the quinoline scaffold of 3 incorporating small, polar aminoheterocycles in place of the 4-(3-fluorobenzyloxy)aniline substituent. We report the biological activity of these inhibitors against Trypanosoma brucei (HAT), T. cruzi (Chagas disease), and Leishmania major (cutaneous leishmaniasis) and describe the identification of N-(5-chloropyrimidin-2-yl)-6-(4-(morpholinosulfonyl)phenyl)quinolin-4-amine (13t) as a promising inhibitor of L. major proliferation and 6-(4-(morpholinosulfonyl)phenyl)-N-(pyrimidin-4-yl)quinolin-4-amine (13j), a potent inhibitor of T. brucei proliferation with improved drug-like properties.

Identification of "Preferred" Human Kinase Inhibitors for Sleeping Sickness Lead Discovery. Are Some Kinases Better than Others for Inhibitor Repurposing?

A kinase-targeting cell-based high-throughput screen (HTS) against Trypanosoma brucei was recently reported, and this screening set included the Published Kinase Inhibitor Set (PKIS). From the PKIS was identified 53 compounds with pEC50 ≥ 6. Utilizing the published data available for the PKIS, a statistical analysis of these active antiparasitic compounds was performed, allowing identification of a set of human kinases having inhibitors that show a high likelihood for blocking T. brucei cellular proliferation in vitro. This observation was confirmed by testing other established inhibitors of these human kinases and by mining past screening campaigns at GlaxoSmithKline. Overall, although the parasite targets of action are not known, inhibitors of this set of human kinases displayed an enhanced hit rate relative to a random kinase-targeting HTS campaign, suggesting that repurposing efforts should focus primarily on inhibitors of these specific human kinases. We therefore term this statistical analysis-driven approach "preferred lead repurposing".

Assessment of the Worldwide Antimalarial Resistance Network Standardized Procedure for In Vitro Malaria Drug Sensitivity Testing Using SYBR Green Assay for Field Samples with Various Initial Parasitemia Levels.

The malaria SYBR green assay, which is used to profilein vitrodrug susceptibility ofPlasmodium falciparum, is a reliable drug screening and surveillance tool. Malaria field surveillance efforts provide isolates with various low levels of parasitemia. To be advantageous, malaria drug sensitivity assays should perform reproducibly among various starting parasitemia levels rather than at one fixed initial value. We examined the SYBR green assay standardized procedure developed by the Worldwide Antimalarial Resistance Network (WWARN) for its sensitivity and ability to accurately determine the drug concentration that inhibits parasite growth by 50% (IC50) in samples with a range of initial parasitemia levels. The initial sensitivity determination of the WWARN procedure yielded a detection limit of 0.019% parasitemia.P. falciparumlaboratory strains and field isolates with various levels of initial parasitemia were then subjected to a range of doses of common antimalarials. The IC50s were comparable for laboratory strains with between 0.0375% and 0.6% parasitemia and for field isolates with between 0.075% and 0.6% parasitemia for all drugs tested. Furthermore, assay quality (Z') analysis indicated that the WWARN procedure displays high robustness, allowing for drug testing of malaria field samples within the derived range of initial parasitemia. The use of the WWARN procedure should allow for the inclusion of more malaria field samples in malaria drug sensitivity screens that would have otherwise been excluded due to low initial parasitemia levels.

Protozoan Parasite Growth Inhibitors Discovered by Cross-Screening Yield Potent Scaffolds for Lead Discovery.

Tropical protozoal infections are a significant cause of morbidity and mortality worldwide; four in particular (human African trypanosomiasis (HAT), Chagas disease, cutaneous leishmaniasis, and malaria) have an estimated combined burden of over 87 million disability-adjusted life years. New drugs are needed for each of these diseases. Building on the previous identification of NEU-617 (1) as a potent and nontoxic inhibitor of proliferation for the HAT pathogen (Trypanosoma brucei), we have now tested this class of analogs against other protozoal species: T. cruzi (Chagas disease), Leishmania major (cutaneous leishmaniasis), and Plasmodium falciparum (malaria). Based on hits identified in this screening campaign, we describe the preparation of several replacements for the quinazoline scaffold and report these inhibitors' biological activities against these parasites. In doing this, we have identified several potent proliferation inhibitors for each pathogen, such as 4-((3-chloro-4-((3-fluorobenzyl)oxy)phenyl)amino)-6-(4-((4-methyl-1,4-diazepan-1-yl)sulfonyl)phenyl)quinoline-3-carbonitrile (NEU-924, 83) for T. cruzi and N-(3-chloro-4-((3-fluorobenzyl)oxy)phenyl)-7-(4-((4-methyl-1,4-diazepan-1-yl)sulfonyl)phenyl)cinnolin-4-amine (NEU-1017, 68) for L. major and P. falciparum.

EVALUATION OF AROMATIC 6-SUBSTITUTED THIENOPYRIMIDINES AS SCAFFOLDS AGAINST PARASITES THAT CAUSE TRYPANOSOMIASIS, LEISHMANIASIS, AND MALARIA.

Target repurposing is a proven method for finding new lead compounds that target Trypanosoma brucei, the causative agent of human African trypanosomiasis. Due to the recent discovery of a lapatinib-derived analog 2 with excellent potency against T. brucei (EC50 = 42 nM) and selectivity over human host cells, we have explored other classes of human tyrosine kinase inhibitor scaffolds in order to expand the range of chemotypes for pursuit. Following library expansion, we found compound 11e to have an EC50 of 84 nM against T. brucei cells while maintaining selectivity over human hepatocytes. In addition, the library was tested against causative agents of Chagas' disease, leishmaniasis, and malaria. Two analogs with sub-micromolar potencies for T. cruzi (4j) and Plasmodium falciparum (11j) were discovered, along with an analog with considerable potency against Leishmania major amastigotes (4e). Besides identifying new and potent protozoan growth inhibitors, these data highlight the value of concurrent screening of a chemical library against different protozoan parasites.

Repurposing human Aurora kinase inhibitors as leads for anti-protozoan drug discovery.

Hesperadin, an established human Aurora B inhibitor, was tested against cultures of Trypanosoma brucei, Leishmania major, and Plasmodium falciparum, and was identified to be a potent proliferation inhibitor. A series of analogs was designed and tested to establish the initial structure-activity relationships for each parasite. In this study, we identified multiple non-toxic compounds with high potency against T. brucei and P. falciparum with good selectivity. These compounds may represent an opportunity for continued optimization.

3,5-bis(benzylidene)-4-piperidones and related N-acyl analogs: a novel cluster of antimalarials targeting the liver stage of Plasmodium falciparum.

Drug resistance is a major challenge in antimalarial chemotherapy. In addition, a complete cure of malaria requires intervention at various stages in the development of the parasite within the host. There are only a few antimalarials that target the liver stage of the Plasmodium species which is an essential part of the life cycle of the malarial parasite. We report a series of antimalarial 3,5-bis(benzylidene)-4-piperidones and related N-acyl analogs 1-5, a number of which exhibit potent in vitro growth-inhibiting properties towards drug-sensitive D6 and drug-resistant C235 strains of Plasmodium falciparum as well as inhibiting the liver stage development of the malarial life cycle. The compounds 2b (IC50: 165 ng/mL), 3b (IC50: 186 ng/mL), 5c (IC50: 159 ng/mL) and 5d (IC50: 93.5 ng/mL) emerged as lead molecules that inhibit liver stage Plasmodium berghei and are significantly more potent than chloroquine (IC50: >2000 ng/mL) and mefloquine (IC50: >2000 ng/mL) in this screen. All the compounds that showed potent inhibitory activity against the P. berghei liver stage were nontoxic to human HepG2 liver cells (IC50: >2000 ng/mL). The compounds 5a and 5b exhibit comparable metabolic stability as chloroquine and mefloquine in human plasma and the most potent compound 5d demonstrated suitable permeability characteristics using the MDCK monolayer. These results emphasize the value of 3,5-bis(benzylidene)-4-piperidones as novel antimalarials for further drug development.

Modification of triclosan scaffold in search of improved inhibitors for enoyl-acyl carrier protein (ACP) reductase in Toxoplasma gondii.

Through our focused effort to discover new and effective agents against toxoplasmosis, a structure-based drug design approach was used to develop a series of potent inhibitors of the enoyl-acyl carrier protein (ACP) reductase (ENR) enzyme in Toxoplasma gondii (TgENR). Modifications to positions 5 and 4' of the well-known ENR inhibitor triclosan afforded a series of 29 new analogues. Among the resulting compounds, many showed high potency and improved physicochemical properties in comparison with the lead. The most potent compounds 16 a and 16 c have IC50 values of 250 nM against Toxoplasma gondii tachyzoites without apparent toxicity to the host cells. Their IC50 values against recombinant TgENR were found to be 43 and 26 nM, respectively. Additionally, 11 other analogues in this series had IC50 values ranging from 17 to 130 nM in the enzyme-based assay. With respect to their excellent in vitro activity as well as improved drug-like properties, the lead compounds 16 a and 16 c are deemed to be excellent starting points for the development of new medicines to effectively treat Toxoplasma gondii infections.

Development of a triclosan scaffold which allows for adaptations on both the A- and B-ring for transport peptides.

The enoyl acyl-carrier protein reductase (ENR) enzyme is harbored within the apicoplast of apicomplexan parasites providing a significant challenge for drug delivery, which may be overcome through the addition of transductive peptides, which facilitates crossing the apicoplast membranes. The binding site of triclosan, a potent ENR inhibitor, is occluded from the solvent making the attachment of these linkers challenging. Herein, we have produced 3 new triclosan analogs with bulky A- and B-ring motifs, which protrude into the solvent allowing for the future attachment of molecular transporters for delivery.

Design, synthesis, and biological activity of diaryl ether inhibitors of Toxoplasma gondii enoyl reductase.

Triclosan is a potent inhibitor of Toxoplasma gondii enoyl reductase (TgENR), which is an essential enzyme for parasite survival. In view of triclosan's poor druggability, which limits its therapeutic use, a new set of B-ring modified analogs were designed to optimize its physico-chemical properties. These derivatives were synthesized and evaluated by in vitro assay and TgENR enzyme assay. Some analogs display improved solubility, permeability and a comparable MIC50 value to that of triclosan. Modeling of these inhibitors revealed the same overall binding mode with the enzyme as triclosan, but the B-ring modifications have additional interactions with the strongly conserved Asn130.

From a cytotoxic agent to the discovery of a novel antimalarial agent.

A novel cytotoxin 3,5-bis(4-chlorobenzylidene)-1-[4-{2-(4-morpholinyl)ethoxy}phenyl-carbonyl]-4-piperidone hydrochloride 2 demonstrated potent antimalarial properties with IC(50) values of 0.60 and 1.97 µM against the drug sensitive D6 strain and the C235 drug-resistant strain of Plasmodium falciparum. This compound concentrates in red blood cells, lowers glutathione concentrations in erythrocytes and permeates across CACO-2 cells. These data reveal 2 to be a promising lead compound in the quest for novel antimalarial agents.