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A novel series of anti-malarials, based on a hydroxy-ethyl-amine scaffold, initially identified as peptidomimetic protease inhibitors is described. Combination of the hydroxy-ethyl-amine anti-malarial phramacophore with the known Mannich base pharmacophore of amodiaquine (57) resulted in promising in vivo active novel derivatives.

The increasing prevalence of multidrug-resistant strains of the malarial parasite Plasmodium falciparum requires the urgent development of new therapeutic agents with novel modes of action. The vacuolar malarial aspartic proteases plasmepsin (PM) I, II, and IV are involved in hemoglobin degradation and play a central role in the growth and maturation of the parasite in the human host. We report the structure-based design, synthesis, and in vitro evaluation of a new generation of PM inhibitors featuring a highly decorated 7-azabicyclo[2.2.1]heptane core. While this protonated central core addresses the catalytic Asp dyad, three substituents bind to the flap, the S1/S3, and the S1' pockets of the enzymes. A hydroformylation reaction is the key synthetic step for the introduction of the new vector reaching into the S1' pocket. The configuration of the racemic ligands was confirmed by extensive NMR and X-ray crystallographic analysis. In vitro biological assays revealed high potency of the new inhibitors against the three plasmepsins (IC(50) values down to 6 nM) and good selectivity towards the closely related human cathepsins D and E. The occupancy of the S1' pocket makes an essential contribution to the gain in binding affinity and selectivity, which is particularly large in the case of the PM IV enzyme. Designing non-peptidic ligands for PM II is a valid route to generate compounds that inhibit the entire family of vacuolar plasmepsins.

More than 40 % of the world's population is at risk of being infected with malaria. Most malaria cases occur in the countries of sub-Saharan Africa, Central and South America, and Asia. Resistance to standard therapy, including artemisinin combinations, is increasing. There is an urgent need for novel antimalarials with new mechanisms of action. In a phenotypic screen, we identified a series of phenylalanine-based compounds that exhibit antimalarial activity via a new and yet unknown mechanism of action. Our optimization efforts culminated in the selection of ACT-451840 [(S,E)-N-(4-(4-acetylpiperazin-1-yl)benzyl)-3-(4-(tert-butyl)phenyl)-N-(1-(4-(4-cyanobenzyl)piperazin-1-yl)-1-oxo-3-phenylpropan-2-yl)acrylamide] for clinical development. Herein we describe our optimization efforts from the screening hit to the potential drug candidate with respect to antiparasitic activity, drug metabolism and pharmacokinetics (DMPK) properties, and in vivo pharmacological efficacy.

Background Artemisinin resistance observed in Southeast Asia threatens the continued use of artemisinin-based combination therapy in endemic countries. Additionally, the diversity of chemical mode of action in the global portfolio of marketed antimalarials is extremely limited. Addressing the urgent need for the development of new antimalarials, a chemical class of potent antimalarial compounds with a novel mode of action was recently identified. Herein, the preclinical characterization of one of these compounds, ACT-451840, conducted in partnership with academic and industrial groups is presented. Method and Findings The properties of ACT-451840 are described, including its spectrum of activities against multiple life cycle stages of the human malaria parasite Plasmodium falciparum (asexual and sexual) and Plasmodium vivax (asexual) as well as oral in vivo efficacies in two murine malaria models that permit infection with the human and the rodent parasites P. falciparum and Plasmodium berghei, respectively. In vitro, ACT-451840 showed a 50% inhibition concentration of 0.4 nM (standard deviation [SD]: ± 0.0 nM) against the drug-sensitive P. falciparum NF54 strain. The 90% effective doses in the in vivo efficacy models were 3.7 mg/kg against P. falciparum (95% confidence interval: 3.3–4.9 mg/kg) and 13 mg/kg against P. berghei (95% confidence interval: 11–16 mg/kg). ACT-451840 potently prevented male gamete formation from the gametocyte stage with a 50% inhibition concentration of 5.89 nM (SD: ± 1.80 nM) and dose-dependently blocked oocyst development in the mosquito with a 50% inhibitory concentration of 30 nM (range: 23–39). The compound’s preclinical safety profile is presented and is in line with the published results of the first-in-man study in healthy male participants, in whom ACT-451840 was well tolerated. Pharmacokinetic/pharmacodynamic (PK/PD) modeling was applied using efficacy in the murine models (defined either as antimalarial activity or as survival) in relation to area under the concentration versus time curve (AUC), maximum observed plasma concentration (Cmax), and time above a threshold concentration. The determination of the dose–efficacy relationship of ACT-451840 under curative conditions in rodent malaria models allowed prediction of the human efficacious exposure. Conclusion The dual activity of ACT-451840 against asexual and sexual stages of P. falciparum and the activity on P. vivax have the potential to meet the specific profile of a target compound that could replace the fast-acting artemisinin component and harbor additional gametocytocidal activity and, thereby, transmission-blocking properties. The fast parasite reduction ratio (PRR) and gametocytocidal effect of ACT-451840 were recently also confirmed in a clinical proof-of-concept (POC) study., Sergio Wittlin and colleagues report in vivo and in vitro modelling of antimalarial activity and dose-efficacy of a new drug candidate., Author Summary Why Was This Study Done? The limited diversity of chemical mode of action in the global portfolio of marketed antimalarials along with recently observed artemisinin resistance that threatens the current first-line treatment highlights the urgent need for development of new antimalarials. In accordance with target product profiles defined by the Medicines for Malaria Venture ([MMV]; www.mmv.org), a new model of not-for-profit public–private partnership providing guidance to the development of new drugs, ACT-451840, a new chemical class of potent compounds with a novel mode of action, was selected for early development. This manuscript integrates a number of studies performed to characterize preclinically ACT-451840 and illustrates the new antimalarial drug development paradigm. What Did the Researchers Do and Find? This study used in vitro models to investigate compound activity on sexual and asexual blood stages and a mouse model to study the human parasite P. falciparum in vivo. While being fully active against artemisinin-resistant strains, ACT-451840 shares many of the favorable properties of artemisinins like its fast onset of action, activity against all asexual blood stage forms, and a PRR of >4 log per parasite cycle. Pharmacodynamic properties of ACT-451840 were interpreted in two mouse models of malaria with respect to the pharmacokinetic properties, with the objective to establish the portable PK/PD parameters to estimate efficacious exposure. What Do These Findings Mean? Modeling the dose–efficacy relationship of ACT-451840 supported prediction of the human efficacious exposure and therefore laid the groundwork for the new clinical development phases. Confirmed in an experimental human malaria infection model, the properties of ACT-451840, including its fast action observed in in vitro and in vivo models and its transmission-blocking activity, suggest this compound may be able to replace the artemisinin component in artemisinin-based combination therapy.

Starting from the structure of bosentan (1), we embarked on a medicinal chemistry program aiming at the identification of novel potent dual endothelin receptor antagonists with high oral efficacy. This led to the discovery of a novel series of alkyl sulfamide substituted pyrimidines. Among these, compound 17 (macitentan, ACT-064992) emerged as particularly interesting as it is a potent inhibitor of ET(A) with significant affinity for the ET(B) receptor and shows excellent pharmacokinetic properties and high in vivo efficacy in hypertensive Dahl salt-sensitive rats. Compound 17 successfully completed a long-term phase III clinical trial for pulmonary arterial hypertension.

The increasing spread of drug-resistant malaria strains underscores the need for new antimalarial agents with novel modes of action (MOAs). Here, we describe a compound representative of a new class of antimalarials. This molecule, ACT-213615, potently inhibits in vitro erythrocytic growth of all tested Plasmodium falciparum strains, irrespective of their drug resistance properties, with half-maximal inhibitory concentration (IC50) values in the low single-digit nanomolar range. Like the clinically used artemisinins, the compound equally and very rapidly affects all 3 asexual erythrocytic parasite stages. In contrast, microarray studies suggest that the MOA of ACT-213615 is different from that of the artemisinins and other known antimalarials.ACT-213615 is orally bioavailable in mice, exhibits activity in the murine Plasmodium berghei model and efficacy comparable to that of the reference drug chloroquine in the recently established P. falciparum SCID mouse model.ACT-213615 represents a new class of potent antimalarials that merits further investigation for its clinical potential. Published Version

The design and synthesis of novel fluorinated building blocks is of major interest in the development of new pharmaceuticals and agrochemicals. A quantitative search in the Protein Data Bank (PDB) manifests the use of di- and trifluoro hydrates for binding to hydrophilic enzyme active sites. Hydrated alicyclic α,α-difluorinated ketones attract attention since they provide suitable functionalities for binding to the pair of catalytically active aspartate (Asp) side chains at the active site of aspartic proteases. This article expands the synthetic availability of this novel class of binding elements. Enantiomerically pure alicyclic α,α-difluoro ketones are efficiently accessed by a straightforward route involving the separation of diastereoisomeric Mosher esters. The transformation into Mosher esters also enables the determination of the absolute configuration of the enantiomeric α,α-difluoro ketones. The synthetic protocol was further expanded to the preparation of highly substituted cyclohexyl-based α,α-difluoro ketones bearing two exit vectors to fill the corresponding side pockets in the malarial aspartic proteases, the plasmepsins. Moderate biological activities toward these enzymes were determined, with IC 50 (median inhibitory concentration) values in the lower micromolar range.

The development of new therapeutic agents against malaria has become urgent during the past few decades, due to an increased prevalence of drug-resistant strains of malaria-causing Plasmodium parasites. Possible targets are the hemoglobin-degrading aspartic proteases, the plasmepsins. While acyclic alpha,alpha-difluoroketone hydrates have been introduced into peptidomimetics to bind to the catalytic Asp dyad of aspartic proteases, alicyclic derivatives were unknown. This paper describes a versatile synthesis of hydrated alicyclic alpha,alpha-difluoro-cyclopentanones and -cyclohexanones, decorated with appropriate substituents to fill the S1/S3 and the "flap-open" pocket at the enzyme active sites. Their biological activity was tested against plasmepsin II and IV, revealing an IC(50) value (concentration of an inhibitor at which 50% maximum initial velocity is observed) of 7 microM for the best ligand. Reference inhibitors with a protonated secondary ammonium centre to address the catalytic dyad showed similar binding affinities. The X-ray crystal structure of a cyclic alpha,alpha-difluoroketone hydrate revealed the ability of these novel building blocks to participate in H-bonding networks. The hydration of difluoroketones was also investigated in solution. An exemplary study showed that the equilibrium constants for the hydration of alpha,alpha-difluorinated cyclohexanones are much higher than those for the corresponding cyclopentanones.

The increasing prevalence of drug-resistant strains of malaria-causing Plasmodium parasites necessitates the development of therapeutic agents that inhibit new biochemical targets. We herein describe the design, synthesis, and in vitro evaluation of a class of inhibitors that target the malarial aspartic proteases known as the plasmepsins. The title compounds feature a 7-azanorbornane skeleton that bears an exo-amino function, which was designed to interact with the catalytic dyad of aspartic proteases while providing vectors for the attachment of binding elements that target the flap and S1/S3 binding pockets at the enzyme active site. Their synthesis takes advantage of a solvent-free and highly diastereoselective conjugate addition of amines to bicyclic vinyl sulfones. Structural optimization based on a little-known conformational preference of aryl sulfones produced the most potent inhibitors of this new class. In vitro assays demonstrate that the title compounds are capable of potent (IC50 ≥ 10 nM) inhibition of plasmepsins, while remaining relatively weak inhibitors of the closely related human enzymes cathepsins D and E. The ideal occupation of the flap pocket is crucial for both potency and selectivity over the human proteases. Differently functionalized compounds were synthesized to gain new insights into the molecular recognition properties of this cavity. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Two bis-trifluoromethyl pepstatin A analogues, carboxylic acid 1 and its methyl ester 2, have been synthesised in order to probe the properties and size of the trifluoromethyl (Tfm) group and compare it to the "bigger" isobutyl that is present in pepstatin A. The results demonstrate that Tfm can effectively replace the isobutyl chain as far as inhibitory activity against plasmepsin II (PM II), an aspartic proteinase from Plasmodium falciparum, is concerned. On the other hand, replacement of isobutyl by Tfm selectively affected activity against other aspartic proteinases tested. Two lines of evidence led to these conclusions. Firstly, compounds 1 and 2 retained single-digit nanomolar inhibitory activity against PM II, but were markedly less active against PM IV, cathepsin D and cathepsin E. Secondly, the X-ray crystal structures of the three complexes of PM II with 1, 2 and pepstatin A were obtained at 2.8, 2.4 and 1.7 Å resolution, respectively. High overall similarity among the three complexes indicated that the central Tfm was well accommodated in the lipophilic S1 pocket of PM II, where it was involved in tight hydrophobic contacts. The interaction of PM II with Phe111 appeared to be crucial. Comparison of the crystal structures presented here, with X-ray structures or structural models of PM IV and cathepsin D, allowed an interpretation of the inhibition profiles of pepstatin A and its Tfm variants against these three enzymes. Interactions of the P1 side chain with amino acids that point into the S1 pocket appear to be critical for inhibitory activity. In summary, Tfm can be used to replace an isobutyl group and can affect the selectivity profile of a compound. These findings have implications for the design of novel bioactive molecules and synthetic mimics of natural compounds.

The malaria parasite Plasmodium falciparum degrades host cell hemoglobin inside an acidic food vacuole during the blood stage of the infectious cycle. A number of aspartic proteinases called plasmepsins (PMs) have been identified to play important roles in this degradation process and therefore generated significant interest as new antimalarial targets. Several x-ray structures of PMII have been described previously, but thus far, structure-guided drug design has been hampered by the fact that only inhibitors comprising a statine moiety or derivatives thereof have been published. Our drug discovery efforts to find innovative, cheap, and easily synthesized inhibitors against aspartic proteinases yielded some highly potent non-peptidic achiral inhibitors. A highly resolved (1.6 A) x-ray structure of PMII is presented, featuring a potent achiral inhibitor in an unprecedented orientation, contacting the catalytic aspartates indirectly via the "catalytic" water. Major side chain rearrangements in the active site occur, which open up a new pocket and allow a new binding mode of the inhibitor. Moreover, a second inhibitor molecule could be located unambiguously in the active site of PMII. These newly obtained structural insights will further guide our attempts to improve compound properties eventually leading to the identification of molecules suitable as antimalarial drugs.

Previously, we reported on the discovery of a novel series of bicyclo[3.1.0]hexane fused thiophene derivatives that serve as potent and selective S1P1 receptor agonists. Here, we discuss our efforts to simplify the bicyclohexane fused thiophene head. In a first step the bicyclohexane moiety could be replaced by a simpler, less rigid cyclohexane ring without compromising the S1P receptor affinity profile of these novel compounds. In a second step, the thiophene head was simplified even further by replacing the cyclohexane ring with an isobutyl group attached either to position 4 or position 5 of the thiophene. These structurally much simpler headgroups again furnished potent and selective S1P1 agonists (e.g., 87), which efficiently and dose dependently reduced the number of circulating lymphocytes upon oral administration to male Wistar rats. For several compounds discussed in this report lymphatic transport is an important route of absorption that may offer opportunities for a tissue targeted approach with minimal plasma exposure.

In order to overcome the problem of drug resistance in malaria, it appears wise to concentrate drug discovery efforts toward new structural classes and new mechanisms of action. We report our results, targeting Plasmepsin II, a Plasmodium falciparum aspartic protease active in hemoglobin degradation, a parasite specific catabolic pathway. The results show that the new structural class is not only inhibiting PMII in vitro but is also active in a P. falciparum infected human red blood cell assay.