Daniel Bur

The Discovery of N-[5-(4-Bromophenyl)-6-[2-[(5-bromo-2-pyrimidinyl)oxy]ethoxy]-4-pyrimidinyl]-N′-propylsulfamide (Macitentan), an Orally Active, Potent Dual Endothelin Receptor Antagonist

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.

2-Imino-thiazolidin-4-one Derivatives as Potent, Orally Active S1P1 Receptor Agonists

Sphingosine-1-phosphate (S1P) is a widespread lysophospholipid which displays a wealth of biological effects. Extracellular S1P conveys its activity through five specific G-protein coupled receptors numbered S1P(1) through S1P(5). Agonists of the S1P(1) receptor block the egress of T-lymphocytes from thymus and lymphoid organs and hold promise for the oral treatment of autoimmune disorders. Here, we report on the discovery and detailed structure-activity relationships of a novel class of S1P(1) receptor agonists based on the 2-imino-thiazolidin-4-one scaffold. Compound 8bo (ACT-128800) emerged from this series and is a potent, selective, and orally active S1P(1) receptor agonist selected for clinical development. In the rat, maximal reduction of circulating lymphocytes was reached at a dose of 3 mg/kg. The duration of lymphocyte sequestration was dose dependent. At a dose of 100 mg/kg, the effect on lymphocyte counts was fully reversible within less than 36 h. Pharmacokinetic investigation of 8bo in beagle dogs suggests that the compound is suitable for once daily dosing in humans.

Substituted piperidines: Highly potent renin inhibitors due to induced fit adaptation of the active site

The identification, synthesis and activity of a novel class of piperidine renin inhibitors is presented. The most active compounds show activities in the picomolar range and are among the most potent renin inhibitors ever identified.

A distinct member of the aspartic proteinase gene family from the human malaria parasite Plasmodium falciparum.

A gene (hap) transcribed during the intra‐erythrocytic life cycle stages of the human malaria parasite Plasmodium falciparum was cloned and sequenced. It was found to encode a protein belonging to the aspartic proteinase family but which carried replacements of catalytically crucial residues in the hallmark sequences contributing to the active site of this type of proteinase. Consideration is given as to whether this protein is the first known parasite equivalent of the pregnancy‐associated glycoproteins that have been documented in ungulate mammals. Alternatively, it may be operative as a new type of proteinase with a distinct catalytic mechanism. In this event, since no counterpart is known to exist in humans, it affords an attractive potential target against which to develop new anti‐malarial drugs.

X-ray Structure of Plasmepsin II Complexed with a Potent Achiral Inhibitor

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 Å) 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.

Piperidine-renin inhibitors compounds with improved physicochemical properties

Piperidine renin inhibitors with heterocyclic core modifications or hydrophilic attachments show improved physical properties (lower lipophilicity, improved solubility). Tetrahydroquinoline derivative rac-30 with a molecular weight of 517 and a log D(pH 7.4) of 1.9 displays potent and long lasting blood pressure lowering effects after oral administration to sodium depleted conscious marmosets.

Design and Preparation of Potent, Nonpeptidic, Bioavailable Renin Inhibitors

Starting from known piperidine renin inhibitors, a new series of 3,9-diazabicyclo[3.3.1]nonene derivatives was rationally designed and prepared. Optimization of the positions 3, 6, and 7 of the diazabicyclonene template led to potent renin inhibitors. The substituents attached at the positions 6 and 7 were essential for the binding affinity of these compounds for renin. The introduction of a substituent attached at the position 3 did not modify the binding affinity but allowed the modulation of the ADME properties. Our efforts led to the discovery of compound (+)-26g that inhibits renin with an IC(50) of 0.20 nM in buffer and 19 nM in plasma. The pharmacokinetics properties of this and other similar compounds are discussed. Compound (+)-26g is well absorbed in rats and efficacious at 10 mg/kg in vivo.

Effects of drug interactions on biotransformation and antiplatelet effect of clopidogrel in vitro

BACKGROUND AND PURPOSE The conversion of clopidogrel to its active metabolite, R‐130964, is a two‐step cytochrome P450 (CYP)‐dependent process. The current investigations were performed to characterize in vitro the effects of different CYP inhibitors on the biotransformation and on the antiplatelet effect of clopidogrel.

Human napsin A: expression, immunochemical detection, and tissue localization

A novel aspartic proteinase, called napsin, has recently been found in human and mouse. Due to high similarity with cathepsin D a structural model of human napsin A could be built. Based on this model a potential epitope SFYLNRDPEEPDGGE has been identified, which was used to immunize rabbits. The resulting antibody was employed in monitoring the expression of recombinant human napsin A in HEK293 cell line. Western blot analysis confirmed the specificity of the antibody and showed that human napsin A is expressed as a single chain protein with the molecular weight of approximately 38 kDa. Immunohistochemical studies revealed high expression levels of napsin A in human kidney and lung but low expression in spleen.

The aspartic proteinase from the rodent parasite Plasmodium berghei as a potential model for plasmepsins from the human malaria parasite, Plasmodium falciparum

The gene encoding an aspartic proteinase precursor (proplasmepsin) from the rodent malaria parasite Plasmodium berghei has been cloned. Recombinant P. berghei plasmepsin hydrolysed a synthetic peptide substrate and this cleavage was prevented by the general aspartic proteinase inhibitor, isovaleryl pepstatin and by Ro40‐4388, a lead compound for the inhibition of plasmepsins from the human malaria parasite Plasmodium falciparum. Southern blotting detected only one proplasmepsin gene in P. berghei. Two plasmepsins have previously been reported in P. falciparum. Here, we describe two further proplasmepsin genes from this species. The suitability of P. berghei as a model for the in vivo evaluation of plasmepsin inhibitors is discussed.