Potent inhibitors of the Plasmodium falciparum enzymes plasmepsin I and II devoid of cathepsin D inhibitory activity.

The hemoglobin-degrading aspartic proteases plasmepsin I (Plm I) and plasmepsin II (Plm II) of the malaria parasite Plasmodium falciparum have lately emerged as putative drug targets. A series of C(2)-symmetric compounds encompassing the 1,2-dihydroxyethylene scaffold and a variety of elongated P1/P1' side chains were synthesized via microwave-assisted palladium-catalyzed coupling reactions. Binding affinity calculations with the linear interaction energy method and molecular dynamics simulations reproduced the experimental binding data obtained in a Plm II assay with very good accuracy. Bioactive conformations of the elongated P1/P1' chains were predicted and agreed essentially with a recent X-ray structure. The compounds exhibited picomolar to nanomolar inhibition constants for the plasmepsins and no measurable affinity to the human enzyme cathepsin D. Some of the compounds also demonstrated significant inhibition of parasite growth in cell culture. To the best of our knowledge, these plasmepsin inhibitors represent the most selective reported to date and constitute promising lead compounds for further optimization.

C2-symmetric inhibitors of Plasmodium falciparum plasmepsin II: synthesis and theoretical predictions.

A series of C(2)-symmetric compounds with a mannitol-based scaffold has been investigated, both theoretically and experimentally, as Plm II inhibitors. Four different stereoisomers with either benzyloxy or allyloxy P1/P1' side chains were studied. Computational ranking of the binding affinities of the eight compounds was carried out using the linear interaction energy (LIE) method relying on a complex previously determined by crystallography. Within both series of isomers the theoretical binding energies were in agreement with the enzymatic measurements, illustrating the power of the LIE method for the prediction of ligand affinities prior to synthesis. The structural models of the enzyme-inhibitor complexes obtained from the MD simulations provided a basis for interpretation of further structure-activity relationships. Hence, the affinity of a structurally similar ligand, but with a different P2/P2' substituent was examined using the same procedure. The predicted improvement in binding constant agreed well with experimental results.

Design and synthesis of plasmepsin I and plasmepsin II inhibitors with activity in Plasmodium falciparum-infected cultured human erythrocytes.

A series of protease inhibitors targeted at the malarial enzymes plasmepsin I and II, and encompassing a basic hydroxyethylamine transition state isostere scaffold, was prepared. The substituents in the P1' position were varied and the biological activities expressed in K(i)-values ranged from 60 to >2000 nM. A more than 4-fold selectivity for either of the plasmepsins could be achieved. All of the active compounds exhibited high preference for the plasmepsins over cathepsin D, the most closely related human protease. A few active compounds were shown to inhibit parasite growth in cultured infected human erythrocytes. An ED(50) value as low as 1.6 microM was observed for one of the inhibitors despite K(i) values of 115 nM (Plm I) and 121 nM (Plm II).