Design and Synthesis of Potent Inhibitors of Plasmepsin I and II:  X-ray Crystal Structure of Inhibitor in Complex with Plasmepsin II

Abstract

New and potent inhibitors of the malarial aspartic proteases plasmepsin (Plm) I and II, from the deadliest malaria parasite Plasmodium falciparum, have been synthesized utilizing Suzuki coupling reactions on previously synthesized bromobenzyloxy-substituted statine-like inhibitors. The enzyme inhibition activity has been improved up to eight times by identifying P1 substituents that effectively bind to the continuous S1-S3 crevice of Plasmepsin I and II. By replacement of the bromo atom in the P1 p-bromobenzyloxy-substituted inhibitors with different aryl substituents, several inhibitors exhibiting K(i) values in the low nanomolar range for both Plm I and II have been identified. Some of these inhibitors are also effective in attenuating parasite growth in red blood cells, with the best inhibitors, compounds 2 and 4, displaying 70% and 83% inhibition, respectively, at a concentration of 5 microM. The design was partially guided by the X-ray crystal structure disclosed herein of the previously synthesized inhibitor 1 in complex with plasmepsin II.

Plasmepsins as potential targets for new antimalarial therapy

“…A recently determined crystal structure of inhibitor 11 in complex with Plm II (1W6H) supports the observation that the pbromobenzyloxy substituent efficiently fills the S1 and S3 subsites. 132 Several beneficial hydrophobic interactions, including van der Waals contacts and stacking, were identified between the p-bromobenzyloxy P1 substituent and the S1 pocket in Plm II. 132 A phenethyl group in the P1 position resulted in a significant decrease in activity (inhibitor 10) possibly due to the shorter ethyl linkage forcing the phenyl into a sterically less advantageous position in the S1-S3 groove.…”

The Potential of P1 Site Alterations in Peptidomimetic Protease Inhibitors as Suggested by Virtual Screening and Explored by the Use of CC‐Coupling Reagents

“…The research groups of Hallberg and Samuelsson reported a 50-fold inhibitory enhancement toward plasmepsin I of a statine (hydroxyethyl) transition-state isostere exhibiting a biphenyl moiety in the opened S1 pocket. [45] Furthermore, the research group at Actelion Pharmaceuticals communicated a crystal structure exhibiting a biphenyl moiety in the S1 pocket. [44] Our results could be realized by a novel synthetic strategy using phosphoranes as linker reagents for C-acylation reactions, thereby allowing a direct incorporation of peptide isostere synthesis in peptide chemistry workflow.…”

Development of Plasmodium falciparum Protease Inhibitors in the Past Decade (2002–2012)

“…Like these HIV-1 aspartyl protease inhibitors, most Plm I and II inhibitors mimic the tetrahedral intermediate formed during the aspartyl protease catalysis. There are several transition state analogue cores used for the design of Plm inhibitors [35,[51][52][53][54][55][56][57][58][59][60][61][62][63][64][65], but the most important include the statine core [54,56,57,64], the reversed-statine core [53,54], or a hydroxyethylamine motif (Fig. 4) [56,61,62].…”

“…(4). Transition-state mimicking groups in peptidomimetic plasmepsin inhibitors: reduced amine [52,59], statine [54,56,57,64], hydroxypropylamine [51], reversed-statine [53,54], dihydroxyethylene (C-and N-duplicated) [35,55,63], norstatine [58,60,65], and hydroxyethylamine [56,61,62].…”