Antimalarial activity enhancement in hydroxymethylcarbonyl (HMC) isostere-based dipeptidomimetics targeting malarial aspartic protease plasmepsin

Abstract

Plasmepsin (Plm) is a potential target for new antimalarial drugs, but most reported Plm inhibitors have relatively low antimalarial activities. We synthesized a series of dipeptide-type HIV protease inhibitors, which contain an allophenylnorstatine-dimethylthioproline scaffold to exhibit potent inhibitory activities against Plm II. Their activities against Plasmodium falciparum in the infected erythrocyte assay were largely different from those against the target enzyme. To improve the antimalarial activity of peptidomimetic Plm inhibitors, we attached substituents on a structure of the highly potent Plm inhibitor KNI-10006. Among the derivatives, we identified alkylamino compounds such as 44 (KNI-10283) and 47 (KNI-10538) with more than 15-fold enhanced antimalarial activity, to the sub-micromolar level, maintaining their potent Plm II inhibitory activity and low cytotoxicity. These results suggest that auxiliary substituents on a specific basic group contribute to deliver the inhibitors to the target Plm.

Figure 1

Binding directions of 1 in HIV protease and Plm IV.

Figure 2

Lead compound 2 and its phenoxyacetyl derivatives.

Figure 3

MD simulated pose of compound 49 bound to Plm II. Asp214 was protonated. (A) Superimposition of pepstatin A (cyan), 1 (yellow) and 49 (red). (B) Pyridinylmethyl group of 49 (green stick) was surrounded by water molecules (lines, shown only 5 Å from the ligand), not in the Plm II binding sites (red surface). (C) Schematic representation of 49. Figures A and B were generated using MacPyMOL (DeLano Scientific LLC, CA, USA).

Scheme 1

Reagents: (a) BrCH₂COOEt, K₂CO₃, DMF; (b) 1M-NaOH, MeOH; (c) 10% Pd–C, H₂, MeOH; (d) (Boc)₂O, Et₃N, THF–H₂O.

Scheme 2

Reagents and condition: (a) BrCH₂COOEt, K₂CO₃, DMF; (b) 10% Pd–C, H₂, MeOH; (c) (Boc)₂O, Et₃N, THF–H₂O; (d) 1 MNaOH, MeOH; (e) 4 MHCl/dioxane; (f) NaNO₂, H₂SO₄, then H₂O, reflux 3 h; (g) HCHO aq, 10% Pd–C, H₂, MeOH; (h) NaH, CH₃I or R–Br, THF.

Scheme 3

Reagents: (a) (1S,2R)-1-amino-2-indanol, BOP, Et₃N, DMF; (b) 4 MHCl/dioxane, anisole; (c) Boc-Apns-OH, EDC·HCl, HOBt·H₂O, Et₃N, DMF; (d) 4a–4w or 8–15, BOP, Et₃N, DMF.

Scheme 4

Reagents: (a) Et₃N, MeOH, butanal or pentanal, NaBH₃CN.