Development of Urea-Bond-Containing Michael Acceptors as Antitrypanosomal Agents Targeting Rhodesain

Abstract

Human African Trypanosomiasis (HAT) is a neglected tropical disease widespread in sub-Saharan Africa. Rhodesain, a cysteine protease of Trypanosoma brucei rhodesiense, has been identified as a valid target for the development of anti-HAT agents. Herein, we report a series of urea-bond-containing Michael acceptors, which were demonstrated to be potent rhodesain inhibitors with K _i values ranging from 0.15 to 2.51 nM, and five of them showed comparable k _2nd values to that of K11777, a potent antitrypanosomal agent. Moreover, most of the urea derivatives exhibited single-digit micromolar activity against the protozoa, and the presence of substituents at the P3 position appears to be essential for the antitrypanosomal effect. Replacement of Phe with Leu at the P2 site kept unchanged the inhibitory properties. Compound 7 (SPR7) showed the best compromise in terms of rhodesain inhibition, selectivity, and antiparasitic activity, thus representing a new lead compound for future SAR studies.

Figure 1

Chemical structures of K11777, K11002, RK-52, and PS-1. Acronyms: RD, rhodesain; BSF, bloodstream form.

Figure 2

Chemical structure of designed urea derivatives 1–9.

Scheme 1. Synthesis of Compounds 1–9

Reagents and conditions: (a) TBTU, DIPEA, DCM, 30 min, rt, then 11, rt, on; (b) TFA/DCM 1:1, rt, TLC monitoring; (c) Boc-Phe-OH, TBTU, DIPEA, DCM, 10 min, rt, then 13, rt, on; (d) TEA, appropriate Ar-NCO, rt, on; (e) LiAlH₄, dry THF, 0 °C, TLC monitoring; (f) DCM, Ph₃PCHCOCH₃, rt, 2 h.

Scheme 2. Synthesis of Compounds 25 and 26

Reagents and conditions: (a) Boc-Leu-OH, TBTU, DIPEA, DCM, 30 min, rt, then 13, rt, on; (b) TFA/DCM 1:1, rt, TLC monitoring; (c) TEA, appropriate Ar-NCO, rt, on; (d) LiAlH₄, dry THF, 0 °C, TLC monitoring; (e) DCM, Ph₃PCHCOCH₃, rt, 2 h.

Figure 3

(A) Superimposition of the docked conformations of K11002 (pink) and 7 (orange) in the rhodesain binding site (blue). (B) Predicted theoretical binding pose of 7/rhodesain and their interactions. The enzyme is depicted in blue ribbons and sticks, and the ligand in orange sticks. Important residues are labeled. H-bonds are shown as red dashed lines. The images were rendered using UCSF Chimera.