Benzo-ring modification on Malaria Box hit MMV008138: effects on antimalarial potency and microsomal stability

Abstract

Tetrahydro-β-carboline 1 (MMV008138) controls growth of asexual blood-stage Plasmodium falciparum by inhibiting IspD, an enzyme in the MEP pathway for synthesis of a critical metabolite, isopentenyl pyrophosphate (IPP). We have previously investigated the structure activity relationship (SAR) of three of its four rings (B, C, and D). In this report we investigate the SAR of the benzo- (i.e. A-ring) of 1, with the goal of increasing its in vitro antimalarial potency and metabolic stability. As in our previous studies of the B- and C-ring substitution, extreme sensitivity to substitution was also seen in the benzo-ring. In total, 19 benzo-ring substitution variants of 1 were prepared. When tested against multidrug-resistant (Dd2 strain) P. falciparum, only three derivatives (20a, c, d) possessed asexual blood stage (ABS) activity with EC₅₀ values within 3-fold of the parent. As hoped, one analog (20c) showed a marked improvement in microsomal stability. However, this improvement unfortunately did not improve plasma exposure relative to 1, and did not lead to oral efficacy in a mouse model of malaria.

Fig. 1. Structures of tetrahydro-β-carboline antimalarial MMV008138 (1), D-ring analogs 2–5, and amide derivatives 6–10. Reported growth inhibition values (EC₅₀) are derived from 72 h exposure of multidrug-resistant Dd2 strain Plasmodium falciparum.

Fig. 2. Inactive analogs of 1 (EC₅₀ ≥ 8000 nM) featuring methyl and spiro-substitution at C1 (11, 12), and methyl substitution at N2, C3, and N9 (13–15).

Scheme 1. Synthesis of enantiopure A-ring variants of 1. i) NiCl₂ (10 mol%), BPhen (10 mol%), Mn (3 equiv.), 25 (1.2 equiv.), NMP. ii) Zn (2.9 equiv.), I₂ (0.1 equiv.), 25 (1.3 equiv.), DMF; add iodoindole, Pd₂dba₃ (2 mol%), SPhos (4 mol%). iii) HCl in MeOH. iv) HCl in EtOAc. v) SOCl₂, MeOH. vi) 4 Å MS, CH₂Cl₂; TFA; isolate trans-diastereomer. vii) Ti(Oi-Pr)₄, TFAA, TFA, 70 °C; MeOH, NaOH (aq); isolate trans-diastereomer. viii) Amberlyst hydroxide, THF/MeOH/H₂O, 16 h; AcOH (aq).