Iboga Inspired N-Indolylethyl-Substituted Isoquinuclidines as a Bioactive Scaffold: Chemoenzymatic Synthesis and Characterization as GDNF Releasers and Antitrypanosoma Agents

Abstract

The first stage of the drug discovery process involves the identification of small compounds with biological activity. Iboga alkaloids are monoterpene indole alkaloids (MIAs) containing a fused isoquinuclidine-tetrahydroazepine ring. Both the natural products and the iboga-inspired synthetic analogs have shown a wide variety of biological activities. Herein, we describe the chemoenzymatic preparation of a small library of novel N-indolylethyl-substituted isoquinuclidines as iboga-inspired compounds, using toluene as a starting material and an imine Diels-Alder reaction as the key step in the synthesis. The new iboga series was investigated for its potential to promote the release of glial cell line-derived neurotrophic factor (GDNF) by C6 glioma cells, and to inhibit the growth of infective trypanosomes. GDNF is a neurotrophic factor widely recognized by its crucial role in development, survival, maintenance, and protection of dopaminergic neuronal circuitries affected in several neurological and psychiatric pathologies. Four compounds of the series showed promising activity as GDNF releasers, and a leading structure (compound 11) was identified for further studies. The same four compounds impaired the growth of bloodstream Trypanosoma brucei brucei (EC₅₀ 1-8 μM) and two of them (compounds 6 and 14) showed a good selectivity index.

Keywords: GDNF; anti-trypanosoma; iboga alkaloid; imino Diels–Alder; isoquinuclidine; toluenedioxigenase.

Figure 1

Iboga alkaloids have been investigated for their variety of pharmacological effects, such as induction of GDNF expression in the central nervous system (ibogaine) and anti-protozoal activity (18-MC). In this work we study these activities in inspired simplified analogs such as enantiomerically pure N-indolylethyl-substituted isoquinuclidines.

Figure 2

(A) Chemoenzymatic synthesis of the isoquinuclidine core 4 using toluene as starting material. (B) Preparation of N-indolylethyl isoquinuclidines 6 and 7. ^a Yield estimated by ¹H-NMR of the crude reaction using trichloroethylene as internal standard.

Figure 3

(A) Preparation of free amines 10 exo and 10 endo from esters mixture 4. (B) Preparation of derivative 11. (C) Preparation of derivative 12.

Figure 4

Synthesis of compounds 14 and 15 using analog 11 as starting material.

Figure 5

(A) Chemical structure of iboga-like compounds tested on their ability to promote GDNF release. (B) GDNF released by C6 cells after 48 h treatment with compounds shown in Figure 5A at 10 µM concentration. (C) 11 dose-response curve after 48 h treatment. (D) 11′s effect on cell lysis determined by LDH assay. (E) 11′s effect on cell viability determined by WST-1 assay. (F) 15′s dose-response curve after 48 h treatment. (G) 15′s effect on cell lysis determined by LDH assay. (H) 15′s effect on cell viability determined by WST-1 assay. Statistics: data represent mean ± SEM of 3 biological replicates. One-way ANOVA followed by Dunnett’s multiple comparisons test is shown (* p < 0.05, *** p < 0.001, **** p < 0.0001).