Structural Rigidification of N-Aryl-pyrroles into Indoles Active against Intracellular and Drug-Resistant Mycobacteria

Abstract

A series of indolyl-3-methyleneamines incorporating lipophilic side chains were designed through a structural rigidification approach and synthesized for investigation as new chemical entities against Mycobacterium tuberculosis (Mtb). The screening led to the identification of a 6-chloroindole analogue 7j bearing an N-octyl chain and a cycloheptyl moiety, which displayed potent in vitro activity against laboratory and clinical Mtb strains, including a pre-extensively drug-resistant (pre-XDR) isolate. 7j also demonstrated a marked ability to restrict the intracellular growth of Mtb in murine macrophages. Further assays geared toward mechanism of action elucidation have thus far ruled out the involvement of various known promiscuous targets, thereby suggesting that the new indole 7j may inhibit Mtb via a unique mechanism.

Figure 1

Previously identified antitubercular pyrroles 1 and 2 and the rationale behind this study.

Scheme 1. Main Synthetic Routes Followed for the Synthesis of the Desired Indole Derivatives 5a–l and 7a–m

Figure 2

Best docked pose of N-alkyl indole 7j into the Mtb MmpL3 homology model. Amino acids of the hydrophobic cage surrounding the heptylamino side chain are represented by thin tubes, and some of them are also labeled; Asp251 and Asp640, which are responsible for two salt bridges with the basic amino group of 7j, are represented by thick tubes, while the remaining amino acids that constitute the ligand binding site are in wire representation. The ligand is in ball and stick representation.

Figure 3

Intramacrophagic activity of 7j and moxifloxacin (Mox) against Mtb-infected murine macrophages (J774A.1 cell line). Macrophages grew in the presence of 7j and Mox at the concentration tested to ensure 100% cell growth. The concentration was 1 × MIC (0.17 μg/mL for Mox and 0.96 μg/mL for 7j). According to one-way ANOVA with Newman-Keuls’s post-test (p < 0.05), the pound sign (#) and asterisk (*) indicate significantly different results. The results are the mean ± standard deviation of at least two independent assays.