Review maps indole derivatives targeting mycolic acid enzymes (MmpL3, InhA, KasA/B), their SAR and synthetic routes, and notes most series still lack enzymatic and genetic target validation

The review organizes indole antitubercular molecules around three nodes of the mycolic acid pathway: MmpL3 (the trehalose monomycolate transporter), InhA (enoyl-ACP reductase catalyzing the final trans-enoyl reduction of FAS-II), and KasA/KasB (β-ketoacyl-ACP synthases responsible for condensation during chain elongation). Compared with prior reports focused on a single target or a single chemical series, this work groups dispersed indole series by target and adds a comparison table against isoniazid (INH), thiolactomycin (TLM), and quinolone/quinoxaline scaffolds covering target, mechanism type, activity against INH-resistant strains, potency trend, synthetic flexibility, resistance risk, pharmacokinetic issues, and clinical status. The basis is review-level synthesis and the in-text comparison table; target assignments come from docking, SAR trends, and whole-cell activity data in the original studies rather than new experiments performed here.

In the MmpL3 direction, several indole-2-carboxamide and adamantane/adamantanol-coupled series report submicromolar to nanomolar whole-cell activity, for example compounds 20-22 with MIC 0.0195 µg/mL against M. tb H37Rv mc26206, compound 36a with MIC 0.024 µM against H37Rv (INH control 0.29 µM), compound 43a with MIC 0.125 µg/mL (ethambutol control 1 µg/mL), and compound 82 with MIC50 0.015 µM. These data push indole scaffold potency in the MmpL3 direction into a range comparable to some first-line or research-stage agents and map substituents to activity, such as the 4,6-difluoro and adamantane amine substituents of compound 36a, the 4,6-dichloro substituent of compound 43a, and the R1/R2 chloro with R3 4-dimethylcyclohexane of compound 82. Evidence is in vitro MIC determination (REMA, 7H9 medium serial dilution, and similar) plus molecular docking (e.g., PDB 6AJJ, 7NVH); the text states that docking and in vitro activity supported each other, but no biochemical inhibition constants against purified MmpL3 are provided.

In the InhA direction, indole compounds are positioned as direct inhibitors that can bypass KatG/EthA prodrug activation and may therefore act against INH- and ethionamide-resistant strains, with representative results of compound 122a at MIC 0.39 µM (INH control 0.91 µM) and compound 143f at MIC 3.99 µM (ethambutol control 15.3 µM). Relative to KatG-dependent INH, direct InhA inhibition does not require prodrug activation, which is the differentiated advantage repeatedly emphasized for the indole scaffold; the text also gives SAR substituent patterns for several series (e.g., the -OMe and 4,5-dimethyl-1,2,3-thiadiazole of 122a, and the 2-aminophenyl of 143f). Evidence is in vitro MIC (REMA, MABA, agar dilution, and similar) with docking scores (e.g., MOE, SYBYL-X, Schrödinger; PDB 4TZK, 4U0J, 1ZIM), plus cytotoxicity/selectivity index data for some series (e.g., SI between >1978.83 and 12.04 for the 122a series).

In the KasA direction, the review includes weak activity for an indole-chalcone series (best compound 179 with MIC 143 µM versus INH at 1.4 µM) and a mechanistic dissection of the indazole sulfonamide GSK3011724A: MIC 0.5 µM in M. bovis BCG, with a 2.13 Å co-crystal structure showing it binds the broad acyl channel that holds the extending meromycolic acid chain and traps the enzyme's open conformation, unlike TLM. The GSK3011724A co-crystal structure supplies molecular detail of KasA inhibition (sulfonamide amide hydrogen bonding to Glu199, indazole ring over the flat hydrophobic surface of Gly200/Pro201, butyl chain entering a narrow lipophilic channel lined by Ile347, Ile202, and Phe239) that whole-cell activity data for indole-chalcones cannot provide. Evidence includes sensitivity testing across eighteen Gram-positive and Gram-negative species to show selectivity, [14C]-acetate labeling of mycolic acid production with INH and TLM as positive controls, and the 2.13 Å co-crystal structure.

This work is aimed at researchers in antitubercular drug discovery and indole chemistry, and applies to settings where one is seeking lead scaffolds along the mycolic acid biosynthesis pathway (MmpL3, InhA, KasA/KasB) and wants SAR and docking results to guide substituent optimization; the synthetic routes given (such as EDC.HCl/HOBt/DMF amide coupling, microwave-assisted synthesis, and one-pot green synthesis) and the pharmacokinetic/cytotoxicity summary table can support early compound screening and prioritization.

The text repeatedly emphasizes that for many indole series target identification relies on molecular docking, SAR trends, and whole-cell activity, while enzymatic IC50/Ki against purified InhA or KasA/KasB and genetic validation such as resistance mutation analysis or target overexpression are still lacking; for membrane-protein targets like MmpL3, indirect or pleiotropic effects cannot be fully excluded. In addition, most indole inhibitors suffer from poor solubility, metabolic instability, and restricted penetration through the lipid-rich cell wall and granulomatous lesions, and whole-cell efficacy may drop due to efflux or intracellular degradation. Readers should also note that this is a review without new experimental data, and that in parts of the tables the 'most potent compound' column does not display a specific compound number in the loaded text, so pharmacokinetic/cytotoxicity entries cannot be matched one-to-one to compounds; some MIC values are written inconsistently (e.g., compound 179 as '1.4 (2 µg/mL)') and some units vary, so citations should be checked against the original literature.