Indolyl Azaspiroketal Mannich Bases Are Potent Antimycobacterial Agents with Selective Membrane Permeabilizing Effects and in Vivo Activity

J Med Chem. 2018 Jul 12;61(13):5733-5750. doi: 10.1021/acs.jmedchem.8b00777. Epub 2018 Jun 25.


The inclusion of an azaspiroketal Mannich base in the membrane targeting antitubercular 6-methoxy-1- n-octyl-1 H-indole scaffold resulted in analogs with improved selectivity and submicromolar activity against Mycobacterium tuberculosis H37Rv. The potency enhancing properties of the spiro-fused ring motif was affirmed by SAR and validated in a mouse model of tuberculosis. As expected for membrane inserting agents, the indolyl azaspiroketal Mannich bases perturbed phospholipid vesicles, permeabilized bacterial cells, and induced the mycobacterial cell envelope stress reporter promoter p iniBAC. Surprisingly, their membrane disruptive effects did not appear to be associated with bacterial membrane depolarization. This profile was not uniquely associated with azaspiroketal Mannich bases but was characteristic of indolyl Mannich bases as a class. Whereas resistant mycobacteria could not be isolated for a less potent indolyl Mannich base, the more potent azaspiroketal analog displayed low spontaneous resistance mutation frequency of 10-8/CFU. This may indicate involvement of an additional envelope-related target in its mechanism of action.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Antitubercular Agents / chemical synthesis
  • Antitubercular Agents / chemistry
  • Antitubercular Agents / pharmacology*
  • Aza Compounds / chemistry
  • Aza Compounds / pharmacology*
  • Cell Death / drug effects
  • Cell Membrane Permeability / drug effects*
  • Indoles / chemistry
  • Indoles / pharmacology*
  • Mannich Bases / chemical synthesis
  • Mannich Bases / chemistry
  • Mannich Bases / pharmacology*
  • Mice
  • Molecular Structure
  • Mycobacterium tuberculosis / drug effects*
  • Spiro Compounds / chemistry
  • Spiro Compounds / pharmacology*


  • Antitubercular Agents
  • Aza Compounds
  • Indoles
  • Mannich Bases
  • Spiro Compounds