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

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

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Department of Pharmacy , National University of Singapore , 18 Science Drive 4 , 117543 , Singapore.
Department of Medicine , National University of Singapore , 14 Medical Drive , 117599 , Singapore.
Public Health Research Institute, New Jersey Medical School, Rutgers, The State University of New Jersey , 225 Warren Street , Newark , New Jersey 07103-2714 , United States.
Department of Microbiology and Immunology , National University of Singapore , 5 Science Drive 2 , 117545 , Singapore.


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.

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