New fluoroquinolones active against fluoroquinolones-resistant Mycobacterium tuberculosis strains

A set of 21 new fluoroquinolones bearing an aromatic or heteroaromatic moiety at C-7 and an alkyl group at N-1 were synthesized based on the lead structure of pirfloxacin and tested in vitro against Mycobacterium tuberculosis (M. tuberculosis) H37Rv by MIC determination in liquid medium. Among the synthesized compounds, 1-(tert-butyl)-6-fluoro-7-(4-hydroxyphenyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (2o) and 1-(tert-butyl)-6-fluoro-7-(pyridin-3-yl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (2n) were found to be the most active ones against M. tuberculosis H37Rv with the same MICs of reference compounds ciprofloxacin (CFX) and levofloxacin (LFX). MICs of 2o and 2n were determined for fluoroquinolone-sensitive and fluoroquinolone-resistant M. tuberculosis clinical isolates and 2o was the most active compound with up 4-fold difference of MIC with respect to CFX. The activity of 2o was also tested at the concentration of 16 μg/mL against M. tuberculosis H37Rv in infected murine macrophages. The results showed a 4-fold decrease in viable count of cell-associated mycobacteria with respect to untreated controls after 48 h of drug incubation.