Activity of Moxifloxacin Against Ofloxacin-ResistantMycobacterium Tuberculosis: A Study of Cross-Resistance Between Ofloxacin and Moxifloxacin

Background : Moxifloxacin is an 8-methoxyquinolone compound which has been shown to have the best activity of the quinolones against M. tuberculosis but there is no literature showing the rate of cross-resistance between moxifloxacin and the other quinolones such as ofloxacin. Therefore, we tested the activity of moxifloxacin against ofloxacin resistant M. tuberculosis by a study of cross-resistance. Methods : We tested MIC's of moxifloxacin and ofloxacin by proportion method against 34 M. tuberculosis isolates showing resistance against ofloxacin at 2.5/ concentration and 13 ofloxacin susceptible isolates from specimens submitted to clinical laboratory of National Masan Hospital from March 2003 to March 2004. Results : For ofloxacin susceptible isolates, MIC50 and MIC90 of ofloxacin were all 1.25/, and MIC50 and MIC90 of moxifloxacin were 0.31/ and 0.63/ respectively. For ofloxacin resistant isolates, MIC50 of ofloxacin was over 10/ and MIC50 of moxifloxacin was 5/ ,MIC90 of ofloxacin and moxifloxacin were all over 10/. The rate of cross-resistance between the two was 67.6%(23/34) at 2.5/ concentration. Conclusions : Moxifloxacin showed activity against 82.4%(28/34) of ofloxacin resistant M. tuberculosis at 10/, but more studies are needed so that moxifloxacin will be used for patient with multi-drug resistant tuberculosis including oflokacin resistance. (