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.

We tested MIC's of moxifloxacin and ofloxacin by proportion method against 34 M. tuberculosis isolates showing resistance against ofloxacin at 2.5µg/ml concentration and 13 ofloxacin susceptible isolates from specimens submitted to clinical laboratory of National Masan Hospital from March 2003 to March 2004.

For ofloxacin susceptible isolates, MIC₅₀ and MIC₉₀ of ofloxacin were all 1.25 µg/ml, and MIC₅₀ and MIC₉₀ of moxifloxacin were 0.31 µg/ml and 0.63µg/ml respectively. For ofloxacin resistant isolates, MIC₅₀ of ofloxacin was over 10µg/ml and MIC₅₀ of moxifloxacin was 5µg/ml,MIC₉₀ of ofloxacin and moxifloxacin were all over 10µg/ml. The rate of cross-resistance between the two was 67.6%(23/34) at 2.5µg/ml concentration.

Moxifloxacin showed activity against 82.4%(28/34) of ofloxacin resistant M. tuberculosis at 10µg/ml, but more studies are needed so that moxifloxacin will be used for patient with multi-drug resistant tuberculosis including ofloxacin resistance.