Sulfamethoxazole enhances the antimycobacterial activity of rifampicin.

Objectives: To investigate the effect of trimethoprim/sulfamethoxazole on the survival of Mycobacterium tuberculosis and trimethoprim and sulfamethoxazole individually and combined with the first-line tuberculosis drugs (isoniazid, rifampicin and ethambutol).

Methods: M. tuberculosis strains were exposed to either trimethoprim/sulfamethoxazole combination or sulfamethoxazole and trimethoprim alone at various concentrations. The strains were also exposed to sulfamethoxazole in combination with existing antibiotics to assess the combined effect on the growth of M. tuberculosis in the BACTEC 460TB system. The effect of the drugs was compared with vehicle-treated controls. Drug interactions were interpreted using quotient values obtained from the growth index of cultures treated with a single drug or the combination.

Results: Trimethoprim showed a negligible effect on the growth of M. tuberculosis while sulfamethoxazole inhibited 80% of the growth of M. tuberculosis at 4.75 mg/L. There was no synergistic activity between sulfamethoxazole and trimethoprim, although an additive effect was observed. A statistically significant synergistic effect was observed between sulfamethoxazole and rifampicin. Sulfamethoxazole also had an additive effect with ethambutol, but there was no interaction with isoniazid.

Conclusions: Sulfamethoxazole is the main active compound against M. tuberculosis in the combination trimethoprim/sulfamethoxazole and has a synergistic effect with rifampicin. These findings suggest that sulfamethoxazole has potential in the multidrug regimen against M. tuberculosis.