The combined effect of rifampin and pyrazinamide within the human macrophage.

A recent study in the murine model suggested that a combination of rifampin and pyrazinamide used as preventive therapy might shorten the duration of treatment time. Clinical trials using this combination have been initiated, but significant results will not be available for many years. The ex vivo human macrophage model has been instructive in expanding our knowledge of the activity of chemotherapeutic agents against intracellular virulent tubercle bacilli. Prior studies have shown rifampin to have a bactericidal effect in this model while even at clinically unachievable levels, pyrazinamide had only a bacteriostatic impact. This study finds an enhanced bacteriostatic effect when low, nonbactericidal levels of rifampin are combined with clinically achievable levels of pyrazinamide but not with higher bactericidal levels of rifampin. Adding pyrazinamide 2 days after the introduction of rifampin clearly enhanced the combined killing effect. However, reversing the order and adding rifampin 2 days after the introduction of pyrazinamide produced a result weaker than introducing the agents simultaneously. Our findings do not support the use of these agents as a potentially effective preventive therapy combination, but they suggest that the timing of the administration of these chemotherapeutic agents could be an important factor in their effectiveness.