Selective Inactivity of Pyrazinamide against Tuberculosis in C3HeB/FeJ Mice Is Best Explained by Neutral pH of Caseum

Abstract

Pyrazinamide (PZA) is one of only two sterilizing drugs in the first-line antituberculosis regimen. Its activity is strongly pH dependent; the MIC changes by several orders of magnitude over a range of pH values that may be encountered in various in vivo compartments. We recently reported selective inactivity of PZA in a subset of C3HeB/FeJ mice with large caseous lung lesions. In the present study, we evaluated whether such inactivity was explained by poor penetration of PZA into such lesions or selection of drug-resistant mutants. Despite demonstrating similar dose-proportional PZA exposures in plasma, epithelial lining fluid, and lung lesions, no dose response was observed in a subset of C3HeB/FeJ mice with the highest CFU burden. Although PZA-resistant mutants eventually replaced the susceptible bacilli in BALB/c mice and in C3HeB/FeJ mice with low total CFU burdens, they never exceeded 1% of the total population in nonresponding C3HeB/FeJ mice. The selective inactivity of PZA in large caseous lesions of C3HeB/FeJ mice is best explained by the neutral pH of liquefying caseum.

FIG 1

Dose-ranging PZA concentration-time profiles in uninfected C3HeB/FeJ mice in plasma. The data are plotted as means and SD.

FIG 2

Dose-ranging PZA concentration-time profiles in infected C3HeB/FeJ mice in plasma (solid symbols) and lesions (open symbols) for doses of 150 and 450 mg/kg BID. Panels A and B represent 2 different sets of experiments. The data are plotted as means and SD.

FIG 3

Dose-response profiles in C3HeB/FeJ mice (A) and BALB/c mice (B) after 3 weeks (blue) and 8 weeks (red) of treatment (W3 and W8, respectively). C3HeB/FeJ mice excluded from the curve fit (outliers) are circled.

FIG 4

Total CFU counts (solid symbols) and PZA-resistant CFU counts (open symbols) in C3HeB/FeJ mice treated with increasing PZA doses. Each symbol shape represents an individual mouse in its dose group. Blue, CFU counts from mice considered to fit the dose-response curve shown in the inset; red, CFU counts from mice considered nonresponsive to the dose increase; solid symbols, total CFU counts determined on drug-free medium for each individual mouse; open symbols, resistant-CFU counts determined on PZA-containing plates for the same mouse.