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

Area of Science:

• Microbiology
• Pharmacology
• Tuberculosis Research

Background:

• Pyrazinamide (PZA) is a crucial sterilizing agent in first-line tuberculosis treatment.
• PZA's efficacy is highly dependent on pH, with significant activity loss at neutral pH.
• Previous studies noted PZA's selective inactivity in specific mouse lung lesions.

Purpose of the Study:

• To investigate the reasons for PZA inactivity in C3HeB/FeJ mice with large caseous lung lesions.
• To differentiate between poor drug penetration and the selection of drug-resistant mutants as causes of PZA failure.

Main Methods:

• Comparative pharmacokinetic analysis of PZA in plasma, epithelial lining fluid, and lung lesions.
• Assessment of PZA-resistant mutant prevalence in different mouse models (C3HeB/FeJ and BALB/c) with varying bacterial burdens.
• Correlation of PZA activity with lesion pH and drug concentration.

Main Results:

• PZA exposure was similar across tissues, ruling out poor penetration as the primary cause of inactivity.
• Drug-resistant PZA mutants were selected in BALB/c and low-burden C3HeB/FeJ mice but remained <1% in nonresponding C3HeB/FeJ mice.
• No dose-response relationship was observed in a subset of C3HeB/FeJ mice with high bacterial loads.

Conclusions:

• The neutral pH within large caseous lesions in C3HeB/FeJ mice is the most likely explanation for PZA's selective inactivity.
• This finding highlights the importance of the microenvironmental pH in determining the efficacy of antitubercular drugs.
• Understanding pH-dependent drug activity is critical for optimizing tuberculosis treatment strategies.