Pyrazinamide (PZA) is a critical component of first-line drug combination therapy for
Mycobacterium tuberculosis complex (MTBC) including both susceptible and multi-drug resistant
tuberculosis (MDR TB). Inclusion of PZA has shortened the previous 9–12 month chemotherapy
regimen to 6 months. PZA has also become an essential part of MDR TB treatment regimens
that include novel compounds now clinically available, such as bedaquiline. PZA is inactive
against organisms in the growth phase during standard culture conditions at neutral pH. PZA has
a sterilizing effect due to its significant activity against non-replicating “persister” organisms or
semi-dormant slowly replicating bacilli at acid pH conditions (pH 5.5), killing bacilli that are not
eliminated by other TB drugs, such as those found in acidic regions of acute inflammation.
The mechanism of action of PZA and resistance to PZA by Mycobacterium tuberculosis (MTBC)
is not well understood. Pyrazinamide is a pro-drug which requires conversion to its active form
of pyrazinoic acid (POA) by MTBC. Pyrazinamide enters mycobacteria by passive diffusion and is
then transformed in the cytoplasm by a nicotinamidase that has pyrizinamidase (PZase) activity,
encoded by the pncA gene of MTBC. Pyrazinoic acid accumulates in the cytoplasm and is actively
expelled by a putative efflux pump. Outside of the bacilli, POA is protonated and then re-enters
the organism and release of the protons occurs, resulting in an increasingly acidic cytoplasm
and the accumulation of POA. This disrupts membrane permeability and transport, resulting
in cellular damage. Recently, the ribosomal protein S1 (translated from the rpsA gene) was
identified as a target of POA, which interferes with trans-translation activity, which is required for
efficient protein synthesis.
The primary mechanism of PZA resistance is due to mutations in the pncA gene resulting in loss
of PZase activity, thus preventing conversion of PZA to POA. In particular, mutations in specific
amino acid locations in the protein affecting catalytic sites of the PZase enzyme and Fe2+ ion
binding site cause loss of PZase activity and are associated with MTBC PZA resistance.