Pyrazinamide: the importance of uncovering the mechanisms of action in mycobacteria

Abstract

Pyrazinamide (PZA) is still one of the key drugs used in current therapeutic regimens for tuberculosis (TB). Despite its importance for TB therapy, the mode of action of PZA remains unknown. PZA has to be converted to its active form pyrazinoic acid (POA) by the nicotinamidase PncA and is then excreted by an unknown efflux pump. At acidic conditions, POA is protonated to HPOA and is reabsorbed into the cell where it causes cellular damage. For a long time, it has been thought that PZA/POA has no defined target of action, but recent studies have shown that both PZA and POA have several different targets interfering with diverse biochemical pathways, especially in the NAD⁺ and energy metabolism. PZA resistance seems to depend not only on a defective pyrazinamidase but is also rather a result of the interplay of many different enzyme targets and transport mechanisms.

Keywords:

• drug susceptibility
• Mycobacterium tuberculosis
• nicotinamidase
• POA
• pyrazinamide
• PZA
• PZase

Financial & competing interests disclosure

M Stehr and AA Elamin are employees of LIONEX Diagnostics and Therapeutics GmbH, Braunschweig. Professor Mahavir Singh is CEO of LIONEX Diagnostics and Therapeutics GmbH, Braunschweig. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

Key issues

• Multidrug-resistant tuberculosis (MDR-TB) is a major public health problem that threatens progress made in TB care and control worldwide. The number of people diagnosed with MDR-TB tripled between 2009 and 2013.

• Pyrazinamide (PZA) is still a key first-line anti-tuberculosis drug used in current therapeutic regimen due to its sterilizing activity against persisters.

• PZA is a prodrug that has to be converted by mycobacterial PZase to its active form pyrazinoic acid (POA). It appears that its bactericidal effect is based on the disruption of the membrane energetics and acidification of the cytoplasm by re-import of protonated (POA).

• Despite a majority (72–97%) of PZA-resistant isolates of M. tuberculosis exhibit mutations in pncA, several PZA-resistant M. tuberculosis isolates have no mutations in pncA, suggesting additional mechanism(s) of PZA resistance.

• Conventional culture-based PZA susceptibility testing of M. tuberculosis is technically challenging and often unreliable. Novel molecular assays for the detection of PZA resistance are urgently needed to overcome the limitations of conventional testing.

• Latest studies revealed that PZA and POA interfere with protein translation and interact with pyridine nucleotide-binding sites of several enzymes such as quinolinate phosphoribosyl transferase and fatty acid synthase.

• Several key mechanisms of bacterial nicotinamide/PZA metabolism are still unknown, such as import and export systems for nicotinamide, PZA and POA.

• Resistance to PZA may result from the complex interplay of several enzymes and transport mechanisms involved in PZA/nicotinamide metabolism. The knowledge of the complex mechanism underlying PZA resistance may help to develop a novel molecular multiparameter diagnostic tool for reliable detection of PZA resistance.

Notes