69. Nitroimidazoles. RNA polymerase inhibitors

Page created on January 7, 2022. Not updated since.

RNA polymerase inhibitors include the macrocyclines and rifamycins.



  • Metronidazole
  • Tinidazole

The most important nitroimidazole is metronidazole, but tinidazole is very similar.

Mechanism of action:

Nitroimidazoles are prodrugs which are metabolized into reactive metabolites by enzymes inside the pathogen cell. The reactive metabolites then bind to and damage DNA and proteins. This effect is bactericidal.

Mechanism of resistance:

Decreased expression of the enzymes which activate them. Decreased penetration into the pathogen.


They are orally absorbed and penetrates tissues well (including the CNS). They are metabolised in the liver and excreted by the kidneys. They inhibit CYP2C9 and CYP3A4.

Adverse effects:

  • Headache
  • Metallic taste

Nitroimidazoles inhibit aldehyde dehydrogenase and therefore give a disulfiram-like effect if alcohol is consumed.

Clinical use:

  • Entamoeba histolytica
  • Trichomoniasis
  • Giardia
  • C. difficile
  • H. pylori

Metronidazole is the second choice in treating C. difficile colitis. It is also used in combination therapy to treat H. pylori.


The only important macrocycline is fidaxomicin.


Clostridium difficile infection.

Mechanism of action:

Fidaxomycin inhibits RNA polymerase, causing a bactericidal effect. There’s a long postantibiotic effect.

Its antimicrobial spectrum is extremely narrow, targeting almost exclusively C. diff while sparing the normal intestinal flora.


It’s not absorbed from the gut, only acting locally.


The most important rifamycins are rifampin/rifampicin and rifabutin.


Rifampin is part of the first-line therapy for TB. Rifabutin is used in rifampin-resistant mycobacteria and to treat people with HIV and TB.

These drugs can also be used to treat H. influenzae type b and leprosy.

Mechanism of action:

These drugs inhibit RNA polymerase, making them bactericidal.


Rifamycins are orally absorbed and well distributed.

They’re partially metabolized in the liver by CYP3A4 and mainly excreted by bile. Rifampin is a strong CYP inducer, also for CYP3A4, thereby inducing its own elimination.


Rifampin is a strong CYP inducer for many of the isozymes, including the isozyme which is most involved in metabolism of drugs (CYP3A4). This causes it to interact with a large number of drugs. The maybe most clinically relevant is that it interacts with anti-HIV drugs.

Rifabutin is not a CYP inducer. For this reason, it’s preferred over rifampin in people who take anti-HIV drugs.

Side effects:

  • Hepatotoxicity
  • Harmless orange discoloration of body fluids (urine, tears)
    • Due to the strong red-orange colour of these drugs

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