Antimicrobials · Antimycobacterial Agents
The facts most likely to be tested
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Pyrazinamide is a prodrug converted to pyrazinoic acid by the mycobacterial enzyme pyrazinamidase.
The exact mechanism is debated; pyrazinoic acid is thought to disrupt membrane transport and energetics in the acidic phagolysosome, with inhibition of mycobacterial fatty acid synthase I proposed as one contributing mechanism.
Pyrazinamide is most effective in acidic environments found within intracellular phagolysosomes of macrophages.
The most common dose-dependent side effect is hyperuricemia, which can precipitate gouty arthritis.
Pyrazinamide is associated with hepatotoxicity; obtain baseline liver function tests, with periodic monitoring for patients with abnormal baseline values, symptoms, or risk factors (e.g., alcohol use, viral hepatitis, HIV, pre-existing liver disease).
Patients may experience non-gouty polyarthralgia due to the drug's effect on uric acid excretion.
Pyrazinamide is contraindicated in patients with severe hepatic impairment or acute gout.
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A 34-year-old male with newly diagnosed pulmonary tuberculosis is started on a four-drug regimen including isoniazid, rifampin, ethambutol, and pyrazinamide. After four weeks of therapy, he presents to the clinic complaining of severe pain and swelling in his right first metatarsophalangeal joint. Physical examination reveals erythema and tenderness at the joint. Laboratory studies demonstrate an elevated serum uric acid level of 10.2 mg/dL.
Which of the following medications is the most likely cause of this patient's current symptoms?
Pyrazinamide
The patient is experiencing an acute gout flare secondary to pyrazinamide-induced hyperuricemia, which is a classic high-yield side effect tested on board exams.
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High yield triage
Classification
Antimycobacterial agent; part of the RIPE regimen.
Indications
First-line treatment for Tuberculosis.
Mechanism of Action
Converted to pyrazinoic acid by pyrazinamidase to disrupt membrane transport.
Side Effects
Hyperuricemia, hepatotoxicity, and polyarthralgia.
Contraindications / Monitoring
Gout and severe hepatic disease. Monitor uric acid and LFTs.
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Mechanism of Action
Pyrazinamide is a prodrug converted into active pyrazinoic acid by the bacterial enzyme pyrazinamidase. This active metabolite accumulates in the acidic environment of the phagolysosome, disrupting membrane energetics/coenzyme A metabolism (exact target debated; FAS-I inhibition is one proposed mechanism). The net effect is disrupted membrane transport and energetics rather than a proven block of mycobacterial cell wall synthesis.
Unique Properties
It is uniquely effective against persister organisms residing in the acidic environment of macrophages. It allows for the shortening of the total tuberculosis treatment duration from 9 months to 6 months.
Indications
Used as part of the RIPE (Rifampin, Isoniazid, Pyrazinamide, Ethambutol) regimen for active Tuberculosis, and it is also frequently an important component of individualized regimens for drug-resistant TB. It is typically discontinued after the first 2 months of therapy.
Pharmacokinetics
Extensively metabolized by the liver; dosage adjustments are required in patients with renal failure due to the accumulation of metabolites. It is well-absorbed orally and achieves excellent penetration into the cerebrospinal fluid.
Side Effects & Adverse Events
The most common adverse effect is hyperuricemia from decreased renal urate excretion, which is usually asymptomatic; symptomatic polyarthralgia occurs less frequently. Hyperuricemia is frequent and may precipitate acute gouty arthritis. Hepatotoxicity is a dose-dependent risk, ranging from asymptomatic transaminitis to severe drug-induced hepatitis.
Contraindications
Severe hepatic disease is an absolute contraindication due to the risk of fulminant liver failure. Acute gout is a contraindication per the FDA label, as the drug significantly inhibits renal urate excretion.
Monitoring
Baseline LFTs are recommended to screen for hepatotoxicity, with periodic monitoring for patients with abnormal baseline values, symptoms, or risk factors (e.g., alcohol use, viral hepatitis, HIV, pre-existing liver disease). Serum uric acid levels should be monitored if the patient has a history of gout or develops symptomatic joint pain.
Clinical Pearls
Always suspect pyrazinamide when a patient on RIPE therapy presents with new-onset joint pain. Remember that it is the most common cause of hyperuricemia among the first-line anti-TB drugs.