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 primary mechanism of action involves inhibiting mycobacterial fatty acid synthase I, which disrupts mycolic acid synthesis.
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, necessitating baseline and periodic monitoring of liver function tests.
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, inhibiting fatty acid synthase I. This disruption leads to the inhibition of mycobacterial cell wall synthesis and membrane transport.
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 exclusively as part of the RIPE (Rifampin, Isoniazid, Pyrazinamide, Ethambutol) regimen for active Tuberculosis. 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 side effect is polyarthralgia due to increased serum urate levels. 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 relative contraindication, as the drug significantly inhibits renal urate excretion.
Monitoring
Baseline and periodic LFTs are mandatory to screen for hepatotoxicity. 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.