Antimicrobials · Sulfonamides
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Trimethoprim-sulfamethoxazole is the first-line treatment for Pneumocystis jirovecii pneumonia (PJP) in both immunocompromised and HIV-positive patients.
The mechanism of action involves the sequential inhibition of folate synthesis by targeting dihydropteroate synthase and dihydrofolate reductase.
Trimethoprim-sulfamethoxazole is a common cause of Type 4 Renal Tubular Acidosis (RTA) due to the trimethoprim component acting as a potassium-sparing diuretic that blocks epithelial sodium channels.
Patients taking trimethoprim-sulfamethoxazole are at significant risk for hyperkalemia, particularly when combined with ACE inhibitors, ARBs, or spironolactone.
Sulfonamides are associated with Type IV (delayed) hypersensitivity reactions, including severe Stevens-Johnson syndrome (SJS) or toxic epidermal necrolysis (TEN).
Trimethoprim-sulfamethoxazole is a potent CYP450 inhibitor and significantly increases the risk of bleeding in patients concurrently taking warfarin.
The drug is contraindicated in patients with a glucose-6-phosphate dehydrogenase (G6PD) deficiency due to the risk of acute hemolytic anemia.
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A 34-year-old male with HIV and a CD4 count of 45 cells/mm³ presents with a 2-week history of progressive exertional dyspnea and a non-productive cough. Physical examination reveals bilateral diffuse interstitial infiltrates on chest X-ray and an arterial blood gas showing hypoxemia with an increased alveolar-arterial (A-a) gradient. The patient is started on empiric therapy for his suspected pulmonary infection. Two days later, his serum potassium increases from 4.2 mEq/L to 5.6 mEq/L.
Which mechanism best explains the patient's new-onset hyperkalemia?
Blockade of epithelial sodium channels in the collecting tubule
The patient is being treated for PJP with trimethoprim-sulfamethoxazole, which acts like amiloride to block epithelial sodium channels, leading to decreased potassium excretion and hyperkalemia.
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High yield triage
Classification
Synergistic folate antagonist combination.
Indications
UTI, MRSA skin infections, Pneumocystis jirovecii pneumonia.
Mechanism of Action
Inhibits dihydropteroate synthase and dihydrofolate reductase.
Side Effects
Rash, Stevens-Johnson syndrome, hyperkalemia, bone marrow suppression.
Contraindications / Monitoring
Sulfa allergy, pregnancy (3rd trimester). Monitor creatinine and potassium.
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Mechanism of Action
Sulfamethoxazole inhibits dihydropteroate synthase, blocking the conversion of PABA to dihydrofolic acid. Trimethoprim inhibits dihydrofolate reductase, preventing the reduction of dihydrofolate to tetrahydrofolate. This dual blockade creates a synergistic bactericidal effect. It effectively starves bacteria of the folate required for DNA synthesis.
Unique Properties
This combination is uniquely effective against MRSA and Pneumocystis jirovecii. It is the drug of choice for PCP prophylaxis and treatment in immunocompromised patients. Unlike monotherapy, the combination prevents the rapid development of bacterial resistance.
Indications
First-line for uncomplicated cystitis and pyelonephritis. Used for MRSA skin and soft tissue infections. Essential for the treatment and prophylaxis of Pneumocystis jirovecii pneumonia in HIV patients. Also used for prostatitis and shigellosis.
Pharmacokinetics
Renally excreted; requires dose adjustment in renal insufficiency. It is a potent inhibitor of CYP2C9, significantly increasing the effect of warfarin. Patients on anticoagulants require frequent INR monitoring.
Side Effects & Adverse Events
Common effects include nausea, vomiting, and photosensitivity. Stevens-Johnson syndrome and toxic epidermal necrolysis are rare but life-threatening dermatologic emergencies. Hyperkalemia occurs due to trimethoprim's structural similarity to the potassium-sparing diuretic amiloride. Bone marrow suppression (leukopenia, thrombocytopenia) can occur with prolonged use.
Contraindications
Sulfa allergy is an absolute contraindication due to risk of anaphylaxis. Pregnancy (3rd trimester) is contraindicated due to the risk of kernicterus in the neonate. Avoid in patients with folate deficiency or megaloblastic anemia.
Monitoring
Monitor serum creatinine and potassium levels, especially in elderly patients or those on ACE inhibitors. Obtain a CBC if therapy exceeds two weeks to screen for pancytopenia. Monitor INR closely in patients on warfarin.
Clinical Pearls
Always suspect PCP in an HIV patient with a CD4 count <200 presenting with dry cough and exertional dyspnea; TMP-SMX is the gold standard treatment. If a patient develops a rash, discontinue immediately to avoid progression to SJS. Remember the sulfa allergy cross-reactivity with other sulfonamide-containing drugs like thiazides and sulfonylureas.