Antimicrobials · Tetracyclines
The facts most likely to be tested
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Doxycycline is the first-line treatment for Lyme disease (Borrelia burgdorferi), Rocky Mountain spotted fever (Rickettsia rickettsii), and Chlamydia trachomatis infections.
Doxycycline acts by binding to the 30S ribosomal subunit, effectively inhibiting bacterial protein synthesis.
Doxycycline is the preferred tetracycline in patients with renal impairment because it is primarily eliminated via the fecal route rather than the kidneys.
Common adverse effects include esophageal ulceration (pill esophagitis), photosensitivity, and gastrointestinal distress.
Doxycycline is contraindicated in pregnant women and children under 8 years old due to the risk of permanent tooth discoloration and enamel hypoplasia.
Absorption of doxycycline is significantly reduced when administered concurrently with divalent or trivalent cations, such as calcium, magnesium, aluminum, or iron supplements.
Doxycycline is a recommended first-line option for outpatient community-acquired pneumonia, including coverage of atypical pathogens like Mycoplasma pneumoniae.
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A 24-year-old female presents to the clinic complaining of a fever, headache, and a maculopapular rash that began on her wrists and ankles before spreading to her trunk. She reports a recent hiking trip in North Carolina where she recalls removing a tick from her leg. Physical examination reveals petechiae on the palms and soles. Laboratory studies show thrombocytopenia and hyponatremia.
What is the most appropriate pharmacologic treatment for this patient?
Doxycycline
The patient's presentation is classic for Rocky Mountain spotted fever, for which doxycycline is the gold-standard treatment regardless of age, as the benefits outweigh the risks of dental staining.
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High yield triage
Classification
Tetracycline antibiotic; bacteriostatic protein synthesis inhibitor.
Indications
Lyme disease, Chlamydia, RMSF, CAP, and MRSA skin infections.
Mechanism of Action
Binds 30S ribosomal subunit to inhibit tRNA binding to mRNA.
Side Effects
Photosensitivity, esophagitis; teratogenic, tooth discoloration.
Contraindications / Monitoring
Pregnancy, children <8 years; monitor for esophageal ulceration.
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Mechanism of Action
Doxycycline reversibly binds to the 30S ribosomal subunit of susceptible organisms. This action prevents the attachment of aminoacyl-tRNA to the mRNA-ribosome complex. By blocking this step, it effectively halts bacterial protein synthesis. It is considered bacteriostatic in its primary mechanism of action.
Unique Properties
Unlike other tetracyclines, it is primarily excreted via the fecal route, making it the preferred agent in patients with renal failure. It exhibits excellent tissue penetration, including the blood-brain barrier, and has unique anti-inflammatory properties.
Indications
It is the first-line treatment for Lyme disease (early localized), Rocky Mountain Spotted Fever, and Chlamydia trachomatis. It is also utilized for community-acquired pneumonia, acne vulgaris, and as an alternative for MRSA skin and soft tissue infections.
Pharmacokinetics
Doxycycline is highly lipophilic with excellent oral bioavailability. It is primarily eliminated via the biliary/fecal route, requiring no dose adjustment in renal impairment. Avoid co-administration with divalent cations (calcium, iron, magnesium) which chelate the drug and reduce absorption.
Side Effects & Adverse Events
Common side effects include photosensitivity and GI upset. Patients must be warned about esophagitis; advise taking with a full glass of water and remaining upright. Tooth discoloration and enamel hypoplasia occur in developing teeth. Teratogenicity is a major concern due to skeletal growth inhibition.
Contraindications
Pregnancy and children <8 years are absolute contraindications due to permanent bone and tooth development interference. Hypersensitivity to tetracyclines is a contraindication. Use caution in patients with a history of esophageal strictures.
Monitoring
Monitor for signs of superinfection (e.g., C. difficile colitis) during prolonged therapy. Assess for photosensitivity reactions and ensure patient compliance with upright positioning to prevent esophageal ulceration. No routine laboratory monitoring is required for short-course therapy.
Clinical Pearls
Always suspect RMSF in a patient with a fever and rash starting on the wrists/ankles; start Doxycycline immediately regardless of age. It is the classic board answer for tick-borne illnesses. Remember the tetracycline class trap: do not take with milk or antacids.