A 64-year-old man comes to the office for follow-up after a recent hospitalization. Ten days ago, he was admitted due to fever, dysuria, urinary urgency, and perineal pain. Evaluation revealed acute prostatitis, and urine culture was positive for Escherichia coli that was sensitive to all tested antibiotics. He received intravenous ciprofloxacin with improvement in his symptoms and was subsequently discharged on oral ciprofloxacin. The patient has been taking the antibiotic as prescribed, but over the past 2 days he has had recurrent fever and dysuria. He takes diltiazem for palpitations, atorvastatin for hyperlipidemia, and an over-the-counter antacid for frequent heartburn. Temperature is 37.3 C (99.1 F), blood pressure is 118/68 mm Hg, and pulse is 90/min. Physical examination shows a mildly enlarged and tender prostate. Repeat urine culture grows E coli. Which of the following is the most likely cause of this patient's recurrent symptoms?

Question

A 64-year-old man comes to the office for follow-up after a recent hospitalization.  Ten days ago, he was admitted due to fever, dysuria, urinary urgency, and perineal pain.  Evaluation revealed acute prostatitis, and urine culture was positive for Escherichia coli that was sensitive to all tested antibiotics.  He received intravenous ciprofloxacin with improvement in his symptoms and was subsequently discharged on oral ciprofloxacin.  The patient has been taking the antibiotic as prescribed, but over the past 2 days he has had recurrent fever and dysuria.  He takes diltiazem for palpitations, atorvastatin for hyperlipidemia, and an over-the-counter antacid for frequent heartburn.  Temperature is 37.3 C (99.1 F), blood pressure is 118/68 mm Hg, and pulse is 90/min.  Physical examination shows a mildly enlarged and tender prostate.  Repeat urine culture grows E coli.  Which of the following is the most likely cause of this patient's recurrent symptoms?

Quizplus · Accepted Answer

11ec160b_3bfb_9f0f_9249_2b86a6ca0017_MD0001_00 This patient has had a relapse of acute prostatitis despite an adequate initial response and good medication adherence. This treatment failure is likely due to a drug-drug interaction with his home medications. Fluoroquinolones normally have very high bioavailability and near-complete absorption. However, they can form insoluble chelate complexes with polyvalent cations (eg, calcium, iron, aluminum, magnesium) in the gastrointestinal tract. This leads to impaired absorption and a significant decrease in drug bioavailability. In this patient's case, his initial response to the antibiotic reflects the parenteral dosing, which bypasses the gastrointestinal tract and avoids this potential interaction. Over-the-counter antacids typically contain calcium, magnesium, and/or aluminum salts and are used by many patients for treatment of heartburn due to gastroesophageal reflux. Calcium salts (eg, calcium carbonate, calcium citrate) are also widely used in the treatment of osteoporosis. However, since these medications are available without a prescription and are often taken intermittently, the potential for interaction may not be apparent at the time antibiotic treatment is initiated. (Choice A) Antimicrobial sensitivity testing of this patient's initial culture showed no significant fluoroquinolone resistance. In addition, he had a positive initial clinical response. Loss of effectiveness after changing to an oral formulation is more consistent with decreased bioavailability rather than antibiotic resistance. (Choice C) Prostatitis due to sexually transmitted organisms is typically seen with Neisseria gonorrhoeae and Chlamydia trachomatis and is more common with younger rather than older men. Reinfection with E coli via sexual contact is unlikely. (Choice D) Ciprofloxacin is primarily cleared by renal excretion, and the dosage may need to be decreased in patients with significant renal impairment. However, switching this patient from parenteral to oral antibiotic therapy would not affect renal drug clearance. (Choice E) Diltiazem is a cytochrome P450 (CYP3A4) inhibitor (not inducer) that can cause increased systemic levels of numerous drugs, leading to drug toxicity (eg, myopathy due to statins). Regardless, ciprofloxacin is primarily eliminated by renal excretion, and drug levels are unlikely to be significantly affected by P450 inducers. Educational objective: Fluoroquinolones can form insoluble chelate complexes with polyvalent cations (eg, calcium, iron, aluminum, magnesium) in the gastrointestinal tract, leading to impaired absorption. This effect can be seen with over-the-counter antacids containing calcium, magnesium, and/or aluminum salts and in patients taking calcium salts (eg, calcium carbonate, calcium citrate) for treatment of osteoporosis.

A 64-Year-Old Man Comes to the Office for Follow-Up After