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QTc Prolongation Associated with Combination Therapy of Levofloxacin, Imipramine, and Fluoxetine

Professor, Department of Clinical and Administrative Sciences, Mercer School of Pharmacy, Atlanta, GA

Casey L Blackmon, PharmD

Student, Mercer School of Pharmacy

Paul E Schmidt, BSPharm

Clinical Pharmacist, St. Joseph's Hospital, Atlanta

Andrea G Roberson, PharmD

Clinical Pharmacy Coordinator, St. Joseph's Hospital

Reprints: Dr. Nykamp, Mercer School of Pharmacy, 3001 Mercer University Dr., Atlanta, GA 30341-4155, fax 678/547-6384, Nykamp_D{at}Mercer.edu

OBJECTIVE: To report QTc interval prolongation associated with combination therapy including levofloxacin, imipramine, and fluoxetine.

CASE SUMMARY: A 49-year-old white female presented to the emergency department with fever, aches, and pains for the past 3 days. She was diagnosed and treated for pyelonephritis in the hospital. Therapy included intravenous levofloxacin 500 mg every 24 hours and ceftriaxone 2 g every 24 hours, along with her medications upon admission, including imipramine 50 mg at bedtime and fluoxetine 10 mg/day. She was discharged after 5 days and returned the next day with chest tightness and shortness of breath. Upon the second admission, a 12-lead electrocardiogram showed a QTc interval of 509 msec. Levofloxacin was discontinued and the QTc interval fell to 403 msec. The patient was discharged 3 days later and instructed to consult with her primary care physician about discontinuing imipramine.

DISCUSSION: This adverse drug reaction is thought to be a pharmacodynamic additive effect among fluoxetine, imipramine, and levofloxacin. Fluoxetine is a potent inhibitor of CYP2D6, and imipramine is metabolized by CYP2D6. Therefore, fluoxetine is able to increase the plasma concentrations of imipramine, leading to QT interval prolongation. Taken with imipramine, levofloxacin can lead to even greater prolongation of the QT interval. Based on the Naranjo probability scale, levofloxacin was possibly associated with cardiac arrhythmias in our patient.

CONCLUSIONS: The use of levofloxacin alone, or more often in concomitant therapy with other medications that are known to prolong the QT interval, may cause QT interval prolongation; however, additional studies/case reports are needed to validate this conclusion.

Key Words: levofloxacin, QT prolongation, torsade de pointes

Published Online, February 1, 2005. www.theannals.com, DOI 10.1345/aph.1E513

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