Loxoprofen Overdose without Toxicity: Pharmacokinetics of Loxoprofen and Metabolites Analysed by Liquid Chromatography–Mass Spectrometry: A Case Report

Background: Loxoprofen is a widely used nonsteroidal anti-inflammatory drug in Japan and a prodrug that is converted to its active trans-hydroxylated metabolite. Reports of loxoprofen overdose are extremely limited, and asymptomatic cases despite massive ingestion have not been previously documented. Case presentation: A 43-year-old woman ingested 8,280 mg of loxoprofen. She arrived at the hospital two hours later in a clinically stable condition. Laboratory tests and electrocardiography were unremarkable, and abdominal computed tomography revealed no gastric contents. She remained asymptomatic, with normal laboratory findings throughout the hospitalisation period, and was discharged on day 3. Pharmacokinetic analysis using liquid chromatography–mass spectrometry revealed markedly elevated plasma concentrations of loxoprofen and its metabolites two hours after ingestion (loxoprofen 159 μg/mL, trans-OH 64 μg/mL, cis-OH 29 μg/mL), which declined rapidly thereafter. Conclusion: This case suggests that markedly elevated plasma concentrations of loxoprofen and its metabolites do not necessarily result in toxicity. Pharmacokinetic findings indicate that rapid metabolism and excretion may prevent organ damage.