Identification of urinary metabolites of (±)-2-(p-isobutylphenyl)propionic acid (Ibuprofen) by routine organic acid screening

Ibuprofen [(±)-2-(p-isobutylphenyl)propionic acid] has recently been introduced as a pediatric anti-inflammatory agent. To determine how this agent interferes with urine organic acid analysis, an important pediatric investigation, we have analyzed urine from two subjects pre- and post-Ibuprofen dosage and two subjects on chronic Ibuprofen therapy. A distinctive pattern of drug interference on the organic acid profile was detected. There were three major components, corresponding to unmetabolized Ibuprofen and to the oxidation products hydroxy Ibuprofen and carboxy Ibuprofen. Therefore, the major mechanism of Ibuprofen metabolism appears to be microsomal, although mitochondrial and peroxisomal routes cannot be excluded. Laboratories carrying out routine organic acid analysis should be aware of the nature and magnitude of the organic aciduria caused by Ibuprofen.