URINARY EXCRETION OF LUMIRUBIN DURING PHOTOTHERAPY

Abstract

Lumirubin (LR) is a stable photoproduct of bilirubin (BR), formed by an essentially irreversible intramolecular cyclization reaction. Unlike other BR photoisomers, LR is detected in both urine and bile in infants under phototherapy (PT). Clinical studies have suggested that the production and excretion of LR may contribute to the reduction of serum BR levels produced by PT. We therefore sought to determine the urinary LR excretion. We measured urinary LR clearance (clr) rate in 9 preterm infants under PT studied at a mean age of 4 days (range 1-6). Mean birthweight was 1.8 kg (range .7-2.9); mean gestational age (GA) was 32 wks (range 25-37). Urine and a serum sample obtained during the 24 hour collection period were analyzed for BR photoproducts by HPLC. Creatinine (Cr) clr was measured on the sample. Mean LR clr was .16±.19 ml/min (mean±S.D.) which is equivalent to 8±4% of Cr clr (2.6±3.2 ml/min). LR clr increased with GA in parallel with Cr clr. The amount of LR excreted per 24 hours was 1.4±1.5 mg (range .9-4). Daily BR production far exceeds the amount of LR excreted in the urine of preterm infants, so this cannot account for the efficacy of PT. However, were the clearance rate of LR to be increased to that of Cr through pharmacologic means (i.e. by blocking tubular reabsorption), urinary clearance of LR could become quantitatively important. As the urinary excretion of LR does not account for the decrease in serum BR with PT, the bile is presumably the major site of excretion of BR photoproducts.