Remifentanil Degradation in Umbilical Cord Blood of Preterm Infants

Background: No pharmacokinetic data about remifentanil in preterm infants exist, although remifentanil is increasingly used in this especially vulnerable subgroup of pediatric patients. Unfortunately, ethical restrictions in the volume of blood that can be withdrawn for kinetic sampling nearly prohibit pharmacokinetic studies in preterm infants. Methods: Because remifentanil is rapidly metabolized by nonspecific blood esterases, we collected umbilical cord serum of preterm and term infants to investigate whether the activity of nonspecific blood esterases depends on gestational age. Umbilical cord serum, buffer solution, ascorbic acid, and remifentanil were mixed in a glass vial placed in a shaking water bath at 37 C. Subsequently, serum samples were subjected to liquid chromatography-mass spectrometry-based analysis of remifentanil and its metabolite GR90291 after 0, 30, 60, 100, and 150 min. Results: We analyzed umbilical cord serum samples of 34 preterm infants (24-36 gestational weeks) and six term infants. The degradation rates of remifentanil to its major metabolite GR90291 were comparable in preterm and term infants. The overall median degradation half-life of remifentanil was 143 +/- (interquartile range) 47 min (minimum, 76 min; maximum, 221 min) without significant differences between very preterm infants (less than 28 gestational weeks) and term infants. The remifentanil concentration remained stable in control runs without serum. Conclusions: Our study demonstrates that very preterm infants exhibit a high nonspecific esterase activity in umbilical cord blood that is comparable with that of term infants. These results support clinical experiences that remifentanil is rapidly metabolized by preterm infants without prolonged side effects.