Exogenous surfactant kinetics in infant respiratory distress syndrome: A novel method with stable isotopes

Little is known about surfactant metabolism in newborn infants, since radioactive isotopes cannot be used in humans. We describe here a new method for studying exogenous surfactant pharmacokinetics in vivo. We measured surfactant half-life, pool size, and turnover time in eight preterm infants (gestational age: 30 ± 2 wk; birth weight: 1,416 ± 202 g) who were mechanically ventilated because of infant respiratory distress syndrome. We administered two doses of 100 mg/kg each of a natural porcine surfactant with 13C-labeled dipalmitoylphosphatidylcholine as a tracer. The 13C enrichment of surfactant disaturated phosphatidylcholine (DSPC) was measured in serial tracheal aspirates by gas chromatography-mass spectrometry. The DSPC half-life was 34.2 ± 9.4 h (mean ± SD; range: 21.8 to 45.9 h). The apparent DSPC pool sizes were 5.8 ± 6.1 mg/kg (range: 0.1 to 17.0 mg/kg) and 17.3 ± 13.6 mg/kg (range: 3.3 to 41.0 mg/kg) at the time of the first and second surfactant doses, respectively. We present a novel and safe method that allows the tracing of exogenous surfactant phosphatidylcholine, the major lipid component of pulmonary surfactant, in infants who receive exogenous surfactant. This method could be a valuable tool for studying: (1) therapies that enhance lung/surfactant maturation; (2) the dosing and timing of surfactant therapy in different lung diseases; and (3) the comparison of different surfactant preparations.