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 (13)C-labeled dipalmitoylphosphatidylcholine as a tracer. The (13)C 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.

Little is known about endogenous surfactant metabolism in infants, because radioactive isotopes used for this purpose in animals cannot be used in humans. We developed a novel and safe method to measure the endogenous surfactant kinetics in vivo in humans by using stable isotope labeled fatty acids. We infused albumin-bound [U-13C]palmitic acid (PA) and [U-13C]linoleic acid (LLA) for 24 h in eight critically ill infants (mean+/-SD: weight: 3.7+/-1.3 kg: age: 51.3+/-61.6 d) who required mechanical ventilation. The 13C enrichment of PA and LLA in surfactant phosphatidylcholine (PC), obtained from tracheal aspirates, was measured by gas chromatography combustion interface-isotope ratio mass spectrometry. We measured a significant incorporation of both 13C-PA and 13C-LLA into surfactant PC. PC-PA and PC-LLA became enriched after 8.7+/-4.9 h (range: 3.4-17.3) and 10.0+/-7.2 h (range: 3.0-22.4), respectively; the times at maximum enrichment were 49.2+/-8.9 and 45.6+/-19.3 h, respectively. The fractional synthesis rate of surfactant PC-PA ranged from 0.4 to 3.4% per h, whereas the fractional synthesis rate of PC-LLA ranged from 0.5 to 3.8% per h. The surfactant PC-PA and PC-LLA half-lives ranged from 16.8 to 177.7 and 23.8 to 144.4 h, respectively. This method provides new data on surfactant metabolism in infants requiring mechanical ventilation. We found that synthesis of surfactant from plasma PA and LLA is a slow process and that there were marked differences in PC kinetics among infants. This variability could be related to differences in lung disease and could affect the clinical course of the respiratory failure.

Surfactant-associated protein B (SP-B) is critical to the biophysical function of pulmonary surfactant. No information is available on SP-B synthesis and kinetics in humans. We administered a 24-h i.v. infusion of 13 C-valine as metabolic precursor of SP-B to six newborn infants (weight 3.5 Ϯ 0.5 kg; age 12 d, range 1-43 d). Three of the study infants also received i.v. 2 H-palmitate to label surfactant disaturated phosphatidylcholine (DSPC). SP-B and DSPC were isolated from tracheal aspirates, and their respective 13 C and 2 H enrichments were measured by gas chromatography-mass spectrometry. SP-B kinetics was measured successfully in all six infants. SP-B median (range) fractional synthesis rate was 30% per day (20 -78% per day), secretion time was 4.5 h (1-9 h), time to peak was 24 h (12-36 h), and half-life was 21 h (8 -35 h). The ascending part of the SP-B kinetic curve was similar to the DSPC curve, suggesting similar secretion pathways. SP-B half-life seemed to be shorter than DSPC half-life. These results agree with existing animal data. We conclude that the measurement of SP-B kinetics is feasible in vivo in humans by stable isotope technology. Surfactant-associated proteins B and C (SP-B and SP-C, respectively) represent Ͻ5% of surfactant by weight, but they play a pivotal role in maintaining surfactant function (1). Surfactant deficiency is the hallmark of respiratory distress syndrome in preterm infants. In respiratory distress syndrome, there is a reduced amount of phospholipids (2) and of SP-B and SP-C (3). The key role of SP-B became evident in congenital SP-B deficiency, which is characterized by severe respiratory failure leading ultimately to death unless lung transplant is performed (1,4). To date, there are no data on SP-B kinetics in humans, but information is available in adult and newborn animals, in which radioactive labels can be used. These studies showed differences in SP-B clearance and kinetics depending on animal species and postnatal age (5-7).We recently described methods based on stable isotopes to measure surfactant disaturated-phosphatidylcholine (DSPC) kinetics in humans (8,9). The main objective of this study was to demonstrate the feasibility of measuring SP-B kinetics in vivo in newborn infants by means of stable isotopes. (Table 1) who were admitted to the neonatal intensive care unit, Department of Paediatrics, University of Padua. Inclusion criteria were 1) gestational age Ͼ37 wk, 2) respiratory failure requiring endotracheal intubation for an estimated length of at least 48 h, 3) arterial and venous lines placed for clinical monitoring, and 4) written parental consent. Exclusion criteria were congenital chromosomal abnormalities and exogenous surfactant given at Ͻ48 h before study start. The local Ethics Committee approved the study protocol. METHODS SP-B kinetics was studied in six infantsAll patients received a 24-h constant i.v. infusion of 2 mg/kg/h 1-13 C valine (Mass Trace, Woburn, MA), dissolved in normal saline, while they were on a lipid-free i.v. infusion. In ...