In this randomized, blinded, controlled, pilot study, we have demonstrated that late administration of an SP-B-containing surfactant transiently increases SP-B content in the lung aspirates in treated infants as compared to controls. Respiratory status, as assessed in terms of RSS, was also improved; however, both biochemical and clinical effects waned by day 2 after surfactant dosing. Late dosing of surfactant was well tolerated, and there was no evidence of adverse effects as regards the incidence of neonatal comorbidities of prematurity or of mortality.
This study was designed to take advantage of the transient effects observed in our previous open-label study of late administration of surfactant, wherein infants received 2 or 3 doses of calfactant (13
). We hoped to augment the clinical response to surfactant treatment by administering up to 5 doses of surfactant. As we had previously increased the number of doses from 2 to 3, we subsequently decreased the dosing interval to 3 d in this study. However, we found that this dosing interval was in fact insufficient to ensure persistent restoration of adequate SP-B levels (and therefore surfactant function) among preterm infants with acquired surfactant dysfunction. Although the acute effect we demonstrated on RSS was modest, it was similar to that observed in our earlier pilot study and the experience reported by Katz and Klein (13
). Bissinger described a more pronounced improvement in RSS with late administration of surfactant when it was carried out during an acute respiratory decompensation (16
). It is not surprising that the response to replacement surfactant during an acute deterioration would be greater than the effect in chronically ventilated infants who are free of acute decompensation. These findings are consistent with those from experimental studies in chronically ventilated preterm lambs, which had a greater amount of water in the lungs as compared with controls, and showed further increases in fluid and protein during episodes of acute infection (consistent with worsening pulmonary edema) (18
We found that late administration of surfactant doses increased surfactant recovery and SP-B content, regardless of whether SP-B was normalized either to phospholipid (PL) or to total protein in surfactant. However, as noted, this effect was transient. An additional interesting finding lends insight into the mechanism of acquired surfactant deficiency in ventilated extremely low BW infants. We expected that the incremental increases in SP-B at day 1 after treatment would be equal, regardless of SP-B levels. However, we found instead that the change in SP-B content with dosing was dependent on mean SP-B content levels, whereas SP-B content was not related to surfactant recovery during treatment. These data are consistent with a faster rate of SP-B turnover as a contributor to low SP-B in these infants, suggesting that additional proteins in the surfactant fraction could be impairing surfactant function through SP-B degradation. In contrast, on the basis of surfactant recovery, there was no indication of faster PL turnover associated with low SP-B increase after treatment. Although specific degradation pathways for SP-B have not been identified, imbalances between proteases and their inhibitors are noted in the ventilated immature lung in experimental models (19
). This situation may be analogous to specific changes in the surfactant lipid profile mediated by elevated phospholipase A2
activity after oleic acid–induced lung injury in rodents (20
). Another consideration is that the biophysical interaction between SP-B and surfactant lipids, which determines lipid organization and surface activity, is influenced by both the concentration of peptide and composition of the lipids (21
). Alterations in surfactant lipid composition occur in newborn premature infants who have surfactant deficiency (22
) as well as in those with later-acquired dysfunction, as we describe in this study (11
). It is therefore likely that variability in the response to late therapy with surfactant depends on both the level of SP-B (which is determined by synthetic and clearance rates) and the lipid composition of endogenous surfactant.
It was also interesting to note that total protein in the surfactant fraction fell with surfactant treatment (albeit not to a statistically significant extent), suggesting that there was an amelioration of pulmonary edema or epithelial injury and sloughing with surfactant therapy. Pulmonary edema has been described in chronically ventilated preterm lambs (18
); it occurs in experimental SP-B deficiency and is associated with increase alveolar protein levels (23
). However, the relationship between alveolar protein content and surfactant is variable per data from animal studies. In the setting of high tidal volume ventilation in rodents, the increase in total lavage protein was associated with decreased surfactant function, despite increase in surfactant levels (24
). Other studies of various forms of augmented surfactant administration have shown anti-inflammatory effects but no differences in total aspirate protein levels (25
). In our previous open-label study of the effects of calfactant administration, late administration surfactant had no sustained effect on inflammatory mediators in lung aspirate fluid (13
). However, in that study, we did not collect daily tracheal aspirate samples during dosing and we had no control group to assess the effects of surfactant administration on the inflammatory profile in chronically ventilated infants.
Pathophysiologic factors that probably contribute to the development of BPD include lung injury secondary to hyperoxia, barotrauma from mechanical ventilation, and atelectasis (18
). Given the multifactorial nature of BPD, a dosing regimen that failed to achieve persistent maintenance of SP-B content, and the smallness of our sample for this feasibility study, we were not surprised to observe only a modest, nonsignificant effect on clinical outcome in the infants at PMA of 36
wk. to compare these outcomes with those of infants enrolled in our previous iNO-only study (nitric oxide for the prevention of chronic lung disease), we extracted unpublished outcome data relating to infants who were similar to those enrolled in this pilot study (BW ≤1,000
g, intubated at the time of study enrollment, and having study treatment initiated at day of life 7–14) (6
). There were 92 infants meeting these criteria in the iNO group and 97 infants in the placebo group; these represented only one-third of the total study enrollment. At a PMA of 36
wk, 46% of the iNO-treated infants and 25% of the placebo-treated infants had survived and without a diagnosis of BPD. Although our outcomes in our study approached those of iNO-treated infants in nitric oxide for the prevention of chronic lung disease, due to the multifactorial nature of BPD, we had hoped to improve upon these outcomes with combination therapy. Data from our various studies have suggested that there has been an evolution in this patient population. In our earlier (1997–2001) descriptive study, 47% of the infants had surfactant dysfunction (11
). The prevalence of surfactant dysfunction in a subset of infants enrolled in the nitric oxide for the prevention of chronic lung disease study (2000–2005) was only 43%, whereas it was 76% in our later surfactant study (2004–2007) (12
). In the earlier descriptive study, SP-B content averaged 0.98% PL with normal surface tension and 0.20% PL with abnormal surface tension (11
), whereas in this later study, SP-B content at baseline trended even lower than in our previous open-label study: median values were 0.23% PL and 0.30% PL in the Surfactant and Control arms, respectively (
), as compared to 0.34% PL in our recent open-label study (13
). These differences may reflect trends toward increasing use of non-invasive ventilation in neonatal intensive care, which could have an impact on the characteristics of infants who remain intubated at day of life 7–14 (29
). In two additional cohorts of extremely preterm infants born between the years 2000 and 2004, mechanical ventilation at days 7–14 was an important and independent risk factor for the development of BPD (30
). Although the level of SP-B required to prevent BPD is not known, experimental models of SP-B deficiency demonstrate that, although lung dysfunction is attenuated when SP-B is restored to ~30% of control levels, normal surface tension is not restored until SP-B content returns to normal (23
In this pilot study, we did not assess the later effects of late treatment with surfactant on pulmonary morbidity, an important finding of the impact of iNO in the nitric oxide for the prevention of chronic lung disease study, even in the subsets of infants who showed less evidence of treatment benefit at a PMA of 36
). Other studies have demonstrated decreased morbidity at later follow-up, despite there having been no significant benefit at a PMA of 36
). Our study found that the safety profile of late administration of calfactant was reassuring. When compared with data from a similar trial of lucinactant administration, bradycardia episodes were less frequent in our study; however, the threshold for reporting such instances was somewhat lower in our study (<80 bpm with a duration of at least 60 s) (17
The change in urinary excretion of NOx from baseline of 10–20 ppm in the iNO group was similar in magnitude to changes in plasma NOx that we have previously reported at these iNO doses (33
). We had not previously reported changes in cyclic guanosine monophosphate. These demonstrated a similar effect size to that of NOx, indicating the biological activity of iNO in this patient population. We hypothesized that we might see improved iNO delivery and biological activity with late administration of surfactant, through recruitment of additional lung surface area. However, any differences that might have occurred were not detectable with our sampling scheme, which coincided with the timing of our late dosing of surfactant.
In conclusion, in this pilot study we found that a dosing interval of 3 d was insufficient to restore and maintain adequate SP-B levels. These data suggest that a more appropriate dosing interval for late administration of surfactant would be 1–2 d. Although the continuous effects of late administration of surfactant may not be a prerequisite for long-term benefit, the additive effects of recurrent dosing at these shortened dosing intervals, with the resultant improvements in lung function, could allow for more rapid weaning from mechanical ventilation, leading to improved pulmonary outcomes, both short-term and long-term. We are studying this modified regimen in an ongoing trial powered to identify an improvement in the rate of BPD at a PMA of 36
wk in extremely low GA newborns. We intend to follow up the pulmonary and developmental outcomes in these infants up to 2 y of corrected age (NCT01022580).