Beyond pulmonary hypertension: sildenafil for chronic lung disease of prematurity.

Bronchopulmonary dysplasia (BPD) is the most common chronic lung disease of infancy in the United States, and complicates the course of 30% or more of extremely preterm infants (those born before 28 wk gestation). While prenatal steroids and surfactant reduce acute respiratory failure and improve survival, they do not diminish the frequency or severity of BPD (1, 2). Multiple therapies such as hydrocortisone, vitamin A, caffeine, superoxide dismutase, and inhaled nitric oxide have been tested to prevent BPD, but success has been inconsistent and generally modest (3). Extremely preterm infants are born in the late canalicular to early saccular stage of lung development, and must therefore complete a substantial amount of lung development in an extrauterine environment. BPD is often regarded as a form of arrested lung development that occurs when the preterm lung attempts to adapt to air breathing during this developmentally sensitive time period. Fetal lung morphogenesis is a complex process that is orchestrated by multiple transcription factors, growth factors, and environmental influences. The fetus thrives in utero under hypoxic conditions that favor lung growth. In contrast, preterm birth exposes the lung to relative hyperoxia at a critical developmental juncture, which disrupts the normal parenchymal and vascular lung development that is fostered by the presence of hypoxia (4). While supplemental oxygen is frequently required for preterm infants, the preterm lung is poorly equipped to handle it. High levels of inspired oxygen promote oxidative stress in the lung, and are clearly associated with worse pulmonary outcomes (5). On the other hand, a recent clinical trial showed that maintaining oxygen saturation limits at a somewhat lower range (85–89%) reduced the incidence of BPD and retinopathy of prematurity, but increased all-cause mortality by 27% (6). This narrow therapeutic window for oxygen administration further emphasizes the need for lung-targeted therapies that minimize systemic effects. Hypoxia-inducible factors (HIFs) are master regulators of the transcriptional response to hypoxia involved in angiogeneisis, survival, andmetabolic pathways.HIFs are constitutively expressed in multiple fetal pulmonary cell types, including epithelial, smooth muscle, and endothelial cells (7). When activated, HIF accumulates in the nucleus and binds to hypoxia response elements of multiple hypoxia-inducible genes including VEGF, SOD2, iNOS, erythropoietin, and others (4, 8). HIF activity is regulated largely at the post-translational level by prolyl hydroxylases (PHDs) that control stability of its a subunit. While HIF-a subunits are rapidly degraded under normoxic conditions, low levels of HIF activation persist and contribute to the basal expression of a large number of genes. Several downstream targets of HIFs likely contribute to abnormal lung development in animal models of BPD and in preterm neonates. One important example is VEGF and VEGF receptor expression, which is decreased in various animal models of BPD induced by hyperoxia or antenatal endotoxin (9–12), as well as in the lungs of premature infants who die with BPD (13, 14). Disruption of pulmonary VEGF leads to abnormal vascular and alveolar development, lung hypoplasia, pulmonary hypertension, and BPD (12, 15–17). The importance of HIFs in fetal lung development has been highlighted by studies revealing that deletion of HIF-1 causes embryonic lethality, and that deletion of HIF-2 reduces VEGF levels and leads to early death due to respiratory failure (16). Untimely exposure to oxygen after preterm birth could promote abnormal lung development by accelerating PHD activity (18, 19) and rapidly degrading HIF (20). In support of this theory, short-term hyperoxic ventilation of preterm lambs increased PHD2 expression and disrupted pulmonary expression of HIF and its downstream target VEGF (21). If restricting oxygen therapy is not feasible for protecting the preterm lung, then selectively stabilizing pulmonary HIF activity under normoxic conditions could duplicate the desirable effects of hypoxia on lung development. Attempts to stabilize HIF through inhibition of PHD activity showed early promise in baboon neonates with early BPD, although worrisome side effects of rash and pulmonary bleeding were noted (22). Accumulated evidence also shows that preterm birth, hyperoxia, and lung injury disrupt NO-cGMP signaling, which has generated great interest in therapies targeted toward this pathway (23, 24). Inhaled nitric oxide has been particularly well studied in large clinical trials of extremely preterm infants, but its ability to prevent BPD has been surprisingly modest (3, 25). Interestingly, pulmonary hypertension and therapies such as oxygen increase expression and activity of cGMP-specific phosphodiesterases, which would be expected to lower cGMP concentrations and exaggerate pulmonary hypertension (26–28). In rodent models of BPD, sildenafil, a selective cGMP-phosphodiesterase inhibitor, improved alveolarization and decreased remodeling of the pulmonary vasculature (29, 30). However, the mechanism driving the improved lung and vascular development in response to PDE5 inhibition has not yet been determined. In the January issue of the Journal, Park and coworkers (pp. 105–113) begin to fill this knowledge gap (31). They administered sildenafil to neonatal rats with BPD created by the combination of antenatal administration of LPS and one week of neonatal hyperoxia. The sildenafil dose was considerably higher than appropriate for humans, but reflects a higher rate of drug metabolism in the rat, and was equivalent to a human dose of approximately 1 mg/kg/day (31, 32). Endpoints of lung alveolarization and angiogenesis were assessed after the 1-week period of hyperoxia, and again after an additional week of recovery in room air. The study by Park and colleagues confirms the previous reports showing that sildenafil improves lung growth and alveolarization (29, 31), and creates an interesting link between sildenafil and increased expression of HIF and its downstream target VEGF. The restoration of lung alveolarization, HIF levels, and VEGF levels was most striking after the second week of “recovery” in room air, which may be pertinent to the common clinical scenario of early respiratory failure followed by a recovery phase. Additional in vitro experiments in epithelial cells further demonstrated that the sildenafil effect on HIF is most likely mediated by cGMP. Together these findings highlight the crucial link between preservation of HIF levels and lung development, and provide an intriguing new explanation This work was supported in part by U01HL102235 from the National Heart, Lung, and Blood Institute.