Persistent pulmonary hypertension of the newborn (PPHN) is a serious disorder that results from the failure of normal pulmonary vascular adaptation at or soon after birth, and its estimated incidence is approximately 2 per 1,000 live births [1]. High-frequency ventilation, inhaled nitric oxide (iNO), and extracorporeal membrane oxygenation have improved outcomes. However, several facilities do not offer these therapies due to their high costs and insufficient infrastructure. Furthermore, the Food and Drug Administration (FDA) recently issued a warning regarding the use of sildenafil to treat pediatric pulmonary arterial hypertension after an unexpectedly high mortality rate was observed among children receiving the drug in high doses in a clinical trial [2].

Therefore, there is a significant need for new, alternative modalities to treat PPHN in neonates. Inhaled iloprost, a stable analogue of prostacyclin (PGI₂), may be an adjunctive or rescue therapy option for infants with PPHN. However, case reports regarding its use as an emergency treatment for PPHN are still preliminary [3-7].

In our center, we used iloprost and other medications to treat PPHN at a time when iNO was not available because medically approved NO gas (iNOmax) was not yet introduced in Korea, and for other NO gases, we were not convinced of their safety. This report aimed to describe the effects of rescue attempts with inhaled iloprost on mechanically ventilated PPHN patients. Additionally, we have evaluated the therapeutic efficacy and safety of inhaled iloprost for the treatment of PPHN when inhaled iNO was not available.

The demographic and clinical characteristics of the patients are presented in Table 1. All nine full-term neonates were referred from primary care, and six were boys. The median gestational age and birth weight were 38⁺⁴ weeks (37⁺¹ to 40⁺²) and 3.42 kg (2.84 to 4.39), respectively. Three patients were born by spontaneous vaginal delivery, and six were delivered by cesarean section. Severe respiratory failure caused by meconium aspiration syndrome, sepsis, asphyxia, and respiratory distress syndrome were associated with the pathogenesis of PPHN. Patient no. 8 was transferred after recurrence of PPHN due to aspiration pneumonia noted at the age of 21 days.

Iloprost was inhaled at an initial dose of 1 to 2 μg/kg/day and subsequently administered every 3 to 6 hours for 2 to 8 days in survivors by connecting an ultrasonic nebulizer to the inspiratory limb of the ventilator circuit. Inotropic support with dopamine and dobutamine was started to maintain systemic blood pressure at normal values for gestational ages in five patients [1]. Surfactant administration and oral sildenafil were combined with iloprost in five and eight patients, respectively. The dose of oral sildenafil for each patient ranged from 1.0 to 2.8 mg/kg/dose.

Fentanyl and midazolam sedation was administered in six patients who showed intolerance to mechanical ventilation (Table 1). Table 2 describes the two-dimensional echocardiographic findings before and after treatment with iloprost. Echocardiography demonstrated a right-to-left shunt or a bidirectional shunt through the PDA or PFO. The inhalation of aerosolized iloprost showed not only a permanent conversion of the initial right-to-left shunting or bidirectional shunt, which leads to a substantial improvement in oxygenation, but also a simultaneous decrease in right ventricular pressure.

Aerosolized iloprost decreased FiO₂ and improved oxygen saturation in eight of the nine patients, who normalized to baseline values within 1 week after iloprost. FiO₂ decreased by 32%, from 0.63±0.17 (pre-iloprost) to 0.43±0.20 (24 hours after iloprost; P=0.007) (Figure 1A). RSS decreased from 11.0±2.6 to 8.7±2.3 at 24 hours after iloprost (P=0.011) (Figure 1B). The use of iloprost in the emergency treatment of pulmonary hypertension was associated with a lifesaving clinical improvement with no evidence of side effects in eight of the nine patients. There were no significant changes in heart rate and mean blood pressure after iloprost inhalation (Figure 2). Moreover, no adverse effects that could be attributed to iloprost were observed.

Details of the outcomes after iloprost inhalation therapy are shown in Table 3. All patients were supported by high-frequency oscillation ventilation or pressure-controlled ventilation. The mean duration of oxygen therapy was 8.4±3.0 days. Enteral feeding was started in infants who were at least 6 days old, and the number of days to full enteral feeding was 9.4±1.3 days, ranging from 8 to 11 days. Eight of the nine patients made a full recovery and were discharged from the hospital within 3 weeks, ranging from 10 to 18 days. However, one patient with severe hypoxic damage and multi-organ failure when he was referred showed a tension pneumothorax in both sides of the lungs and a large PDA. He died due to a lack of response to treatment 2 days after his birth.

The main reason why inhaled iloprost is more preferred than iNO is its readily availability and comparable clinical effects, while iNO is cost prohibitive and requires special monitoring for side effects. However, there is limited experience regarding inhaled iloprost therapy, specifically in neonates. Our study suggests that inhaled iloprost is a feasible therapy for mechanically ventilated neonates with PPHN. No clinically significant complications were observed during the nebulization of iloprost, and we achieved significant oxygenation and hemodynamic improvements.

PPHN is a complex clinical syndrome with significant morbidity and mortality [9]. iNO, inhaled iloprost, and phosphodiesterase inhibitors (sildenafil) are known treatment options that are used along with supportive care such as alkalinization and high-frequency oscillatory ventilation in neonates with PPHN. iNO has a selective effect on the pulmonary vasculature through the guanosine 3’,5’-cyclic monophosphate and has become well established as the first-line therapy for PPHN [10,11]. However, iNO therapy is still unavailable in many developing countries. Although pulmonary hypertension in adults is commonly treated with PGI₂ analogues (treprostinil, iloprost, or epoprostenol), endothelin receptor antagonists (bosentan and ambrisentan), and phosphodiesterase type 5 inhibitors (sildenafil) [12], few reports have addressed the use of these treatments in neonates and preterm infants [3,4,13-17]. The pharmacokinetics of oral sildenafil in neonates with decreased gastric motility remain uncertain. Moreover, the FDA issued a warning regarding the pediatric-specific side effects of sildenafil, including hypotension and hemoptysis [2]. Therefore, there is an increasing interest in the potential benefit of inhaled iloprost in neonates. In 2014, Kahveci et al. [18] compared the effectiveness and safety of oral sildenafil and inhaled iloprost therapy for PPHN in a referral center located in a low-income region of Turkey. Results showed that iloprost was considered to be more effective than sildenafil in the treatment of PPHN with regard to time to clinical response, ventilator/respiratory parameters, and combined drug administration [18]. However, it seems to be impossible to evaluate the isolated effects of both drugs in the clinical field due to ethical issues.

Iloprost is a PGI₂ analogue, and PGI₂ is a member of the prostanoid group of eicosanoids, which regulate homeostasis, smooth muscle function, and inflammation [19]. PGI₂ is produced by endothelial cells and influences several cardiovascular processes by increasing adenosine 3’,5’-cyclic monophosphate levels. This mechanism could explain the beneficial effect of iloprost on nonresponders to inhaled iNO and the additive effect of combining both agents [11]. Several drawbacks are encountered regarding the clinical applications of inhaled iloprost in neonates. There are insufficient prospective clinical studies, practical difficulties of aerosolized application into a mechanical ventilator, and uncertainties about optimal dosage and the time to achieve a significant effect in neonates [5,7]. Han et al. [20] reported the therapeutic effect of inhaled iloprost in newborn infants with severe PPHN refractory to iNO but enrolled only a small number of cases retrospectively. Although clinical data are few, inhaled iloprost, single or combined, has been useful and effective as a rescue or additional treatment for improving oxygenation in cases of acute pulmonary hypertension in children, bronchopulmonary dysplasia, postoperative pulmonary hypertensive crisis in children with congenital heart disease, and pulmonary hypertension in preterm infants in addition to PPHN [4,8,11,15,16,21].

Our study could provide useful guidelines on the clinical trials of inhaled iloprost in terms of available dosage and the timing of inhaled iloprost for neonates. Moreover, iloprost might be an alternative drug treatment that improves oxygenation in the absence of iNO. Adverse effects of bronchoconstriction or unstable hemodynamics such as hypotension have sometimes been observed during iloprost nebulization in older children [13]. However, compared to that with the systemic application of PGI₂ [22], the rate of adverse effects with iloprost seems to be negligible. In our study, adverse effects were not observed.

This study has several limitations, that is, the total number of cases may be considered insufficient and they were all from a single center. Given that a control group is not set up, we cannot exclude the positive effects from other applied drugs such as a surfactant and sildenafil.

In conclusion, our clinical experience supports that nebulized iloprost, with or without concomitant oral sildenafil, is a safe and promising therapy for PPHN, specifically when inhaled NO is unavailable. However, larger controlled studies are required to confirm the clinical usefulness of inhaled iloprost and to define its safety and efficacy alone or in combination with other vasodilator drugs.