###### Abstract  
High-flow nasal cannula (HFNC) is a non-invasive respiratory support (NRS) modality to treat premature infants with respiratory distress syndrome (RDS). The delivery of nebulized surfactant during NRS would represent a truly non-invasive method of surfactant administration and could reduce NRS failure rates. However, the delivery efficiency of nebulized surfactant during HFNC has not been evaluated in vitro or in animal models of respiratory distress. We, therefore, performed first a benchmark study to compare the surfactant lung dose delivered by commercially available neonatal nasal cannulas (NCs) and HFNC circuits commonly used in neonatal intensive care units. Then, the pulmonary effect of nebulized surfactant delivered via HFNC was investigated in spontaneously breathing rabbits with induced respiratory distress. The benchmark study revealed the surfactant lung dose to be relatively low for both types of NCs tested (Westmed NCs 0.5 \(\pm\) 0.45%; Fisher & Paykel NCs 1.8 \(\pm\) 1.9% of a nominal dose of 200 mg/kg of _Poractant alfa_). The modest lung doses achieved in the benchmark study are compatible with the lack of the effect of nebulized surfactant in vivo (400 mg/kg), where arterial oxygenation and lung mechanics did not improve and were significantly worse than the intratracheal instillation of surfactant. The results from the present study indicate a relatively low lung surfactant dose and negligible effect on pulmonary function in terms of arterial oxygenation and lung mechanics. This negligible effect can, for the greater part, be explained by the high impaction of aerosol particles in the ventilation circuit and upper airways due to the high air flows used during HFNC. P 2022141093100/pharmacecutsci14051093111 April 202218 May 2022141093100/pharmacecutsci1405109311 April  
of the nasopharyngeal dead space, the reduced work of breathing, and the delivery of a distending pressure have been endorsed as the main advantages of this NRS modality [4; 5]. Contrary to pressure-controlled NRS devices, such as nasal continuous positive airway pressure (CPAP), the pressure delivered during HFNC is not continuously measured, although it correlates with the preset air flow rate [6; 7]. The use of NRS reduces the need for mechanical ventilation and intratracheal surfactant administration. Unfortunately, a significant number of spontaneously breathing preterm infants fail NRS and require mechanical ventilation and/or late intratracheal surfactant. For instance, CPAP failure has been reported to occur in over 40% of infants with a gestational age (GA) below 29 weeks [8; 9]. Similarly, [PERSON] et al. reported a 30% HFNC failure in a cohort of infants (n = 97) with a GA > 30 weeks, who eventually required other types of NRS or mechanical ventilation [10]. These preterm infants usually receive intratracheal surfactant later in the course of RDS, when the benefits of therapy may be attenuated compared with the early surfactant administration [11]. The aerosol delivery of surfactant during NRS would represent a non-invasive method of surfactant administration, enabling timely surfactant treatment [12]. Studies in animal models of induced respiratory distress supported with CPAP have demonstrated that nebulized surfactant can be as effective as surfactant instillation in improving arterial gas exchange and lung mechanics [13; 14; 15]. Moreover, scintigraphy studies confirmed the feasibility of surfactant nebulization during CPAP, revealing a mean lung deposition of 11.4% in healthy non-human primates [16] and 15.9% in newborn piglets [17]. Early pilot clinical studies on surfactant nebulization during CPAP demonstrated the feasibility and the safety of this therapy [18; 19; 20]. However, these initial studies were rather heterogeneous, used jet nebulizers, different surfactant preparations and administration protocols, and included a small number of patients with various GAs.