Development of a New Dry Powder Aerosol Synthetic Lung Surfactant Product for Neonatal Respiratory Distress Syndrome (RDS) - Part II: In vivo Efficacy Testing in a Rabbit Surfactant Washout Model

Abstract

Purpose: Surfactant therapy incorporates liquid bolus instillation via endotracheal tube catheter and a mechanical ventilator in preterm neonates with respiratory distress syndrome (RDS). Aerosolized surfactants have generated interest and conflicting data on the efficacy of phospholipid (PL) dose requirements. We developed and characterized a synthetic lung surfactant excipient enhanced growth (SLS-EEG) dry powder aerosol product. In this study, we compare the in vivo performance of the new aerosol product with standard-of-care liquid instillation.

Methods: Juvenile rabbits were sedated, anesthetized, intubated, and ventilated. Endogenous surfactant was depleted via whole lung lavage. Animals received either a standard dose of liquid Curosurf (200 mg PL/kg) instilled via a tracheal catheter, SLS-EEG powder aerosol (60 mg device loaded dose; equivalent to 24 mg PL/kg), or sham control. Gas exchange, lung compliance, and indices of disease severity were recorded every 30 min for 3.5 h and macro- and microscopy images were acquired at necropsy.

Results: While aerosol was administered at an approximately tenfold lower PL dose, both liquid-instilled and aerosol groups had similar, nearly complete recoveries of arterial oxygenation (PaO₂; 96-100% recovery) and oxygenation index, and the aerosol group had superior recovery of compliance (P < 0.05). The SLS-EEG aerosol group showed less lung tissue injury, greater uniformity in lung aeration, and more homogenous surfactant distribution at the alveolar surfaces compared with liquid Curosurf.

Conclusions: The new dry powder aerosol SLS product (which includes the delivery strategy, formulation, and delivery system) has the potential to be a safe, effective, and economical alternative to the current clinical standard of liquid bolus surfactant instillation.

Keywords: infant aerosol therapy; respiratory distress syndrome; surfactant aerosol; surfactant replacement therapy; synthetic lung surfactant.

Fig. 1

Schematic of the Surfactant Aerosol Delivery System (iDP-ADS) used in Rabbits. During aerosol delivery, the animals were removed from the ventilator and delivered aerosol under positive pressure, generated with the DPI actuator (electromechanical timer) and oxygen gas source. The D2 air-jet DPI connected directly to the rabbit ET tube, enabling precise control over the PIP (25 ± 2 cmH₂O), PEEP of 5 cmH₂O, V_T of 7 mL/kg, flow of 3 L/min, and actuation time of ~ 0.20–0.22 s. Each positive pressure actuation of the DPI was controlled by manually depressing a PIP/PEEP valve manifold attached to the outlet of the DPI. The DPI employs an adjustable pressure relief valve (> 25 cm H2O) and an analog pressure manometer, akin to a manual resuscitator, to prevent barotrauma. The aerosolization chamber was manually filled with formulation, and the DPI was actuated to deliver the aerosol and inflate the lungs, followed by an approximately 1-s breath hold, after which exhalation was allowed. The animal was promptly returned to the ventilator following dosing.

Fig. 2

Time schedule for animal experimental procedures. The sequence of procedures for lung lavage to establish surfactant deficiency and lung injury, treatment groups, time schedule for procedures, surfactant dosing, and physiologic measurements are shown. Additional details on the dose delivery protocol and use of the iDP-ADS are provided in Part I of this two-part study.

Fig. 3

Gas Exchange. Gas exchange data are shown as mean ± SEM for control (black squares) and surfactant treatment groups during the 210-min study period. The grey dashed line represents the timepoint (0 min) following the completion of liquid Curosurf (black circles) administration and the initial 30 mg dose of SLS-EEG surfactant aerosol (white circles). Among the surfactant groups, arterial oxygenation was not different at any timepoint following administration, and PaO₂ values fully recovered to the pre-lavage basal levels (96–100%) at 3.5 h. The PaO₂ values at 30 min did not differ between the surfactant groups, but the liquid Curosurf group had higher PaO₂ than did the controls. Both surfactant treatment groups had higher PaO₂ than the sham controls from 60 min until the 3.5-h endpoint. The PaCO₂ values in the controls were increased throughout the experiment, and they did not differ between the groups. The liquid Curosurf group had a higher pH than the controls at 180 and 210-min time points. Two-way ANOVA and post-hoc Tukey’s test compared mean differences at each time point between the groups. *p < 0.05, SLS-EEG aerosol vs. liquid CuroSurf. ⁺p < 0.05, control vs SLS-EEG aerosol. ^#p < 0.05, control vs liquid Curosurf.

Fig. 4

Lung Compliance and Disease Severity. Gas exchange data are shown as mean ± SEM for control (black squares) and surfactant treatment groups during the 210-min study period. The grey dashed line represents the timepoint (0 min) following the completion of liquid Curosurf (black circles) administration and the initial 30 mg dose of SLS-EEG surfactant aerosol (white circles). The SLS-EEG aerosol group had greater compliance than the liquid Curosurf and control groups from the 150-min timepoint until the end of the experiments. At the 3.5-h timepoint, the compliance values in the liquid Curosurf and sham control group were not different, with only 14% improvement from baseline in the liquid group. In contrast, the surfactant SLS-EEG compliance had doubled and recovered to 60% of the pre-washout value. The OI remained low for the control cases over the 3.5-h observation period. In contrast, surfactant SLS-EEG aerosol and liquid Curosurf groups provided values below five within 90 min and near two at 3.5 h. The VEI was higher in the SLS-EEG surfactant group than the controls at 30 and 120-min time points and did not vary between the surfactant groups. Two-way ANOVA and post-hoc Tukey’s test compared mean differences at each time point between the groups. *p < 0.05, SLS-EEG aerosol vs. liquid CuroSurf. ⁺p < 0.05, control vs SLS-EEG aerosol. ^#p < 0.05, control vs liquid Curosurf.

Fig. 5

Postmortem Histological Evaluation of Lung Tissue. Macro and Microscopic Images are shown between the different groups. The macro and microscopic images of the lungs for the surfactant and control groups are illustrated in Fig. 5. The gross macroscopic appearance of the anterior lungs is shown while being ventilated ex vivo with peak inspiratory pressures of 20 cmH₂O (5a-c). The control lungs are underinflated with large areas of discoloration suggestive of pulmonary collapse, edema, and hemorrhage (5a). The liquid Curosurf-treated lungs (5b) show moderate inflation with areas of heterogeneous ventilation and petechial hemorrhage. The surfactant aerosol SLS-EEG-treated lungs are well-inflated with very little tissue damage (5c). The gross appearance of the excised tracheas following medial incision (5d-f), shows the control animal with a well hydrated and patent trachea (5d). The Curosurf-treated animal (5e) shows abundant foamy liquid assumed to be surfactant obstructing the trachea with overflow out of the airway. The aerosol SLS-EEG-treated animal (5f) reveals a patent trachea free from powder residual or agglomeration and well hydrated. Following dissection, the macroscopic appearance of the excised rabbit lungs is a cross-sectional slice of the lung lobes (g-i). In the control subject (g), diffuse hemorrhage with a singular surfactant pool (black triangle) or lung edema fluid on the cut surface of the lung lobe section. The liquid Curosurf (h) lung sample shows a heterogeneous distribution of what appears to be foamy surfactant at the level of the lobar bronchi and in the lung periphery liquid (black triangles). The SLS-EEG aerosol (5i) sample shows a homogenous layer of less foamy surfactant evenly distributed across the internal surfaces of the transected lung lobe.