The lungs of premature and term babies are structurally different from the adult lungs. Preterm lungs are underdeveloped, non-compliant in terms of breathing, often need mechanical ventilation and these patients commonly develop syndromes as a consequence of their prematurity, such as bronchopulmonary dysplasia (BPD). Surfactant protein SP-D could be a therapy for BPD. However, there is a need for an animal model that resembles the structural characteristics of premature lungs to test SP-D and future molecules that will target the newborn population. The aim of this study was to develop and validate a pre-clinical model of early alveolarization and structurally premature lungs in 10-day-old rats, and establish the dose safety and distribution of rhSP-D administered intratracheally to premature lungs.
The stages of lung development in newborn rats are very similar to those in the human lung.1 Rats exhibit lungs at the saccular stage (without alveoli) from postnatal day 0 to 4. Classical “bulk” alveolarization takes place from day 4 to day 21, but it continues until adulthood as a moderate process called “continued alveolarization”.2 During the first days of the early alveolarization, the lungs are transitioning and still exhibit low number of alveoli, and low alveolar and ductal volume.3 Parallel to alveolar development, microvascular maturation takes place throughout the different stages of alveolarization, with a stronger development at the initial period postnatal day 13 to 21.4 The main difference observed between humans and rats in lung development is that humans are born with their lungs in the alveolarization stage, instead of saccular stage as is the case for rats; significant extra alveolarization also takes place after birth in humans.5 Therefore, rat lungs at early postnatal days can be compared to the human lungs from premature infants, which are in a transition from saccular to alveolar stage.6 All these characteristics postulate the lungs of early postnatal age rats as a good animal model of premature and newborn human lungs.
SP-D plays its main role in the innate immune defense of the lungs, although it is also involved in lung surfactant homeostasis7 and the maturation of the postnatal lung surfactant pool regarding lipid sizes.8 In response to an infectious insult in the lung, such as bacterial lipopolysaccharide (LPS), SP-D can decrease pulmonary inflammation by decreasing the number of inflammatory cells, specifically the neutrophilic inflammation, as well as, reducing the pulmonary and circulating levels of pro-inflammatory cytokines (such as IL-6, TNF, IL-1).9 In addition, SP-D also inhibits the lung inflammation caused by mechanical ventilation in premature lambs, which is a relevant animal model of bronchopulmonary dysplasia (BPD).10 Interestingly, SP-D levels in BALF of preterm human babies with neonatal respiratory distress syn-drome (RDS) that lately evolved to BPD were lower compared to healthy babies.11 The anti-inflammatory effects observed with SP-D and its lower concentration in the BALF of BPD patients have postulated rhSP-D as a potential therapy for babies suffering from BPD.
Although other studies have utilized prematurely delivered animals, such as sheep and rabbits, as model systems that mimic premature infants, these models can only be applied for brief studies due to high mortality rates in the first few hours of life. Currently, there is not a reliable animal model for pre-clinical testing of formulations and drugs in the premature infant lung that can be maintained for several days to weeks.12 The establishment of an animal model that mimics the lung structure observed in the premature lung of the human new-born with scarce alveolarization would allow higher quality, more robust and translational pre-clinical studies with investigational drugs that will target that population. The authors hypothesized that the10-day-old rat would serve as a good pre-clinical animal model to test intratracheal administration of compounds. Several pilot and small-scale experiments were performed to test a formulation vehicle, dose safety, repetitive dosing tolerance, and pharmacokinetics distribution using rhSP-D as our study drug, and to probe the suitability of 10-day-old rats as an animal model for pre-clinical studies.
Arroyo et. al.
Annals of Anatomy
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- Burri,2006; Schittny, 2017; Schittny et al., 2008; Tschanz et al., 2014;Zeltner et al., 1987
- Tschanz et al., 2014
- Schittny, 2017; Schittny et al., 2008
- Schittny, 2017
- Burri, 2006; Zoetis and Hurtt, 2003
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- Arroyo et al.,2019; Ikegami et al., 2006; Ikegami et al., 2007; King and Kingma,2011; Yamazoe et al., 2008
- Sato et al., 2010
- Beresford and Shaw, 2003; Bersani et al., 2012;Kotecha et al., 2013
- Downes, 2012