Assessing lung function and pharmacokinetics in neonatal and young rats
Introduction: Young, developing and mature, adult organisms differ drastically in their physiological abilities and adaptive responses. Furthermore, organ development occurs in different time scales in different species which enables the researcher to cover human development in relatively short animal studies. In the context of chemical risk assessment and pediatric drug approval, new methods need to be developed and basic data need to be generated to enable appropriate study designs especially focused on juvenile animal-toxicity. Methods: Pulmonary function was determined noninvasively in male and female juvenile rats (Wistar WU) with a head-out body plethysmograph system at an age of 2-50 days. The conscious, spontaneously breathing rats were placed in the body plethysmographs while the head of each animal protruded through a neck collar into a ventilated head chamber. After reaching a steady state in respiration, typical lung function parameters were recorded for approximately 15 min. In a second approach, the feasibility of nose-only inhalation exposure (2 h/day) was tested with verapamil hydrochloride as a model compound in neonatal rats at an age of 4-21 days. End points for the evaluation were clinical signs, body weight gain during development, substance serum levels for kinetic analysis and possible cross-contamination between dams and littermates. Results: In the lung function measurements, tidal volume as well as tidal midexpiratory flow EF50 and respiratory minute volume increased markedly (~ 20 fold) between day 2 and 50 post partum (pp). Lower changes were found in respiratory frequency, inspiratory and expiratory time. All physiological changes were based on increasing age and body weight and no major differences between male and female rats were observed. The measurements proved to be feasible, no injuries or mortalities occurred among the tested animals. In the nose-only inhalation, the results gave no indication of adverse clinical signs in neonates due to daily maternal deprivation during exposure. No increased mortality was observed. The cumulative body weight gain in the treatment groups vs. cage control was comparable. An efficient serum recovery of verapamil as early as day 4 pp was achieved. Surprisingly, cross-contamination analysis gave no evidence of a carry-over via lactation and maternal behaviour or between littermates. Conclusions: Two methods have been developed to noninvasively assess juvenile rat lung function starting at a neonatal stage. A plethysmographic method has been modified and longitudinal data are presented which describe the functional development of the lungs in male and female rats starting at an age of 2 days, showing the possibility to use this technique at very early stages of development. Furthermore, the practicability of nose-only inhalation treatment in juvenile rats as young as day 4 pp has been proven. Both techniques permit new insights into human neonatal risk assessment and therefore these animal models are suitable for regulatory studies.