INHALATION ADMINISTRATION OF ALL-TRANS-RETINOIC ACID FOR TREATMENT OF ELASTASE-INDUCED PULMONARY EMPHYSEMA IN FISCHER 344 RATS

Abstract

A past study demonstrated that all-trans-retinoic acid (ATRA) treatment by intraperitoneal injection in a rat model of elastase-induced emphysema caused tissue regeneration as evidenced by a decrease in alveolar size and lung volume and an increase in alveolar number. We postulated that treatment with this retinoid by nose-only inhalation exposure would be a more efficient means of targeting damaged lung tissue. Emphysema was induced in male Fischer 344 rats by intratracheal instillation of pancreatic elastase (0.5 IU/g body weight). Four weeks after elastase instillation, animals were treated once daily, 4 days/week, for 3 weeks by exposing them nose-only to aerosolized ATRA (target concentration-time of 3000 or 15,000 mg-min/m3) or by injecting them intraperitoneally with ATRA in cottonseed oil (0.5 or 2.5 mg/kg). Based on estimates of particle deposition in the respiratory tract, inhalation doses were chosen to be consistent with injected doses. Lungs were fixed by inflation with formalin (constant pressure for 6 hours followed by >48 hours of immersion) and were embedded in paraffin. Sections were evaluated by histopathology and stereology. Inhalation exposure to ATRA at both aerosol concentrations caused significant elevations of ATRA in the lung, whereas only the high-dose injection treatment was associated with an elevation of lung ATRA. The mean ATRA concentration from lungs of rats in the high-dose inhalation exposure groups as measured by liquid chromatography--mass spectrometry was approximately 12-fold greater than that of high-dose injection-treated rats. Elastase instillation caused increased lung volumes, irregular alveolar air space enlargement, and fragmentation and attenuation of alveolar septa. Neither inhaled nor injected ATRA reduced the enlarged lung volumes associated with this emphysema model. Stereology demonstrated that alveolar air space enlargement in ATRA-treated rats was similar to that in sham-treated emphysematous animals. Thus, while inhalation treatment caused greater levels of the drug in lung tissue in comparison to that of injection-treated animals, treatment with ATRA by either route of administration did not cause a reversal of lung tissue damage in this model of elastase-induced emphysema.

Mechanisms of Lung Development: Contribution to Adult Lung Disease and Relevance to Chronic Obstructive Pulmonary Disease

“…However, there are also studies that failed to show benefits of RA in emphysema (45)(46)(47)(48)(49). The reason for the conflicting findings is unclear.…”

All-trans retinoic acid results in irregular repair of septa and fails to inhibit proinflammatory macrophages

“…Some studies in the elastase-or cigarette smoke-induced rat emphysema model reported the reversal of airspace enlargement [6][7][8] which, however, were mainly based on mean linear intercept measurements, a highly controversially discussed parameter [36]. Focussing on elastase-and cigarette smoke-exposure models of emphysema, only one study using design-based stereology convincingly demonstrated the regeneration of lost alveolar septa after ATRA-therapy [7], whereas others did not find an effect of ATRA on emphysema relevant histological parameters in mice, rats or guinea pigs [9][10][11][12][13]. Limited reversal of functional parameters and of an aerosol-derived measure of airspace size was seen in a papain-induced model of emphysema in dogs [38].…”

“…In addition, several experimental studies in rats and mice suggested that regeneration of lost alveolar septa can be achieved by ATRA in elastase-or cigarette smoke-induced emphysema [6][7][8]. However, a number of studies failed to demonstrate a beneficial effect of ATRA in other emphysema models [9][10][11][12][13][14], and beneficial effects in human patients have been marginal so far [15][16][17][18]. The reason for these discrepancies is still unclear [19].…”

Animal models of pulmonary emphysema: a stereologist's perspective

“…To date, systemic application of all-trans-retinoic acid is the only therapeutic intervention that has been demonstrated by means of unbiased stereological tools to reverse some of the effects of elastaseinduced emphysema in rats [31]. However, subsequent studies from other laboratories failed to reproduce these findings in rat [105], mouse [69,106,107] and rabbit [108]. No effect of alltrans-retinoic acid was seen in guinea pigs exposed to smoke [70].…”