SELECTIVE CLARA CELL INJURY IN MOUSE LUNG FOLLOWING ACUTE ADMINISTRATION OF COUMARIN

Coumarin is a known hepatotoxicant in laboratory animals, particularly rats. However, the mouse lung was identified as a major target organ in a chronic bioassay, with an oral gavage dosage of 200 mg/kg coumari n increasing the incidence of alveolar/bronchiolar adenomas and carcin omas. The purpose of the present work was to determine whether coumari n was acutely toxic in the mouse and rat lung. Male and female B6C3F1 mice were dosed orally by gavage with coumarin at 0, 10, 20, 50, 100, 150, and 200 mg/kg and lung toxicity was determined 24 h later by hist ological evaluation. The results indicated that coumarin dosages great er than or equal to 150 mg/kg caused selective injury to Clara cells i n the distal bronchiolar epithelium. The time course of this injury wa s studied from 6 h to 7 days after a single dosage of coumarin (200 mg /kg). At 12 h after dosing, Clara cell swelling was apparent along wit h the onset of necrosis and bronchiolar epithelial disorganization. At 24-48 h, necrotic Clara cells were observed sloughed into the lumens of the terminal bronchioles, with concomitant thinning of the epitheli um and flattening of the remaining ciliated cells. By 72-96 h, there w as epithelial hypertrophy and hyperplasia, and by 7 days after dosing, the Clara cells had regenerated and the bronchiolar epithelial archit ecture appeared nearly normal. Unlike the mouse, oral administration o f coumarin (200 mg/kg) caused severe hepatotoxicity in male F344 rats, seen histologically as centrilobular necrosis and associated with inc reases, up to 140-fold, in serum ALT, AST, and SDH levels. Clara cell toxicity was not observed in the distal bronchioles of treated rats. H owever, in the upper airways, coumarin treatment produced generalized epithelial necrosis involving both ciliated and nonciliated cells. 3,4 -Dihydrocoumarin (DHC), which is not a mouse lung carcinogen, did not cause Clara cell injury when dosed to mice at 800 mg/kg. This finding suggests, because DHC lacks a 3,4-double bond, that bioactivation of c oumarin to a 3,4-epoxide intermediate may contribute to mouse lung Cla ra cell toxicity. Collectively, the results indicate that coumarin is a Clara cell toxicant and establish the mouse lung as a target organ f or coumarin toxicity. These new findings lay the foundation for studie s to determine the mechanisms of coumarin-induced toxicity and carcinc ogenicity and to define the relevance of these effects to humans. (C) 1998 Academic Press.