Enalapril is an angiotensin converting enzyme (ACE) inhibitor that is
frequently used in human, feline and canine patients with cardiac disease. Its
use has been associated with impotence in human patients. The purpose of this
study was to evaluate if enalapril (0.5 mg/kg PO, q24h) is likely to alter behavior
in stallions and to assess its effect on ACE activity at the standard dose used in dogs
and cats. Twelve pony stallions were evaluated by physical examination and echocardiography
followed by treatment with enalapril (n ¼ 6) or placebo (n ¼ 6) for 2
months. After one month, blood was drawn and stored to evaluate ACE activity
in the 2 groups. At the end of the study, repeat physical examination and echocardiography
were performed. Physical examination, echocardiographic indices, and
reproductive performance were unchanged and there was no suppression of ACE
activity. Results of this study suggest that enalapril (0.5 mg/kg PO, q24h) is either
poorly absorbed in the horse or is inadequately converted to the active form of the
drug, enalaprilat.