Lack of presystemic metabolism of nifedipine in the rabbit.

In humans, oral bioavailability of nifedipine has been reported to be around 60%, although the organ(s) contributing to its first-pass metabolism have not been determined. The aim of this study was to determine in vivo, in anesthetized and conscious rabbits the role of the intestine, liver, and lungs in the first-pass metabolism of nifedipine. To assess the extraction of nifedipine by the intestine, liver, and lungs, nifedipine was administered before and after each organ, and serial blood samples were withdrawn from an artery. In conscious rabbits, the systemic clearance of nifedipine injected into a lateral vein of an ear was 14.6 +/- 1.6 ml/min per kg, a value that was slightly decreased by anesthesia. In anesthetized rabbits, compared to the clearance estimated when nifedipine was administered into the thoracic aorta, the administration of nifedipine into a jugular vein, into the portal vein, or into the portal vein, or into the duodenum did not increase the value of the systemic clearance. In conscious rabbits, the clearance of nifedipine estimated when the drug was administered into the duodenum, the peritoneum, the portal vein, or into the jugular vein was identical to the clearance calculated when the drug was injected into the thoracic aorta. In vitro, nifedipine was metabolized in liver and intestinal epithelial cells homogenates but not in lungs or kidneys. We concluded that in the rabbit, oral nifedipine is not subjected to a first-pass metabolism, even though the intestine and the liver may contribute to nifedipine systemic clearance.