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The extent of "first-pass" elimination of racemic propranolol and dextropropranolol in doses of 0.25 or 0.50 mg was investigated in relation to the site of drug administration in the rectum of rats. The compounds were given orally, i.v., and rectally at distances of 2 and 1 cm from and directly at the anus by low volume zero-order 30 min infusion. Unchanged propranolol was determined in blood, and propranolol and three metabolites were measured in urine. The systemic availability of propranolol after oral administration was approximately 6 %. Rectal administration at 2 cm, at 1 cm and directly at the anus (0.2 cm) gave two, three and six times higher values, respectively. The more distal application site produced urinary metabolite profiles that were comparable to those observed after oral administration, while application directly at the anus was similar to i.v. dosing. In all experiments log-linear elimination phases with comparable elimination half-lives (range 12-18 min) were found, except with the 0.50 mg dose after i.v. and rectal administration close to the anus which showed a non-linear profile. The mean systemic availability after rectal administration of 0.25 mg dextro-propranolol close to the anus was 50 and 64 % as compared to a 0.25 and 0.125 mg i.v. dose, respectively. The rectal route may be used for propranolol to partially prevent hepatic first-pass metabolism. However, avoidance of presys-temic elimination is maximal only in the immediate vicinity of the anus as the venous blood supply of the upper part of the rectum of rats appears to be connected to the portal system and the lower part to the general circulation. |
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