Rectal and intravenous propranolol infusion to steady state: kinetics and beta-receptor blockade

The effects of intravenous propranolol infusion for 24 hr was compared with those of zero-order rectal administration by an osmotic delivery system in six healthy subjects. In plasma and urine, levels of propranolol, 4-OH-propranolol (4-OH-P), and conjugates were determined just before and at 6 hr and at 20 hr during drug administration. With the rectally applied osmotic delivery system providing zero-order release, fairly constant steady-state levels of propranolol in plasma were produced within 12 to 15 hr (four to five times elimination t1/2). The mean steady-state levels were 25 ng/ml after 1.1 micrograms/min/kg rectally and 60 ng/ml after 0.8 micrograms/min/kg IV. The mean rectal systemic availability was 33%; the elimination t1/2s for the two routes did not differ. The results of analysis of plasma for metabolites indicate that after rectal propranolol different metabolic pathways are followed and that there is partial avoidance of first-pass elimination. The isoproterenol challenge resulted in a reproducible assessment of beta-receptor blockade that was closely related to the propranolol concentration in plasma in all subjects and for both routes of administration. With rate-controlled release of propranolol from an osmotic delivery device, the rectal route provides an alternative to intravenous infusion to achieve constant steady-state propranolol concentrations. This may be useful for research purposes or during the perioperative period in surgical patients.