Pharmacokinetics and multiple peaking of acebutolol enantiomers in rats

Acebutolol (AC) is a chiral beta-blocker which is extensively metabolized to an active, chiral metabolite, diacetolol (DC). Similar to some other beta-adrenoceptors, AC exhibits multiple peaks in plasma concentration-time curves after oral doses to humans. We examined the suitability of the rat as an animal model and studied the effect of various factors including the route of administration, food, and bile on the pharmacokinetics of AC enantiomers. Pharmacokinetics of AC were delineated after oral (fed and fasted), i.v., and i.p. doses, and after oral and i.v. doses, to intact and bile-duct-ligated female Sprague-Dawley rats, respectively. The possibility of intestinal metabolism or saturable absorption was studied in vitro using everted rat gut. Multiple peaks were present but only after oral doses independent of food intake, suggestive of gastrointestinal tract involvement. Oral absorption of AC enantiomers was incomplete as bioavailability was lower after oral (R, 0.59; S, 0.63) as compared to i.p. off(R, 0.86; S, 0.84) doses. Food reduced bioavailability by 60%. A 250-fold increase in the dose did not alter the absorption kinetics of AC through the everted gut, ruling out the possibility of saturable absorption. No intestinal metabolism was detected in vitro. Enterohepatic recirculation cannot be responsible as ligation of the bile duct did not alter the pattern or route dependence of the multiple peaking. The rat appears to be a suitable animal model; a bile- and food-independent erratic absorption is probably responsible for the observed multiple peaking of AC.