Absorption of Valproic Acid Suppositories in Human Volunteers

The absorption of valproic acid administered by rectal suppository was studied in six male volunteers. Valproic acid was incorporated into a synthetic lipid base in our pharmacy. Each subject received 500 mg of valproic acid by rectal suppository and 500 mg of oral valproate sodium syrup one week apart; 13 blood samples were drawn to determine serum concentrations over 48 hours after administration of each formulation. Significant differences were evident in the amount absorbed, maximum serum concentration, and time to achieve maximum serum concentration between the oral and rectal formulations. Mean absorption after rectal suppository was 80%. Maximum serum concentration was 43.4 mg/L after oral administration and 29.2 mg/L after rectal suppository. The time to achieve maximum serum concentration was 1.0 hour after oral syrup and 3.1 hours after rectal suppository. Absorption of the rectal suppository was consistent and complete within 3 hours. The use of valproate sodium in rectal suppository form can be a more convenient and satisfactory method of administering valproic acid when the oral route is impossible. Dosage increases may be necessary, and serum concentrations should be monitored.