The bioavailability and pharmacokinetics of guanfacine after oral and intravenous administration to healthy volunteers.

Guanfacine is a centrally acting alpha-2 adrenergic agonist. The absolute bioavailability, pharmacokinetics, and renal clearance of this antihypertensive drug were investigated in healthy male volunteers. Eighteen subjects received a single oral or intravenous dose of guanfacine 3 mg in a two-way cross-over study design. Blood samples were obtained before dosing and up to 72 hours after dosing for determination of drug levels in plasma. Additional blood samples were obtained for protein binding studies. Urine was collected and pooled for specific intervals up to 96 hours after dosing. The absolute bioavailability of guanfacine after a single oral dose was 81.1%. The elimination half-lives were 13.8 hours and 13.4 hours after oral and intravenous administration, respectively. The volume of distribution results were approximately 6 L/kg by both routes of administration. The mean plasma protein binding results were 71.6%, not influenced by plasma concentration or route of administration. The urinary recovery of guanfacine was 44.3% after oral dosing and 50% after intravenous dosing. Renal clearance of guanfacine was 50% of total body clearance and appeared to be due to a net renal tubular secretory process.