METABOLISM In experimental studies using human liver S9 Arbaclofen placarbil was not shown to be a substrate for CYP1A2, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. ; ; Arbaclofen placarbil, the acyloxyalkyl carbamate prodrug of R-arbaclofen, is believed to undergo hydrolysis by the esterase enzyme human carboxylesterase-2 into the parent amine, R-baclfen. Carbon dioxide, isobutyric acid, isobutyraldehyde, are also expected to be produced in equimolar quantities. ; ; The productions of isobutyric acid has been confirmed in vitro untilizing mass spectrometry and gas chromatography.

ABSORPTION Unlike baclofen, absorption of R-baclofen(arbaclofen) is not limited to the upper small intestine. The ability of arbaclofen to be absorbed throughout the gastrointestinal tract allowed for the development of the sustained release formulation, arbaclofen placarbil (AP). ; ; In one study of AP absorption in 10 healthy volunteers, one 20mg oral dose of AP, in the presence of food, resulted in a Tmax of 5.05h. ; ; The oral bioavailability of R-baclofen in rats when AP was dosed at 10mg/kg was 44 ± 12%, and when dosed at 1mg/kg, oral bioavailability was 68 ± 6%. ; ; In monkeys and dogs, the oral bioavailability of R-baclofen when AP was orally dosed was high: 94 ± 16%, and 92 ± 7%, respectively. In comparison, when oral R-balofen was dosed oral bioavailability was 39 ± 21% in monkeys and 49 ± 20% in dogs.; ; Colonic absorption studies measuring R-baclofen bioavailability post intracolonic dosing in rats and monkeys, have revealed low bioavailability with the administration of R-baclofen (7 ± 3% and 3 ± 2%, respectively), and significantly higher R-baclofen bioavailability with intracolonic dosing of AP suspension ( 37 ± 9% and 37 ± 15%, in rats and monkeys respectively). ; Intracolonic dosing of AP suspension also resulted in high biolavailability of R-baclofen in dogs (77 ± 23%). ; ; Absorption throughout the intestine is both passive and active and occurs via the monocarboxylate type 1 transporter.