Relative bioavailability of carbinoxamine and phenylephrine from a retard capsule after single and repeated dose administration in healthy subjects.

The plasma pharmacokinetics of carbinoxamine (CA, CAS 486-16-8) and phenylephrine (PE, CAS 59-42-7) after single dose administration of a retard capsule (Rhinopront) containing 20 mg PE hydrochloride and 4 mg CA maleate, were compared to those of the same active principles given as an aqueous solution. The study was performed in 20 healthy subjects according to a standard crossover design with a one-week wash-out. The pharmacokinetic profile of the active ingredients of the retard capsule was also investigated in the same subjects under repeated dosing conditions (one capsule b.i.d. during 4 days). Blood samples were collected before each administration and up to 36 h after the first and last doses. CA and total PE (free + conjugated) were assayed in the plasma samples by HPLC with coulometric detection and by gas chromatography with electron-capture detection, respectively. The pharmacokinetic parameters obtained after single dose administration indicated an effective slow release of PE and CA with the retard capsule, compared to the solution. Significantly dampened Cmax, delayed tmax and prolonged plateau time were observed. Despite the clear decrease in absorption rate, the two formulations yielded a similar extent of absorption for CA (90% confidence interval of AUC ratio: 61-111%), but not for PE (90% confidence interval of AUC ratio: 56-69%). At steady-state, accumulation of the two active principles apparently followed simple superposition (accumulation index R = 1.6 for PE and 3.9 for CA). The slow absorption pattern of the formulation was maintained at steady-state with tmax and plateau time similar to single dose conditions.