Bioavailability of enteral tramadol formulations. 1st communication: capsules.

The absolute bioavailability of tramadol hydrochloride (rac-1(e)-(m-methoxyphenyl)-2-(e)-(dimethylaminomethyl)cyclohexan- 1(a)-ol hydrochloride, CG 315) after the oral administration of Tramal capsules was determined in a balanced cross-over study in 10 male volunteers. Each volunteer received two single doses of 100 mg tramadol hydrochloride, one by oral (2 Tramal capsules) and one by intravenous route (2 ampoules of Tramal 50 solution for injection). The formulations were administered in the morning on an empty stomach, and the interval between the two applications was one week. Serum concentrations of tramadol were determined by gas chromatography-mass spectrometry and the bioavailability was ascertained by calculation of the areas under the serum concentration curves. The absolute bioavailability of tramadol in Tramal capsules was 68 +/- 13% (means +/- SD; n = 10) with a range of 41-84% and a 95% confidence interval of 55.0-79.2%. The areas under the serum concentration curves of tramadol hydrochloride (AUC) were 2488 +/- 774 ng X h/ml (p.o.) and 3709 +/- 977 ng X h/ml (i.v.). Peak serum concentrations of 280 +/- 49 ng/ml were reached 2 h after oral administration of two Tramal capsules; a serum concentration of 100 ng/ml (assumed as the threshold value for analgesic efficacy) was reached after 0.68 +/- 0.17 h and was maintained for 9.0 +/- 2.2 h. The half-life of absorption was 0.38 +/- 0.18 h and the lag-time 0.48 +/- 0.14 h. In the terminal phase the biological half-lives of tramadol were 5.1 +/- 0.8 h (p.o.) and 5.2 +/- 0.8 h (i.v.).(ABSTRACT TRUNCATED AT 250 WORDS)