Short-period double-dosing for simultaneous evaluation of intestinal absorption and hepatic disposition in a single conscious rat using cephalexin as test drug.

A new method has been developed for simultaneous evaluation of local absorption from the intestine into the portal system and local disposition through the liver, and for assessment of the bioavailability of a drug in a single conscious rat. The method is based on the difference between plasma concentrations in portal and systemic blood (PS method). Because cephalexin is known to be absorbed completely from the intestine and not to be eliminated through the liver, it was used as test drug to confirm the validity of the new method. The portal vein and the femoral artery of a rat were simultaneously cannulated and blood samples were obtained from both sites. Two methods of administration, single-dosing and double-dosing, were investigated and the efficacy of double-dosing (DD) was demonstrated. Rats received an intra-arterial (group A) or oral (group B) dose in single-dosing, whereas rats used for double-dosing received an oral dose 3 h after an intra-arterial dose (group C). After administration of cephalexin, the portal and arterial plasma concentrations were determined by HPLC. Groups A and B were monitored for 4 h and group C for 8 h. The portal-blood flow rate was measured by means of an electromagnetic flow-meter. Global and local moments were calculated by trapezoidal integration with extrapolation to infinite time. On the basis of the PS method, the local absorption ratio (Fa) and the mean local absorption time (t(a)) were estimated to be 0.975 +/- 0.104 and 2.19 +/- 0.51 h, respectively, in group B. By comparing the averaged moments between groups A and B, the extent of bioavailability (F), the mean absorption time (MAT) and the hepatic recovery ratio (FH) were calculated to be 1.01, 1.92 h and 1.04, respectively. The mean hepatic transit time (tH) was negligible. In group C, Fa = 0.936 +/- 0.107, tH = 1.55 +/- 0.32 h, F = 1.08 +/- 0.07, MAT = 1.55 +/- 0.40 h and F(H) = 1.17 +/- 0.14 h, the mean values being close to those from groups A and B. In conclusion, the PS method with short-period double-dosing (PS-DD method) can offer an effective means of evaluating the local absorption kinetics of drugs, because F, MAT and F(H) are obtained from a single conscious rat, and consequently the standard deviations of the quantities can be quickly estimated.