Stereospecific Absorption and Degradation of Cephalexin

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Abstract
Stereospecific absorption and degradation of two stereoisomers about the α-amino group at the 7-position of cephalexin (CEX) have been investigated in the rat intestine. The L-isomer (L-CEX) was not found to be present either in serum or urine after oral administration, but the D-isomer (D-CEX) was well absorbed. In contrast to the saturable uptake of D-CEX (Kt = 10.54 ± 1.73 mM, pH = 6.0) by the in-vitro everted intestinal sac, no appreciable uptake of L-CEX was observed. However, L-CEX competitively inhibited the uptake of D-CEX by the in-vitro everted intestine and the inhibitory constant (Ki) of L-CEX was determined to be 0.67 ± 0.09 mM. L-CEX was rapidly degraded in-vitro in the intestinal tissue homogenate, serum and urine, while there was no appreciable degradation of D-CEX. Analysis of the major metabolite of L-CEX by high-performance liquid chromatography identified it as 7-aminodeacetoxycepha-losporanic acid (7-ADCA). Furthermore, 7-ADCA was detected in serum after oral administration of L-CEX, indicating significant absorption of L-CEX as well as D-CEX. The results obtained suggest that both L- and D-CEX can be absorbed through the intestinal brush-border membrane via the same mechanism, most likely through the dipeptide transport system, and that the affinity of L-CEX to the carrier system is higher than that of D-CEX. However, owing to L-CEX having a higher affinity to the hydrolysing enzymes that are present in the intestinal tissue, serum, and urine, intact L-CEX was immediately hydrolysed after absorption and was not detectable in these samples. These results, demonstrating stereospecific transport and metabolism of CEX, provide strong evidence for a mechanism of carrier-mediated uptake in the absorption of CEX.