Gastrointestinal absorption and metabolism of two 35S-labelled ampicillin esters

Two ampicillin esters, 35S-pivampicillin and 35S-carampicillin and polyethylene glycol (nonabsorbable marker) were given orally to healthy subjects with gastrointestinal tubes. The cumulative absorption of radioactivity in both compounds (60-90%) was higher than (25-67%) previously found after administration of 35S-ampicillin. The pek plasma levels of radioactivity were reached earlier and were about twice as high as in the latter study. The amount of radioactivity excreted in urine was about the same as that absorbed from the proximal part of the gastrointestinal tract. Both 35S-pivampicillin and 35S-carampicillin were partly hydrolyzed in the stomach and upper small intestine and labelled ampicillin was released. They were also decomposed after absorption since all the radioactivity recovered from blood and urine appeared to be attached to ampicillin and ampicillin metabolites. Studies in vitro indicated that ampicillin esters absorbed intact may be hydrolyzed not only in the blood but also in the intestinal wall and the liver.