Pharmacology: Pharmacokinetics: Ampicillin is relatively stable in the acid gastric section and is moderately well absorbed from the gastro-intestinal tract after oral administration. Food can interfere with the absorption of Ampicillin so doses should preferable be taken at least 30 minutes before meals. Peak concentrations in plasma are obtained in about 1 to 2 hours and following a dose of 500 mg by mouth are reported to range from 2 to 6 μg per mL. Ampicillin is given by injection as the sodium salt and following the intramuscular administration of 500 mg peak plasma concentrations occur within about 1 hour and reported to range from 7 to 14 μg per mL.
Ampicillin is widely distributed and therapeutic concentration can be achieved in ascetic, pleural and joint fluids. It crosses the placental into the fetal circulation and small amounts are excreted in breast milk. There is little diffusion into the cerebrospinal fluid except when the meninges are inflamed. About 20% is bound to plasma proteins and the half-life is about 1 to 1.5 hours, but this may be increased in neonates and the elderly; in renal failure half-lives or 7 to 20 hours have been reported.
Ampicillin is metabolized to some extent to penicilloic acid which is excreted in the urine.
Renal clearance of Ampicillin occurs partly by glomerular filtration and party by tubular secretion; it is retarded by the concomitant administration of probenecid. About 20 to 40% of an orally administered dose may be excreted unchanged in the urine in 6 hours; urinary concentrations have ranged from 0.25 to 1 mg per mL following a dose of 500 mg, following parenteral administration about 60 to 80% is excreted in the urine within 6 hours. Ampicillin is removed by haemodialysis. High concentrations are reached in bile; it undergoes enterohepatic recycling and some is excreted in the feces.