Disposition of S-1108, a new oral cephem antibiotic, and metabolic fate of pivalic acid liberated from [pivaloyl-14C]S-1108 in rats and dogs.

[pivaloyl-14C]S-1108, which is 14C labeled at the pivalic acid moiety of the pivaloyloxymethyl side chain of S-1108, was administered orally to rats and dogs, and the disposition of pivalic acid cleft from S-1108 was examined. Besides pivaloylcarnitine and pivaloylglucuronide, pivaloylglycine was identified in dog urine as a metabolite of pivalic acid by thin-layer chromatography and high-performance liquid chromatography analysis. The concentrations in the plasma of rats to which doses of 6.65, 26.6, and 532 mg/kg of body weight were administered showed dose-proportionate levels. The radioactivity was eliminated rapidly, with a half-life of approximately 3 h until 24 h at both the 6.65- and 26.6-mg/kg doses. Free pivalic acid in plasma accounted for more than 80% of the concentration of radioactivity. Radioactivity was distributed throughout the body and was eliminated quickly at a rate similar to that of radioactivity from plasma. Most of the absorbed radioactivity was excreted in the urine, and it was completed within 24 h after administration. In dogs, the half-life of radioactivity in plasma was longer than that in the rats. The ratio of free pivalic acid in plasma was 60 to 70% of the radioactivity in plasma. The concentration of radioactivity in the liver, cortex of the kidney, and skeletal muscle 144 h after oral dosing was more than 10 times higher than the concentration in plasma for all doses. Urinary excretion in dogs was slower than that in rats. The differences in the disposition of pivalic acid between dogs and rats may account for differences in the degree of skeletal muscle disorders. The safety in humans of S-1108 given at 200 mg three times a day is discussed in relation to the metabolic formation of the carnitine conjugate of pivalic acid and the reduction of the carnitine concentration in plasma.