Metabolism of butalbital, 5-allyl-5-isobutylbarbituric acid, in the dog.

Butalbital, 5-allyl-5-isobutylbarbituric acid, labeled in the 2-position with 14C, was administered to dogs. Ninety-two percent of the radioactivity of the dose was excreted in the urine. The drug and three major urinary metabolites were identified in the urinary excretion of the dog. The major metabolite was 5-isobutyl-5-(2,3-dihydroxypropyl)barbituric acid, which accounted for 50.2% of the dose. Smaller amounts of the unchanged drug (2.6% of the dose) and urea (8.6% of the dose) were present. 5-Allyl-5-(3-hydroxy-2-methyl-1-propyl)barbituric acid, formed by omega-hydroxylation, accounted for 10.1% of the dose; the optical rotation of the 1,3-diethyl derivative was [alpha]D20 = +10.5. Five minor and unidentified metabolities accounted for an additional 10.7% of the dose. A total of 82.2% of the dose was accounted for.