Metabolic fate of phenobarbital. A quantitative study of p-hydroxyphenobarbital elimination in man.

Biotransformation of phenobarbital (PB) to p-hydroxyphenobarbital (PHPB) was studied quantitatively by gas-liquid chromatography in 8 epileptic patients who were receiving an established regimen of antiepileptic drugs including PB. PB and both conjugated and unconjugated PHPB were present in each patient's urine; m-hydroxyphenobarbital (MHPB) was not detected despite an assay sensitivity of 0.25 mug/ml. Incubation of the urine with beta-glucuronidase, but not with arylsulfatase, liberated PHPB which was, therefore, presumed to be conjugated with glucuronic acid. In general, the patients' urine contained more PB than total PHPB. Recovery of the patients' total daily dose of PB ranged from 24 to 77% (mean, 42%). After receiving a single iv dose of PB, PB and both conjugated and unconjugated PHPB were found in a normal volunteer's urine throughout a 16-day collection period; 30% of the dose was recovered. PB excretion was proportional to urine volume in the volunteer and in two additional patients who were made to vary their daily fluid intake. PHPB was not detected in the cerebrospinal fluid of 10 patients receiving PB. Neither PB, PHPB, nor MHPB were detected in the feces of four patients. These results suggest that metabolites other than PHPB or MHPB may be important in the elimination of PB in man.