Isomerisation and urinary excretion of proxibarbal and valofan in man; a preliminary study

Proxibarbal and valofan are tautomeric drugs which interconvert rapidly in solution. In the present study, the urinary excretion of these two drugs was investigated in two subjects after separate oral administration. In both cases, only proxibarbal was found in urine, while the excretion of valofan was negligible or non-detectable. More than half of a dose remains unaccounted for after proxibarbal administration, and more than three quarters after valofan administration. A simple chemical equilibrium of tautomerism as found in vitro is insufficient to account for the excretion kinetics of the two drugs in humans.