1. Administration of dichloralphenazone, a complex of chloral hydrate and phenazone (antipyrine) caused a fall in steady-state plasma warfarin concentration and loss of anticoagulant control in five subjects.

2. This effect of dichloralphenazone is due to stimulation of the drug-oxidizing enzymes of the liver endoplasmic reticulum by antipyrine, the non-hypnotic part of the complex. Administration of antipyrine caused a fall in steady-state plasma warfarin concentration in five subjects, a shortening of the plasma warfarin half-life, with increased urinary excretion of the metabolites of 14C-labelled warfarin in two subjects and increased urinary excretion of 6β-hydroxycortisol which is formed in the liver endoplasmic reticulum.

3. Administration of chloral hydrate, the hypnotic part of dichloralphenazone, caused no change in anticoagulant control but a fall in steady-state plasma warfarin concentration in five subjects. This is due to the accumulation of trichloroacetic acid which displaces warfarin from plasma protein binding sites.

4. Individual differences in the extent of enzyme induction have been shown to be related to the subjects' rates of drug oxidation.

5. In the rat administration of dichloralphenazone and antipyrine, but not chloral hydrate, caused shortening of pentobarbitone sleeping time and of the plasma [14C]pentobarbitone half-life, shortening of the zoxazolamine paralysis time and increase in the maximal velocity of N-demethylation of ethylmorphine.

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