Biphasic effect of pyrazolone derivatives on drug-metabolizing enzyme in rat liver.

The pyrazolone derivatives aminophenazone, phenazone, and propyphenazone known as inducers are capable of inhibiting initially monooxygenase-dependent biotransformation steps. In the dose of 1.5 mmol X kg-1 they prolong the hexobarbital narcosis (max. 1 h after administration) due to a reduced hexobarbital metabolism in the liver. An influence on the N-demethylation (aminophenazone as substrate) ist not substantial. The catalytic binding site of cytochrome P-450 is not influenced. In aminophenazone- and phenazone-treated animals the inhibitory phase is followed by a stimulatory one. After the repeated administration the inhibitory phase continues up to the third measured 1 h after administration. The initially inhibitory effect of inducers seems to be caused by a mutual interference of a complete or partial binding to various forms of cytochrome P-450. However, effects on the CNS cannot be excluded.