Effects of Cimetidine and Ranitidine on Hepatic Monooxygenase Activity in Humans.

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The effect of cimetidine and ranitidine administration on hepatic drug-metabolizing enzyme activity was investigated in 7 healthy volunteers using the ratio of 6-hydroxycortisol (6-OHF) to total 17-hydroxycorticosteroids (17-OHCS) in 24 hr urine as an index of the hepatic P-450-dependent enzyme system. Although the ratio of 6-OHF to 17-OHCS in different individuals varied from 0.043 to 0.074, the values obtained in three trials at one month intervals are highly reproducible within the same subjects. In contrast with very little effect of ranitidine ingestion at a 150 mg dose on hepatic P-450 systems, oral cimetidine administration at a dose of 200 mg four times a day immediately reduced the ratio from 0.058±0.008 to 0.045±0.009 (mean±S. D., P<0.02). This reduction continued at the same degree as the initial decrease during the cimetidine treatment for 15 consecutive days, whereas the ratio returned to the control value as soon as cimetidine was discontinued. The extent of reduction by a 200mg dose twice a day is slightly smaller than by the same dose four times a day. These results suggest that the rise of plasma concentration of some drugs may occur immediately following cimetidine administration, even at lower dosage, that drug interaction with cimetidine is to be avoided if cimetidine therapy is stopped, and that ranitidine dose not influence drug-metabolizing enzymes.
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