¹Department of Medical Laboratory Sciences and Technology, Division of Clinical Pharmacology, Karolinska Institutet, Huddinge University Hospital, Huddinge Sweden, and the Department of Pharmacology and Clinical Pharmacology, University of Turku, Turku, Finland.

Correspondence: Kari Laine, MD, PhD, Department of Clinical Pharmacology, Huddinge Hospital C1 68, S-14186 Huddinge, Sweden. E-mail: karlai@utu.fi

^*Supported by grants from the Swedish Medical Research Council (3902), the Academy of Finland (66938), the Finnish Medical Foundation, and the Orion Corporation Research Foundation.

Received 5 April 2000; Accepted 28 May 2000.

Abstract

Background: Although it is known that the use of oral contraceptives inhibits oxidative drug metabolism, there is little information regarding their effect on CYP2C19 activity. Moreover, earlier reports suggest that there may be differences in CYP2C19 activity between men and women.

Objective: We sought to assess the effect of sex and intake of oral contraceptives on CYP2C19 activity as measured by the probe drugs mephenytoin and omeprazole.

Methods: To determine CYP2C19 activity in white Swedish subjects, 644 subjects previously phenotyped with mephenytoin and 175 subjects phenotyped with omeprazole were investigated. The 8-hour urinary mephenytoin S/R ratio after ingestion of 100 mg mephenytoin and the plasma concentration ratio of omeprazole/hydroxyomeprazole at 3 hours after ingestion of 20 mg omeprazole were used as measures of CYP2C19 activity. Differences in these ratios and in their frequency distributions were then examined among women with and without oral contraceptives and men. In addition, nearly all subjects in the omeprazole group had been genotyped with regard to the CYP2C19*2 (m₁) allele. Subjects homozygous for the CYP2C19*2 allele were excluded from the study.

Results: The median mephenytoin S/R ratio was 2.5-fold higher in the subgroup of women taking oral contraceptives compared with either women not taking oral contraceptives (P < .001) or men (P < .001). Similarly, the mean omeprazole/hydroxyomeprazole ratio was twice as high in the oral contraceptive group compared with women not taking oral contraceptives (P < .001) or men (P < .001). However, no differences were evident between women not taking oral contraceptives and men in either the mephenytoin group (P = .48) or the omeprazole group (P = .77). The oral contraceptive–induced inhibitory effect on CYP2C19 activity was similar between the CYP2C19*1/*1 and *1/*2 genotypes, and they were independent of age.

Conclusions: Intake of oral contraceptives significantly inhibits CYP2C19 activity, but there is no true sex-related difference in CYP2C19 activity in healthy, white, Swedish subjects.