Increase of cytochrome P450IA2 activity by omeprazole: Evidence by the 13C‐[N‐3‐methyl]‐caffeine breath test in poor and extensive metabolizers of S‐mephenytoin

Omeprazole has been shown to induce cytochrome P450IA1 and P450IA2 activity in vitro. To reflect cytochrome P450IA2 (CYP1A2) activity in vivo, the 13C‐[N‐3‐methyl]‐caffeine breath test was conducted in 18 volunteers: 12 extensive metabolizers, one intermediate metabolizer, and five poor metabolizers of S‐mephenytoin. Breath tests were performed before treatment with an oral dose of 40 mg omeprazole, on the seventh day of treatment, and after a 7‐day washout period. The mean percentage exhalation of the 13C test dose, as determined by 13CO2 in breath during 8 hours, was 23.0% ± 8.0% (n = 18) before treatment. The largest increases in exhalation rate of 13CO2 were observed in the poor metabolizers and the intermediate metabolizers (range, 12.8% to 62.9%; median, 38.9%); median area under the plasma concentration—time curves (AUC) of omeprazole was four times higher than in the extensive metabolizers. The change after omeprazole treatment in extensive metabolizers ranged from −9.8% to +47.7% (median, 12.3%; n = 12) of pretreatment values. In both groups exhalation rates of 13CO2 returned to near pretreatment values within the 7‐day washout period (24.2% ± 7.8%; n = 17). Changes in the 13C‐caffeine breath test correlated well with both the pretreatment value (R = −0.67, p = 0.003; n = 18) and the plasma AUC of omeprazole (R = 0.61, p = 0.007; n = 18). Therapeutic doses of omeprazole seem to induce CYP1A2 activity in poor metabolizers, whereas they exert minor inducing effects in extensive metabolizers of S‐mephenytoin.