Inhibition of human CYP1A2 activity in vitro by methylxanthines: potent competitive inhibition by 8-phenyltheophylline.. S Murray; A O Odupitan; B P Murray; A R Boobis; R J Edwards (2001) Xenobiotica; the fate of foreign compounds in biological systems display abstract
1. Humans are exposed in vivo to methylxanthines by dietary ingestion, as well as from their use as therapeutic agents. The inhibitory effect of a series of these compounds on high-affinity phenacetin O-deethylase activity in the human liver microsomal fraction, a measure of CYP1A2 activity, has been evaluated. 2. Paracetamol, the product of phenacetin O-deethylase activity, was analysed by gas chromatography/negative-ion mass spectrometry using a novel bistrifluoromethylbenzoyl/ trimethylsilyl derivative, and incubation conditions for assessing high-affinity phenacetin O-deethylase activity were examined and optimized. 3. 1-Methylxanthine, caffeine, theophylline, 8-methylxanthine, pentoxyfylline and 3isobutyl-1-methylxanthine caused moderate inhibition with IC50 = 260, 140, 120, 100, 62 and 36 microM respectively. 4. 8-Phenyltheophylline was a potent competitive inhibitor of high-affinity phenacetin O-deethylase activity with an IC50 = 0.7 microM and Ki = 0.11 microM. 5. The specificity of inhibition by 8-phenyltheophylline was assessed by measuring its effect on debrisoquine 4-hydroxylase (CYP2D6), terfenadine hydroxylase (CYP3A4), chlorzoxazone 6-hydroxylase (CYP2E1) and tolbutamide 4-hydroxylase (CYP2C9) activities in human liver microsomal fraction. No inhibition of any of these activities was observed. 6. The potency and specificity of 8-phenyltheophylline as an inhibitor of human hepatic CYP1A2 indicate that the compound may be useful as a chemical inhibitor of this enzyme for further in vitro studies.