1 Furafylline (1,8-dimethyl-3-(2'-furfuryl)methylxanthine) is a methylxanthine derivative that was introduced as a long-acting replacement for theophylline in the treatment of asthma. Administration of furafylline was associated with an elevation in plasma levels of caffeine, due to inhibition of caffeine oxidation, a reaction catalysed by one or more hydrocarbon-inducible isoenzymes of P450. We have now investigated the selectivity of inhibition of human monooxygenase activities by furafylline. 2 Furafylline was a potent, non-competitive inhibitor of high affinity phenacetin 0-deethylase activity of microsomal fractions of human liver, a reaction catalysed by P450IA2, with an IC50 value of 0.07 FLM.3 Furafylline had either very little or no effect on human monooxygenase activities catalysed by other isoenzymes of P450, including P4501ID1, P4501IC, P450IIIA. Of particular interest, furafylline did not inhibit P4501A1, assessed from aryl hydrocarbon hydroxylase activity of placental samples from women who smoked cigarettes. 4 It is concluded that furafylline is a highly selective inhibitor of P450IA2 in man. 5 Furafylline was a potent inhibitor of the N3-demethylation of caffeine and of a component of the Ni-and N7-demethylation. This confirms earlier suggestions that caffeine is a selective substrate of a hydrocarbon-inducible isoenzyme of P450 in man, and identifies this as P450IA2. Thus, caffeine N3-demethylation should provide a good measure of the activity of P450IA2 in vivo in man. 6 Although furafylline selectively inhibited P450IA2, relative to P4501A1, in the rat, this was at 1000-times the concentration required to inhibit the human isoenzyme, suggesting a major difference in the active site geometry between the human and the rat orthologues of P50IA2.

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.

1970) J. Rid. C'hem. 245,6649-6656 Kes. Comrnun. 24, 79-84 ( 1 986) Insect Hiochem. 16,749-756 Vol. I6

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