Objective: Constituents of grapefruit juice are known to interfere with mammalian cytochrome P450 isozymes such as intestinal CYP3A4 and hepatic CYP2A6, lowering the biotransformation of drugs and increasing their bioavailability. The aim of this study was to investigate whether the presence of naringin is demanded for the inhibition of the coumarin 7-hydroxylase in man or other compounds are responsible for it.
Methods: In cross-over studies, doses of 10 mg coumarin, together with combinations of grapefruit juice, water and naringin, were given orally to one healthy male volunteer, We investigated increasing amounts of grapefruit juice, keeping the volume of liquid constant at 1 L; increasing doses of naringin given in water; increasing amounts of juice, keeping the dose of naringin constant; or increasing doses of naringin, keeping the amount of juice constant. Urine samples were collected up to 24 h after dosing and 7-hydroxycoumarin was quantified fluorimetrically in urine hydrolysates after HPLC separation to determine the excretion rates.
Results: While increasing amounts of grapefruit juice delay the excretion of 7-hydroxycoumarin by 2 h, increasing doses of naringin in water up to twofold (i.e. naringin content of 2 L grapefruit juice) do not cause any alteration in the time course of excretion. Experiments with increasing amounts of juice, keeping the dose of naringin constant, indicate that the inhibitory potency of small amounts of grapefruit juice can be amplified by naringin. The same is true when the ratio between juice constituents and naringin is enhanced up to threefold by adding naringin.
Conclusion: As naringin alone is ineffective, the inhibitory effect of grapefruit juice on the metabolism of coumarin is caused by at least one compound other than naringin. The persistency of the primary inhibitor not identified yet can obviously be modulated by the naring(en)in-system.