After looking into the information available it is definitely possible that grapefruit can have a negative affect on warfarin metabolism. Grapefruit, in general, can affect the metabolism of many different medications by acting on the cytochrome P-450 enzyme system.2 This system is mainly located in the small intestine and the liver.2 Grapefruit affects many different enzymes within this system including 3A4, 1A2, and 2A6.2 Some of these enzymes are also involved in the metabolism of warfarin and they are CYP1A2 and CYP3A4.2 The enzyme CYP3A4 appears to be the main concern and there are a couple compounds found within grapefruit that are potential contributors to altered metabolism of CYP3A4.2
The two compounds that are thought to have an affect on CYP3A4 metabolism are naringin and dihydroxybergamottin (DHB).2 Naringin is a flavonoid found in grapefruit and one of its metabolites, naringenin, has been found to mainly inhibit the synthesis of dihydropyridine.2 The second compound, DHB, is a furanocoumarin, which is a natural substance known to affect the metabolism of CYP3A4.2 It is thought that these two compounds, and possibly others, work together to inhibit CYP3A4 and thus affect the metabolism of warfarin.2 Subsequently, studies have shown that the amount of CYP3A4 in the small intestines is reduced after consuming grapefruit or grapefruit juice.2
Warfarin is a drug with a narrow therapeutic index and is composed of two enantiomers named R and S.2,3,4 Warfarin’s R enantiomer is mainly metabolized by CYP1A2 and CYP3A4 while the S enantiomer is metabolized by CYP2C9.2,3,4 It is stated that the R enantiomer plays only a small role in metabolism while the S enantiomer is the main contributor.3 It is thought that the S enantiomer is 3-6 times as potent as the R enantiomer when it comes to metabolism.4 As stated above, it has been found that grapefruit affects the metabolism of CYP3A4, which is part of the metabolism of the R enantiomer of warfarin. This inhibition of first pass metabolism of the CYP 3A4 substrates leads to an increase in plasma drug concentration and total amount of drug present in the body.3 The half life and overall distribution of warfarin seems to be unaffected by this interaction.3
This interaction between warfarin and grapefruit remains somewhat of a mystery and is mainly based on the inhibition of CYP3A4. The overall...