Organic compounds that are derivatives of furanocoumarin interfere with liver and intestinal enzyme CYP3A4 and are believed to be primarily responsible for the effects of grapefruit on the enzyme. Cytochrome isoforms affected by grapefruit components also include CYP1A2, CYP2C9, and CYP2D6. Bioactive compounds in grapefruit juice may also interfere with MDR1 (multidrug resistance protein 1) and OATP (organic anion transporting polypeptides), either increasing or decreasing the bioavailability of a number of drugs. Drugs that are metabolized by these enzymes may have interactions with citrus chemicals.