Polymethoxylated flavones in orange juice are inhibitors of P-glycoproteinbut not cytochrome P450 3A4

The presence in orange juice of compounds that specifically inhibit the P-g lycoprotein (P-gp) drug efflux transporter, but not the cytochrome P450 (CY P) isozyme CYP3A4, was investigated. The uptake of [H-3]vinblastine, a subs trate of P-gp, by Caco-2 cells was measured. An ethyl acetate extract of or ange juice did not affect the initial uptake rate of [H-3]vinblastine but s ignificantly increased the steady-state uptake, as did cyclosporin A (20 mu M), an inhibitor of P-gp. No significant effect on the uptake of 9-O-[H-3] methylglucose or [C-14]phenylalanine by Caco-2 cells was found, compared wi th the control. When the extract was separated on a Cosmosil column, the el uate with 70% methanol showed the most potent ability to increase [H-3]vinb lastine uptake. Additional separation of the 70% methanol eluate on a silic a gel column with hexane-acetone (3:1) gave 3,3',4',5,6,7,8-heptamethoxyfla vone (HMF) and 4',5,6,7,8-penta-methoxyflavone (tangeretin). HMF, tangereti n, and 3',4',5,6,7,8-hexamethoxyflavone (nobiletin), another methoxyflavone contained in orange juice, all increased the steady-state uptake of [H-3]v inblastine by Caco-2 cells in a concentration-dependent manner. The order o f potency of these compounds at the concentration of 50 mu M was tangeretin > HMF > nobiletin. None of these methoxyflavones inhibited 6 beta-hydroxyl ation of testosterone catalyzed by CYP3A4. The ethyl acetate extract of ora nge juice and these methoxyflavones also increased steady-state [H-3]vinbla stine uptake by LLC-GA5-COL300 cells (a cell line transfected with human MD R1 cDNA). We conclude that these methoxyflavones enhanced vinblastine uptak e by specifically inhibiting drug efflux via P-gp. They may have potential as agents for reversing multidrug resistance or for recovering the bioavail ability of certain drugs.