In vitro and in vivo reversal of P-glycoprotein-mediated multidrug resistance by a novel potent modulator, XR9576

The overexpression of P-glycoprotein (P-gp) on the surface of tumor cells c auses multidrug resistance (MDR), This protein acts as an energy-dependent drug efflux pump reducing the intracellular concentration of structurally u nrelated drugs. Modulators of P-gp function can restore the sensitivity of MDR cells to such drugs. XR9576 is a novel anthranilic acid derivative deve loped as a potent and specific inhibitor of P-gp, and in this study we eval uate the irt vitro and in vivo modulatory activity of this compound. The in vitro activity of XR9576 was evaluated using a panel of human (H69/LX4, 27 80AD) and murine (EMT6 AR1.0, MC2B) MDR cell lines. XR9576 potentiated the cytotoxicity of several drugs including doxorubicin, paclitaxel, etoposide, and vincristine; complete reversal of resistance was achieved in the prese nce of 25-80 nM XR9576, Direct comparative studies with other modulators in dicated that XR9576 was one of the most potent modulators described to date , Accumulation and efflux studies with the P-gp substrates, [H-3]daunorubic in and rhodamine 123, demonstrated that XR9576 inhibited P-gp-mediated drug efflux, The inhibition of P-gp function was reversible, but the effects pe rsisted for >22 h after removal of the modulator from the incubation medium . This is in contrast to P-gp substrates such as cyclosporin A and verapami l, which lose their activity within (10 min, suggesting that XR9576 is not transported by P-gp, Also, XR9576 was a potent inhibitor of photoaffinity l abeling of P-gp by [H-3]azidopine implying a direct interaction with the pr otein, In mice bearing the intrinsically resistant MC26 colon tumors, coadm inistration of XR9576 potentiated the antitumor activity of doxorubicin wit hout a significant increase in toxicity; maximum potentiation was observed at 2.5-4.0 mg/kg dosed either i.v. or p.o. In addition, coadministration of XR9576 (6-12 mg/kg p.o.) fully restored the antitumor activity of paclitax el, etoposide, and vincristine against two highly resistant MDR human tumor xenografts f2780AD, H69/LX4) in nude mice, Importantly all of the efficaci ous combination schedules appeared to be well tolerated, Furthermore, i.v. coadministration of XR9576 did not alter the plasma pharmacokinetics of pac litaxel, These results demonstrate that XR9576 is an extremely potent, sele ctive, and effective modulator with a long duration of action. It exhibits potent i.v. and p.o., activity without apparently enhancing the plasma phar macokinetics of paclitaxel or the toxicity of coadministared drugs. Hence, XR9576 holds great promise for the treatment of P-gp-mediated MDR cancers.