sdz-psc-833 and azidopine

The overexpression of P-glycoprotein (P-gp) on the surface of tumor cells causes multidrug resistance (MDR). This protein acts as an energy-dependent drug efflux pump reducing the intracellular concentration of structurally unrelated drugs. Modulators of P-gp function can restore the sensitivity of MDR cells to such drugs. XR9576 is a novel anthranilic acid derivative developed as a potent and specific inhibitor of P-gp, and in this study we evaluate the in vitro and in vivo modulatory activity of this compound. The in vitro activity of XR9576 was evaluated using a panel of human (H69/LX4, 2780AD) and murine (EMT6 AR1.0, MC26) MDR cell lines. XR9576 potentiated the cytotoxicity of several drugs including doxorubicin, paclitaxel, etoposide, and vincristine; complete reversal of resistance was achieved in the presence of 25-80 nM XR9576. Direct comparative studies with other modulators indicated that XR9576 was one of the most potent modulators described to date. Accumulation and efflux studies with the P-gp substrates, [3H]daunorubicin and rhodamine 123, demonstrated that XR9576 inhibited P-gp-mediated drug efflux. The inhibition of P-gp function was reversible, but the effects persisted 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 verapamil, which lose their activity within 60 min, suggesting that XR9576 is not transported by P-gp. Also, XR9576 was a potent inhibitor of photoaffinity labeling of P-gp by [3H]azidopine implying a direct interaction with the protein. In mice bearing the intrinsically resistant MC26 colon tumors, coadministration of XR9576 potentiated the antitumor activity of doxorubicin without 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 paclitaxel, etoposide, and vincristine against two highly resistant MDR human tumor xenografts (2780AD, H69/LX4) in nude mice. Importantly all of the efficacious combination schedules appeared to be well tolerated. Furthermore, i.v. coadministration of XR9576 did not alter the plasma pharmacokinetics of paclitaxel. These results demonstrate that XR9576 is an extremely potent, selective, and effective modulator with a long duration of action. It exhibits potent i.v. and p.o. activity without apparently enhancing the plasma pharmacokinetics of paclitaxel or

Topics: Acridines; Animals; Antibiotics, Antineoplastic; Antineoplastic Agents; ATP Binding Cassette Transporter, Subfamily B, Member 1; Azides; Binding, Competitive; Cell Division; Cyclosporins; Daunorubicin; Dihydropyridines; Dose-Response Relationship, Drug; Doxorubicin; Drug Resistance, Multiple; Drug Resistance, Neoplasm; Drug Synergism; Female; Humans; Isoquinolines; Mice; Mice, Inbred BALB C; Mice, Nude; Neoplasm Transplantation; Neoplasms, Experimental; Paclitaxel; Quinolines; Tetrahydroisoquinolines; Time Factors; Treatment Outcome; Tritium; Xenograft Model Antitumor Assays

Using three different cell lines exhibiting the MDR phenotype, we have studied the ability of eight different modulators to restore doxorubicin intracellular accumulation and to inhibit azidopine binding to membrane extracts. One cell line was of human origin (KB VI) and two of murine origin, overexpressing two different isoforms of the mdrl gene (C6 IV and C6 0.5). The modulators were distributed in different drug classes: cyclosporine A and PSC-833, quinine and quinidine, nifedipine and nicardipine, and verapamil and S-9788. We observed that there was no strict parallelism between restoration of doxorubicin intracellular accumulation and inhibition of azidopine binding. However, when considering separately each group of drugs, it appeared that the most potent drug in inhibiting azidopine labelling of P-glycoprotein (P-gp) was also the most potent in restoring doxorubicin accumulation. This indicates that azidopine binding cannot be used as a general screening test for the identification of new modulators, but rather at the level of the selection of potent analogues within a chemical family. The three cell lines behaved similarly, indicating that the structural diversity of P-pgs did not influence the efficiency and binding of modulators.

Topics: Affinity Labels; Animals; Antibiotics, Antineoplastic; Antineoplastic Agents; ATP Binding Cassette Transporter, Subfamily B, Member 1; Azides; Cyclosporine; Cyclosporins; Dihydropyridines; Doxorubicin; Drug Resistance, Multiple; Humans; Mice; Nicardipine; Piperidines; Triazines; Tumor Cells, Cultured; Verapamil