The molecular interaction of the high affinity reversal agent XR9576 with P-glycoprotein

Br J Pharmacol. 1999 Sep;128(2):403-11. doi: 10.1038/sj.bjp.0702807.


1 The kinetics and nature of equilibrium binding were used to characterize the molecular interaction of the anthranilic acid derivative [3H]-XR9576 with the multidrug resistance P-glycoprotein (P-gp). XR9576 displayed specific high-affinity binding to P-gp (Bmax = 275 pmol mg-1, Kd = 5.1 nM). The transport substrates [3H]-vinblastine and [3H]-paclitaxel displayed 4 fold and 20 fold lower affinity respectively for P-gp. The duration of action of XR9576 with P-gp was increased in comparison to that of vinblastine which displayed a slower rate of association and a faster dissociation rate. 2 The relative affinities of several modulators and transport substrates to interact with P-gp were determined from displacement drug equilibrium binding assays. Vinblastine and paclitaxel could only fractionally displace [3H]-XR9576 binding, displaying Ki values significantly different from their measured Kd values. This suggests a non-competitive interaction between XR9576 and the P-gp substrates vinblastine and paclitaxel. 3 XR9576 was shown to be a potent modulator of P-gp mediated [3H]-vinblastine and [3H]-paclitaxel transport as it increased the steady-state accumulation of these cytotoxics in CHrB30 cells to levels observed in non-P-gp-expressing AuxB1 cells (EC50 = 487+/-50 nM). This inhibition of drug transport is not mediated through competition for transport since [3H]-XR9576 accumulation was not influenced by P-gp expression or function. 4 These results demonstrate that the P-gp modulator XR9576 exhibits greater selectivity, duration of inhibition and potency of interaction with this transporter than any other reported modulators. Several lines of evidence suggest that XR9576 inhibits P-gp function by binding at a site which is distinct from the site of interaction of transport substrates. The two sites may be classified as serving modulatory or transport functions.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • ATP Binding Cassette Transporter, Subfamily B / drug effects*
  • Adenosine Triphosphate / metabolism
  • Animals
  • Antineoplastic Agents, Phytogenic / pharmacology
  • CHO Cells
  • Cell Membrane / drug effects
  • Cell Membrane / enzymology
  • Cricetinae
  • Drug Resistance, Neoplasm*
  • Kinetics
  • Paclitaxel / pharmacology
  • Protein Binding
  • Quinolines / pharmacology*
  • Vinblastine / pharmacology


  • ATP Binding Cassette Transporter, Subfamily B
  • Antineoplastic Agents, Phytogenic
  • Quinolines
  • Vinblastine
  • Adenosine Triphosphate
  • tariquidar
  • Paclitaxel