1 The kinetics and nature of equilibrium binding were used to characterize
the molecular interaction of the anthranilic acid derivative [H-3]-XR9576 w
ith the multidrug resistance P-glycoprotein (P-gp). XR9576 displayed specif
ic high-affinity binding to P-gp (B-max=275 pmol mg(-1), K-d=5.1 nM). The t
ransport substrates [H-3]-vinblastine and [H-3]-paclitaxel displayed 4 fold
and 20 fold lower affinity respectively for P-gp. The duration of action o
f 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 bind
ing assays. Vinblastine and paclitaxel could only fractionally displace [H-
3]-XR9576 binding, displaying K-i values significantly different from their
measured K-d values. This suggests a non-competitive interaction between X
R9576 and the P-gp substrates vinblastine and paclitaxel.
3 XR9576 was shown to be a potent modulator of P-gp mediated [H-3]-vinblast
ine and [H-3]- paclitaxel transport as it increased the steady-state accumu
lation of these cytotoxics in CH(r)B30 cells to levels observed in non-P-gp
-expressing AuxB1 cells (EC50= 487+/-50 nM). This inhibition of drug transp
ort is not mediated through competition for transport since [H-3]-XR9576 ac
cumulation 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 t
ransporter than any other reported modulators. Several lines of evidence su
ggest that XR9576 inhibits P-gp function by binding at a site which is dist
inct from the site of interaction of transport substrates. The two sites ma
y be classified as serving modulatory or transport functions.