Correlation between functional and molecular analysis of mdr1 P-glycoprotein in human solid-tumor xenografts

The contribution of P-glycoprotein (Pgp) to multidrug resistance in human solid tumors is generally estimated from bulk mRNA measurements or immunohistochemistry, while direct measurement of the effect of Pgp on intracellular drug concentrations has not been pursued. We investigated the feasibility and sensitivity of a method for probing Pgp-mediated drug transport in cells isolated from solid tumors, using xenograft models. Human tumor xenografts (XG) were grown by s.c. injection of Pgp-expressing cell lines 2780AD, BRO/mdr1 and KB8-5. Tumor uptake of doxorubicin (DOX) after administration of DOX to the mice was determined. XG from untreated mice were enzymatically dissociated. The effect of the Pgp modulator bepridil on steady-state cellular daunorubicin (DNR) and vincristine (VCR) accumulation and chemosensitivity of these XG cells was compared with its effects in the cell lines (CL). mdr1 mRNA and Pgp (by flow cytometry) were measured. Also, the dependence on intracellular ATP concentration, [ATP]i, of the modulator effect was determined in intact KB8-5 cells. The results showed that i.v. administration of DOX to the mice led to lower DOX levels in the Pgp-expressing XG than in the "sensitive" XG, suggesting the presence of an in vivo functional Pgp in these XG tumor models. Dissociated, viable XG cells appeared to have ATP levels sufficient to sustain Pgp-ATPase-coupled drug transport. This was inferred from experiments using KB8-5 CL, which showed half-maximal inhibition of DNR transport at an [ATP]i of 1 to 2 mM. The effect of bepridil on DNR and VCR accumulation and chemosensitivity in the XG cells was in accordance with the XG expression of mdr1/Pgp. In KB8-5 XG cells, Pgp function was hardly detectable, in accordance with decreased mdr1/Pgp expression in vivo. In conclusion, Pgp activity can be determined in freshly dissociated XG human tumor cells. The results obtained with the more necrotic KB8-5 XG may represent some of the interpretation problems arising when low levels of Pgp expression occur within a heterogeneous cell population, such as may be expected in clinical human tumors. Also our results indicate that Pgp activity may be impaired in vivo at [ATP]i below 2 mM, which are realistic values for human solid tumors.