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Development of an intrinsic P‐glycoprotein‐mediated doxorubicin resistance in quiescent cell layers of large, multicellular prostate tumor spheroids

Growing multicellular prostate tumor spheroids develop quiescent cell subpopulations in central regions with features of intrinsic multicell‐mediated drug resistance. Doxorubicin (dox) uptake was significantly reduced in large spheroids (diameter 400 ± 70 μm), which consist predominantly of quiescent cells, as compared to small spheroids (diameter 100 ± 50 μm), which consist entirely of proliferating cells. After removal of dox from the incubation medium, dox fluorescence declined more efficiently in large spheroids, which led to a decreased dox toxicity as revealed by colony‐forming assays. Verapamil significantly increased dox retention in large spheroids and, consequently, augmented dox toxicity. At a depth 80 μm from the spheroid periphery, a significantly decreased dox fluorescence was observed in the deep, quiescent cell layers of large spheroids. The P‐glycoprotein‐mediated multidrug resistance (MDR)‐reversing agents verapamil, cyclosporin A, quinidine, sodium orthovanadate and tamoxifen significantly increased dox fluorescence at this depth, whereas genistein, indomethacin, probenecid and brefeldin A, which reverse multidrug‐resistance‐associated protein (MRP) function, exerted no effect. Anti‐P‐glycoprotein immunohistochemistry of multicellular tumor spheroids revealed an increase of P‐glycoprotein expression in large speroids as compared to small spheroids, which was most prominent in the Ki‐67‐negative, quiescent cell layers 60 to 100 μm distant from the periphery of the spheroid, indicating that the MDR phenotype is related to cell quiescence. This was corroborated by whole‐cell patchclamp experiments, where the C219 antibody, which is directed against the ATP‐binding site of P‐glycoprotein, significantly inhibited P‐glycoprotein‐associated, volume‐activated chloride currents in quiescent, but not proliferating cells from multicellular tumor spheroids. Int. J. Cancer 75:855–863, 1998. © 1998 Wiley‐Liss, Inc.