DECREASED INTRACELLULAR COMPARTMENTALIZATION OF DOXORUBICIN IN CELL-LINES EXPRESSING P-GLYCOPROTEIN

After initial rapid [C-14]doxorubicin distribution into drug-sensitive HL-60 and SU-4 cells, slow uptake continues for more than 4 hr, accou nting for up to 80% of the total intracellular drug. In contrast, in P -glycoprotein-expressing drug-resistant HL-GOR and SU-4R cells, doxoru bicin distribution rapidly approaches equilibrium. The simplest kineti c model of this behavior consists of rapid diffusion from extracellula r fluid into the cell, followed by uptake into a nonexchangeable intra cellular pool. At 3.4 mu M doxorubicin, transmembrane diffusion cleara nce was similar for all cell lines (0.78-0.98 mu l sec(-1)). There was no decrease in the normalized apparent volume of distribution in the P-glycoprotein-expressing cell lines, as would be expected if an activ e, unidirectional efflux were present. However, in resistant cells, do xorubicin accumulation in the nonexchangeable pool was up to 15-fold s lower than in sensitive cells (0.004 vs. 0.050 mu l sec(-1) in HL-GOR vs. HL-60; 0.004 vs. 0.058 mu l sec(-1) in SU-4R vs. SU-4). No pool in flow could be detected in sec either SU-4 or SU-4R cells exposed to do xorubicin at 0 degrees C, indicating that the nonexchangeable accumula tion requires energy. The process preventing accumulation began to sat urate in SU-4R cells at 20 mu M doxorubicin, whereas no evidence of sa turation was seen with HL-60R, which is more highly resistant than SU4 R. We propose that alteration in compartmentalization is primarily res ponsible for the doxorubicin resistance observed in these cell lines.