Partial circumvention of P-glycoprotein-mediated multidrug resistance by doxorubicin-14-O-hemiadipate

Previously, we have reported partial circumvention ofP-glycoprotein (Pgp)-associated resistance to doxorubicin(Dox) in MCF7/R human breast carcinoma and P388/R murineleukemia cell lines by doxorubicin-14-O-hemiadipate (H-Dox)Povarov L.S. et al. (1995) Russian J. Bioorganic Chemistry21: 797–803]. We felt that these changes were due toalterations in the cellular pharmacokinetics of the analog inmultidrug (MDR) resistant cells, as compared to that of Dox.To address this hypothesis, we performed comparative studiesof the accumulation, retention and intracellular localizationof H-Dox and Dox in Dox-sensitive murine leukemia cell lineP388/S and its Dox-selected, Pgp-positive drug resistantP388/R subline. These studies were performed in the presenceor absence of cyclosporin A (CsA), a competitive inhibitor ofPgp. Flow cytometric analysis revealed significant differencesin Dox and H-Dox accumulation in P388/R cells when compared toP388/S cells. In P388/R versus P388/S cells, there was a 38-fold decrease in Dox accumulation, but only a 5-fold decreasein H-Dox accumulation, indicating over a 7-fold increase in H-Dox buildup in resistant cells. CsA did not affect uptake orretention of either drug by sensitive cells. However,coincubation with CsA resulted in a 54-fold increase in Doxaccumulation and only a 5-fold increase in H-Dox uptake inP388/R cells, restoring anthracycline levels in P388/R to100% of that found in P388/S cells. Once internalized by theresistant cells, H-Dox was retained better than Dox regardlessof presence or absence of CsA. Confocal microscopic analysisrevealed the presence of H-Dox but no Dox in cellular nucleiof P388/R cells. Thus, increased activity of H-Dox towardP388/R cells was correlated with its enhanced ability to enterand be retained in these cells, and also with redistributionof H-Dox into the nuclei of the resistant cells as compared toDox. Overall, our findings support our initial hypothesis andprovide evidence that H-Dox, a 14-O-hemiadipate ofdoxorubicin, is affected by Pgp-mediated MDR to a lesserextent than parental Dox due to changes in the cellularpharmacokinetics of the analog.