Figure 2.

Effect of doxorubicin treatment and drug holiday on mouse P388 lymphoblastic leukemia cells. (a) Parental P388 cells were treated with 13 nM DOX. After 3 cycles of DOX treatment (42 days) P388 D cells showed a significant increase in P-glycoprotein activity (MAF 0.6 vs. MAF 0.04), which was significantly reduced after a 32-day-long drug holiday (MAF 0.47). Flow cytometry histograms show the results of the calcein assay of cells assayed in the presence (red) or absence (black) of the Pgp inhibitor verapamil. (b) Changes of doxorubicin sensitivity as a result of drug treatment and drug holiday. Sequential DOX treatments of P388 cells resulted in a 9.9-fold increase of doxorubicin tolerance (P388 D), which was significantly reduced following a drug holiday (P388 D/DH). Resistance of P388 D cells was abrogated in the presence of tariquidar (P388 D + TQ) (c) Abcb1a and b mRNA expression and DOX IC₅₀ values (red dots) of P388 parental cells (P388) after DOX treatment (D) and following drug holiday (D/DH). Statistical analysis was performed on mRNA samples, ** p < 0.01, *** p < 0.001, **** p < 0.0001.