Time course of MDR gene amplification during in vivo selection for doxorubicin-resistance and during reversal in murine leukemia L 1210.

MDR gene expression in murine leukemia L 1210 cells was investigated during treatment in vivo with 0.5 mg doxorubicin/kg body weight (BW). Drug resistance (measured by an in vitro short-term test and immunohistochemistry) increased with the number of treatments and the maximum resistance reached after 8 treatments was similar with that of an established multidrug- resistant cell line (20 treatments, 2 mg/kg BW). Southern-blot and DNA dot-blot analyses show that development of MDR is associated with MDR-gene amplification and correlates with the degree of drug resistance and P-glycoprotein-expression. After cessation of doxorubicin treatment, resistance decreased continuously and disappeared after 20 passages. This decrease in resistance is accompanied by a loss of MDR gene amplification and P-glycoprotein expression. Furthermore, P-glycoprotein expression was analyzed in the first hours after treatment with doxorubicin in vivo (0.5 mg/kg BW). Expression was markedly increased and peaked at about 24 hours after treatment. In contrast, only slightly increased resistance and no MDR gene amplification could be detected.