Cytogenetic and Phenotypic Analysis of a Human Colon Carcinoma Cell Line Resistant to Mitoxantrone1

A human colon carcinoma cell line selected for a 21-fold resistance to mixtoxantrone was cross-resistant to the anthracycline, doxorubicin, but not to the anthracene, bisantrene. A 2-fold resistance was observed with vinblastine, another drug associated with multidrug resistance. Net intracellular mitoxantrone and doxorubicin accumulation were decreased at 1 h for all dose levels in the resistant cell line compared to the sensitive cell line. Although the resistant cells were more resistant to mitoxantrone than doxorubicin, the net accumulation of mitoxantrone was only 19% less than the sensitive cell line; whereas doxorubicin accumulation was decreased by 49%. No significant difference between the sensitive and resistant cell lines was observed in the initial accumulation of mitoxan trone; however, the efflux of mitoxantrone was increased in the resistant cell line. Verapamil did not overcome the resistance to mitoxantrone and did not increase the net accumulation of drug. No alterations in the electrophoretic mobility of membrane proteins were observed. Using immunoblotting techniques, the resistant cell line did not express Pglycoprotein which is frequently observed for cells resistant to anthra cycline antibiotics. Cytogenetic analysis showed a putative homogenously staining region on the short arm of chromosome 7 in the resistant cell line. The limited cross-resistant phenotype, lack of verapamil reversal, nondetection of P-glycoprotein, and Cytogenetic evidence of gene ampli fication suggests the involvement of a novel drug-resistant gene associated with resistance to mitoxantrone.