Cytosine arabinoside (ara-C) resistance confers cross-resistance or collateral sensitivity to other classes of anti-leukemic drugs.

The major limitation of treatment with antimetabolite drugs is that they produce resistant clones both in vitro and in patients who either do not respond to treatment or relapse soon after response has been documented. To better understand the phenomenon of cross-resistance, we developed seven CEM/ara-C-resistant leukemic clones from the CEM/0 (wt) cell line. These clones ranged from 4- to 3.5 x 10(8)-fold more resistant to ara-C than the wt CEM/0 cell line. Using this model, we determined IC50 concentrations to several chemotherapeutic agents and gamma radiation, and we also studied pro- (p53) and anti-apoptotic (bcl-2) proteins, as well as P-glycoprotein (P-gp) and multidrug resistance related protein (MRP). The cell viability assays showed that these clones were cross-resistant to 6-thioguanine (6-TG) or 6-mercaptoguanosine (6-TGuo) from 1.1- to 8.8-fold with ara-C; cross-resistance to vincristine (VCR) was from 200- to 1 x 10(4)-fold with ara-C. Taxotere (TXR) showed cross-resistance with ara-C from 1.39- to 3.03 x 10(3)-fold; dexamethasone (DEX) also showed a significant degree of cross-resistance from 27.4- to 3.87 x 10(7)-fold. Gamma radiation treatments from 0.77 Gy to 12.3 Gy showed a radiation dose-dependent cross-resistance with ara-C from 1.43- to 2.93-fold. Idarubicin was collaterally sensitive with ara-C from 4.6- to 1 x 10(9)-fold in these cell lines. The CEM/ara-C/G resistant cell line was 3-fold more sensitive to 6-TG or VCR than CEM/0 (wt), and 5-fold more sensitive to 6-TGuo. This cell clone expressed p53 and did not overexpress bcl-2 protein. All of the cell lines studied, CEM/0 (wt) and the ara-C resistant clones, showed functional p53 protein. The cell treatment with 0.1, 1 and 10 microM ara-C for 48 hours showed increased p53 protein expression in most of these lines. No increase in bcl-2 protein expression was seen in the wt cell line after ara-C treatment for 48 hours. Three cell lines resistant to ara-C (CEM/ara-C/B, CEM/ara-C/D and CEM/ara-C/I) showed an important increased expression of bcl-2 protein after treatment with 1 microM ara-C, but not after 10 microM. This alteration may lead to resistance to apoptosis and enhanced cell survival. The ratio of bcl-2 to p53 was increased significantly in these three clones, thus favoring an anti-apoptotic drive. All of the cell lines examined were negative for MRP expression and only two, CEM/ara-C/B and CEM/ara-C/J, were positive for MRP functional activity. However, three ara-C resistant cell clones, CEM/ara-C/7A, CEM/ara-C/B and CEM/ara-C/G, were positive for P-gp expression and functional activity. It is apparent that selection for ara-C resistance confers cross-resistance to many other classes of drugs and gamma radiation, probably due to bcl-2 protein overexpression or P-gp and MRP expression, as independent mechanisms.