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 pr oduce resistant clones both in vitro and in patients who either do not resp ond to treatment or relapse soon after response has been documented To bett er understand the phenomenon of cross-resistance we developed seven CEM/ara -C-resistant leukemic clones from the CEM/0 (wt) cell line. These clones ra nged 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 m ultidrug resistance related protein (MRP). The cell viability assays showed that these clones were cross-resistant to 6-thioguanine (6-TG) or 6-mercap toguanosine (6-TGuo) from 1.1- to 8.8-fold with ara-C; cross-resistance to vincristine (VCR) was from 200- to 1x10(4)-fold with ara-C Taxotere (TXR) s howed cross-resistance with ara-C from 1.39- to 3.03x10(3)-fold; dexamethas one (DEX) also showed a significant degree of cross-resistance fr om 27.4- to 3.87x10(7)-fold. Gamma radiation treatments from 0.77 Gy to 12.3 Gy show ed a radiation dose-dependent cross-resistance with ara-C from 1.43- to 2.9 3-fold. Idarubicin was collaterally sensitive with ara-C from 4.6- to 1x10( 9)-fold in these cell lines. The CEM/ara-C/G resistant cell line was S-fold more sensitive to 6-TG or VCR than CEM/0 (wt), and 5-fold more sensitive t o 6-TGuo. This cell clone expressed p53 and did not overexpress bcl-2 prote in. 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, I and 10 mu M ara-C for 48 hours showed increased p53 protein expression in most of thes e lines. No increase in bcl-2 protein expression was seen in the wt cell li ne 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 ex pression of bcl-2 protein after treatment with I mu M ara-C, but not after 10 mu M. This alteration may lend to resistance to apoptosis and enhanced c ell 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 r esistant cell clones, CEM/ara-C/7A, CEM/ara-CIB and CEM/ara-C/G, were posit ive for P-gp expression and functional activity. It is apparent that select ion 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.