CROSS-RESISTANCE TO ANTIFOLATES IN MULTIDRUG-RESISTANT CELL-LINES WITH P-GLYCOPROTEIN OR MULTIDRUG-RESISTANCE PROTEIN EXPRESSION

Resistance to some (lipophilic) antifolates has been associated with P -glycoprotein (P-gp)mediated multidrug resistance (MDR). A possible re lationship with non-P-gp MDR has not been established. We studied resi stance to antifolates in SW-1573 human lung carcinoma cells, a P-gp ov erexpressing variant SW-1573/2R160 and a multidrug resistance protein (MRP) overexpressing Variant SW-1573/2R120. In this study, thymidylate synthase (TS) inhibitors with different properties concerning the eff iciency of membrane transport and the efficiency of polyglutamylation were tested for cross-resistance in SW-1573/2R120 and SW-1573/2R160 ce lls. Growth inhibition patterns in this cell line panel were measured by the Sulforhodamine B (SRB) assay. Resistance factors for TS inhibit ors were: 2.4 and 0.4 for 5-fluorouracil (5FU), 18.8 and 8.8 for 2D169 4, 17 and 0.7 for AG337, and 40 and 8.3 for BW1843U89 in SW-1573/2R160 and SW-1573/2R120, respectively. This study showed changes in the TS enzyme kinetics during the induction of doxorubicin resistance in both SW-1573 variants, resulting in 2-fold lower K-m values for 2'-deoxyur idine-5'-monophosphate (dUMP) in both resistant variants compared to t he parental cell line. TS activity, TS protein induction and TS mRNA e xpression all had 2-fold increased in the SW-1573/2R120 compared to th e SW-1573/2R160. H-3 MTX influx was 2-fold lower in SW-1573/2R160 cell s compared to SW-1573/2R120 and SW-1573 cells. In the SW-1573/2R160 ce ll line, an aberrant intracellular trafficking towards the target TS w as observed, compared to SW-1573/2R120 and SW-1573 cells as measured b y the TS in situ assay. The rate of TS inhibition by the TS inhibitors used in this study was similar in all cell lines. In conclusion, coll ateral sensitivity to 5FU and the lipophilic AG337 and cross-resistanc e to other antifolates were observed in non-P-gp MDR SW-1573/2R120 cel ls, as well as resistance to all antifolates in P-gp SW-1573/2R160 cel ls. The mechanism of resistance in SW-1573/2R160 cells possibly involv es reduced influx and changes in intracellular trafficking routes. For the SW-1573/2R120 cell line, several changes related to the TS enzyme possibly play a role in the observed cross-resistance and collateral sensitivity pattern. (C) 1997 Elsevier Science Inc.