The H82 “variant” and the H69 “classic” small cell lung cancer (SCLC) cell lines were treated with low levels of epirubicin (69 and 14 nM) which caused little cell death but produced the H82/E8 and H69/E8 extended‐multidrug resistant sublines. Both were resistant to drugs associated with multidrug resistance (MDR), and to chlorambucil (9.5‐ and 5.6‐fold, respectively) and cisplatin (2.3‐ and 8.5‐fold, respectively). There was increased expression of the multidrug resistance‐associated protein (MRP1) in the H82/E8 subline while P‐glycoprotein expression was not detected in any cells or sublines. Treatment of the H82 cells for 1 hr with 69 nM epirubicin increased MRP1‐mRNA expression within 4 hr and this was associated with an increase in the resistance to epirubicin, chlorambucil, cisplatin and paclitaxel. Further, a 1 hr treatment with non‐cytotoxic doses of chlorambucil (2.5 μM), cisplatin (1.3 μM) or paclitaxel (13 nM), drugs not normally associated with MRP1‐mediated MDR, also increased MRP1‐mRNA expression in the H82 cells with paclitaxel causing the highest increase (4.5‐fold). For chlorambucil treatment, this increased MRP1‐mRNA expression was accompanied by increased drug resistance while paclitaxel treatment had no effect on drug resistance in the H82 cells. For the drug resistant H82/E8 subline, these drug treatments had no effect on the MRP1‐mRNA expression and little effect on increasing the subline drug resistance. However, pre‐treatment with paclitaxel sensitised the H82/E8 subline to chlorambucil and cisplatin returning the subline to the sensitivity of the H82 cell line. We conclude that treatment with low levels of MDR and non‐MDR drugs can induce extended‐multidrug resistance in SCLC cells, a process that probably involves the co‐ordinate upregulation of MRP1 and other resistance mechanisms. The results also suggest paclitaxel may have a role as a response modifier in the treatment of refractory SCLC. Int. J. Cancer 76:702–708, 1998.© 1998 Wiley‐Liss, Inc.

No abstract

In an attempt to mimic clinical conditions for the treatment of leukaemia, the HL60 promyelocytic cell line was treated for 18 h with low, clinically relevant, levels of the anthracycline epirubicin and the Vinca alkaloid vinblastine. The resulting drug-resistant sublines not only expressed P-glycoprotein and the MDR phenotype but were also cross-resistant to chlorambucil, methotrexate and cisplatinum, and had increased resistance to radiation. Development of resistance was associated with an aberrant differentiation phenotype with decreased expression of myeloid antigens and expression of glycophorin A, an antigen normally associated with erythroid differentiation. The ability of HL60 cells to terminally differentiate in response to all-trans-retinoic acid (vitamin A acid) was lost in the sublines. These results suggest that either a single novel mechanism is responsible for multiple drug resistance or the initial response to drug treatment is the co-induction of multiple mechanisms. These cells and the method by which they were generated therefore provide a clinically relevant model for the study of the initial events in the development of not only multidrug resistance but also the extended multiple drug resistance usually encountered in the treatment of leukaemia.

The development of drug resistance was studied in a series of haemopoietic cells to determine its relationship to cell lineage. Treatment of the U937 monocytic cell line with epirubicin (15 ng/ml) or vinblastine (8 ng/ml) induced drug-resistant sublines with cross-resistance to epirubicin (8- and 16-fold respectively), vinblastine (5- and 20-fold), paclitaxel (15- and 42-fold) and etoposide (19- and 13-fold). However, sublines were also 3-5-fold resistant to the alkylating agent chlorambucil, cis-platinum and methotrexate, demonstrating an extended multidrug resistance (MDR) phenotype. These cells over-expressed P-glycoprotein, but decreased drug accumulation was not restored in the presence of verapamil, suggesting that the P-glycoprotein was not functional. Similar drug treatment of the HL60 promyelocytic cell line also produced sublines exhibiting an extended MDR phenotype. The KG1a and the HEL cell lines expressed functional P-glycoprotein and were resistant to the drug concentrations used for treatment. Multidrug resistance as mediated by P-glycoprotein cannot explain the resistance of CML patients to chemotherapy, especially in blast crisis. The induction of an extended MDR phenotype specifically in myeloid cells in response to drug treatment may explain the resistance observed in the treatment of CMI.