Development of alkylating agent-resistant human tumor cell lines

Summary Survival curves and dose escalation studies of four representative human tumor cell lines exposed to the various alkylating agents are presented. With HN2, at a level of one log of cell kill there was a fivefold range in drug concentration required to achieve this degree of cell kill among the cell lines, from 4.5 M for the SL6 lung adenocarcinoma to 22 M for the SW2 small-cell lung carcinoma. Four logs of SCC-25 squamous carcinoma cells were killed by 100 M CDDP; however, there was only about one log of SL6 cells killed by 500 M CDDP. To kill one log of G3361 melanoma cells required 380 M 4-HC and to kil one log of SCC-25 cells required 24 M, approximately a 16-fold difference. The curves for cell kill by L-PAM appeared to be biphasic, with a break at about 100 M. There was about a threefold range in drug concentration required to achieve one log of cell kill with L-PAM, from 60 M in the SCC-25 cell line to 18 M in the SW2 line. To kill one log of SCC-25 cells required 295 M BCNU and to kill one log of SW2 cell required 120 M, about a 2.5-fold difference. The range of maximally tolerated HN2 concentrations were from 1200M for the SL6 cell line, 48 times the initial concentration, to 300 M for the SCC-25 line, 16 times the initial concentration. The G3361 line tolerated the highest level of CDDP, 1900 M, 48 times the initial concentration. The SCC-25 line, on the other hand, tolerated only 600 M, 30 times the initial concentration. The SL6 cell line maximally tolerated 36 times the initial concentration of 4-HC (1450 M), whereas the SCC-25 cell line tolerated only 18 times the initial concentration (720 M). The G3361 melanoma tolerated 1555 M, 30 times the initial concentration of L-PAM, and the SCC-25 cell line tolerated 700 M, 14 times the initial concentration. The SL6 cell line tolerated the highest concentration of BCNU, 4200 M, 24 times the initial concentration. The SCC-25 cell line tolerated 1450 M, 8 times the initial concentration. In all cases, the SCC-25 cell line was least able to tolerate exposure to increasing concentrations of alkylating agents. The SL6 and G3361 cell lines showed the greatest tolerance for increasing concentrations of alkylating agents. With maximal selection pressure, in terms of intensity and duration, 5-to 15-fold resistance at best could be produced to these alkylating agents. This contrasts with other drugs, indicates that alkylating agents are more like X-rays, and has implications for high-dose clinical treatments. The importance of these findings to the clinical treatment of cancer is discussed.Abbreviations NH2 Nitrogen mustard (mustragen) - CDDP cis-diamminedichloroplatinum (II) (cisplatin) - BCNU N,N-bis(2-chloroethyl)-N-nitrosourea - L-PAM L-phenylalanine mustard (melphalan) - 4-HC 4-hydroperoxycyclophosphamide - DMEM Dulbecco's modified Eagle's medium - FBS fetal bovine serum - PBS phosphate-buffered salineThis work was supported by NCI grant #5PO1-CA38493 and a grant from the Mathers Foundation