title: Nitrosourea and nitrosocarbamate derivatives of the antiestrogen tamoxifen as potential estrogen receptor-mediated cytotoxic agents in human breast cancer cells.
abstract: We have prepared two analogs of the antiestrogen tamoxifen that incorporate known DNA-crosslinking functions, a chloroethyl nitrosourea and a nitrosocarbamate moiety, and we have tested their bioactivities in cultures of human breast cancer cells. Both compounds bind to the estrogen receptor from MCF-7 cells, with relative binding affinities of 0.18% for the nitrosocarbamate derivative and 0.35% for the nitrosourea derivative, while the affinity of tamoxifen is 1.8%, and that of estradiol is set at 100%. The tamoxifen-nitrosocarbamate compound demonstrated a dose-related cytotoxicity by the colony formation and cell proliferation assays that was not blocked by estradiol in either estrogen receptor-positive MCF-7 cells or estrogen receptor-negative MDA-MB-231 cells, and thus, was not studied further. Tamoxifen-nitrosourea (TAM-NU) showed dose-related cytotoxicity in MCF-7 cells that was blocked by estradiol, whereas its activity in MDA-MB-231 cells was unaffected by estradiol. N-2-(4-t-butylphenoxy)ethyl-N'-chloroethyl-N'-nitrosourea (BPE-NU), a control compound which contains the nitrosourea moiety but does not bind to the estrogen receptor, had no effect on cell proliferation or colony formation in MCF-7 cells, but was very inhibitory in the receptor-negative MDA-MB-231 cells. In contrast, TAM-NU was more active in the receptor-positive MCF-7 cells than in the MDA-MB-231 line. Thus, because TAM-NU appears to be active selectively against the receptor-positive cell line, and because this activity is suppressible by estradiol, its cytotoxic effect seems to be mediated via the estrogen receptor. However, since TAM-NU is active only in prolonged treatment protocols, it appears likely that its cytotoxic activity results from the hormone antagonistic effect of the hydrolysis product of TAM-NU (bis-desmethyltamoxifen), rather than from a direct receptor-mediated, DNA-directed cytotoxic action of TAM-NU itself. This study stresses the need for the use of appropriate control compounds and cell systems in order to assess whether the toxic activity displayed by hormone-cytotoxic conjugates is mediated by receptor interactions and whether it operates through the intended toxic mechanism.
mesh_terms: D001943:Breast Neoplasms; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects*; D055598:Chemical Phenomena; D002621:Chemistry; D003432:Cross-Linking Reagents / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D009607:Nitrosourea Compounds / Q000494:pharmacology*; D011960:Receptors, Estrogen / Q000187:drug effects; D013329:Structure-Activity Relationship; D013629:Tamoxifen / Q000031:analogs & derivatives / Q000494:pharmacology; D014410:Tumor Stem Cell Assay
chemical_list: D003432:Cross-Linking Reagents; D009607:Nitrosourea Compounds; D011960:Receptors, Estrogen; D013629:Tamoxifen; C049601:N-(2-chloroethyl)-2-(4-(1,2-diphenylbutenyl)phenoxy)ethyl carbamate; C049602:N-(2-chloroethyl)-N-nitroso-2-(4-(1,2-diphenylbutenyl)phenoxy)ethyl carbamate; C049603:N-(2-chloroethyl)-N'-2-(4-(1,2-diphenylbutenyl)phenoxy)ethyl-N-nitrosourea