Acquired multicellular-mediated resistance to alkylating agents in cancer.

EMT-6 murine mammary tumor sublines highly resistant to cyclophosphamide, cis-diamminedichloro-platinum(II), or N,N',N"-triethylenethiophosphoramide were generated in vivo by sequential treatment of tumor-bearing mice with the respective drugs. Previous studies demonstrated the drug-resistant phenotypes of the sublines were not expressed in vitro when the cells were grown as monolayer cultures. We now show that expression of drug resistance--including patterns of cross-drug resistance observed in vivo--can be fully recapitulated in vitro when the cells are grown under in vivo-like, three-dimensional conditions--namely, as multicellular tumor spheroids. Moreover, the spheroids generated from all of the drug-resistant sublines manifested a much more compact structure. Immediate drug-sensitivity testing of single cells released by trypsin treatment from compact drug-resistant spheroids revealed that such cells lost much of their drug-resistant properties. The results suggest a possible mechanism of acquired drug resistance in tumors based on the response of a cell population (i.e., multicellular or tissue resistance) as opposed to classic (uni)cellular resistance mechanisms.