Selection and Characterization of Chinese Hamster Ovary Cell Mutants Resistant to Melphalan (l-Phenylalanine Mustard)¹

Melphalan (Alkeran) is an alkylating agent commonly used in the treatment of multiple myeloma and other neoplasia. We have isolated melphalan-resistant (Mel^R) Chinese hamster ovary cells in vitro. Stably resistant clones were observed with a frequency of about 10^-7 after a single drug exposure. Two clones, Mel^R1 and Mel^R6, were studied in detail, and their phenotypes were compared to a colchicine-resistant membrane permeability mutant (CH^RC5) isolated previously by Ling and Thompson (J. Cell Physiol., 83: 103–116, 1974), which was cross-resistant to melphalan.

The Mel^R cones were cross-resistant to the nitrogen mustard class of alkylating agents, such as melphalan, nitrogen mustard, and chlorambucil, but not to alkylating agents, such as methanesulfonic acid:ethyl ester or mitomycin C. The Mel^R clones differed from CH^RC5 in their lack of cross-resistance to puromycin and Adriamycin. Studies with [¹⁴C]melphalan showed CH^RC5, but not the Mel^R clones, to be defective in drug accumulation into whole cells. The mechanism of melphalan resistance in CH^RC5 is attributed to reduced drug accumulation due to a plasma membrane alteration. In the Mel^R clones, investigation of drug distribution into cell subfractions revealed reduced accumulation into the nucleus compared to the parental cell line. We therefore propose that the mechanism of resistance in the Mel^R cells results from a nuclear alteration(s) specific to a subclass of alkylating agents.