The mechanism of differential sensitivity to methotrexate of normal and malignant human epidermal cells.

Squamous carcinoma cells are much more sensitive (greater than 10(4) times) to the cytotoxic effects of methotrexate (MTX) and 5-fluorodeoxyuridine (FUDR) than are normal human keratinocytes as measured by cell growth. Among the drugs tested, this phenomenon was found to be specific for MTX and FUDR, since arabinosylcytidine (ARA-C), 13-bis-chloroethylnitrosourea (BCNU), and daunomycin failed to show differences in inhibition between the normal and malignant cell lines. Drug uptake studies did not reveal a significant difference in MTX intracellular levels between malignant and normal epidermal cell lines at 60 min. Thymidine (TdR) salvage was assessed by examining the effects of the presence of 3 microM TdR on MTX-induced cytotoxicity. On the withdrawal of TdR, normal cells demonstrated an increased level of inhibition amounting to 4 orders of magnitude, whereas the squamous-cell carcinoma cells showed no change in sensitivity. Interestingly, the immortal nontumorigenic keratinocyte line (NM-110) was similarly not rescued by the addition of TdR. The high degree of sensitivity TO MTX shown by squamous-cell carcinoma (SCC) and NM-110 cells is attributable to a significant diminution of their ability to use exogenous TdR as compared with that of normal keratinocytes and might be indicative of a biochemical change associated with cellular immortality.