Determination of multidrug resistance levels in cultured mammalian cells using ornithine decarboxylase activity

We describe an ornithine decarboxylase (ODC) activity-based assay for the quantitation of multidrug resistance (MDR) and its reversal by MDR modulators in cultured mammalian cells. ODC catalyzes the first and rate-limiting step in polyamine biosynthesis. The activity of this enzyme rises rapidly after growth initiation, such as after addition of serum-containing medium to quiescent mammalian cells. This increase in enzyme activity is prevented when growth is arrested, such as after treatment with cytotoxic drugs. In this assay cultures of drug-sensitive animal and human carcinoma cells as well as their MDR sublines were exposed to various concentrations of different cytotoxic agents for 6-48 h. A dose-dependent decrease in ODC activity was obtained with a variety of chemotherapeutic agents including anthracyclines, vinca alkaloids, epipodophyllotoxins, actinomycin D, antifolates, and cisplatinum. Anticancer drug resistance levels were calculated as the 50% inhibitory concentration of ODC activity obtained with drug-resistant cells divided by that obtained with sensitive cells. These cytotoxicity determinations correlated favorably with those obtained by the well-established colony formation assay. The ODC assay also proved useful in the assessment of MDR reversal with modulators of the MDR phenotype. Therefore, these studies show that the ODC assay could be useful for the reliable determination of drug resistance levels in cultured mammalian cells and for the assessment of drug resistance reversal by various modulators of the MDR phenotype.