Methotrexate cytotoxicity for L5178Y/Asn- lymphoblasts: relationship of dose and duration of exposure to tumor cell viability

Abstract

To determine the relative contribution of dose and duration of exposure to methotrexate (MTX) cytotoxicity, suspension cultures of L5178Y/Asn- murine leukemic cells were exposed to 0.1 to 100 microM MTX for 3 to 42 hr. Viability was determined by cloning in soft agar. While there was a linear relationship between dose and MTX cytotoxicity for exposure duration of 3 and 6 hr (r = -0.66), there was a pronounced flattening of this curve at exposure durations of 18 to 42 hr (r = -0.48). Furthermore, there was an excellent correlation (r = -0.85) between MTX cytotoxicity and durations of exposure for 6 to 42 hr (dose range, 1 to 100 microM). Using the linear least-squares method, a best-fit equation for the kinetics of MTX cytotoxicity was determined to be: log viability = 2.25 = 1.76 (log duration) - 0.31 (log dose). In practice, this equation predicts that a 1-log increase in duration of exposure results in almost a 2-log increase in cytotoxicity, whereas a 1-log increase in dose results in only a 0.3-log increase in cytotoxicity. The clinical utility of these data suggest that protracted infusions of lower doses of MTX would be equally as useful as or more useful than short-term high-dose infusions.