In Vitro Evaluation of in Vivo Brain Tumor Chemotherapy with 1,3-bis(2-chloroethyl)-1-nitrosourea

An in vitro colony formation assay was used to determine the efficacy of in vitro therapy with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) on a rat brain tumor. The fraction of clonogenic cells surviving in vivo therapy was determined by a comparison between the in vitro colony-forming capacity of cells derived from previously treated and untreated tumors. With this intracerebral solid tumor a direct correlation was found between the surviving fraction of cells and animal survival, implying that the in vitro assay system is a reliable test of therapeutic effect. The BCNU dose-response curve was exponential up to a dose of 0.75 times the LD10 dose with little additional cell kill noted at higher drug levels. This plateau does not appear to represent a resistant subpopulation of cells, since retreatment of tumors derived from cells surviving an LD10 dose were as sensitive to BCNU as those with no prior drug exposure. Instead, it may represent, at least in part, failure of the drug to reach and/or enter cells in all parts of solid tumors. On the average BCNU doses of 0.75 times the LD10 dose or greater resulted in slightly more than a 3-log cell kill and doubled the life-span for our tumor-bearing animals. The finding that an increase in animal life-span requires at least a 1-log tumor cell kill indicates that survival studies with intracranial tumor models may be insensitive to single courses of many chemotherapeutic agents with modest but significant antitumor activity.