Brain-tumor therapy

Quantitative analysis using a model system

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✓ A recently developed colony-formation assay has been used to evaluate in vivo 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) therapy of a transplantable rat brain-tumor model. A comparison of the in vitro colony-forming capacity of treated and untreated tumor cells permits calculation of the fraction of clonogenic tumor cells surviving in vivo therapy. The plateau that we previously observed on the BCNU dose-response curve is not the result of repair of potentially lethal damage, since no change in the 0.1% of surviving clonogenic tumor cells occurs during the first 2 to 4 days after treatment. Although reanalysis of the dose-response curve indicates that sublethal damage exists, its repair is probably minimal. The most likely explanation for the observed limitation of the BCNU effect is the drug's failure to reach all clonogenic cells. A dose of BCNU that kills more than 99.9% of clonogenic tumor cells within 30 minutes of treatment results in only a 60% decrease in tumor weight by Day 14. This disparity is explained by retarded removal of dead cells, and, along with a previously determined 90% cell-kill threshold necessary to appreciate increased animal survival, demonstrates the inherent limitations of measurements of tumor size (including brain scans and clinical patient evaluations) in evaluating the efficacy of brain-tumor therapy. Following an LD10 dose of BCNU the surviving clonogenic tumor cells increase in number after a latency period of 2 to 4 days; during regrowth the cell doubling time is 40 hours. Marked variability in tumor response and regrowth was noted. The determination of information regarding disturbed tumor cell kinetics and tumor heterogeneity is essential for the proper planning of combination chemotherapy and multimodality regimens.

Article Information

Dr. Rosenblum is a recipient of American Cancer Society Clinical Fellowship CF3537.

Address reprint requests to: Mark L. Rosenblum, M.D., Department of Neurosurgery, University of California Medical Center, San Francisco, California 94143.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Schematic representation of colony formation assay procedure.

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    Graph shows analysis of untreated tumor growth in vivo. Each point represents the mean weight for six tumors obtained from animals sacrificed at various intervals after transplantation. The error bars represent the 95% confidence intervals.

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    Graph shows determination of the surviving fraction of clonogenic tumor cells from 118 animals in 19 separate experiments treated in vivo with various doses of BCNU 6 to 24 hours before colony-formation assay. The resulting dose-response curve was determined by least squares regression analysis. The inserted numbers represent the number of tumors evaluated at each dose, and each point represents their mean values; the error bars indicate the 95% confidence intervals.

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    Graph shows changes in tumor weight after an LD10 dose of BCNU. Each solid circle represents the mean value for eight to 23 tumors in three experiments (± 1 SE). The open circles represent the untreated situation. Tumor weight is alternatively expressed as tumor cell number (assuming 106 cells/mg). Extrapolation of the posttreatment tumor regrowth to the day of BCNU administration estimates the tumor cell kill.

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    Graphs show posttreatment tumor weight changes contrasted to clonogenic tumor cell kinetics for 20 days following an LD10 dose of BCNU. The surviving fraction of clonogenic cells (triangles and dashed curve) was calculated as the product of the measured surviving fraction (circles and solid curve) and the relative total tumor cell number at various times after animal treatment (see text).

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