Therapeutic perturbation of the tumor ecosystem in reconstructed heterogeneous mouse mammary tumors.

We have measured the response to methotrexate in vivo of paired mixtures of sister subpopulation lines from a mouse mammary tumor, as a model of drug response of a heterogeneous tumor. The subpopulation lines differed in intrinsic sensitivity to methotrexate. Response was measured both as growth delay and as a shift in tumor cell population distribution toward the more resistant cell line. We found differences between two pairs of cell lines in growth delay: line 66 plus 4T07 mixtures tended to be as responsive as was line 4T07 (the more sensitive line) alone, whereas line 168 plus 4T07 mixtures tended to be less responsive than line 4T07 alone. With both paired mixtures, the tumors arising after treatment tended to contain more line 66 or line 168 than did untreated tumors, but this shift was extremely variable among individual tumors. Within most treatment groups, there was no correlation between the growth rate of individual mixed tumors and the final tumor cell distribution. Likewise, between experiments, there was no correlation between the amount of growth delay in mixed tumors and the final tumor cell distribution. Thus, the cellular composition of treated tumors did not directly reflect the response to therapy.