The use of the human
tumor cloning assay as a predictor of clinical response of human
tumors to drugs is predicated on the hypothesis that the in vivo response of a
tumor to a
drug can be correlated with the in vitro response of cells derived from the
tumor. To test this hypothesis, we utilized a murine
tumor model in which the in vivo and in vitro responses of a
tumor can be accurately and reproducibly compared.
Drug activity was assessed in
P388 leukemia with the standard in vivo antitumor assay (i.p.
tumor/i.p.
drug administration) and an in vitro assay wherein the
ascites tumor cells are removed from mice, treated with a
drug, and directly cloned in soft
agar to measure clonogenic capacity. The response of P388 cells to analogues within four separate classes of
antitumor agents,
anthracyclines,
anthraquinones,
platinum(II)
coordination complexes, and phosphinogold(I) complexes was evaluated. The clonogenic assay failed to discriminate between highly active in vivo
antitumor agents and analogues with only marginal in vivo efficacy (i.e.,
doxorubicin and
daunorubicin versus rhodomycins A and B,
ametantrone versus NSC 276740,
cisplatin versus
transplatin, [
Au(dppe)2]Cl versus [Au(depe)2]PF6. Furthermore, the in vitro clonogenic assay failed to detect
carboplatin which was a highly active agent in vivo. The basis for these discrepancies was explored by a more detailed comparison of
doxorubicin and
rhodomycin B. In vivo or in vitro
drug exposure with subsequent measurement of cell kill by the in vitro clonogenic and in vivo tumorigenic assay demonstrated that the in vitro assay overestimated the cytotoxic potency of the drugs relative to the tumorigenic assay. Treatment of
tumors in vivo with
doxorubicin at doses below the maximally tolerated dose in mice resulted in multiple log cell kill as measured in vitro or in vivo, whereas
rhodomycin B was cytotoxic only at dose levels exceeding its maximally tolerated dose. The results indicate that a subset of tumor stem cells capable of forming colonies in soft
agar are significantly more sensitive to the cytotoxic effects of
anthracyclines than are in vivo tumorigenic stem cells. Cytotoxic potency as measured by an in vitro soft
agar clonogenic assay is not an accurate predictor of in vivo antitumor efficacy even in a model in which
ascites tumor cells are directly exposed to i.p.
drug. The in vitro cytotoxicity assay is useful only as a nonselective prescreen and must be used in combination with other indicators of
tumor cell selectivity and dose-limiting organ toxicity.