Different effect of paclitaxel on primary tumor mass, tumor cell contents, and metastases for four experimental human prostate tumors expressing luciferase.

PURPOSE Primary tumor growth is usually assessed by measuring tumor mass or volume, under the assumption that such variables correlate with the contents of tumor cells. However, tumors are complex interacting mixtures of tumor cells and host components. The different sensitivity of such components to cytostatic agents should be taken into consideration when evaluating the effectiveness of antineoplastic agents. We evaluate the effect of the antineoplastic agent paclitaxel on primary tumors expressing luciferase and their metastases using a sensitive luminescence-based procedure to directly asses the number tumor cells, in comparison with traditionally used tumor mass measurement. EXPERIMENTAL DESIGN Nude mice bearing human prostate tumors expressing the luciferase gene, LNCaP.Sluc, DU 145.Sluc, and PC-3.Sluc, i.m. inoculated, and PC-3M.Sluc, orthotopically inoculated, were subjected to a 10-day treatment with either 10 mg/kg/d paclitaxel or saline solution. At the end of the treatment period, primary tumors as well as metastasis target organs were harvested, weighed, and homogenized. The presence of tumor cells in the tissue homogenates was evaluated using a luminometer, following the addition of luciferin. Tumor cell equivalent is defined as the amount of light produced by a single tumor cell in culture. RESULTS Paclitaxel had a different effect on the primary tumor mass and the contents of tumor cells for each tumor type. Whereas LNCaP.Sluc, PC-3.Sluc, and PC-3M.Sluc primary tumor masses were significantly reduced by the action of paclitaxel, their contents in tumor cell equivalents were not significantly affected. In contrast, paclitaxel only reduced significantly the number of tumor cell equivalents in DU 145 primary tumors. In the lymph nodes, paclitaxel reduced the number of DU 145.Sluc metastases significantly, by a factor of 10(3), but had no significant effect on the rest of tumor cells. However, in lungs and muscle, paclitaxel treatment reduced significantly the number of metastatic PC-3.Sluc and PC-3M.Sluc tumor cell equivalents. In the bones, no tumor cell type was significantly affected by paclitaxel. CONCLUSIONS Some components of tumor stroma seem to be more sensitive to antineoplastic agents than the tumor cells themselves and may also contribute to modulate the response to therapy. Our results caution against the use of a single general variable, such as tumor mass, to evaluate the effectiveness of antineoplastic agents and emphasize the effect of the tumor cell environment in their sensitivity to treatment.