Effects of social housing condition on chemotherapeutic efficacy in a Shionogi carcinoma (SC115) mouse tumor model: influences of temporal factors, tumor size, and tumor growth rate.

OBJECTIVE The objective of this study was to investigate 1) whether social housing condition, tumor size, and tumor growth rate alter responses to chemotherapy and 2) whether the timing of tumor cell injection or chemotherapy initiation (relative to housing condition formation) influences tumor growth rate or the efficacy of chemotherapy. METHODS Mice were reared individually (I) or in groups (G). In experiment 1, mice were rehoused (IG or GI) or left in group housing (GG) immediately after tumor cell injection. In experiment 2, housing conditions (II, IG, GG, or GI) were formed when tumors weighed 1 g. Chemotherapy (adriamycin 4 mg/kg and cyclophosphamide 61.5 mg/kg IP) and exposure to acute novelty stress (15 min/d, 5 d/wk) were initiated 1 day after housing condition formation. RESULTS If chemotherapy was initiated when the tumor burden was undetectable (experiment 1), housing condition did not alter tumor response to chemotherapy, although IG mice lost the most weight and overall had the lowest probability of survival. If chemotherapy was initiated when tumors weighed 1 g (experiment 2), both tumor and host responses to chemotherapy were poorest for IG mice. Timing of tumor cell injection relative to housing condition formation also differentially influenced the rate of tumor growth in mice treated with the drug vehicle; in experiment 1, tumor growth rate was faster in GI and GG mice than in IG mice, whereas in experiment 2, the rate of tumor growth was faster in II mice than in GG and IG mice. CONCLUSIONS Altering the temporal relationships among social housing condition formation, tumor cell injection, and chemotherapy initiation differentially influences the rate of tumor growth and the efficacy of chemotherapy. Effects of housing condition are independent of tumor growth rate at chemotherapy initiation and, in terms of host responses, independent of tumor burden.