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 ch emotherapy and 2) whether the timing of tumor cell injection or chemotherap y initiation (relative to housing condition formation) influences tumor gro wth rate or the efficacy of chemotherapy. Methods: Mice were reared individ ually (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 ex periment 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 initiat ed 1 day after housing condition formation. Results: If chemotherapy was in itiated when the tumor burden was undetectable (experiment 1), housing cond ition did not alter tumor response to chemotherapy, although IG mice lost t he most weight and overall had the lowest probability of survival. If chemo therapy was initiated when tumors weighed 1 g (experiment 2), both tumor an d 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 Il mi ce than in GG and IG mice. Conclusions: Altering the temporal relationships among social housing condition formation, tumor cell injection, and chemot herapy initiation differentially influences the rate of tumor growth and th e efficacy of chemotherapy. Effects of housing condition are independent of tumor growth rate at chemotherapy initiation and, in terms of host respons es, independent of tumor burden.