High doses of thymosin alpha 1 enhance the anti-tumor efficacy of combination chemo-immunotherapy for murine B16 melanoma

Background: We have reported previously that combined chemo-immunotherapy with cyclophosphamide (CY), thymosin alpha 1 (T alpha 1) and low dose interferon alpha,beta (IFN alpha beta) has significant anti- tumour effects. Here, using mouse B16 melanoma as a model, we tested whether increasing the dose of Tal could increase the anti-tumour activity of triple combination chemo-immunotherapy. Materials and Methods: C57BL/6 mice were challenged subcutaneously with B16 melanoma cells and injected intraperitoneally with saline, CY (200 mg/kg, day 7), or CY with Tal (200, 600 or 6000 mu g/kg/day, days 10-13) and IFN alpha beta (30,000 I.U., day 13). Results: Chemo-immunotherapy with high dose (HD)-T alpha 1 caused complete tumour regression for 27.5 days after tumour cell injection (3.9 times longer than untreated controls) and delayed tumour relapse compared to all other groups. Moreover, it significantly increased the median survival time of treated mice, and cured an average of 23% of animals, while none was cured in any other group. Splenocytes from HD-Tal-treated mice showed markedly increased cytotoxic activities against both YAC-I and autologous B16 tumour cells. HD-T alpha 1 treatment reversed the tumour-induced reduction in percentages of CD3 and CD4-positive splenocytes to non- tumour levels, and it increased percentages of CD8, B220 and IL-2R beta-positive cells to well beyond non-tumour controls. Conclusions: High doses of Tal improve antitumour efficacy of tnple chemo-immunotherapy against B16 melanoma. These effects appear to be mediated by stimulation of the host immune response.