In human clinical trials of melanoma
patients with multiple subcutaneous metastases treated with intralesional RB, anti-tumor responses were observed in non-treated bystander lesions [12
]. In 42 of 80 patients with designated bystander lesions, 26% experienced a CR in an uninjected bystander lesion,7% a PR, and 17% demonstrated stable disease. Responses correlated with that of patients’ target lesions with a CR or PR in target disease corresponding with a 56% CR and 6% PR in bystander lesions. On the other hand, patients who did not experience an objective response of their target lesions experienced only a 6% CR and 12% PR rate (P=0.023) [14
]. These findings may imply that tumor response in the primary lesion may be able to prime the immune system of patients for activity against distant lesions. Evaluation of peripheral blood samples on days 7-14 after RB treatment showed increased populations of T-cells and natural killer T-cells compared to pre-treatment, corresponding with increased interferon-gamma production from purified T-cells when exposed to autologous tumor, suggesting the possibility of antigen-specific T-cell activation and proliferation [19
In a corresponding murine model with flank B16 melanoma tumors, the authors further demonstrated that tumor draining lymph nodes from RB treated tumors contained greater numbers of dendritic cells than non-draining lymph nodes, implying the possibility of greater antigen
presenting cell activation and proliferation as a result of RB treatment. Additional experiments injected B16 murine melanoma cells in the flank and simultaneously via tail vein for the induction of lung metastases in C57B6 mice. Seven days later, flank tumors were injected with RB and on day 14 lungs were harvested. All 5 control injected animals demonstrated >250 lung metastases, whereas in 3/5 RB treated animals, <4 metastases were established [15
]. Adoptive transfer of T-cells from PV-10 treated mice into sublethally irradiated B16-tumor bearing mice also slowed tumor growth in these animals and T-cells demonstrated increased levels of interferon-gamma (IFN-g) release.
Similarly, splenocytes from mice with PV-10 treated MT-901 murine breast cancer tumors were re-exposed to tumor cells in-vitro and IFN-g levels were assessed with ELISA. Almost 5 times greater IFN-g levels were observed in supernatants when splenocytes were collected from PV-10 treated animals. Increased INF-g production was specific to MT-901 cells as no difference was observed when the splenocytes were exposed to syngenic colorectal cancer cells [15
]. These experiments established that in human and murine melanoma and breast cancer, there appeared to be an antigen-driven T-cell response that has the potential to activate T-cells and impart anti-tumor responses in bystander lesions
and distant metastases.
Furthermore, in syngeneic orthotopic models of murine hepatocellular carcinoma (HCC), RB similarly induced chemoablation in all treated tumors. Twenty-one to 81 days after RB treatment, when re-challenged with the same tumor, durable immunity was demonstrated in 14/14 animals without measurable tumor formation, whereas B16-F10 melanoma tumors, i.e., non-HCC cells, were able to be established in 13/13 animals [20
]. Additionally, immunity to establishment of a new HCC tumor could be created through adoptive transfer of splenocytes from treated animals. This was an interesting finding given that analysis of splenic composition from animals with B16 melanoma that experienced distant lung tumor regression after RB treatment of flank tumors did not demonstrate any difference in the percent of T cells, Tregs
, NK cells, B cells, myeloid derived suppressor cells, or macrophages [15
]. The authors further demonstrated that bystander lesions disappeared or decreased in size in HCC models, whereas, no bystander tumors were ever observed to resolve in nude mice without a competent T-cell immune system. These experiments established that, similar to melanoma, in immunocompetent mice with orthotopic primary hepatocellular carcinoma flank tumors, an anti-tumor response can be induced by priming the animals with RB treatment of tumors. These experiments raise the possibility that RB induced cytotoxicity exposes antigens and mounts an immune response that may protects animals from additional tumor formation that can be adoptively transferred to other animals using splenocytes.