RESUMO
Vascular disrupting agents (VDAs), such as DMXAA, effectively destroy tumor blood vessels and cause the formation of large areas of necrosis in the central parts of the tumors. However, the use of VDAs is associated with hypoxia activation and residues of rim cells on the edge of the tumor that are responsible for tumor regrowth. The aim of the study was to combine DMXAA with radiotherapy (brachytherapy) and find the appropriate administration sequence to obtain the maximum synergistic therapeutic effect. We show that the combination in which tumors were irradiated prior to VDAs administration is more effective in murine melanoma growth inhibition than in either of the agents individually or in reverse combination. For the first time, the significance of immune cells' activation in such a combination is demonstrated. The inhibition of tumor growth is linked to the reduction of tumor blood vessels, the increased infiltration of CD8+ cytotoxic T lymphocytes and NK cells and the polarization of macrophages to the cytotoxic M1 phenotype. The reverse combination of therapeutic agents showed no therapeutic effect and even abolished the effect of DMXAA. The combination of brachytherapy and vascular disrupting agent effectively inhibits the growth of melanoma tumors but requires careful planning of the sequence of administration of the agents.
RESUMO
Radiotherapy (RT) is one of the major methods of cancer treatment. RT destroys cancer cells, but also affects the tumor microenvironment (TME). The delicate balance between immunomodulation processes in TME is dependent, among other things, on a specific radiation dose. Despite many studies, the optimal dose has not been clearly determined. Here, we demonstrate that brachytherapy (contact radiotherapy) inhibits melanoma tumor growth in a dose-dependent manner. Doses of 10Gy and 15Gy cause the most effective tumor growth inhibition compared to the control group. Brachytherapy, at a single dose of ≥ 5Gy, resulted in reduced tumor blood vessel density. Only a dose of 10Gy had the greatest impact on changes in the levels of tumor-infiltrating immune cells. It most effectively reduced the accumulation of protumorogenic M2 tumor-associated macrophages and increased the infiltration of cytotoxic CD8+ T lymphocytes. To summarize, more knowledge about the effects of irradiation doses in anticancer therapy is needed. It may help in the optimization of RT treatment. Our results indicate that a single dose of 10Gy leads to the development of a robust immune response. It seems that it is able to convert a tumor microenvironment into an "in situ" vaccine and lead to a significant inhibition of tumor growth.
