The Proper Administration Sequence of Radiotherapy and Anti-Vascular Agent—DMXAA Is Essential to Inhibit the Growth of Melanoma Tumors

SIMPLE SUMMARY: The growth of tumors depends on the development of their abnormal vasculature. Targeting tumor blood vessels could be an effective approach to anti-cancer therapy. One of the strategies is targeting existing tumor blood vessels—anti-vascular therapy. Anti-vascular drugs destroy the core of the tumor but leave the rim of viable cells at the periphery of the tumor. These viable cells, so-called tumor rim cells, are resistant to elimination and are responsible for tumor regrowth and therapy failure. Our work aimed to find the correct sequencing of the vascular disrupting agent—DMXAA with radiotherapy (brachytherapy) to improve therapeutic efficacy. Throughout the manuscript, we attempt to explain the importance of immune cells’ activation in such therapy and prove the significance of sequential therapeutic agent administration. ABSTRACT: 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.