Background and purpose: Low-dose irradiation and suboptimal drug concentrations may induce unexpected biological responses. Paclitaxel (PTX) is a widely used drug, which has a range of antitumoral effects and which is also regarded as a radiation sensitizer. In this study, it was tested how "suboptimal" short exposure to PTX modifies the biological effects of low-dose irradiation on human epithelial carcinoma cell lines.
Material and methods: 2 Gy irradiation, 7 and 100 nM PTX treatment, and their combinations were tested on squamous and transitional cell carcinoma lines A431, KB and ECV304. Cytoskeleton of interphase cells was investigated with immunocytochemistry and confocal laser scanning microscopy; viability and clonogenicity were assessed with MTT test and standard clonogenic assay. Effects on tumor growth and metastatic potential of A431 cells were tested in vivo using a liver metastasis model in SCID mice.
Results: Exposure of human tumor cells to irradiation induced bundling of microtubules, similar to PTX. Combined treatments suspended each other's effect independently of treatment combinations. Single or combination treatments with low-dose PTX did affect cell proliferation to no relevant extent in vitro or in vivo (primary tumor xenografts of A431 cells). However, exposure to irradiation of A431 cells inhibited while 100 nM PTX stimulated their liver-metastatic potential. Combination of 100 nM PTX with irradiation suspended the effect of 100 nM PTX and exhibited the highest antimetastatic activity.
Conclusion: Short exposure to PTX and irradiation can interfere with each other's effects on tumor cells without significant modulation of proliferation and tumorigenicity. However, PTX and irradiation may significantly modulate the metastatic property of tumor cells.