The in vivo interaction between vincristine and radiation in a C3H mammary carcinoma and the feet of CDF1 mice

G2 checkpoint abrogator abates the antagonistic interaction between antimicrotubule drugs and radiation therapy

BACKGROUND AND PURPOSE

We previously demonstrated that radiation may arrest tumor cells at G2 phase, which in turn prevents the cytotoxicity of antimicrotubule drugs and results in antagonistic interaction between these two modalities. Herein we tested whether G2 abrogators would attenuate the above antagonistic interaction and improve the therapeutic efficacy of combination therapy between radiation and antimicrotubule drugs.

MATERIALS AND METHODS

Breast cancer BCap37 and epidermoid carcinoma KB cell lines were administered with radiation, UCN-01 (a model drug of G2 abrogator), paclitaxel or vincristine, alone or in combinations. The antitumor activities of single and combined treatments were analyzed by a series of cytotoxic, apoptotic, cell cycle, morphological and biochemical assays.

RESULTS

UCN-01 significantly enhanced the cytotoxicity of radiation, antimitotic drugs, and their combined treatments in vitro. Further investigations demonstrated that UCN-01 attenuated radiation-induced G2 arrest, and subsequently repressed the inhibitory effect of radiation on drug-induced mitotic arrest and apoptosis.

CONCLUSIONS

This is the first report demonstrating that G2 checkpoint abrogation represses the inhibitory effect of radiation on antimicrotubule drugs, which may be implicated in cancer combination therapy. Considering that G2 abrogators are under extensive evaluation for cancer treatment, our findings provide valuable information for this class of promising compounds.

Role of cell cycle perturbations in the combination therapy of chemotherapeutic agents and radiation

The combination of radiotherapy with chemotherapeutic agents that sensitize tumor cells to ionizing radiation has long been regarded as a promising strategy to enhance cancer therapy. Many chemotherapeutic agents interact with radiation and enhance the cytotoxic or anti-tumor effect of radiation through a number of mechanisms. These include an increase in initial radiation damage, inhibition of cellular repair, cell cycle redistribution, enhancement of apoptosis, counteracting hypoxia and overcoming accelerated repopulation. This article focuses on the role of cell cycle perturbations in the radiosensitivity of cancer cells.

Combination of γ-radiation antagonizes the cytotoxic effects of vincristine and vinblastine on both mitotic arrest and apoptosis

PURPOSE

Combination therapy with different modalities is a common practice in the treatment of cancer. The promising clinical profile of vincristine and vinblastine has promoted considerable interest in combining these vinca alkaloids with radiation therapy to treat a variety of solid tumors. However, the therapeutic efficacy and the interaction between the vinca alkaloids with radiation is not entirely clear. In this study, we assessed the potential interactions in the combination of vincristine or vinblastine with gamma-radiation against human tumor cells in vitro.

METHODS AND MATERIALS

Vincristine or vinblastine and gamma-radiation were administrated at three different sequences designed as preradiated, coradiated, and postradiated combinations in human breast cancer cells and human epidermoid carcinoma cells. The cytotoxic interactions and mutual influences between these two modalities were analyzed by a series of assays including cytotoxic, morphologic, and biochemical examinations.

RESULTS

Our results showed that the combination of these two modalities did not produce any synergistic or additive effects. Instead, the clonogenic assays showed the survival rates of these combinations were increased up to 2.17-fold and 2.7-fold, respectively, of those treated with vincristine or vinblastine alone (p < 0.01). DNA fragmentation, TalphaT-mediated dUTP nick end labeling (TUNEL) assay, and flow cytometric assays also showed that the combination of gamma-radiation significantly interfered with the ability of these vinca alkaloids to induce apoptosis. Further analyses indicated that addition of gamma-radiation resulted in cell cycle arrest at the G(2) phase, which subsequently prevented the mitotic arrest induced by vincristine or vinblastine. In addition, biochemical examinations revealed that gamma-radiation regulated p34(cdc2)/cyclin B1 and survivin, and inhibited IkappaBalpha degradation and bcl-2 phosphorylation.

CONCLUSIONS

These results suggest that gamma-radiation might specifically block the cell cycle at the G(2) phase, which in turn interferes with the cytotoxic effects of vincristine or vinblastine on mitotic arrest and apoptosis. Thereby, it eventually results in an antagonistic interaction between these two modalities. This finding may be implicated in the clinical application of combination therapy of vinca alkaloids and radiation.