Theanine and glutamate transporter inhibitors enhance the antitumor efficacy of chemotherapeutic agents.

Biochemical modulation has played an important role in the development of cancer chemotherapy. The combined effects of theanine, a specific amino acid in green tea, and glutamate transporter inhibitors on the antitumor activity of doxorubicin (DOX), were investigated and we clarified the biochemical mechanisms of action of these modulators. In M5076 ovarian sarcoma-bearing mice, theanine significantly enhanced the inhibitory effect of DOX on tumor growth and increased the DOX concentration in the tumor, compared to DOX-alone group. Furthermore, the oral administration of theanine or green tea similarly enhanced the antitumor activity of DOX. Moreover, the combination of theanine with DOX suppressed the hepatic metastasis of ovarian sarcoma. In contrast, an increase in DOX concentration was not observed in normal tissues, such as liver and heart. Namely, theanine did not enhance, rather it tended to normalize the increase of lipid peroxide (LPO) levels and reduction of glutathione peroxidase activity as indicators of the DOX-induced side toxicity. On the other hand, in vitro experiments proved that theanine inhibited the efflux of DOX from tumor cells, supporting a theanine-induced increase in the DOX concentration in tumors in vivo. Moreover, theanine significantly inhibited the glutamate uptake by M5076 cells similar to specific inhibitors. Two astrocytic high-affinity glutamate transporters, GLAST and GLT-1, were expressed in M5076 cells. These results suggested that the inhibition of DOX efflux was induced by theanine-mediated inhibition of glutamate transporters. The reduction in the concentration of glutamate in tumor cells caused by theanine induced decreases in the intracellular glutathione (GSH) and GS-DOX conjugate levels. As the expression of MRP5 in M5076 cells was confirmed, it is suggested that the GS-DOX conjugate was transported extracellularly via the MRP5/GS-X pump in M5076 cells and that theanine affected this route. Namely, theanine increases the concentration of DOX in a tumor in vivo through inhibition of the glutamate transporter via the GS-X pump. Similarly, dihydrokainate (DHK) and L-serine-O-sulfate (SOS), specific glutamate transporter inhibitors, indicated the enhancement of the DOX antitumor activity via inhibition of glutamate uptake. Therefore, we revealed the novel mechanism of enhancement of antitumor efficacy of DOX via the inhibition of glutamate transporters. Similarly, theanine enhanced the antitumor activities of other anthracyclines, cisplatin and irinotecan. Consequently, the modulating effect of theanine on the efficacy of antitumor agents is expected to be applicable in clinical cancer chemotherapy.