Stimulation of host resistance against tumors by glycyrrhizin, an active component of licorice roots

The effect of glycyrrhizin (GR), a Chinese herbal drug extracted from licorice roots, on the host resistance to tumors was investigated in a murine system. Administration of GR to BALB/c mice, which were inoculated s.c. with 1 x 10⁶ cells/mouse of Meth A tumors, resulted in either no antitumor effect (8/20, 40%), a delay in tumor growth (9/20, 45%), or elimination of tumor growth (3/20, 15%) in these mice. In addition, the incidence of Meth A solid tumors was inhibited by GR when mice were inoculated with 1.5 x 10⁴ cells/mouse or less of Meth A tumor cells, but not 7.5 x 10⁴ cells/mouse or more. These results indicate that in this murine tumor system GR has a very weak antitumor effect. When GR at a dose of 20 mg/kg was administered to mice immunized with allogeneic lymphocytes 1 to 9 days after the immunization, the generation of suppressor macrophages (S-M∅) was clearly reduced as compared with that of S-M∅ generated in immunized controls. In addition, when allospecific CTL (allo-CTL), which were generated in alloimmunized mice treated with GR followed by in vitro stimulation with allogeneic lymphocytes in a mixed lymphocyte reaction, were adoptively transferred to tumor-bearing mice treated with GR, the antitumor activity of allo-CTL derived from immunized mice treated with GR was markedly enhanced as compared with that of allo-CTL from immunized mice. Furthermore, established solid tumors were completely eliminated when interleukin-2 immunotherapy was performed in these mice in combination with GR treatments, but not interleukin-2 or GR alone. These results suggest that, even though GR has a very weak antitumor activity, the CTL or IL-2 immunotherapy against tumors will be more successful when combined with the compound, because GR acts as an inhibitor of S-M∅.