Effect of silibinin on the growth and progression of primary lung tumors in mice

Background: Silibinin, a flavanone from milk thistle, inhibits the growth of tumors in several rodent models. We examined the effects of dietary silibinin on the growth, progression, and angiogenesis of urethane-induced lung tumors in mice. Methods: A/J mice (15 per group) were injected with urethane (1 mg/g body weight) or saline alone and fed normal diets for 2 weeks, after which they were fed diets containing different doses of silibinin (0%-1% [wt/wt] silibinin) for 18 or 27 weeks. Immunohistochemistry and Western blot analysis were used to examine angiogenesis and enzymatic markers of inflammation, proliferation, and apoptosis. All statistical tests were two-sided. Results: Urethane-injected mice exposed to silibinin had statistically significantly lower lung tumor multiplicities than urethane-injected mice fed the control diet lacking silibinin (i.e., control mice). Mice that received urethane and 1% (wt/wt) dietary silibinin for 18 weeks had 93% fewer large (i.e., 1.5-2.5-mm-diameter) lung tumors than control mice (mean number of tumors/mouse: 27 in the urethane group versus 2 in the urethane + 1% silibinin group, difference = 25 tumors/mouse, 95% confidence interval [CI] = 13 to 37 tumors/mouse, P =.005). Lung tumors of silibininfed mice had 41%-74% fewer cells positive for the cell proliferation markers proliferating cell nuclear antigen and cyclin D I than lung tumors of control mice. Tumor microvessel density was reduced by up to 89% with silibinin treatment (e.g., 56 microvessels/400x field in tumors from control mice versus 6 microvessels/400x field in tumors from urethane + 1% silibinin-treated mice [difference = 50 microvessels/400x field, 95% CI = 46 to 54 microvessels/400x field; P <.0011). Silibinin decreased lung tumor expression of vascular endothelial growth factor (VEGF) and of inducible nitric oxide synthase and cyclooxygenase-2, two enzymes that promote lung tumor growth and progression by inducing VEGF expression. Conclusions: Silibinin inhibits lung tumor angiogenesis in an animal model and merits investigation as a chemopreventive agent for suppressing lung cancer progression.