Inhibitory effects of the combination of rapamycin with gemcitabine plus paclitaxel on the growth of pancreatic cancer tumors.

We previously examined the antitumor effects of short interfering RNA nanoparticles targeting mammalian target of rapamycin (mTOR) in an orthotopic pancreatic cancer mouse model. We herein report the inhibitory effects of the mTOR inhibitor rapamycin on tumor growth in a novel established mouse model of pancreatic cancer using human pancreatic cancer cell line-derived organoids. Gemcitabine, 5-fluorouracil, and gemcitabine plus nab-paclitaxel are clinically used to treat advanced pancreatic cancer. In vitro assays showed that rapamycin strongly inhibited cell invasion, while gemcitabine, 5-fluorouracil, and gemcitabine plus paclitaxel primarily inhibited cell proliferation with minimal effects on invasion. In vivo mouse experiments demonstrated that rapamycin exhibited superior antitumor activity to S-1 (a metabolically activated prodrug of 5-fluorouracil) and another mTOR inhibitor, everolimus, while its efficacy was similar to that of gemcitabine plus paclitaxel (which was used instead of nab-paclitaxel due to concerns about allergic reactions in mice to human albumin) in a mouse model of pancreatic cancer using human pancreatic cancer cell line-derived organoids. Furthermore, the combination of rapamycin with gemcitabine plus paclitaxel exerted synergistic inhibitory effects on the growth of pancreatic cancer tumors. Although the inhibition of tumor growth was significantly stronger in everolimus-treated mice than in control mice, there were no additive anti-growth effects when combined with gemcitabine plus paclitaxel. The present results suggest that the combination of rapamycin with gemcitabine plus paclitaxel achieved the greatest reduction in tumor volumes in the mouse xenograft model and, thus, has significant clinical promise.