Combinatorial treatment with mTOR inhibitors and streptozotocin leads to synergistic In Vitro and In Vivo antitumor effects in insulinoma cells

Streptozotocin-based chemotherapy is the first-line chemotherapy recommended for advanced pancreatic neuroendocrine tumors (pNETs), whereas targeted therapies, including mTOR inhibitors, are available in second-line treatment. Unfortunately, objective response rates to both treatments are limited. Because mTOR pathway activation, commonly observed in pNETs, has been reported as one of the major mechanisms accounting for chemoresistance, we investigated the potential benefit of mTOR inhibition combined with streptozotocin treatment in a subset of pNETs, namely insulinomas. To evaluate the potential of mTOR inhibition in combination with streptozotocin, we selected four different inhibitors acting at various levels of the pathway (everolimus: inhibition of mTORC1; MK-2206: inhibition of AKT; BKM120: inhibition of PI3K, mTORC1, and mTORC2; and BEZ235: inhibition of mTORC1 and mTORC2). Effects on cell viability and apoptosis were assessed in insulinoma cell lines INS-1E (rat) and MIN6 (mouse) in vitro and were confirmed in vivo by using a mouse model of hepatic tumor dissemination after intrasplenic xenograft. In vitro, all four combinations display synergistic effects. These combinations lead to heterogeneous mTOR pathway inhibition, in agreement with their respective target, and increased apoptosis. In vivo, tumor growth in the liver was significantly inhibited by combining streptozotocin with everolimus (P = 0.0014), BKM120 (P = 0.0092), or BEZ235 (P = 0.008) as compared to each agent alone. These results suggest that targeting the mTOR pathway in combination with streptozotocin could be of potential benefit for insulinomas and pNET patients and thus support further clinical investigations.