Abstract P138: nab-Sirolimus improves mTOR pathway suppression and antitumor activity versus oral mTOR inhibitors in PTEN null bladder cancer (UMUC3) and TSC2 null liver cancer (SNU398) xenografts
Authors: Hou Shihe , Du Heng , Schmid Anita N. , Kwiatkowski David J. , Desai Neil P. 
Published: December 2021

Abstract
Background: TSC1, TSC2, and PTEN genes are tumor suppressors in the mTOR pathway and can be inactivated or deleted across many cancers (Kwiatkowski, Clin Cancer Res 2016). The mTORC1 pathway is frequently activated in cancer and causes phosphorylation of downstream targets S6K (activation) and 4EBP1 (inactivation). Nanomolar concentrations of mTOR inhibitors (mTORi) sirolimus and everolimus can effectively inhibit S6K but not 4EBP1 and may lead to therapeutic resistance (Kang, Science 2013). In a registrational phase 2 trial (AMPECT) with malignant PEComa, nab-sirolimus (ABI-009) had a response rate of 64% (9/14) in patients with TSC1 or TSC2 mutations. In an expanded access program (NCT03817515), of 8 patients with TSC1 or TSC2 mutations (2 had prior mTORi) treated with nab-sirolimus, 5 had partial responses (all mTORi naïve). This study investigated mTOR pathway inhibition, tumor drug levels, and antitumor activity of nab-sirolimus vs equal doses of oral mTORi in PTEN-null and TSC2-null xenograft models. Methods: Athymic mice bearing subcutaneous PTEN-null UMUC3 bladder cancer xenografts were treated with either saline or equal weekly doses (15 mg/kg) of nab-sirolimus (IV, 7.5 mg/kg, 2x/wk), and sirolimus or everolimus (PO, 3 mg/kg/day, 5 days/wk). Tumors were harvested and analyzed for tumor drug levels (LC-MS/MS) and pS6 inhibition by immunohistochemistry (IHC). The same treatment conditions were repeated in a subsequent experiment with TSC2-null SNU-398 hepatocellular carcinoma xenografts, which further analyzed pS6K, pS6, and p4EBP1 via western blot (WB). Results: In UMUC3 xenografts, compared with oral mTORi, IV nab-sirolimus resulted in significantly higher drug exposure (AUC 7d) in the tumor (P&lt;0.0001) and greater pS6 inhibition as measured by IHC (P=0.0001 vs sirolimus, P=0.0034 vs everolimus). Correspondingly, nab-sirolimus resulted in significantly greater tumor growth inhibition (TGI) than sirolimus (69.6% vs 24.3%, P&lt;0.0001) and everolimus (36.2%, P=0.0023), and prolonged animal survival vs both oral mTORi (P&lt;0.05 log-rank). Based on WB in SNU-398 xenografts, IV nab-sirolimus consistently inhibited mTOR targets pS6K, pS6, and p4EBP1, whereas oral sirolimus only partially decreased pS6K and pS6 and did not appear to reduce p4EBP1 levels. Correspondingly, nab-sirolimus resulted in significantly greater TGI than sirolimus (67.8% vs 36.2%, P&lt;0.05) and prolonged animal survival (P&lt;0.05 log-rank). Conclusions: The relatively low tumor concentrations achieved with oral mTORi may limit their effectiveness as anticancer therapies. IV nab-sirolimus at equal dose showed significantly higher tumor accumulation and inhibition of pS6 in a PTEN-null bladder cancer xenograft and increased inhibition of mTOR targets pS6K, pS6, and p4EBP1 in a TSC2-null hepatocellular carcinoma xenograft. This was accompanied with significantly greater antitumor activity, suggesting that nab-sirolimus may have a more optimal pharmacologic profile than the oral mTORi. Clinical studies in cancers harboring these alterations are planned.
Citation Format: Shihe Hou, Heng Du, Anita N. Schmid, David J. Kwiatkowski, Neil P. Desai. nab-Sirolimus improves mTOR pathway suppression and antitumor activity versus oral mTOR inhibitors in PTEN null bladder cancer (UMUC3) and TSC2 null liver cancer (SNU398) xenografts [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P138.