Upregulation of Somatostatin Receptor Type 2 in a Receptor-Deficient In Vivo Pancreatic Neuroendocrine Tumor Model Improves Tumor Response to Targeted 177Lu-DOTATATE

PurposeThe goal of this study was to test whether histone deacetylase inhibitors (HDACis) restore somatostatin receptor type 2 (SSTR2) expression in models of high-grade pancreatic neuroendocrine tumors (PNETs), thereby facilitating effective treatment with 177Lu-DOTATATE therapy. MethodsTo assess tumor grade correlation with SSTR2 expression, we assessed human SSTR2 promoter methylation and expression levels in 96 NIH patient samples and merged the GSE149395 and GSE117852 datasets. We used three NET cell lines (QGP-1, BON-1, GOT-1) characterized by variable SSTR2 expression profiles for functional in vitro studies using HDACis. Finally, the QGP-1 xenograft mouse model, with low basal SSTR2 expression, was used to analyze the therapeutic efficacy of combined HDACi and 177Lu-DOTATATE therapies. ResultsHuman PNET SSTR2 promoter methylation showed a significant positive correlation with higher tumor grades (P = 0.000014). We also found a significant negative correlation (P < 0.0001) between SSTR2 promoter methylation and SSTR2 expression in three NET cell lines. In vitro, SSTR2 expression increased significantly in BON-1 and QGP-1 cells at 48 and 72 hours in a dose-dependent fashion using two different HDACis, valproic acid and CI-994. In vivo studies demonstrated a significant increase in 177Lu-DOTATATE tumor uptake in QGP-1-engrafted mice after 10 days of CI-994 pretreatment (P = 0.0175). Treatment with 177Lu-DOTATATE reduced tumor size in mice pretreated with CI-994 compared to 177Lu-DOTATATE alone (at 15 days, P = 0.0028). ConclusionHDACis increase SSTR2 surface expression in models of high-grade, SSTR2-deficient PNETs. This approach has the potential to improve tumor response to targeted therapy with 177 Lu-DOTATATE in patients with receptor-negative, metastatic PNETs. Translational Relevance StatementPancreatic neuroendocrine tumors (PNETs) express high levels of somatostatin receptor type 2 (SSTR2), a unique target for both tumor imaging and therapy. Unfortunately, high-grade PNETs lose SSTR2 surface expression and thus become ineligible for SSTR2-targeted 177Lu-DOTATATE peptide receptor radionuclide therapy (PRRT). Restoring SSTR2 expression through the reversal of inhibitory epigenetic gene silencing mechanisms has the potential for improving tumor responsiveness to PRRT. We demonstrate that histone deacetylase inhibitors (HDACis) upregulate SSTR2 surface expression in three NET cell lines in vitro. In an in vivo PNET xenograft model with low basal SSTR2 expression, our studies validate a significantly higher tumor uptake of SSTR2-targeted 177Lu-DOTATATE in animals pretreated with HDACis compared to controls. Furthermore, we show that this higher tumor uptake results in significant anti-tumor response when compared to standard PRRT alone. Our preclinical results thus provide a rationale for utilizing HDACi pretreatment to improve targeted radionuclide therapy in patients with SSTR2-negative, metastatic PNETs.