Comparing the Effect of Multiple Histone Deacetylase Inhibitors on SSTR2 Expression and [111In]In-DOTATATE Uptake in NET Cells

Abstract

The aim of this study was to increase somatostatin type-2 receptor (SSTR2) expression on neuroendocrine tumor (NET) cells using histone deacetylase inhibitors (HDACis), potentially increasing the uptake of SSTR2-targeted radiopharmaceuticals and subsequently improving treatment efficacy of peptide receptor radionuclide therapy (PRRT). Human NET cell lines BON-1, NCI-H727, and GOT1 were treated with HDACis (i.e., CI-994, entinostat, LMK-235, mocetinostat, panobinostat, or valproic acid (VPA); entinostat and VPA were the HDACis tested in GOT1 cells) to examine SSTR2 mRNA expression levels and uptake of SSTR2-targeting radiotracer [111In]In-DOTATATE. Reversibility of the induced effects was examined after drug-withdrawal. Finally, the effect of VPA on radiosensitivity was investigated. A strong stimulatory effect in BON-1, NCI-H727, and GOT1 cells was observed after HDACi treatment, both on SSTR2 mRNA expression levels and [111In]In-DOTATATE uptake. The effects of the HDACis were largely reversible over a period of seven days, demonstrating largest reductions within the first day. The reversibility profile of the induced effects suggests that proper timing of HDACi treatment is most likely essential for a beneficial outcome. In addition to increasing SSTR2 expression levels, VPA enhanced the radiosensitivity of all cell lines. In conclusion, HDACi treatment increased SSTR2 expression, and radiosensitivity was also enhanced upon VPA treatment.

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

“…These results indicate that both time course and dose play crucial roles in regulating SSTR2 transcription using epidrug-based treatments. Others have reported similar results for upregulation of SSTR2 mRNA in BON-1 and GOT-1 with HDACi treatments (12,14,38). In an in vitro study in BON-1 and NCI-H727 cells (14), the reversibility of SSTR2 gene expression upregulation after HDACi (VPA and CI-994) withdrawal returned to baseline within 24 hours.…”

“…Others have reported similar results for upregulation of SSTR2 mRNA in BON-1 and GOT-1 with HDACi treatments (12,14,38). In an in vitro study in BON-1 and NCI-H727 cells (14), the reversibility of SSTR2 gene expression upregulation after HDACi (VPA and CI-994) withdrawal returned to baseline within 24 hours. In vivo , our results confirm that effects induced by low doses of HDACi treatment (5 mg/kg) for a short duration were largely and rapidly reversible.…”

“…We found that higher NET grades correlated with increased SSTR2 promoter methylation levels in an NIH NET patient cohort, which provides an explanation for the silenced expression of SSTR2, as shown in other datasets (35). In high-grade PNETs, SSTR2 seems lost due to epigenetic gene-silencing mechanisms (14,36–38); and a few studies have reported a reversal of these mechanisms by treating NET cell lines with DNMTis and HDACis (39), thus inducing DNA hypomethylation and histone acetylation, respectively (34,40,41).…”

“…Thus epigenetic regulation may play a dominant role in the oncogenesis and progression of NETs, and account for the lack of SSTR2 in high-grade NETs (11). A few preclinical studies have shown that epidrug treatment significantly increased SSTR2 expression in cell lines with low basal expression (12–14). However, how this translates to in vivo animal models has not been previously demonstrated.…”

Effect of epigenetic treatment on SST2 expression in neuroendocrine tumour patients

“…Several preclinical studies have uncovered that epigenetic drugs can upregulate somatostatin receptor subtype 2 (SST 2 ) expression in neuroendocrine tumour (NET) models, 1 , 2 which could be of eminent importance for NET patients with low tumoural SST expression. In a prospective clinical proof‐of‐concept trial involving nine advanced NET patients with low SST expression, we were able to show that epigenetic treatment with the histone deacetylase (HDAC) inhibitor valproic acid and the DNA methyltransferase (DNMT) inhibitor hydralazine did not lead to an increase in tumour‐uptake of 68 Ga‐DOTATATE, contradicting the in vitro data.…”