Applying HDACis to increase SSTR2 expression and radiolabeled DOTA-TATE uptake: from cells to mice.

AIMS: The aim of our study was to determine the effect of histone deacetylase (HDAC) inhibitors (HDACis) on somatostatin type-2 receptor (SSTR2) expression and [111In]In-/[177Lu]Lu-DOTA-TATE uptake in vitro and in vivo. MATERIALS AND METHODS: The human cell lines NCI-H69 (small-cell lung carcinoma) and BON-1 (pancreatic neuroendocrine tumor) were treated with HDACis (i.e. entinostat, mocetinostat (MOC), LMK-235, CI-994 or panobinostat (PAN)), and SSTR2 mRNA expression levels and [111In]In-DOTA-TATE uptake were measured. Furthermore, vehicle- and HDACi-treated NCI-H69 and BON-1 tumor-bearing mice were injected with radiolabeled DOTA-TATE followed by biodistribution studies. Additionally, SSTR2 and HDAC mRNA expression of xenografts, and of NCI-H69, BON-1, NCI-H727 (human pulmonary carcinoid) and GOT1 (human midgut neuroendocrine tumor) cells were determined. KEY FINDINGS: HDACi treatment resulted in the desired effects in vitro. However, no significant increase in tumoral DOTA-TATE uptake was observed after HDACi treatment in NCI-H69 tumor-bearing animals, whereas tumoral SSTR2 mRNA and/or protein expression levels were significantly upregulated after treatment with MOC, CI-994 and PAN, i.e. a maximum of 2.1- and 1.3-fold, respectively. Analysis of PAN-treated BON-1 xenografts solely demonstrated increased SSTR2 mRNA expression levels. Comparison of HDACs and SSTR2 expression in BON-1 and NCI-H69 xenografts showed a significantly higher expression of 6/11 HDACs in BON-1 xenografts. Of these HDACs, a significant inverse correlation was found between HDAC3 and SSTR2 expression (Pearson r = -0.92) in the studied cell lines. SIGNIFICANCE: To conclude, tumoral uptake levels of radiolabeled DOTA-TATE were not enhanced after HDACi treatment in vivo, but, depending on the applied inhibitor, increased SSTR2 expression levels were observed.

Epigenetic regulation of SST2 expression in small intestinal neuroendocrine tumors.

Background: Somatostatin receptor type 2 (SST2) expression is critical for the diagnosis and treatment of neuroendocrine tumors and is associated with improved patient survival. Recent data suggest that epigenetic changes such as DNA methylation and histone modifications play an important role in regulating SST2 expression and tumorigenesis of NETs. However, there are limited data on the association between epigenetic marks and SST2 expression in small intestinal neuroendocrine tumors (SI-NETs). Methods: Tissue samples from 16 patients diagnosed with SI-NETs and undergoing surgical resection of the primary tumor at Erasmus MC Rotterdam were analysed for SST2 expression levels and epigenetic marks surrounding the SST2 promoter region, i.e. DNA methylation and histone modifications H3K27me3 and H3K9ac. As a control, 13 normal SI-tissue samples were included. Results: The SI-NET samples had high SST2 protein and mRNA expression levels; a median (IQR) of 80% (70-95) SST2-positive cells and 8.2 times elevated SST2 mRNA expression level compared to normal SI-tissue (p=0.0042). In comparison to normal SI-tissue, DNA methylation levels and H3K27me3 levels were significantly lower at five out of the eight targeted CpG positions and at two out of the three examined locations within the SST2 gene promoter region of the SI-NET samples, respectively. No differences in the level of activating histone mark H3K9ac were observed between matched samples. While no correlation was found between histone modification marks and SST2 expression, SST2 mRNA expression levels correlated negatively with DNA methylation within the SST2 promoter region in both normal SI-tissue and SI-NETs (p=0.006 and p=0.04, respectively). Conclusion: SI-NETs have lower SST2 promoter methylation levels and lower H3K27me3 methylation levels compared to normal SI-tissue. Moreover, in contrast to the absence of a correlation with SST2 protein expression levels, significant negative correlations were found between SST2 mRNA expression level and the mean level of DNA methylation within the SST2 promoter region in both normal SI-tissue and SI-NET tissue. These results indicate that DNA methylation might be involved in regulating SST2 expression. However, the role of histone modifications in SI-NETs remains elusive.

The Effect of VPA Treatment on Radiolabeled DOTATATE Uptake: Differences Observed In Vitro and In Vivo.

Background: To improve peptide receptor radionuclide therapy (PRRT), we aimed to enhance the expression of somatostatin type-2 receptors (SSTR2) in vitro and in vivo, using valproic acid (VPA). Methods: Human NCI-H69 small-cell lung carcinoma cells were treated with VPA, followed by [111In]In-DOTATATE uptake studies, RT-qPCR and immunohistochemistry analysis. Furthermore, NCI-H69 xenografted mice were treated with VPA or vehicle, followed by [177Lu]Lu-DOTATATE injection. Biodistribution studies were performed, and tissues were collected for further analysis. Results: VPA significantly increased SSTR2 expression in vitro. In animals, a statistically significant increased [177Lu]Lu-DOTATATE tumoral uptake was observed when VPA was administered eight hours before [177Lu]Lu-DOTATATE administration, but increased tumor SSTR2 expression levels were lacking. The animals also presented significantly higher [177Lu]Lu-DOTATATE blood levels, as well as an elevated renal tubular damage score. This suggests that the enhanced tumor uptake was presumably a consequence of the increased radiotracer circulation and the induced kidney damage. Conclusions: VPA increases SSTR2 expression in vitro. In vivo, the observed increase in tumoral [177Lu]Lu-DOTATATE uptake is not caused by SSTR2 upregulation, but rather by other mechanisms, e.g., an increased [177Lu]Lu-DOTATATE circulation time and renal toxicity. However, since both drugs are safely used in humans, the potential of VPA to improve PRRT remains open for investigation.

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

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.