Somatostatin agonist and mTOR inhibitors as potential radioprotectors or radiosensitizers in neuroendocrine tumor cells

Objectives Overeypression of somatostatin receptors onneuroendocrine tumors (NETs) are currently utilized for diagnostic and therapeutic approaches. For peptide receptor radionuclide therapy (PRRT), synthetic somatostatin analogs like DOTATOC or DOTATATE are coupled to radionuclides like 90Y or 177Lu, which after injection are bound and taken up specifically by NETs. Nevertheless, complete remission is extremely rare, compared to similar radiotherapies for thyroid cancer and non-Hodgkin lymphoma despite achieving very high tumor radiation dose upto 250 Gy. One hypothesis potentially explaining the discrepancies between expected result and observed outcome of PRRT might be that somatostatin analogs induce a G1 arrest in NET cells, thereby rendering them radioresistant. Other medical therapeutics like mTOR inhibitors may also contribute to radiosensitivity towards PRRT.

Methods To investigate this, NET cell lines were incubated with or without somatostatin agonists and mTOR inhibitors and exposed to different radiation doses from a 137Cs source between 0 and 50 Gy. Cells were harvested at distinct timepoints, stained with propidium iodine and the cell cycle distribution was assessed by FACS analysis. Proliferation and vitality were assessed using cell counting, clonogenic assay and Alamar Blue assay.

Results Initially, the somatostatin receptor expression (SSTR1-5) of five neuroendocrine tumour cell lines (BON, H727, QGP-1, LCC18, UMC11) was analyzed by quantitative real time PCR to assess the cell specific expression patterns of the different receptor subtypes. In radiation experiments all of five tested NET cell lines showed a radiation-induced decline in G1 cells as well as a G2/M arrest. Also, the percentage of apoptotic cells in sub-G1 phase increased time- and dose-dependently. Pretreatment with a potential G1 inhibitor did not only result in an expected G1 arrest but also in a decrease of radiation-induced G2/M arrest. Moreover, a reduction of cells in sub-G1 was observed in two of five cell lines. In contrast, the combination of octreotide treatment with radiation showed no difference compared to radiation alone, not supporting the initial hypothesis. While somatostatin analogues showed no influence, mTOR inhibitors were able to enhance radiosensitivity in three NET cell lines.

Conclusions Further experiments are needed to better understand the influence of somatostatin agonists as well as mTOR inhibitors and other intrinsic or extrinsic factors on neuroendocrine tumor cells to pave the way for a significant increase in the overall therapeutic efficacy of PRRT.