Anti-tumor effects of peptide hormone analogs in pheochromocytoma cell lines and preclinical tumor imaging

Objective
Pheochromocytoma (PHEO) is a rare but potentially lethal neuroendocrine tumor arising from catecholamine producing chromaffin cells. Available treatment strategies are limited and, if the tumor has metastasized, not very effective. The abundant expression of peptide hormone receptors on endocrine tumor cells allows specific targeting by highly effective anti-tumor peptide analogs. The present study focuses on the preclinical evaluation of potential therapies in the treatment of pheochromocytoma targeting peptide hormone receptors.

Design and method
Our in vitro evaluation of peptide hormone receptor expression on the mouse pheochromocytoma (MPC) cell line and a more malignant mouse tumor tissue-derived (MTT) cell line was based on RT-PCR and immunohistological analysis. Based on these data, we evaluated the effects of cytotoxic peptide hormone analogs on cell viability, apoptosis and necrosis on MPC and MTT cells. For in vivo studies, we established a subcutaneous mouse model of MPC-derived PHEO and multimodal tumor imaging using PET, MRI, and CT, fluorescence and bioluminescence.

Results and conclusions
Our data reveal significant anti-tumor effects mediated by the cytotoxic peptide hormone analogs AN-162 and AN-238 targeting somatostatin receptor 2 (sst2), by the antagonist Cetrorelix and the cytotoxic analog AN-152 targeting luteinizing hormone-releasing hormone receptor (LHRH-R) as well as by the antagonist MIA-602 targeting growth hormone-releasing hormone receptors (GHRH-R) on MPC cells. Similar anti-tumor effects were evidenced for AN-152 and MIA-602 also on the more aggressive MTT cells. Using our newly established mouse model, we were able to visualize the growth of MPC cell-derived subcutaneous pheochromocytomas in vivo by multimodal molecular imaging including SSTR2 PET. Additionally, ex vivo tumor characterization demonstrated unaltered peptide hormone receptor expression during in vivo tumor growth in mice.
Our current investigation provides strong evidence for a possible future treatment of malignant pheochromocytoma using targeted peptide hormone receptor therapy.