Lutetium-177-octreotate treatment of small intestine neuroendocrine tumors - Radiation biology as basis for optimization

Abstract: Patients with neuroendocrine tumors (NETs) often have metastatic spread at the time of diagnosis. NETs frequently express somatostatin receptors (SSTR) that can be targeted by radiolabeled somatostatin analogs (e.g. 177Lu-octreotate). Despite being highly effective in animal models (e.g. the human small intestine NET GOT1 transplanted to nude mice), 177Lu-octreotate-based therapies have shown low cure rates in clinical studies. The cellular processes that underlie positive treatment response to 177Lu-octreotate are largely unknown. The aim of this work was to study the possibilities to optimize the therapeutic effects of 177Lu-octreotate in the GOT1 model in nude mice. A literature study of available data on radiolabeled somatostatin analogs on NETs in animal models was performed, to identify strategies for treatment optimization. To test these strategies, GOT1-bearing BALB/c nude mice were treated with non-curative amounts of 177Lu-octreotate in different treatment schedules including single administrations, priming (fractionated) administrations and combination treatment with hedgehog inhibitor sonidegib. Biodistribution and dosimetry studies were performed and anti-tumor effects were monitored by measuring tumor volume. Global transcriptional and proteomic responses in tumor samples were evaluated using RNA microarray and liquid chromatography mass spectrometry, respectively. 177Lu-octreotate therapy of GOT1 tumors xenotransplanted in nude mice resulted in tumor volume reduction. Priming administration resulted in increased anti-tumor effects and increased therapeutic window. Combination therapy using sonidegib and 177Lu-octreotate resulted in prolonged time to progression. The global transcriptional and proteomic analyses of 177Lu-octreotate treated tumor samples revealed time-specific responses in terms of affected biological functions. In conclusion, time-dependent changes in p53-related cell cycle regulation and apoptosis, angiogenesis, endoplasmic reticulum stress, and oxidative stress-related processes suggest possible niches for combination therapy at different time points after radionuclide therapy. Priming 177Lu-octreotate therapy and combination therapy using sonidegib and 177Lu-octreotate could be beneficial to patients with NE-tumors.