Gastrointestinal stromal tumors (GISTs) express somatostatin receptors and bind radiolabeled somatostatin analogs

Background. Gastrointestinal stromal tumors (GISTs) can be effectively treated with tyrosine kinase inhibitors (TKIs). However, some patients with GIST develop drug resistance, and alternative treatment strategies are therefore needed. The aim of this study was to analyze the expression of somatostatin receptors (SSTR) in GIST as a target for peptide receptormediated radiotherapy (PRRT). Material and methods. Expression profi ling of SSTR1 – 5 was performed on biopsies from 34 GISTs (16 gastric tumors, 15 small intestinal tumors, and three rectal tumors). SSTR scintigraphy ( 111 In-octreotide) and measurement of 111 In activity in tumor specimens was performed in seven patients. Uptake and internalization of 177 Luoctreotate was studied in primary cell cultures from two patients. Results. Quantitative PCR analysis showed expression of SSTR1 and SSTR2 in the majority of tumors, while SSTR3 – 5 were expressed at low levels. Immunohistochemical analysis confi rmed the presence of SSTR1 and SSTR2 proteins in all GISTs, and SSTR3 – 5 in a subset of tumors. Diagnostic imaging by SSTR scintigraphy, using 111 In-octreotide, demonstrated tumor uptake of 111 In in three of six GIST patients. Measurement of 111 In activity in excised tumor specimens from fi ve patients gave tumor-to-blood (T/B) activity ratios of between eight and 96. Tumor cells in primary culture (gastric and small intestinal GIST) specifi cally bound and internalized 177 Lu when incubated with the therapeutic compound 177 Lu-octreotate for 4 – 48 hours (p 0.05). Conclusion. Peptide receptor-mediated radiotherapy via SSTR may provide a novel treatment strategy in carefully selected GIST patients with TKI-resistant tumors.