Immunohistochemical localization and quantitative expression of somatostatin receptors in normal human spleen and thymus: Implications for the in vivo visualization during somatostatin receptor scintigraphy

Background: [¹¹¹In-DTPA-D-Phe¹]-octreotide scintigraphy allows the visualization of SRIF receptor (SSR)-expressing tumors, including thymic tumors, and normal tissues. While the spleen is clearly visualized, the thymus is not depicted, although both contain SSR. Aim: We evaluated whether the heterogeneity, the type, and the amount of SSR might explain this contrasting finding. Materials, methods, and results: By ligand-binding the number of [¹²⁵I-Tyr¹¹]-SRIF-14 binding sites resulted comparable between the two tissues, whereas the number of [ ¹²⁵I-Tyr³]-octreotide sites was significantly higher in the spleen (p<0.001). Quantitative RT-PCR showed a significantly higher expression of sst_2A mRNA in the spleen, whereas a significantly higher expression of SRIF and sst₃ in the thymus. The highest density of sst_2A in the spleen is in line with the in vivo uptake of [ ¹¹¹In-DTPA-D-Phe¹]-octreotide, which is considered a sst2-preferring ligand. The specificity is confirmed by the evidence that in vivo [¹¹¹In-DT-PA-D-Phe¹]-octreotide uptake can be abolished during chronic administration of "cold" octreotide. Immunohistochemistry confirmed a preferential expression of sst^2A on microenvironmental cells and of sst₃ on lymphoid cells. Conclusions: The heterogeneity of SSR expression and the higher SRIF content explain the lack of thymus visualization during scintigraphy, whereas thymic tumors, which do not express SRIF, are visualized. Apart from the affinity of the radioligand, also the efficacy of the internalization is crucial for the in vivo uptake, and both heterogeneity and SRIF content affect this process. These observations might have an important impact when interpretating in vivo visualization of SSR-positive lesions, and when treatment with novel SRIF analogs is considered.