Potential indications for somatostatin analogues: immune system and limphoproliferative disorders

Among hormones and neuropeptides influencing the immune system, somatostatin seems to play a key role not only in inhibiting specific immune cell activities, but also in promoting selected functions of particular immune cell subsets. Indeed, controversial effects have been observed in experimental conditions where somatostatin seems to stimulate certain cell functions, such as secretion of specific products (immunoglobulin, cytokines), cell migration and adhesion to extracellular matrix components. However, interestingly, cortistatin (CST), a neuropeptide that strongly resembles somatostatin, from both the structural and functional points of view, seems to have potential roles in regulating immune responses, as well as other lymphoid cell functions. The unexpected wide distribution of CST in a number of human organs, but particularly in immune cells, points to a broader physiological role of CST than previously presumed. The actions of somatostatin and its synthetic analogs (SSA) are mediated by five membrane G protein-coupled receptors subtypes (SSTR1-5), displaying a tissue specific distribution. The majority of somatostatin-target tissues, including lymphoid tissues, may co-express multiple somatostatin receptor (SSTR). The number of SSTRs in lymphoid cells is significantly lower compared to neuroendocrine tissues. However, the presence of receptors allowed the localization by in vivo SSTR scintigraphy of lymphoproliferative disorders, as well as granulomatous and autoimmune diseases. In specific cases, this technique may contribute to establishing the diagnosis and staging the disease. Recent studies evaluating the specific and quantitative SSTR distribution in lymphoid organs and cells, in both normal conditions and immune disorders, have largely contributed to better understand the phenomenology of in vivo receptor imaging and also the involvement of the different SSTR in determining the uptake of radiolabeled SSAs. Moreover, since lymphomas are highly radiosensitive malignancies, a promising approach in refractory patients with malignant lymphomas may be represented by radionuclide-targeted therapy with radioactive-coupled SSAs combined with gene therapy. This latter technique seems effective in inducing the expression or increasing the number of given SSTR in order to ameliorate the impact of radionuclide-targeted therapy. Medical treatment of lymphoproliferative diseases with currently available synthetic analogs have produced unsatisfactory and conflicting results. This might be due to the affinity of the current available SSAs for specific SSTR. However, the synthesis of new compounds with distinct properties has reopened a challenge in this field. The application of receptor-based localization and anti-tumor strategies should also be taking into account the new knowledge recently emerged on the physiopathology of neuropeptide receptors: firstly, neuropeptide receptor homo- and heterodimerization, which may involve different subtypes of SSTRs, as well as other neuropetide receptors, and secondly, the role of endogenous SSTR ligands, such as CST.