Value of immunohistochemistry for somatostatin receptor subtype sst2A in cancer tissues : Lessons from the comparison of anti-sst2A antibodies with somatostatin receptor autoradiography

The somatostatin receptor sst2A is highly expressed at the cellular surface of neuroendocrine and other human tumor cells, allowing somatostatin receptor-targeted scintigraphic tumor imaging and tumor therapy. In vitro information on tumoral somatostatin receptor expression is provided mainly by receptor autoradiography, based on binding of radiolabeled somatostatin analogs to tumoral somatostatin receptors. Recent availability of anti-sst2A antibodies opens the possibility of sst2A assessment in human tumors with immunohistochemistry. The aim of the present study was to evaluate the usefulness of several commercial anti-sst2A antibodies for this purpose in comparison with the extensively characterized antibody R2-88 and with receptor autoradiography. sst2A immunohistochemistry was performed in 64 formalin-fixed tumors known to frequently express sst2A (pancreatic, gastrointestinal, pulmonary neuroendocrine tumors, breast carcinomas, meningiomas, pituitary adenomas, neuroblastomas, medulloblastomas, pheochromocytomas, and paragangliomas); receptor autoradiography could be performed simultaneously in 37 of these cases. The commercial antibody SS-800 clearly identified sst2A and correlated excellently with R2-88: compared with R2-88, SS-800 immunohistochemistry generally yielded the same tissular and subcellular staining distribution. Results of R2-88 and SS-800 immunohistochemistry correlated excellently with those obtained with receptor autoradiography; compared with receptor autoradiography, immunohistochemistry with both R2-88 and SS-800 resulted in a slightly lower tumoral sst2A incidence, but a higher resolution, with frequent identification of heterogeneous sst2A distribution at high magnification. Finally, not only membranous, but also cytoplasmic, sst2A immunostaining was simultaneously observed with both antibodies in some tumors. In conclusion, the commercially available SS-800 antibody promises to be useful for the routine immunohistochemical assessment of sst2A in formalin-fixed human tumors.