Somatostatin Receptor Subtype 2A Immunohistochemistry Using a New Monoclonal Antibody Selects Tumors Suitable for In Vivo Somatostatin Receptor Targeting
Meike Korner, Beatrice Waser, Agnes Schonbrunn, Aurel Perren, Jean Claude Reubi. Institute of Pathology of the University of Berne, Berne, Switzerland; Health Science Center Houston, University of Texas, Houston, TX
Background: High over-expression of somatostatin receptors in neuroendocrine tumors allows imaging and radiotherapy with radiolabelled somatostatin analogues. To know if a tumor is suitable for such an in vivo somatostatin receptor targeting, its somatostatin receptor expression has to be determined. There are specific indications to use immunohistochemistry for the somatostatin receptor subtype 2A (sst2A) for this purpose, but this has up to now been limited by the lack of an adequate reliable antibody. The aim of the present study was to correlate results of immunohistochemistry using the new monoclonal anti-sst2A antibody UMB-1 with those obtained by the gold standard in vitro method 125I-[Tyr3]-octreotide autoradiography that quantifies somatostatin receptor levels in tumor tissues.
Design: Eighty-nine tumors were subjected to UMB-1 immunohistochemistry and in vitro 125I-[Tyr3]-octreotide autoradiography. UMB-1 staining levels were semi-quantitatively assessed with respect to the fraction of stained tumor cells and the staining intensity. These UMB-1 staining levels were compared with somatostatin receptor binding site levels quantified with autoradiography.
Results: An immunohistochemical staining threshold was defined to distinguish tumors with somatostatin receptor levels high enough for clinical applications from those with low receptor expression. The presence of more than 10% of tumor cells positive for UMB-1 correctly predicted high autoradiographic somatostatin receptor levels in 95% of cases. Conversely, no UMB-1 staining at all truly reflected low or no autoradiographic somatostatin receptor expression in 96% of tumors. If 1-10% of tumor cells were stained, a weak staining intensity was suggestive of low somatostatin receptor levels.
Conclusions: For the first time, a reliable recommendation concerning the eligibility of an individual patient for in vivo somatostatin receptor targeting can be made based on sst2A immunohistochemistry. Under optimal methodological conditions, UMB-1 immunohistochemistry may be equivalent to in vitro receptor autoradiography.
Monday, March 19, 2012 1:15 PM
Platform Session: Section H, Monday Afternoon