OBJECTIVE:The frequent overexpression of somatostatin receptors (sst) in neuroendocrine tumors provides the molecular basis for the diagnostic and therapeutic application of stable somatostatin analogs. Whereas octreotide acts mainly via the sst(2) receptor, the novel pan-somatostatin analog pasireotide exhibits particular high affinity for the sst(5) receptor. To determine whether a patient is a candidate for octreotide or pasireotide therapy, it is important to evaluate the somatostatin receptor status. However, so far highly specific rabbit monoclonal antibodies have been developed for the sst(2) receptor only (clone UMB-1).METHODS:Here, we have extensively characterized a novel rabbit monoclonal antibody for the human sst(5) receptor (clone UMB-4). In a comparative immunohistochemical study, the expression of sst(5) and sst(2) receptors was assessed using UMB-4 and UMB-1, respectively.RESULTS:Western blot experiments unequivocally demonstrated that UMB-4 selectively detected its cognate sst(5) receptor and did not cross-react with other proteins present in crude tissue homogenates. UMB-4 yielded a highly effective immunostaining of distinct cell populations in formalin-fixed, paraffin-embedded human tissues with a predominance of plasma membrane staining. In the pituitary, sst(5) was present on all growth hormone (GH)- and adrenocorticotropin hormone (ACTH)-producing cells whereas sst(2) was only observed on a subpopulation of GH-positive cells. Consequently, sst(5) was detectable on the majority of GH and ACTH adenomas. In contrast, sst(2) was only seen on GH but not on ACTH adenomas.CONCLUSIONS:The rabbit monoclonal antibodies UMB-4 and UMB-1 will facilitate the assessment of the somatostatin receptor status of human tumors during routine histopathological examinations.