Identification of a Novel SSTR3 Full Agonist for the Treatment of Nonfunctioning Pituitary Adenomas

SIMPLE SUMMARY: Therapy for nonfunctioning pituitary adenomas (NFPAs) is a significant unmet medical need since these adenomas are frequently invasive, are difficult to resect completely and often recur. NFPAs express high levels of somatostatin receptor 3 (SSTR3), and SSTR3 agonists could be a promising new treatment, but SSTR3 agonists with sufficient characterization to allow clinical testing are not available. We unexpectedly discovered that ITF2984, a molecule originally developed as a pan-SSTR agonist, is a full agonist of SSTR3 in in vitro assays. A similar full agonism was not observed with Pasireotide another pan-SSTR agonist approved for the treatment of Cushing’s disease. These unexpected findings prompted us to test ITF2984 in an in vivo model that recapitulates the human disease, where ITF2984 showed significant antitumor activity. ITF2984 has completed phase II clinical trials in acromegaly patients, is well tolerated and can be directly tested in NFPA patients, potentially providing a long-sought-after therapeutic option. ABSTRACT: Somatostatin receptor (SSTR) agonists have been extensively used for treating neuroendocrine tumors. Synthetic therapeutic agonists showing selectivity for SSTR2 (Octreotide) or for SSTR2 and SSTR5 (Pasireotide) have been approved for the treatment of patients with acromegaly and Cushing’s syndrome, as their pituitary tumors highly express SSTR2 or SSTR2/SSTR5, respectively. Nonfunctioning pituitary adenomas (NFPAs), which express high levels of SSTR3 and show only modest response to currently available SSTR agonists, are often invasive and cannot be completely resected, and therefore easily recur. The aim of the present study was the evaluation of ITF2984, a somatostatin analog and full SSTR3 agonist, as a new potential treatment for NFPAs. ITF2984 shows a 10-fold improved affinity for SSTR3 compared to Octreotide or Pasireotide. Molecular modeling and NMR studies indicated that the higher affinity for SSTR3 correlates with a higher stability of a distorted β-I turn in the cyclic peptide backbone. ITF2984 induces receptor internalization and phosphorylation, and triggers G-protein signaling at pharmacologically relevant concentrations. Furthermore, ITF2984 displays antitumor activity that is dependent on SSTR3 expression levels in the MENX (homozygous mutant) NFPA rat model, which closely recapitulates human disease. Therefore, ITF2984 may represent a novel therapeutic option for patients affected by NFPA.