Non-functioning pituitary adenomas (NFPAs), mainly gonadotroph pituitary adenomas (GPAs), are the second most common type of PAs. Given the lack of symptoms secondary to hormone hypersecretion, NFPAs are often diagnosed when they cause mass effects. At this stage they are invasive (50% of cases), they cannot be completely resected and often recur. Despite their frequency, no standard of care drug treatment currently exists for these tumors. SSAs (somatostatin agonists) like octreotide and lanreotide, binding to somatostatin receptor (SSTR) 2 and to a lesser extent to SSTR5 and SSTR3, are poorly efficacious in NFPAs. Similarly, the pan-agonist pasireotide, binding to SSTR 1,2,3,5, showed only modest efficacy in a recent phase II clinical trial of NFPA patients. SSTR3 was found to be frequently and strongly expressed in NFPAs, while SSTR2 only in few patients and SSTR5 only exceptionally. We here report the characterization of ITF2984, a novel cyclic hexapeptide pan-SSTR agonist with high SSTR3 specificity. While structurally analogous to other pan-agonists, ITF2984 shows higher affinity for SSTR3 vs known molecules. Molecular modeling revealed that higher α-II turn probability in ITF2984 correlated with higher SSTR3 affinity, thus providing a rationale for its unique selectivity. ITF2984 inhibited GH release from GHRH-stimulated rat anterior pituitary primary cells similarly to pasireotide. To functionally characterize ITF2984 mode of action, the following assays were performed:
Receptor internalization in HEK293 and U2OS cell lines overexpressing human SSTRs.
SSA-mediated activation of human SSTRs by western blotting using phosphosite-specific antibodies.
Agonist-mediated G protein-signaling of SSTR3 (GIRK activation) in SSTR3 overexpressing HEK293 and AtT20 cells.
Unlike pasireotide and octreotide, ITF2984 induced SSTR3 internalization and phosphorylation, as well as GIRK activation in a pharmacologically relevant concentration range. Thus, ITF2984 behaved as a full SSTR3 agonist. The in vivo activity of ITF2984 was tested in the MENX NFPA rat model. Adenomas developing in this model have a gender-specific SSTR3 expression pattern, with higher expression levels in females. Consistent with its receptor affinity profile and the in vitro data, ITF2984 showed selective antitumor activity in female rats, accompanied by a decrease in proliferation (KI67 positivity). In addition, ITF2984 selectively induced SSTR3 mRNA expression in female rat tumors, suggesting a compensatory gene upregulation. These data are in line with an in vivo engagement of SSTR3 and a predominantly SSTR3-driven antitumor activity of ITF2984 and provide an in vivo proof-of-concept for the potential clinical use of ITF2984 in NFPAs and other SSTR3-driven diseases.
21 May 2022 - 24 May 2022