Human SSTR3 Stable Cell Line-CHO-K1

There is a significant unmet medical need in the treatment of non-functioning pituitary adenomas (NFPAs), as these tumors are often aggressive, difficult to completely resect, and prone to recurrence. NFPAs highly express somatostatin receptor 3 (SSTR3), making SSTR3 agonists a promising therapeutic avenue; however, no SSTR3 agonist with suitable characteristics for clinical trials has yet been identified. Researchers unexpectedly discovered that ITF2984-a molecule originally developed as a pan-SSTR agonist-acts as a full SSTR3 agonist in in vitro assays. In contrast, pasireotide, another pan-SSTR agonist approved for treating Cushing's disease, did not exhibit this full agonist activity. These findings prompted the researchers to evaluate ITF2984 in in vivo models that recapitulate human disease characteristics, revealing significant anti-tumor activity. ITF2984 has already demonstrated a favorable safety profile in Phase II clinical trials involving patients with acromegaly; the drug is now ready for clinical testing in NFPA patients, offering the potential to provide a long-sought therapeutic option.

ITF2984 induced complete phosphorylation of SSTR3 in a dose-dependent manner (Figure 1A, left), with an effective dose in the pharmacologically relevant nanomolar (nM) range; in contrast, pasireotide produced only a weak effect at supra-pharmacological micromolar (µM) doses. Regarding SSTR2, both cyclic hexapeptides induced selective phosphorylation at the S341/S343 sites, whereas SRIF14 and octreotide caused complete phosphorylation of SSTR2 (Figure 1B, left). Finally, regarding the pT333 phosphorylation site of SSTR5, all tested compounds had only a partial effect, with pasireotide showing the most significant impact (Figure 1C, left). Somatostatin receptor signaling is known to activate G protein-coupled inwardly rectifying potassium (GIRK) channels. Therefore, the researchers investigated SSTR agonist-mediated G protein signaling in hSSTR3-overexpressing HEK293 cells using a GIRK-based fluorescence membrane potential assay. Stimulation of HEK293 cells stably expressing the human SSTR3 receptor with SRIF14, octreotide, pasireotide, and ITF2984 resulted in a dose-dependent decrease in the fluorescence signal of the FMP dye, with ITF2984 demonstrating greater potency than octreotide or pasireotide (Figure 1A-C, right).

Figure 1. SSTR phosphorylation (left panel) and agonist-mediated G protein signaling of SSTRs in HEK293 cells (right panel). (Modena D, et al., 2023)