Targeted DNA damage through SSTR2: Preclinical development of a novel peptide-drug conjugate for neuroendocrine tumors

Targeted induction of DNA damage in tumor cells offers a promising strategy to efficiently kill cancer cells while sparing healthy tissues. Precise tumor targeting can be achieved by the exploitation of the differential expression of specific molecular targets on the surface of tumor cells. Somatostatin receptor 2 (SSTR2) has emerged as a valuable tumor-associated target, due to its high expression in gastroenteropancreatic neuroendocrine tumors (GEP-NETs). SSRT2 is not only expressed in these differentiated, slow-proliferating NET, but also in highly proliferative and metastatic neuroendocrine carcinomas, such as small cell lung cancer (SCLC). SSTR2 overexpression occurs in up to 50% of primary tumors from SCLC patients and is correlated with poor prognosis, highlighting SSTR2 as an optimal candidate for targeted therapeutic intervention in a subset of SCLC patients. To exploit this opportunity, we designed a peptide drug conjugate (PDC), named ITF3912 which combines a modified octreotide analog with a novel and potent duocarmicin via a cathepsin-cleavable linker. In preclinical studies in vitro ITF3912 demonstrated high affinity and selectivity for SSTR2, acting as an agonist with an IC₅₀ of 8 nM and a selectivity index of at least 10 over SSTR3/5, with no significant binding observed for SSTR1 or SSTR4. ITF3912 induced cytotoxicity in SSTR2-expressing cells, with efficacy correlated to SSTR2 expression levels. In high level SSTR2 expressing NCI-H524 cells, ITF3912 was rapidly internalized and localized in lysosomes where enzymatic cleavage of the linker released the active toxin. Once in the nucleus, the toxin induced an S-phase replication block and a potent and lethal DNA damage as evidenced by the activation of key biomarkers such as the phosphorylation of ATM, ATR, CHK1, CHK2, RPA32 and gamma-H2AX, leading to cell death with an IC50 of 4 nM. The NCI-H69 cell line that expresses lower level of SSTR2 can still be targeted by ITF3912 but with lower efficiency (60 nM). The activity on NCI-H727 cell line that expresses very low level of SSTR2 diminished as expected (400 nM). Consistent with the in vitro findings, ITF3912 showed strong antitumor efficacy in vivo. It effectively reduced the tumor growth of NCI-H524 with a survival rate of 50% after 80 days of treatment and several mice showed complete tumor remission. In contrast, ITF3912 was less effective on NCI-H727 cell line by inducing a tumor growth reduction of 30%. Collectively, these data establish ITF3912 as a potent and selective SSTR2-targeted PDC, whose efficacy in vitro and in vivo correlates with receptor expression levels. These results support the use of ITF3912 as a therapeutic option for patients with SSTR2-positive neuroendocrine tumors, addressing a significant unmet medical need. Encouraging preliminary toxicology results, further support the progression of ITF3912 toward clinical development.