Comparison of SSTR2 agonist and antagonist peptides incorporating a novel ^99mTc-chelator for targeting neuroendocrine tumors

Objectives Radiolabeled octreotide analogs are used for the detection and treatment of neuroendocrine tumors (NET). We evaluated the affinity, specificity, agonist/antagonist properties, tumor uptake and tissue distribution of a somatostatin receptor 2 (SSTR2) agonist (Tyr3-octreotide (TOC)) and antagonist (Cpa-cyclo[D-Cys-Aph(Hor)-D-Aph(Cbm)-Lys-Thr-Cys]D-TyrNH₂ (ANT)) after N-terminal incorporation of a novel polar single amino acid chelator (2,2'-(2,2'-(5-amino-5-carboxypentylazanediyl)-bis(methylene)bis(1H-imidazole-2,1-diyl))diacetic acid (CIM)) for labeling with ^99mTc(CO)₃.

Methods Peptides were prepared by SPPS to >95% purity. Competitive binding and saturation binding were performed using AR42J cells. Peptides were assessed for SSTR specificity and agonist/antagonist activity via receptor binding, and internalization assays. ^99mTc-labeled peptides were evaluated in SSTR2(+) AR42J xenografts, +/- cold competition to evaluate uptake and specificity of localization.

Results ^99mTc-CIM-TOC and ^99mTc-CIM-ANT bound to AR42J cells with high affinity (K_d = 9.7 nM and 4.2 nM by saturation binding, respectively), similar to ¹¹¹In-DOTA-TOC (34 nM). Re-CIM-TOC and Re-CIM-ANT maintained agonist and antagonist properties, respectively, and SSTR2 subtype specificity. Both peptides exhibited low accumulation in non-target tissues and similar high tumor uptake of >18 %ID/g, superior to ¹¹¹In-DOTA-TOC (8.5 %ID/g), with tumor:blood >8:1 and tumor:muscle >30:1 at 1 h. Receptor-specific tumor localization was demonstrated in vivo via dose-dependent reduction when cold peptide (0.01-1,000 μg/kg) was co-injected.

Conclusions ^99mTc-CIM-TOC and ^99mTc-CIM-ANT bind specifically and with high affinity to SSTR2 on AR42J cells, and exhibit high tumor uptake in vivo. The non-target organ kinetics and high tumor uptake make these peptides attractive candidates for clinical evaluation in patients with NET