A new ⁶⁸Ga-labeled somatostatin analog containing two iodo-amino acids for dual somatostatin receptor subtype 2 and 5 targeting

Background: Somatostatin receptor (SST) targeting, specifically of the subtype 2 (SST2), with radiolabeled somatostatin analogs, is established for imaging and treatment of neuroendocrine tumors. Owing to the concomitant and heterogeneous expression of several subtypes on the same tumor, analogs targeting more subtypes than SST2 potentially target a broader spectrum of tumors and/or increase the uptake of a given tumor. The analog ST8950 ((4-amino-3-iodo)-D-Phe-c[Cys-(3-iodo)-Tyr-D-Trp-Lys-Val-Cys]-Thr-NH₂), bearing 2 iodo-amino acids, exhibits sub-nanomolar affinity to SST2 and SST5. We report herein the development and preclinical evaluation of DOTA-ST8950 labeled with ⁶⁸Ga, for imaging SST2- and SST5-expressing tumors. Comparative in vitro and in vivo studies were performed with the de-iodinated DOTA-ST8951 ((4-amino)-D-Phe-c[Cys-Tyr-D-Trp-Lys-Val-Cys]-Thr-NH₂) and with the reference compounds DOTA-TATE (SST2 selective) and DOTA-NOC (for SST2 and SST5).

Results: Compared with ^natGa-DOTA-NOC, ^natGa-DOTA-ST8950 exhibited higher affinity to SST2 and SST5 (IC₅₀ (95%CI), nM = 0.32 (0.20-0.50) and 1.9 (1.1-3.1) vs 0.70 (0.50-0.96) and 3.4 (1.8-6.2), respectively), while ^natGa-DOTA-ST8951 lost affinity for both subtypes. ^natGa-DOTA-ST8950 had the same potency for inducing SST2-mediated cAMP accumulation as ^natGa-DOTA-TATE and slightly better than ^natGa-DOTA-NOC (EC₅₀, nM = 0.46 (0.23-0.92) vs 0.47 (0.15-1.5) vs 0.59 (0.18-1.9), respectively). [⁶⁷Ga]Ga-DOTA-ST8950 had a similar internalization rate as [⁶⁷Ga]Ga-DOTA-NOC in SST2-expressing cells (12.4 ± 1.6% vs 16.6 ± 2.2%, at 4 h, p = 0.0586). In vivo, [⁶⁸Ga]Ga-DOTA-ST8950 showed high and specific accumulation in SST2- and SST5-expressing tumors, comparable with [⁶⁸Ga]Ga-DOTA-NOC (26 ± 8 vs 30 ± 8 %IA/g, p = 0.4630 for SST2 and 15 ± 6 vs 12 ± 5 %IA/g, p = 0.3282, for SST5, 1 h p.i.) and accumulation in the SST-positive tissues, the kidneys and the liver. PET/CT images of [⁶⁸Ga]Ga-DOTA-ST8950, performed in a dual HEK-SST2 and HEK-SST5 tumor xenografted model, clearly visualized both tumors and illustrated high tumor-to-background contrast.

Conclusions: [⁶⁸Ga]Ga-DOTA-ST8950 reveals its potential for PET imaging SST2- and SST5-expressing tumors. It compares favorably with the clinically used [⁶⁸Ga]Ga-DOTA-NOC in terms of tumor uptake; however, its uptake in the liver remains a challenge for clinical translation. In addition, this study reveals the essential role of the iodo-substitutions in positions 1 and 3 of [⁶⁸Ga]Ga-DOTA-ST8950 for maintaining affinity to SST2 and SST5, as the de-iodinated [⁶⁸Ga]Ga-DOTA-ST8951 lost affinity for both receptor subtypes.