Combination of terbium-161 with somatostatin receptor antagonists-a potential paradigm shift for the treatment of neuroendocrine neoplasms.

Abstract

The β^¯-emitting terbium-161 also emits conversion and Auger electrons, which are believed to be effective in killing single cancer cells. Terbium-161 was applied with somatostatin receptor (SSTR) agonists that localize in the cytoplasm (DOTATOC) and cellular nucleus (DOTATOC-NLS) or with a SSTR antagonist that localizes at the cell membrane (DOTA-LM3). The aim was to identify the most favorable peptide/terbium-161 combination for the treatment of neuroendocrine neoplasms (NENs). The capability of the ¹⁶¹Tb- and ¹⁷⁷Lu-labeled somatostatin (SST) analogues to reduce viability and survival of SSTR-positive AR42J tumor cells was investigated in vitro. The radiopeptides' tissue distribution profiles were assessed in tumor-bearing mice. The efficacy of terbium-161 compared to lutetium-177 was investigated in therapy studies in mice using DOTATOC or DOTA-LM3, respectively. In vitro, [¹⁶¹Tb]Tb-DOTA-LM3 was 102-fold more potent than [¹⁷⁷Lu]Lu-DOTA-LM3; however, ¹⁶¹Tb-labeled DOTATOC and DOTATOC-NLS were only 4- to fivefold more effective inhibiting tumor cell viability than their ¹⁷⁷Lu-labeled counterparts. This result was confirmed in vivo and demonstrated that [¹⁶¹Tb]Tb-DOTA-LM3 was significantly more effective in delaying tumor growth than [¹⁷⁷Lu]Lu-DOTA-LM3, thereby, prolonging survival of the mice. A therapeutic advantage of terbium-161 over lutetium-177 was also manifest when applied with DOTATOC. Since the nuclear localizing sequence (NLS) compromised the in vivo tissue distribution of DOTATOC-NLS, it was not used for therapy. The use of membrane-localizing DOTA-LM3 was beneficial and profited from the short-ranged electrons emitted by terbium-161. Based on these preclinical data, [¹⁶¹Tb]Tb-DOTA-LM3 may outperform the clinically employed [¹⁷⁷Lu]Lu-DOTATOC for the treatment of patients with NENs.