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

Abstract

Purpose: 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).

Methods: 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.

Results: 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.

Conclusion: 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.

Keywords: Auger electrons; NEN; PRRT; Radionuclide therapy; SSTR antagonists; Terbium-161.

Fig. 1

Graphs representing cell uptake and localization of a/e radiolabeled DOTATOC; b/f radiolabeled DOTATOC-NLS, and c/g radiolabeled DOTA-LM3

Fig. 2

Graphs representing AR42J tumor cell viability and survival after treatment. a/b/c Results of the cell viability assessment (MTT assay); d/e/f results of the cell survival assessment (clonogenic assay)

Fig. 3

DSBs quantification (γH2AX) in AR42J cells. a Representative γH2AX-staining in sham-treated cells or cells treated with the respective radiopeptide (scale bar: 50 μm). Quantification of γH2AX staining in b sham-treated cells; or cells treated with c [¹⁶¹Tb]Tb-/[¹⁷⁷Lu]Lu-DOTATOC; d [¹⁶¹Tb]Tb-/[¹⁷⁷Lu]Lu-DOTATOC-NLS or e [¹⁶¹Tb]Tb-/[¹⁷⁷Lu]Lu-DOTA-LM3. The number of positive cells is expressed in percent relative to the 1% positive cases detected in average in sham-treated cells

Fig. 4

SPECT/CT images of AR42J tumor-bearing mice shown as maximum intensity projections 2 h after injection of the radiopeptides (15 MBq, 1.0 nmol per mouse). Mice injected with a [¹⁶¹Tb]Tb-DOTATOC [43]; b [¹⁶¹Tb]Tb-DOTATOC-NLS (b), and c [¹⁶¹Tb]Tb-DOTA-LM3 [43]. AR42J = SSTR-positive xenograft; Ki = kidneys; Li = liver; Bl = urinary bladder

Fig. 5

Biodistribution data obtained in AR42J tumor-bearing mice. Results after injection of a [¹⁶¹Tb]Tb-DOTATOC (0.2 nmol/mouse) and b [¹⁶¹Tb]Tb-DOTA-LM3 (0.2 nmol/mouse) shown in percentage of injected activity per tissue mass (% IA/g) and presented as the average ± SD of n = 3

Fig. 6

Results of the therapy study performed with ¹⁶¹Tb- and ¹⁷⁷Lu-SSTR agonist and antagonist (2 × 10 MBq; 0.2 nmol) in AR42J tumor-bearing mice. a The RTV at day 0 (set as 1) for mice of Group A (sham), B ([¹⁶¹Tb]Tb-DOTATOC), C ([¹⁷⁷Lu]Lu-DOTATOC), D ([¹⁶¹Tb]Tb-DOTA-LM3), and E ([¹⁷⁷Lu]Lu-DOTA-LM3). Data are shown until the first mouse of the respective group reached an endpoint. b/c/d/e/f Absolute TV of single mice (grey lines) and average (colored line) of Groups A–E; g tumor growth delay of Group A–E; h tumor doubling time of Groups A–E; i Kaplan–Meier plot of Groups A–E

Fig. 7

Parameters of potential side effects in therapy mice. a/b/c Indicators for the general health status of the mice: RBW during the therapy (a) shown until the first mouse of the group reached an endpoint, RBW at the endpoint (b), and plasma ALB levels (c). d/e/f Indicators for kidney toxicity: kidney-to-brain mass ratios (d), plasma CRE (e), plasma BUN levels (f). g/h/i Indicators for liver toxicity: liver-to-brain mass ratios (g), plasma ALP levels (h), and plasma TBIL levels (i). Organ-to-brain mass ratios and plasma parameters refer to mice at the endpoint