Preclinical evaluation of [225Ac]Ac-crown-TATE – An alpha-emitting radiopharmaceutical for neuroendocrine tumors

Abstract

Background

Targeted alpha therapy (TAT) of somatostatin receptor-2 (SSTR2) positive neuroendocrine tumors (NETs) involving Ac-225 ([²²⁵Ac]Ac-DOTA-TATE) has previously demonstrated improved therapeutic efficacy over conventional beta particle-emitting peptide receptor radionuclide therapy agents. DOTA-TATE requires harsh radiolabeling conditions for chelation of [²²⁵Ac]Ac³⁺, which can limit the achievable molar activities and thus therapeutic efficacy of such TAT treatments. Macropa-TATE was recently highlighted as a potential alternative to DOTA-TATE, owing to the mild radiolabeling conditions and high affinity toward [²²⁵Ac]Ac³⁺; however, elevated liver and kidney uptake were noted as a major limitation and a suitable imaging radionuclide is yet to be reported, which will be required for patient dosimetry studies and assessment of therapeutic benefit. Previously, [¹⁵⁵Tb]Tb-crown-TATE has shown highly effective imaging of NETs in preclinical SPECT/CT studies, with high tumor uptake and low non-target accumulation; these favourable properties and the versatile coordination behavior of the crown chelator may therefore show promise for combination with Ac-225 for TAT.

Methods

Crown-TATE was labeled with Ac-225, and radiochemical yield was analyzed as the function of crown-TATE concentration. LogD_7.4 was measured as the indication of hydrophilicity. Free [²²⁵Ac]Ac³⁺ release from [²²⁵Ac]Ac-crown-TATE in human serum was studied. Biodistribution studies of [²²⁵Ac]Ac-crown-TATE in mice bearing AR42J tumors was evaluated at 1, 4, 24, 48, and 120 h, and the absorbed dose to major organs calculated. Therapy-monitoring studies with AR42J tumor bearing mice were undertaken using 30 kBq and 55 kBq doses of [²²⁵Ac]Ac-crown-TATE and compared to controls treated with PBS or crown-TATE.

Results

[²²⁵Ac]Ac-crown-TATE was successfully prepared with high molar activity (640 kBq/nmol), and characterized as a moderately hydrophilic radioligand (LogD_7.4 = −1.355 ± 0.135). No release of bound Ac-225 was observed over 9 days in human serum. Biodistribution studies of [²²⁵Ac]Ac-crown-TATE showed good initial tumor uptake (11.1 ± 1.7% IA/g at 4 h) which was sustained up to 120 h p.i. (6.92 ± 2.03% IA/g). Dosimetry calculations showed the highest absorbed dose was delivered to the tumors. Therapy monitoring studies demonstrated significant (log-rank test, P < 0.005) improved survival in both treatment groups compared to controls.

Conclusions

This preclinical study demonstrated the therapeutic efficacy of [²²⁵Ac]Ac-crown-TATE for treatment of NETs, and highlights the potential of using crown chelator for stable chelation of Ac-225 under mild conditions.