Preclinical evaluation of 225Ac-labeled minigastrin analog DOTA-CCK-66 for Targeted Alpha Therapy

Abstract

The recently developed metabolically more stable minigastrin derivative, DOTA-CCK-66, displayed promising preclinical data when labeled either with ⁶⁸Ga or ¹⁷⁷Lu. First positron emission tomography/computed tomography (PET/CT) imaging using [⁶⁸Ga]Ga-DOTA-CCK-66 in two patients suffering from medullary thyroid carcinoma (MTC) displayed a favorable biodistribution profile. Here, we aim to investigate the therapeutic potential of [²²⁵Ac]Ac-DOTA-CCK-66 as a targeted α-therapy (TAT) agent in a comparative treatment study of [¹⁷⁷Lu]Lu- versus [²²⁵Ac]Ac-DOTA-CCK-66.

Methods: Treatment studies were performed (3 groups, n = 5, AR42J tumor-bearing 394-NOD SCID mice). Control group animals were injected with [⁶⁸Ga]Ga-DOTA-CCK-66 (1.1 MBq, PET/CT imaging), while treatment group animals received a single dose of either [¹⁷⁷Lu]Lu-DOTA-CCK-66 (37 MBq, radioligand therapy (RLT)) or [²²⁵Ac]Ac-DOTA-CCK-66 (37 kBq, TAT). All animals' tumor volume and body weight were monitored twice a week until end-point criteria were reached. Blood samples were evaluated (VetScan VS2, Abaxis) once mice were sacrificed.

Results: Upon treatment, an initial decline in tumor volume, followed by a significantly delayed tumor growth of treated cohorts, was observed. Mean survival of ¹⁷⁷Lu- as well as ²²⁵Ac-treated animals was increased by 3- (37 ± 3 d) and 4.5-fold (54 ± 6 d), respectively, when compared to non-treated animals (12 ± 3 d). Blood sample analysis did not indicate toxic side effects to the liver, kidney, or stomach upon ¹⁷⁷Lu and ²²⁵Ac-treatment.

Conclusion: We demonstrated a substantial therapeutic efficacy of ¹⁷⁷Lu- and ²²⁵Ac-labeled DOTA-CCK-66. As expected, treatment with the latter resulted in the highest mean survival rates. These results indicate a high therapeutic potential of ²²⁵Ac-labeled DOTA-CCK-66 for TAT in MTC patient management.

Keywords: Actinium-225; CCK-2R; MTC; Targeted Alpha Therapy (TAT).

Fig. 1

In vitro characterization of [²²⁵Ac]Ac-DOTA-CCK-66. A Stability in human serum after incubation at 37 °C for 1–10 days (n = 3, depicted in green colors), as well as a quality control chromatogram of the ²²⁵Ac-labeled peptide (light grey) and a reference chromatogram of free ²²⁵Ac (dark grey). B Lipophilicity depicted as distribution coefficients at pH = 7.4 (logD_7.4) of [²²⁵Ac]Ac-DOTA-CCK-66, [¹⁷⁷Lu]Lu-DOTA-CCK-66 and [⁶⁷Ga]Ga-DOTA-CCK-66. *Data taken from Günther et al. [24]. C CCK-2R expression of AR42J cells as determined by flow cytometry using a polyclonal CCK antibody

Fig. 2

Study design of animal experiments (5 × 5 animals). AR42J tumors are depicted as black dots. Blood sample analysis of ¹⁷⁷Lu- and ²²⁵Ac-treated tumor-bearing and naïve mice was performed

Fig. 3

Representative PET/CT scan ([⁶⁸Ga]Ga-DOTA-CCK-66; 1.1 MBq, 30 pmol) of a control group AR42J tumor-bearing mouse on day 0 of the experiment. Tumor is indicated with a white arrow

Fig. 4

A Tumor growth inhibition, B body weight and (C) Kaplan-Meyer curves of AR42J tumor-bearing mice treated with [²²⁵Ac]Ac-DOTA-CCK-66 (37 kBq, n = 5) or [¹⁷⁷Lu]Lu-DOTA-CCK-66 (37 MBq, n = 5), compared to untreated control (n = 5)

Fig. 5

Blood values of untreated control (AR42J tumor, n = 5) versus AR42J tumor-bearing 394-NOD SCID treatment group mice ([²²⁵Ac]Ac-DOTA-CCK-66: 37 kBq, n = 5 and [¹⁷⁷Lu]Lu-DOTA-CCK-66: 37 MBq, n = 5) as well as naïve animals injected either with [²²⁵Ac]Ac-DOTA-CCK-66 (37 kBq, n = 5) or [¹⁷⁷Lu]Lu-DOTA-CCK-66 (37 MBq, n = 4)

Fig. 6

Representative images of (A) HE as well as (B) IHC stains of different organs isolated from the untreated control (10–17 d after injection) and [²²⁵Ac]Ac-DOTA-CCK-66-treated animals (49–64 d after injection). All images are shown at a 20-fold magnification. Brown staining indicates CCK-2R positivity