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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. [ABSTRACT FROM AUTHOR]

Purpose: Fibroblast Activation Protein (FAP) is an emerging theranostic target that is highly expressed on cancer-associated fibroblasts and on certain tumor cells including sarcoma. We investigated the anti-tumor efficacy of [225Ac]Ac-FAPI-46 as monotherapy or in combination with immune checkpoint blockade (ICB) in immunocompetent murine models of sarcoma sensitive or resistant to ICB. Methods: [68Ga]Ga- and [225Ac]Ac-FAPI-46 were tested in subcutaneous FAP+ FSA fibrosarcoma bearing C3H/Sed/Kam mice. The efficacy of up to three cycles of 60 kBq [225Ac]Ac-FAPI-46 was evaluated as monotherapy and in combination with an anti-PD-1 antibody. Efficacy of [225Ac]Ac-FAPI-46 and/or ICB was further compared in FAP-overexpressing FSA (FSA-F) tumors that were sensitive to ICB or rendered ICB-resistant by tumor-induction in the presence of Abatacept. Results: [225Ac]Ac-FAPI-46 was well tolerated up to 3 × 60 kBq but had minimal effect on FSA tumor growth. The combination of three cycles [225Ac]Ac-FAPI-46 and ICB resulted in growth delay in 55% of mice (6/11) and partial tumor regression in 18% (2/11) of mice. In FSA-F tumors with FAP overexpression, both [225Ac]Ac-FAPI-46 and ICB were effective without additional benefits from the combination. In locally immunosuppressed and ICB resistant FAP-F tumors, however, [225Ac]Ac-FAPI-46 restored responsiveness to ICB, resulting in significant tumor regression and tumor-free survival of 56% of mice in the combination group up to 60 days post treatment. Conclusion: [225Ac]Ac-FAPI-46 efficacy is correlated with tumoral FAP expression levels and can restore responsiveness to PD-1 ICB. These data illustrate that careful patient selection based on target expression and rationally designed combination therapies are critically important to maximize the therapeutic impact of FAP-targeting radioligands. [ABSTRACT FROM AUTHOR]

The costimulatory molecule CD80 is an early marker for immune activation. It is upregulated on activated antigen-presenting cells. We aimed at developing a tracer for imaging CD80 by positron emission tomography (PET). Novel CD80 ligands were synthesized and tested by SPR for affinity to human CD80 (hCD80) and displacement of endogenous ligands. Several compounds bound with one-digit nanomolar affinity to hCD80 and displaced CTLA-4 and CD28 at nanomolar concentrations. A structure-affinity relationship study revealed relevant moieties for strong affinity to hCD80 and positions for further modifications. Lead compound MT107 ( 7f ) was radiolabeled with carbon-11. In vitro, [ 11 C]MT107 showed specific binding to hCD80-positive tissue and high plasma protein binding. In vivo, [ 11 C]MT107 accumulated in liver, gall bladder, and intestines but only scarcely in hCD80-positive xenografts. The unfavorable in vivo performance may result from high plasma protein binding and extensive biliary excretion.