Preclinical Evaluation of a Radiotheranostic Single-Domain Antibody Against Fibroblast Activation Protein α

Fibroblast activation protein α (FAP) is highly expressed on cancer-associated fibroblasts of epithelial-derived cancers. Breast, colon, and pancreatic tumors often show strong desmoplastic reactions, which result in a dominant presence of stromal cells. FAP has gained interest as a target for molecular imaging and targeted therapies. Single-domain antibodies (sdAbs) are the smallest antibody-derived fragments with beneficial pharmacokinetic properties for molecular imaging and targeted therapy. Methods: We describe the generation, selection, and characterization of a sdAb against FAP. In mice, we assessed its imaging and therapeutic potential after radiolabeling with tracer-dose 131I and 68Ga for SPECT and PET imaging, respectively, and with 131I and 225Ac for targeted radionuclide therapy. Results: The lead sdAb, 4AH29, exhibiting picomolar affinity for a distinct FAP epitope, recognized both purified and membrane-bound FAP protein. Radiolabeled versions, including [68Ga]Ga-DOTA-4AH29, [225Ac]Ac-DOTA-4AH29, and [131I]I-guanidinomethyl iodobenzoate (GMIB)-4AH29, displayed radiochemical purities exceeding 95% and effectively bound to recombinant human FAP protein and FAP-positive GM05389 human fibroblasts. These radiolabeled compounds exhibited rapid and specific accumulation in human FAP-positive U87-MG glioblastoma tumors, with low but specific uptake in lymph nodes, uterus, bone, and skin (∼2-3 percentage injected activity per gram of tissue [%IA/g]). Kidney clearance of unbound [131I]I-GMIB-4AH29 was fast (<1 %IA/g after 24 h), whereas [225Ac]Ac-DOTA-4AH29 exhibited slower clearance (8.07 ± 1.39 %IA/g after 24 h and 2.47 ± 0.18 %IA/g after 96 h). Mice treated with [225Ac]Ac-DOTA-4AH29 and [131I]I-GMIB-4AH29 demonstrated prolonged survival compared with those receiving vehicle solution. Conclusion: [68Ga]Ga-DOTA-4AH29 and [131I]I-GMIB-4AH29 enable precise FAP-positive tumor detection in mice. Therapeutic [225Ac]Ac-DOTA-4AH29 and [131I]I-GMIB-4AH29 exhibit strong and sustained tumor targeting, resulting in dose-dependent therapeutic effects in FAP-positive tumor-bearing mice, albeit with kidney toxicity observed later for [225Ac]Ac-DOTA-4AH29. This study confirms the potential of radiolabeled sdAb 4AH29 as a radiotheranostic agent for FAP-positive cancers, warranting clinical evaluation.

Targeted α-Therapy Using 225Ac Radiolabeled Single-Domain Antibodies Induces Antigen-Specific Immune Responses and Instills Immunomodulation Both Systemically and at the Tumor Microenvironment

Targeted radionuclide therapy (TRT) using targeting moieties labeled with α-particle-emitting radionuclides (α-TRT) is an intensely investigated treatment approach as the short range of α-particles allows effective treatment of local lesions and micrometastases. However, profound assessment of the immunomodulatory effect of α-TRT is lacking in literature. Methods: Using flow cytometry of tumors, splenocyte restimulation, and multiplex analysis of blood serum, we studied immunologic responses ensuing from TRT with an antihuman CD20 single-domain antibody radiolabeled with ²²⁵Ac in a human CD20 and ovalbumin expressing B16-melanoma model. Results: Tumor growth was delayed with α-TRT and increased blood levels of various cytokines such as interferon-γ, C-C motif chemokine ligand 5, granulocyte-macrophage colony-stimulating factor, and monocyte chemoattractant protein-1. Peripheral antitumoral T-cell responses were detected on α-TRT. At the tumor site, α-TRT modulated the cold tumor microenvironment (TME) to a more hospitable and hot habitat for antitumoral immune cells, characterized by a decrease in protumoral alternatively activated macrophages and an increase in antitumoral macrophages and dendritic cells. We also showed that α-TRT increased the percentage of programmed death-ligand 1 (PD-L1)-positive (PD-L1^pos) immune cells in the TME. To circumvent this immunosuppressive countermeasure we applied immune checkpoint blockade of the programmed cell death protein 1-PD-L1 axis. Combination of α-TRT with PD-L1 blockade potentiated the therapeutic effect, however, the combination aggravated adverse events. A long-term toxicity study revealed severe kidney damage ensuing from α-TRT. Conclusion: These data suggest that α-TRT alters the TME and induces systemic antitumoral immune responses, which explains why immune checkpoint blockade enhances the therapeutic effect of α-TRT. However, further optimization is warranted to avoid adverse events.

Preclinical Evaluation of 225Ac-Labeled Single-Domain Antibody for the Treatment of HER2pos Cancer

Human epidermal growth factor receptor type 2 (HER2) is overexpressed in various cancers; thus, HER2-targeting single-domain antibodies (sdAb) could offer a useful platform for radioimmunotherapy. In this study, we optimized the labeling of an anti-HER2-sdAb with the α-particle-emitter 225Ac through a DOTA-derivative. The formed radioconjugate was tested for binding affinity, specificity and internalization properties, whereas cytotoxicity was evaluated by clonogenic and DNA double-strand-breaks assays. Biodistribution studies were performed in mice bearing subcutaneous HER2pos tumors to estimate absorbed doses delivered to organs and tissues. Therapeutic efficacy and potential toxicity were assessed in HER2pos intraperitoneal ovarian cancer model and in healthy C57Bl/6 mice. [225Ac]Ac-DOTA-2Rs15d exhibited specific cell uptake and cell-killing capacity in HER2pos cells (EC50 = 3.9 ± 1.1 kBq/mL). Uptake in HER2pos lesions peaked at 3 hours (9.64 ± 1.69% IA/g), with very low accumulation in other organs (<1% IA/g) except for kidneys (11.69 ± 1.10% IA/g). α-camera imaging presented homogeneous uptake of radioactivity in tumors, although heterogeneous in kidneys, with a higher signal density in cortex versus medulla. In mice with HER2pos disseminated tumors, repeated administration of [225Ac]Ac-DOTA-2Rs15d significantly prolonged survival (143 days) compared to control groups (56 and 61 days) and to the group treated with HER2-targeting mAb trastuzumab (100 days). Histopathologic evaluation revealed signs of kidney toxicity after repeated administration of [225Ac]Ac-DOTA-2Rs15d. [225Ac]Ac-DOTA-2Rs15d efficiently targeted HER2pos cells and was effective in treatment of intraperitoneal disseminated tumors, both alone and as an add-on combination with trastuzumab, albeit with substantial signs of inflammation in kidneys. This study warrants further development of [225Ac]Ac-DOTA-2Rs15d.

Labeling of Anti-HER2 Nanobodies with Astatine-211: Optimization and the Effect of Different Coupling Reagents on Their in Vivo Behavior

The use of nanobodies (Nbs) as vehicles in targeted alpha therapy (TAT) has gained great interest because of their excellent properties. They combine high in vivo affinity and specificity of binding with fast kinetics. This research investigates a novel targeted therapy that combines the α-particle emitter astatine-211 (²¹¹At) and the anti-HER2 Nb 2Rs15d to selectively target HER2+ cancer cells. Two distinctive radiochemical methodologies are investigated using three different coupling reagents. The first method uses the coupling reagents, N-succinimidyl 4-(1,2-bis-tert-butoxycarbonyl)guanidinomethyl-3-(trimethylstannyl)benzoate (Boc₂-SGMTB) and N-succinimidyl-3-(trimethylstannyl)benzoate (m-MeATE), which are both directed to amino groups on the Nb, resulting in random conjugation. The second method aims at obtaining a homogeneous tracer population, via a site-specific conjugation of the N-[2-(maleimido)ethyl]-3-(trimethylstannyl)benzamide (MSB) reagent onto the carboxyl-terminal cysteine of the Nb. The resulting radioconjugates are evaluated in vitro and in vivo. 2Rs15d is labeled with ²¹¹At using Boc₂-SGMTB, m-MeATE, and MSB. After astatination and purification, the binding specificity of the radioconjugates is validated on HER2+ cells, followed by an in vivo biodistribution assessment in SKOV-3 xenografted mice. α-camera imaging is performed to determine uptake and activity distribution in kidneys/tumors. 2Rs15d astatination resulted in a high radiochemical purity >95% for all radioconjugates. The biodistribution studies of all radioconjugates revealed comparable tumor uptake (higher than 8% ID/g at 1 h). [²¹¹At]SAGMB-2Rs15d showed minor uptake in normal tissues. Only in the kidneys, a higher uptake was measured after 1 h, but decreased rapidly after 3 h. Astatinated Nbs consisting of m-MeATE or MSB reagents revealed elevated uptake in lungs and stomach, indicating the presence of released ²¹¹At. α-Camera imaging of tumors revealed a homogeneous activity distribution. The radioactivity in the kidneys was initially concentrated in the renal cortex, while after 3 h most radioactivity was measured in the medulla, confirming the fast washout into urine. Changing the reagents for Nb astatination resulted in different in vivo biodistribution profiles, while keeping the targeting moiety identical. Boc₂-SGMTB is the preferred reagent for Nb astatination because of its high tumor uptake, its low background signals, and its fast renal excretion. We envision [²¹¹At]SAGMB-2Rs15d to be a promising therapeutic agent for TAT and aim toward efficacy evaluation.

Targeted alpha therapy using short-lived alpha-particles and the promise of nanobodies as targeting vehicle

Introduction: The combination of a targeted biomolecule that specifically defines the target and a radionuclide that delivers a cytotoxic payload offers a specific way to destroy cancer cells. Targeted radionuclide therapy (TRNT) aims to deliver cytotoxic radiation to cancer cells and causes minimal toxicity to surrounding healthy tissues. Recent advances using α-particle radiation emphasizes their potential to generate radiation in a highly localized and toxic manner because of their high level of ionization and short range in tissue.

Areas covered: We review the importance of targeted alpha therapy (TAT) and focus on nanobodies as potential beneficial vehicles. In recent years, nanobodies have been evaluated intensively as unique antigen-specific vehicles for molecular imaging and TRNT.

Expert opinion: We expect that the efficient targeting capacity and fast clearance of nanobodies offer a high potential for TAT. More particularly, we argue that the nanobodies’ pharmacokinetic properties match perfectly with the interesting decay properties of the short-lived α-particle emitting radionuclides Astatine-211 and Bismuth-213 and offer an interesting treatment option particularly for micrometastatic cancer and residual disease.