Pretargeting: A Path Forward for Radioimmunotherapy

Harnessing nanoparticles and bioorthogonal chemistries for improving precision of nuclear medicine

The convergence of nanotechnology, radiopharmaceutical development and molecular imaging has unveiled exciting opportunities for the progress of innovative diagnostic and therapeutic strategies, paving the way for significant advancements in biomedical research, especially in relation to cancer. For example, the use of highly sensitive and quantitative nuclear imaging techniques including PET and SPECT, together with nanoparticles for tumour imaging and therapy has recently expanded rapidly. While the long circulating properties of many nanomaterials are beneficial for prodrug chemotherapy formulations, due to the constant decay processes involved in nuclear medicines, directly labelled materials result in prolonged systemic radiation exposure and reduced therapeutic indices due to the unfavourable target-to-background ratios. This is due to the tendency for long circulating nanomaterials to distribute within the blood to other organs, such as the liver and spleen. The recent integration of bioorthogonal chemistry with nanotechnology and molecular imaging/radiotherapy has revolutionized the field by allowing the decoupling of the targeting molecule (i.e. nanomaterial with a bioorthogonal tag) and the imaging/therapeutic radioisotope. In this way, the detection/therapeutic element can be administered as a secondary "chase" molecule that contains the bioorthogonal partner, thereby creating an avenue to improve therapeutic index and provide imaging and treatments with reduced risk. This review will provide an overview of the progress made thus far in the field of nuclear imaging and radiotherapy for cancer using the combination of nanomaterials and bioorthogonal chemistry. We also provide a critical evaluation of the challenges and opportunities for using these approaches to better understand disease and treatment mechanisms, with the potential for downstream clinical translation.

Radioimmunotherapy for peritoneal carcinomatosis: Preclinical proof of concept to clinical translation

Peritoneal carcinomatosis (PC), characterized by the dissemination of metastatic tumor cells throughout the peritoneal cavity from several gastrointestinal and gynecological malignancies, has significantly compromised patient survival. The standard of care is cytoreductive surgery with or without intraperitoneal chemotherapy. However, surgical resection often leaves behind microscopic or clinically occult disease due to the complex anatomy of the peritoneum, where intraperitoneal chemotherapy and systemic chemotherapy have shown limited success. To improve the therapeutic outcome, targeted therapy using radionuclides such as alpha, beta, and Auger emitters delivered by antibodies is actively being investigated. While preclinical murine models of PC have shown the potential of radioimmunotherapy (RIT) using various radioisotopes across a wide spectrum of antigen targets and tumor diagnoses with acceptable toxicities, successful clinical trials are lacking. Here, we retrospectively summarize preclinical and clinical PC studies, consider their translational potential, and examine paths to development that maximize the clinical benefit of RIT in this context.

The Advancement of Targeted Alpha Therapy and the Role of Click Chemistry Therein

Recent years have seen a swift rise in the use of α-emitting radionuclides such as 225Ac and 223Ra as various radiopharmaceuticals to treat (micro)metastasized tumors. They have shown remarkable effectiveness in clinical practice owing to the highly cytotoxic α-particles that are emitted, which have a very short range in tissue, causing mainly double-stranded DNA breaks. However, it is essential that both chelation and targeting strategies are optimized for their successful translation to clinical application, as α-emitting radionuclides have distinctly different features compared to β--emitters, including their much larger atomic radius. Furthermore, upon α-decay, any daughter nuclide irrevocably breaks free from the targeting molecule, known as the recoil effect, dictating the need for faster targeting to prevent healthy tissue toxicity. In this review we provide a brief overview of the current status of targeted α-therapy and highlight innovations in α-emitter-based chelator design, focusing on the role of click chemistry to allow for fast complexation to biomolecules at mild labeling conditions. Finally, an outlook is provided on different targeting strategies and the role that pre-targeting can play in targeted alpha therapy.

Toward Realization of Bioorthogonal Chemistry in the Clinic

In the last decade, the use of bioorthogonal chemistry toward medical applications has increased tremendously. Besides being useful for the production of pharmaceuticals, the efficient, nontoxic reactions open possibilities for the development of therapies that rely on in vivo chemistry between two bioorthogonal components. Here we discuss the latest developments in bioorthogonal chemistry, with a focus on their use in living organisms, the translation from model systems to humans, and the challenges encountered during preclinical development. We aim to provide the reader a broad presentation of the current state of the art and demonstrate the numerous possibilities that bioorthogonal reactions have for clinical use, now and in the near future.

Comparison of radiobiological effects induced by radiolabeled antibodies in human cancer cells and fungal cells

PURPOSE

Acute myeloid leukemia (AML) is a deadly form of leukemia, and its treatment often leaves patients immunocompromised, making them vulnerable to fungal infections. Radioimmunotherapy (RIT) is explored for both AML and fungal infections. This study compares the radiobiological effects of alpha emitter Actinium-225 (225Ac) and beta emitter Lutetium-177 (177Lu)-labeled antibodies on AML and Cryptococcus neoformans cells.

MATERIALS AND METHODS

AML OCI-AML3 and C. neoformans Cap-67 cells were treated with anti-(1-3)-beta-glucan antibody 400-2 and anti-CD33 antibody HuM-195, conjugated to DOTA and radiolabeled with 225Ac or 177Lu. Clonogenic survival, γH2A/X staining, and micronuclei assays were conducted. Antibody internalization was assessed by flow cytometry.

RESULTS

Both 225Ac- and 177Lu-enabled RIT resulted in decreased clonogenic survival in Cap-67 and OCI-AML3 cells, with Cap-67 recovering more rapidly. DNA double-strand breaks and micronuclei formation revealed DNA damage, with fewer micronuclei in OCI-AML3 cells due to radiation destruction. HuM-195 antibody internalized into OCI-AML3 cells, whereas 400-2 did not internalize into Cap-67 cells.

CONCLUSIONS

While both cell lines showed similar responses to 225Ac- and 177Lu-enabled RIT, variations were observed based on cellular structure, doubling times and DNA repair mechanisms. This study offers insights for future in vivo research on fungal infections in cancer setting.

Optimizing Cancer Treatment: Exploring the Role of AI in Radioimmunotherapy

Radioimmunotherapy (RIT) is a novel cancer treatment that combines radiotherapy and immunotherapy to precisely target tumor antigens using monoclonal antibodies conjugated with radioactive isotopes. This approach offers personalized, systemic, and durable treatment, making it effective in cancers resistant to conventional therapies. Advances in artificial intelligence (AI) present opportunities to enhance RIT by improving precision, efficiency, and personalization. AI plays a critical role in patient selection, treatment planning, dosimetry, and response assessment, while also contributing to drug design and tumor classification. This review explores the integration of AI into RIT, emphasizing its potential to optimize the entire treatment process and advance personalized cancer care.

Enhanced mRNA delivery using ultrasound-delivered click reactive anchors

Therapeutic nucleic acid delivery has many potential applications, but it remains challenging to target extrahepatic tissues in a flexible and image-guided manner. To address this issue, we report a bioorthogonal pre-targeting strategy that uses focused ultrasound to promote the delivery of mRNA-loaded lipid nanoparticles (mRNA-LNP). We synthesized amphiphilic click reactive anchors (ACRAs) consisting of a phospholipid PEG-conjugate functionalized with transcyclooctene (TCO) or its companion reactive partner methyltetrazine (mTz), yielding ACRA-TCO and ACRA-mTz. ACRA derivatives were screened for cellular activity, yielding functionalized DOPE-PEG (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N- (polyethylene glycol)) derivatives outperforming those containing saturated lipid or branched PEG. Nanobubbles encapsulating ultrasound-responsive gas precursor delivered ACRA-TCO to targeted cells and tissues using focused ultrasound, and this pre-targeting promoted the subsequent delivery of mRNA- LNP functionalized with companion ACRA-mTz. In cell cultures and in mice, ultrasound pre-targeting enhanced the accumulation of mTz-functionalized small molecule and nanoparticle compounds by 75% and 3.6-fold, respectively, and increased gene expression using mRNA-LNP in vivo . Taken together, this report presents a modular, ultrasound-enabled strategy for enhancing nucleic acid delivery in targeted tissues.

Expression-Dependent Tumor Pretargeting via Engineered Avidity

Selective delivery of therapeutic modalities to tumor cells via binding of tumor-selective cell-surface biomarkers has empowered substantial advances in cancer treatment. Yet, tumor cells generally lack a truly specific biomarker that is present in high density on tumor tissue while being completely absent from healthy tissue. Rather, low but nonzero expression in healthy tissues results in on-target, off-tumor activity with detrimental side effects that constrain the therapeutic window or prevent use altogether. Advanced technologies to enhance the selectivity for tumor targeting are sorely needed. We have engineered a binding platform that is quantitatively dependent upon expression levels, via avidity-driven specificity, rather than binarily reliant on the presence or absence of a biomarker. We systematically varied monomeric binding affinity by engineering affibodies to target carcinoembryonic antigen-related cell adhesion molecule 5 (CEACAM5) and folate receptor 1 (FolR1). Two identical affibody ligands were tethered, with varying polypeptide linker lengths, to a nanobody that binds Alfa peptide to create a bispecific, trivalent protein for use in pretargeted radioligand therapy. Expression-dependent targeting was achieved in both systems: with 110 nM monomeric affinity to CEACAM5 with a two-amino-acid linker or with 250 nM monomeric affinity for FolR1 and a 10 amino acid linker. The latter bispecific, trivalent achieved over 25-fold differentiation between FolR1high and FolR1low cells in a mixed culture. Similar selectivity was achieved in a size-efficient bivalent molecule lacking a central nanobody. Moreover, the avid bivalent affibody molecule exhibited minimal inhibition by soluble antigen, whereas high-affinity bivalent antibody was inhibited by 97 ± 2%, which is indicative of serum inhibition of shed antigen. This work advances design principles for achieving expression-dependent tumor targeting via low-affinity, high-avidity ligands.

Pretargeted alpha therapy in MUC16-positive high-grade serous ovarian cancer

BACKGROUND

Peritoneal metastasis with micrometastatic cell clusters is a common feature of advanced ovarian cancer. Targeted alpha therapy (TAT) is an attractive approach for treating micrometastatic diseases as alpha particles release enormous amounts of energy within a short distance. A pretargeting approach - leveraging the inverse-electron-demand Diels-Alder reaction between tetrazines (Tz) and trans-cyclooctene (TCO) - can minimize off-target toxicity related to TAT, often associated with full-length antibodies. We hypothesized that a pretargeting strategy could effectively treat high-grade serous (HGS) ovarian tumors while minimizing toxicity.

METHODS

We utilized the humanized antibody, AR9.6, labeled with actinium-225 (225Ac). AR9.6 targets fully glycosylated and hypoglycosylated isoforms of MUC16. For biodistribution and radioimmunotherapy studies, AR9.6-TCO was injected into OVCAR3-bearing mice 72 h before administering [225Ac]Ac-mcp-PEG8-Tz, e.g. using a 1,2,4,5-tetrazine conjugated to the macropa chelator via a polyethylene glycol (PEG) linker.

RESULTS

Biodistribution data revealed that the pretargeting approach achieved substantial tumor uptake. Cerenkov luminescence imaging confirmed successful in vivo pretargeting during TAT studies. Compared to the control groups, TAT with AR9.6-TCO and [225Ac]Ac-mcp-PEG8-Tz significantly suppressed tumor growth and improved overall survival in OVCAR3 tumor-bearing mice. Renal and ovarian pathology compatible with toxicity was observed in mice in addition to transient hematologic toxicity.

CONCLUSION

We confirmed that pretargeting with AR9.6-TCO and [225Ac]Ac-mcp-PEG8-Tz has durable antitumor effects in high MUC16-expressing tumors. These findings demonstrate great potential for using pretargeting in combination with TAT for the treatment of ovarian cancer.

CLASSIFICATION

Biological Sciences; Applied Biological Sciences.

In Vitro and In Vivo Comparison of Random versus Site-Specific Conjugation of Bifunctional Chelating Agents to the CD33-Binding Antibody for Use in Alpha- and Beta-Radioimmunotherapy

Radiometal chelator conjugation is a cornerstone of radioimmunotherapy (RIT). Continued interest in selective placement of chelators remains an active topic of discussion in the field. With several simple site-specific methods being recently reported, it was of interest to investigate the benefits and potential drawbacks of the site-specific method with a full comparison to a more typical random conjugation method that is currently utilized in clinical applications. In this study, the conjugation methods were evaluated side by side to determine the utility of both methods using commercially available random and site-specific conjugation reagents by performing antigen binding; radiolabeling with 64Cu, 177Lu, and 225Ac radioisotopes; antibody-conjugate stability, cytotoxicity, in vivo distribution, pharmacokinetics analyses, and dosimetry to gather a whole data set for preclinical investigation. Evaluation revealed that both methods performed similarly during most experiments with the site-specific method, resulting in higher binding capacity of the antibody conjugate via flow cytometry. Radiolabeling was not significantly different between two methods, while stability showed that the site-specifically conjugated antibody was somewhat more stable at 37 °C in human serum over 1 week. In vitro experiments demonstrated less cell killing with the random conjugation method, while in vivo experiments showed no statistical differences in tumor uptake between conjugation methods. Dosimetry calculations were performed using the acquired PET/CT data and showed that apart from the liver, there was no significant difference in radiation doses delivered by either antibody conjugate. These results demonstrate that both methods are viable for future work, while the site-specific method offers several potential advantages and, in some cases, improved efficacy.

Molecular imaging of renal cell carcinomas: ready for prime time

The clinical diagnosis of renal cell carcinoma (RCC) is constantly evolving. Diagnostic imaging of RCC relying on enhanced computed tomography (CT) and magnetic resonance imaging (MRI) is commonly used for renal mass characterization and assessment of tumour thrombosis, whereas pathology is the gold standard for establishing diagnosis. However, molecular imaging is rapidly improving the clinical management of RCC, particularly clear-cell RCC. Molecular imaging aids in the non-invasive visualization and characterization of specific biomarkers such as carbonic anhydrase IX and CD70 within the tumours, which help to assess tumour heterogeneity and status. Target-specific molecular imaging of RCCs will substantially improve the diagnostic landscape of RCC and will further facilitate clinical decision-making regarding initial staging and re-staging, monitoring of recurrence and metastasis, patient stratification and selection, and the prediction and evaluation of treatment responses.

Advancing Cancer Theranostics Through Integrin αVβ3-Targeted Peptidomimetic IAC: From Bench to Bedside

Introduction: The expression of alpha-five beta-three (αVβ3) integrins is upregulated in various malignancies undergoing angiogenesis. The development of integrin antagonists as diagnostic probes makes the αVβ3 integrin a suitable candidate for targeting tumor angiogenesis. The goal of this study was to optimize the radiolabeling and evaluate the potential of conjugated integrin antagonist carbamate (IAC), a peptidomimetic, as a theranostic radiopharmaceutical for targeting tumor angiogenesis. Methodology: Radiolabeling of DOTAGA [2,2',2"-{10-(2,6-dioxotetrahydro-2H-pyran-3-yl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl} triacetic-acid]-IAC with [68Ga]Ga, [177Lu]Lu, and [225Ac]Ac was optimized. The binding affinity (Kd) of DOTAGA-IAC for the αVβ3 receptor and cancer cell lines was quantified. The biodistribution studies were conducted in healthy Wistar rats. Dosimetry analysis was performed on [177Lu]Lu-DOTAGA-IAC distribution data. A pilot study of [68Ga]Ga-DOTAGA-IAC and [18F]FDG Positron Emission Tomography (PET/CT) imaging was performed in five patients with histopathologically confirmed breast cancer. PET/CT findings were compared between [68Ga]Ga-DOTAGA-IAC and [18F]FDG in these patients. Results: Radiopharmaceuticals were prepared with high radiochemical purity (>99.9%). Kd and Bmax measurements were 15.02 nM and 417 fmol for αVβ3 receptor protein: 115.7 nM and 295.3 fmol for C6 glioma cells. Biodistribution studies in rats suggested the excretion via kidneys and partially through the hepatobiliary route. The effective dose of [177Lu]Lu-DOTAGA-IAC was found to be 0.17 mSv/MBq. The dynamic study in patients revealed the optimal imaging time to be 30-35 mins postadministration. Out of the cohort, [68Ga]Ga-DOTAGA-IAC detected the primary lesions in all five patients with a mean standard uptake value (SUVmax) of 3.94 ± 0.58 compared with [18F]FDG (SUVmax 13.8 ± 6.53). Conclusion: The study demonstrates that DOTAGA-IAC exhibits strong binding to αVβ3 integrin, positioning it as a promising PET agent for assessing primary and metastatic cancers. The outcomes from the pilot study suggest the potential of [68Ga]Ga-DOTAGA-IAC PET/CT in breast carcinoma diagnosis. While recognizing the theranostic potential of DOTAGA-IAC for αVβ3 integrin-expressing tumors, further clinical investigations are warranted to comprehensively assess therapeutic efficacy.

Trends in cancer imaging

Molecular imaging of cancer is a collaborative endeavor, uniting scientists and physicians from diverse fields. Such collaboration is actively developing and translating cutting-edge molecular imaging approaches to enhance the diagnostic landscape of human malignancies. The advent of positron emission tomography (PET) and PET imaging tracers has realized non-invasive target annotation and tumor characterization at the molecular level. In surgical procedures, novel imaging techniques, such as fluorescence or Cherenkov luminescence, help identify tumors and enhance surgical precision. Simultaneously, progress in imaging equipment, innovative algorithms, and artificial intelligence has opened avenues for next-generation cancer screening and imaging, augmenting the efficiency and accuracy of cancer diagnosis. In this review, we provide a panorama of molecular cancer imaging and ongoing developments in the field.

Anti-IL-1RAP scFv-mSA-S19-TAT fusion carrier as a multifunctional platform for versatile delivery of biotinylated payloads to myeloid leukemia cells

Acute myeloid leukemia (AML) is an aggressive blood cancer with frequently poor clinical outcomes. This heterogeneous malignancy encompasses genetically, molecularly, and even clinically different subgroups. This makes it difficult to develop therapeutic agents that are effective for all subtypes of the disease. Therefore, a selective, universal, and adaptable delivery platform capable of carrying various types of anti-neoplastic agents is an unmet requirement in this area. Two multifunctional fusion proteins were designed for the delivery of biotinylated cargoes to human myeloid leukemia cells by fusing an anti-IL-1RAP single-chain antibody with streptavidin (tetramer or monomer), a cell-penetrating peptide (CPP), and an endosomolytic peptide in a single biomacromolecule. The designed fusions were analyzed primarily in silico, and the biofunctionality of the selected fusion was fully characterized via several binding assays, hemolysis assay, confocal microscopy and cell cytotoxicity assay after production via the Escherichia coli (E. coli) system. The refolded protein exhibited desirable binding activity to leukemic cells, pure antigen and biotinylated BSA. Further analyses revealed efficient cellular uptake, endosomolytic activity, and nuclear penetration without any detectable cytotoxicity toward normal epithelial cells. The described platform seems to have great potential for targeted delivery of different therapeutics to malignant myeloid cells.

Theranostic GPA33-Pretargeted Radioimmunotherapy of Human Colorectal Carcinoma with a Bivalent 177Lu-Labeled Radiohapten

Radiolabeled small-molecule DOTA-haptens can be combined with antitumor/anti-DOTA bispecific antibodies (BsAbs) for pretargeted radioimmunotherapy (PRIT). For optimized delivery of the theranostic γ- and β-emitting isotope 177Lu with DOTA-based PRIT (DOTA-PRIT), bivalent Gemini (DOTA-Bn-thiourea-PEG4-thiourea-Bn-DOTA, aka (3,6,9,12-tetraoxatetradecane-1,14-diyl)bis(DOTA-benzyl thiourea)) was developed. Methods: Gemini was synthesized by linking 2 S-2-(4-isothiocyanatobenzyl)-DOTA molecules together via a 1,14-diamino-PEG4 linker. [177Lu]Lu-Gemini was prepared with no-carrier-added 177LuCl3 to a molar-specific activity of 123 GBq/μmol and radiochemical purity of more than 99%. The specificity of BsAb-177Lu-Gemini was verified in vitro. Subsequently, we evaluated biodistribution and whole-body clearance for [177Lu]Lu-Gemini and, for comparison, our gold-standard monovalent [177Lu]Lu-S-2-(4-aminobenzyl)-DOTA ([177Lu]Lu-DOTA-Bn) in naïve (tumor-free) athymic nude mice. For our proof-of-concept system, a 3-step pretargeting approach was performed with an established DOTA-PRIT regimen (anti-GPA33/anti-DOTA IgG-scFv BsAb, a clearing agent, and [177Lu]Lu-Gemini) in mouse models. Results: Initial in vivo studies showed that [177Lu]Lu-Gemini behaved similarly to [177Lu]Lu-DOTA-Bn, with almost identical blood and whole-body clearance kinetics, as well as biodistribution and mouse kidney dosimetry. Pretargeting [177Lu]Lu-Gemini to GPA33-expressing SW1222 human colorectal xenografts was highly effective, leading to absorbed doses of [177Lu]Lu-Gemini for blood, tumor, liver, spleen, and kidneys of 3.99, 455, 6.93, 5.36, and 14.0 cGy/MBq, respectively. Tumor-to-normal tissue absorbed-dose ratios (i.e., therapeutic indices [TIs]) for the blood and kidneys were 114 and 33, respectively. In addition, we demonstrate that the use of bivalent [177Lu]Lu-Gemini in DOTA-PRIT leads to improved TIs and augmented [177Lu]Lu-Gemini tumor uptake and retention in comparison to monovalent [177Lu]Lu-DOTA-Bn. Finally, we established efficacy in SW1222 tumor-bearing mice, demonstrating that a single injection of anti-GPA33 DOTA-PRIT with 44 MBq (1.2 mCi) of [177Lu]Lu-Gemini (estimated tumor-absorbed dose, 200 Gy) induced complete responses in 5 of 5 animals and a histologic cure in 2 of 5 (40%) animals. Moreover, a significant increase in survival compared with nontreated controls was noted (maximum tolerated dose not reached). Conclusion: We have developed a bivalent DOTA-radiohapten, [177Lu]Lu-Gemini, that showed improved radiopharmacology for DOTA-PRIT application. The use of bivalent [177Lu]Lu-Gemini in DOTA-PRIT, as opposed to monovalent [177Lu]Lu-DOTA-Bn, allows curative treatments with considerably less administered 177Lu activity while still achieving high TIs for both the blood (>100) and the kidneys (>30).

Scission-Enhanced Molecular Imaging (SEMI)

Positron emission tomography (PET) imaging methods have advanced our understanding of human biology, while targeted radiotherapeutic drug treatments are now routinely used clinically. The field is expected to grow considerably based on an expanding repertoire of available affinity ligands, radionuclides, conjugation chemistries, and their FDA approvals. With this increasing use, strategies for dose reduction have become of high interest to protect patients from unnecessary and off-target toxicity. Here, we describe a simple and powerful method, scission-enhanced molecular imaging (SEMI). The technique allows for rapid corporeal elimination of radionuclides once imaging or theranostic treatment is completed and relies on "click-to-release" bioorthogonal linkers.

Abscopal effect: from a rare phenomenon to a new frontier in cancer therapy

Radiotherapy (RT) controls local lesions, meantime it has the capability to induce systemic response to inhibit distant, metastatic, non-radiated tumors, which is referred to as the "abscopal effect". It is widely recognized that radiotherapy can stimulate systemic immune response. This provides a compelling theoretical basis for the combination of immune therapy combined with radiotherapy(iRT). Indeed, this phenomenon has also been observed in clinical treatment, bringing significant clinical benefits to patients, and a series of basic studies are underway to amplify this effect. However, the molecular mechanisms of immune response induced by RT, determination of the optimal treatment regimen for iRT, and how to amplify the abscopal effect. In order to amplify and utilize this effect in clinical management, these key issues require to be well addressed; In this review, we comprehensively summarize the growing consensus and emphasize the emerging limitations of enhancing the abscopal effect with radiotherapy or immunotherapy. Finally, we discuss the prospects and barriers to the current clinical translational applications.

Tetrazine-trans-cyclooctene ligation: Unveiling the chemistry and applications within the human body

Bioorthogonal reactions have revolutionized chemical biology by enabling selective chemical transformations within living organisms and cells. This review comprehensively explores bioorthogonal chemistry, emphasizing inverse-electron-demand Diels-Alder (IEDDA) reactions between tetrazines and strained dienophiles and their crucial role in chemical biology and various applications within the human body. This highly reactive and selective reaction finds diverse applications, including cleaving antibody-drug conjugates, prodrugs, proteins, peptide antigens, and enzyme substrates. The versatility extends to hydrogel chemistry, which is crucial for biomedical applications, yet it faces challenges in achieving precise cellularization. In situ activation of cytotoxic compounds from injectable biopolymer belongs to the click-activated protodrugs against cancer (CAPAC) platform, an innovative approach to tumor-targeted prodrug delivery and activation. The CAPAC platform, relying on click chemistry between trans-cyclooctene (TCO) and tetrazine-modified biopolymers, exhibits modularity across diverse tumor characteristics, presenting a promising approach in anticancer therapeutics. The review highlights the importance of bioorthogonal reactions in developing radiopharmaceuticals for positron emission tomography (PET) imaging and theranostics, offering a promising avenue for diverse therapeutic applications.

Development and Validation of Novel Z-360-Based Macromolecules for the Active Targeting of CCK2-R

The cholecystokinin type 2 receptor (CCK2-R) represents an ideal target for cancer therapy since it is overexpressed in several tumors and is associated with poor prognosis. Nastorazepide (Z-360), a selective CCK2-R antagonist, has been widely investigated as a CCK2-R ligand for targeted therapy; however, its high hydrophobicity may represent a limit to cell selectivity and optimal in vivo biodistribution. Here, we present three new fluorescent Z-360 derivatives (IP-002G-Rho, IP-002L-Rho, and IP-002M-Rho) in which nastorazepide was linked, through spacers bearing different saccharides (glucose (G), lactose (L), and maltotriose (M)), to sulforhodamine B. A fourth compound (IP-002H-Rho) with no pendant sugar was also synthesized as a control. Through two-dimensional (2D) and three-dimensional (3D) in vitro studies, we evaluated the compound association with and selectivity for CCK2-R-overexpressing cells (A431-CCK2-R+) vs CCK2-R-underexpressing cells (A431 WT). 2D in vitro studies highlighted a progressive increase of IP-002x-Rho association with A431-CCK2-R+ cells according to the linker hydrophilicity, that is, maltotriose > lactose > glucose > hydrogen, with IP-002M-Rho showing a 2.4- and a 1.36-fold higher uptake than IP-002G-Rho and IP-002L-Rho, respectively. Unexpectedly, IP-002H-Rho showed a similar cell association to that of IP-002L-Rho but with no difference between the two tested cell lines. On the contrary, association with A431-CCK2-R+ cells as compared to the A431 WT was found to be 1.08-, 1.14-, and 1.37-fold higher for IP-002G-Rho, IP-002L-Rho, and IP-002M-Rho, respectively, proving IP-002M-Rho to be the best-performing compound, as also confirmed by competition studies. Trafficking studies on A431-CCK2-R+ cells incubated with IP-002M-Rho suggested the coexistence of receptor-mediated endocytosis and simple diffusion. On the contrary, a high and selective uptake of IP-002M-Rho by A431-CCK2-R+ cells only was observed on 3D scaffolds embedded with cells, underlining the importance of 3D models in in vitro preliminary evaluation.

Engineered Antibodies as Cancer Radiotheranostics

Radiotheranostics is a rapidly growing approach in personalized medicine, merging diagnostic imaging and targeted radiotherapy to allow for the precise detection and treatment of diseases, notably cancer. Radiolabeled antibodies have become indispensable tools in the field of cancer theranostics due to their high specificity and affinity for cancer-associated antigens, which allows for accurate targeting with minimal impact on surrounding healthy tissues, enhancing therapeutic efficacy while reducing side effects, immune-modulating ability, and versatility and flexibility in engineering and conjugation. However, there are inherent limitations in using antibodies as a platform for radiopharmaceuticals due to their natural activities within the immune system, large size preventing effective tumor penetration, and relatively long half-life with concerns for prolonged radioactivity exposure. Antibody engineering can solve these challenges while preserving the many advantages of the immunoglobulin framework. In this review, the goal is to give a general overview of antibody engineering and design for tumor radiotheranostics. Particularly, the four ways that antibody engineering is applied to enhance radioimmunoconjugates: pharmacokinetics optimization, site-specific bioconjugation, modulation of Fc interactions, and bispecific construct creation are discussed. The radionuclide choices for designed antibody radionuclide conjugates and conjugation techniques and future directions for antibody radionuclide conjugate innovation and advancement are also discussed.

In vitro and in vivo evaluation of a tetrazine-conjugated poly-L-lysine effector molecule labeled with astatine-211

BACKGROUND

A significant challenge in cancer therapy lies in eradicating hidden disseminated tumor cells. Within Nuclear Medicine, Targeted Alpha Therapy is a promising approach for cancer treatment tackling disseminated cancer. As tumor size decreases, alpha-particles gain prominence due to their high Linear Energy Transfer (LET) and short path length. Among alpha-particle emitters, 211At stands out with its 7.2 hour half-life and 100% alpha emission decay. However, optimizing the pharmacokinetics of radiopharmaceuticals with short lived radionuclides such as 211At is pivotal, and in this regard, pretargeting is a valuable tool. This method involves priming the tumor with a modified monoclonal antibody capable of binding both the tumor antigen and the radiolabeled carrier, termed the "effector molecule. This smaller, faster-clearing molecule improves efficacy. Utilizing the Diels Alder click reaction between Tetrazine (Tz) and Trans-cyclooctene (TCO), the Tz-substituted effector molecule combines seamlessly with the TCO-modified antibody. This study aims to evaluate the in vivo biodistribution of two Poly-L-Lysine-based effector molecule sizes (10 and 21 kDa), labelled with 211At, and the in vitro binding of the most favorable polymer size, in order to optimize the pretargeted radioimmunotherapy with 211At.

RESULTS

In vivo results favor the smaller polymer's biodistribution pattern over the larger one, which accumulates in organs like the liver and spleen. This is especially evident when comparing the biodistribution of the smaller polymer to a directly labelled monoclonal antibody. The smaller variant also shows rapid and efficient binding to SKOV-3 cells preloaded with TCO-modified Trastuzumab in vitro, emphasizing its potential. Both polymer sizes showed equal or better in vivo stability of the astatine-carbon bond compared to a monoclonal antibody labelled with the same prosthetic group.

CONCLUSIONS

Overall, the small Poly-L-Lysine-based effector molecule (10 kDa) holds the most promise for future research, exhibiting significantly lower uptake in the kidneys and spleen compared to the larger effector (21 kDa) while maintaining an in vivo stability of the astatine-carbon bond comparable to or better than intact antibodies. A proof of concept in vitro cell study demonstrates rapid reaction between the small astatinated effector and a TCO-labelled antibody, indicating the potential of this novel Poly-L-Lysine-based pretargeting system for further investigation in an in vivo tumor model.

Shorter Peptide Nucleic Acid Probes Improve Affibody-Mediated Peptide Nucleic Acid-Based Pretargeting

Affibody-mediated PNA-based pretargeting shows promise for HER2-expressing tumor radiotherapy. In our recent study, a 15-mer ZHER2:342-HP15 affibody-PNA conjugate, in combination with a shorter 9-mer [177Lu]Lu-HP16 effector probe, emerged as the most effective pretargeting strategy. It offered a superior tumor-to-kidney uptake ratio and more efficient tumor targeting compared to longer radiolabeled effector probes containing 12 or 15 complementary PNA bases. To enhance the production efficiency of our pretargeting system, we here introduce even shorter 6-, 7-, and 8-mer secondary probes, designated as HP19, HP21, and HP20, respectively. We also explore the replacement of the original 15-mer Z-HP15 primary probe with shorter 12-mer Z-HP12 and 9-mer Z-HP9 alternatives. This extended panel of shorter PNA-based probes was synthesized using automated microwave-assisted methods and biophysically screened in vitro to identify shorter probe combinations with the most effective binding properties. In a mouse xenograft model, we evaluated the biodistribution of these probes, comparing them to the Z-HP15:[177Lu]Lu-HP16 combination. Tumor-to-kidney ratios at 4 and 144 h postinjection of the secondary probe showed no significant differences among the Z-HP9:[177Lu]Lu-HP16, Z-HP9:[177Lu]Lu-HP20, and the Z-HP15:[177Lu]Lu-HP16 pairs. Importantly, tumor uptake significantly exceeded, by several hundred-fold, that of most normal tissues, with kidney uptake being the critical organ for radiation therapy. This suggests that using a shorter 9-mer primary probe, Z-HP9, in combination with 9-mer HP16 or 8-mer HP20 secondary probes effectively targets tumors while minimizing the dose-limiting kidney uptake of radionuclide. In conclusion, the Z-HP9:HP16 and Z-HP9:HP20 probe combinations offer good prospects for both cost-effective production and efficient in vivo pretargeting of HER2-expressing tumors.

The present and future of bispecific antibodies for cancer therapy

Bispecific antibodies (bsAbs) enable novel mechanisms of action and/or therapeutic applications that cannot be achieved using conventional IgG-based antibodies. Consequently, development of these molecules has garnered substantial interest in the past decade and, as of the end of 2023, 14 bsAbs have been approved: 11 for the treatment of cancer and 3 for non-oncology indications. bsAbs are available in different formats, address different targets and mediate anticancer function via different molecular mechanisms. Here, we provide an overview of recent developments in the field of bsAbs for cancer therapy. We focus on bsAbs that are approved or in clinical development, including bsAb-mediated dual modulators of signalling pathways, tumour-targeted receptor agonists, bsAb-drug conjugates, bispecific T cell, natural killer cell and innate immune cell engagers, and bispecific checkpoint inhibitors and co-stimulators. Finally, we provide an outlook into next-generation bsAbs in earlier stages of development, including trispecifics, bsAb prodrugs, bsAbs that induce degradation of tumour targets and bsAbs acting as cytokine mimetics.

PET-MR Guided, Pre-targeted delivery to HER2(+) Breast Cancer Model

Purpose: HER2(+) metastatic breast cancer (mBC) is one of the most aggressive and lethal cancer types among females. While initially effective, targeted therapeutic approaches with trastuzumab and pertuzumab antibodies and antibody-drug conjugates (ADC) lack long-term efficacy against HER2(+) mBC and can cause severe systemic toxicity due to off-target effects. Therefore, the development of novel targeted delivery platforms that minimize toxicity and increase therapeutic efficacy is critical to the treatment of HER2(+) breast cancer (BC). A pretargeting delivery platform can minimize the non-specific accumulation and off-target toxicity caused by traditional one-step delivery method by separating the single delivery step into a pre-targeting step with high-affinity biomarker binding ligand followed by the subsequent delivery step of therapeutic component with fast clearance. Each delivery component is functionalized with bioorthogonal reactive groups that quickly react in situ , forming cross-linked clusters on the cell surface, which facilitates rapid internalization and intracellular delivery of therapeutics. Procedures: We have successfully developed a click chemistry-based pretargeting platform for HER2(+) BC enabling PET-MR image guidance for reduced radiation dose, high sensitivity, and good soft tissue contrast. Radiolabeled trastuzumab and superparamagnetic iron-oxide carriers (uSPIO) were selected as pretargeting and delivery components, respectively. HER2(+) BT-474 cell line and corresponding xenografts were used for in vitro and in vivo studies. Results: An enhanced tumor accumulation as well as tumor- to-organ accumulation ratio was observed in pretargeted mice up to 24 h post uSPIO injection. A 40% local T 1 decrease in the pretargeted mice tumor was observed within 4 h, and an overall 15% T 1 drop was retained for 24 h post uSPIO injection. Conclusions: Prolonged tumor retention and increased tumor-to-organ accumulation ratio provided a solid foundation for pretargeted image-guided delivery approach for in vivo applications.

Quantum Dot-based Bio-conjugates as an Emerging Bioimaging Tool for Cancer Theranostic- A Review

Cancer is the most widely studied disorder in humans, but proper treatment has not yet been developed for it. Conventional therapies, like chemotherapy, radiation therapy, and surgery, have been employed. Such therapies target not only cancerous cells but also harm normal cells. Conventional therapy does not result in specific targeting and hence leads to severe side effects. The main objective of this study is to explore the QDs. QDs are used as nanocarriers for diagnosis and treatment at the same time. They are based on the principle of theranostic approach. QDs can be conjugated with antibodies via various methods that result in targeted therapy. This results in their dual function as a diagnostic and therapeutic tool. Nanotechnology involving such nanocarriers can increase the specificity and reduce the side effects, leaving the normal cells unaffected. This review pays attention to different methods for synthesising QDs. QDs can be obtained using either organic method and synthetic methods. It was found that QDs synthesised naturally are more feasible than the synthetic process. Top or bottom-up approaches have also emerged for the synthesis of QDs. QDs can be conjugated with an antibody via non-covalent and covalent binding. Covalent binding is much more feasible than any other method. Zero-length coupling plays an important role as EDC (1-Ethyl-3-Ethyl dimethylaminopropyl)carbodiimide is a strong crosslinker and is widely used for conjugating molecules. Antibodies work as surface ligands that lead to antigen- antibody interaction, resulting in site-specific targeting and leaving behind the normal cells unaffected. Cellular uptake of the molecule is done by either passive targeting or active targeting. QDs are tiny nanocrystals that are inorganic in nature and vary in size and range. Based on different sizes, they emit light of specific wavelengths. They have their own luminescent and optical properties that lead to the monitoring, imaging, and transport of the therapeutic moiety to a variety of targets in the body. The surface of the QDs is modified to boost their functioning. They act as a tool for diagnosis, imaging, and delivery of therapeutic moieties. For improved therapeutic effects, nanotechnology leads the cellular uptake of nanoparticles via passive targeting or active targeting. It is a crucial platform that not only leads to imaging and diagnosis but also helps to deliver therapeutic moieties to specific sites. Therefore, this review concludes that there are numerous drawbacks to the current cancer treatment options, which ultimately result in treatment failure. Therefore, nanotechnology that involves such a nanocarrier will serve as a tool for overcoming all limitations of the traditional therapeutic approach. This approach helps in reducing the dose of anticancer agents for effective treatment and hence improving the therapeutic index. QDs can not only diagnose a disease but also deliver drugs to the cancerous site.

Click Chemistry and Radiochemistry: An Update

The term "click chemistry" describes a class of organic transformations that were developed to make chemical synthesis simpler and easier, in essence allowing chemists to combine molecular subunits as if they were puzzle pieces. Over the last 25 years, the click chemistry toolbox has swelled from the canonical copper-catalyzed azide-alkyne cycloaddition to encompass an array of ligations, including bioorthogonal variants, such as the strain-promoted azide-alkyne cycloaddition and the inverse electron-demand Diels-Alder reaction. Without question, the rise of click chemistry has impacted all areas of chemical and biological science. Yet the unique traits of radiopharmaceutical chemistry have made it particularly fertile ground for this technology. In this update, we seek to provide a comprehensive guide to recent developments at the intersection of click chemistry and radiopharmaceutical chemistry and to illuminate several exciting trends in the field, including the use of emergent click transformations in radiosynthesis, the clinical translation of novel probes synthesized using click chemistry, and the advent of click-based in vivo pretargeting.

Preclinical Development in Radiopharmaceutical Therapy for Prostate Cancer

Prostate cancer is a leading cause of cancer death in men worldwide. Among the various treatment options, radiopharmaceutical therapy has shown notable success in metastatic, castration-resistant disease. Radiopharmaceutical therapy is a systemic approach that delivers cytotoxic radiation doses precisely to the malignant tumors and/or tumor microenvironment. Therapeutic radiopharmaceuticals are composed of a therapeutic radionuclide and a high-affinity, tumor-targeting carrier molecule. Therapeutic radionuclides used in preclinical prostate cancer studies are primarily α-, β--, or Auger-electron-emitting radiometals or radiohalogens. Monoclonal antibodies, antibody-derived fragments, peptides, and small molecules are frequently used as tumor-targeting molecules. Over the years, several important membrane-associated proteases and receptors have been identified, validated, and subsequently used for preclinical radiotherapeutic development for prostate cancer. Prostate-specific membrane antigen (PSMA) is the most well-studied prostate cancer-associated protease in preclinical literature. PSMA-targeting radiotherapeutic agents are being investigated using high-affinity antibody- and small-molecule-based agents for safety and efficacy. Early generations of such agents were developed simply by replacing radionuclides of the imaging agents with therapeutic ones. Later, extensive structure-activity relationship studies were conducted to address the safety and efficacy issues obtained from initial patient data. Recent regulatory approval of the 177Lu-labeled low-molecular-weight agent, 177Lu-PSMA-617, is a significant accomplishment. Current preclinical experiments are focused on the structural modification of 177Lu-PSMA-617 and relevant investigational agents to increase tumor targeting and reduce off-target binding and toxicity in healthy organs. While lutetium-177 (177Lu) remains the most widely used radionuclide, radiolabeled analogs with iodine-131 (128I), yttrium-90 (89Y), copper-67 (67Cu), and terbium-161 (161Tb) have been evaluated as potential alternatives in recent years. In addition, agents carrying the α-particle-emitting radiohalogen, astatine-211 (211At), or radiometals, actinium-225 (225Ac), lead-212 (212Pb), radium-223 (223Ra), and thorium-227 (227Th), have been increasingly investigated in preclinical research. Besides PSMA-based radiotherapeutics, other prominent prostate cancer-related proteases, for example, human kallikrein peptidases (HK2 and HK3), have been explored using monoclonal-antibody-(mAb)-based targeting platforms. Several promising mAbs targeting receptors overexpressed on the different stages of prostate cancer have also been developed for radiopharmaceutical therapy, for example, Delta-like ligand 3 (DLL-3), CD46, and CUB domain-containing protein 1 (CDCP1). Progress is also being made using peptide-based targeting platforms for the gastrin-releasing peptide receptor (GRPR), a well-established membrane-associated receptor expressed in localized and metastatic prostate cancers. Furthermore, mechanism-driven combination therapies appear to be a burgeoning area in the context of preclinical prostate cancer radiotherapeutics. Here, we review the current developments related to the preclinical radiopharmaceutical therapy of prostate cancer. These are summarized in two major topics: (1) therapeutic radionuclides and (2) tumor-targeting approaches using monoclonal antibodies, small molecules, and peptides.

Efficacy of HER2-Targeted Intraperitoneal 225Ac α-Pretargeted Radioimmunotherapy for Small-Volume Ovarian Peritoneal Carcinomatosis

Epithelial ovarian cancer (EOC) is often asymptomatic and presents clinically in an advanced stage as widespread peritoneal microscopic disease that is generally considered to be surgically incurable. Targeted α-therapy with the α-particle-emitting radionuclide 225Ac (half-life, 9.92 d) is a high-linear-energy-transfer treatment approach effective for small-volume disease and even single cells. Here, we report the use of human epidermal growth factor receptor 2 (HER2) 225Ac-pretargeted radioimmunotherapy (PRIT) to treat a mouse model of human EOC SKOV3 xenografts growing as peritoneal carcinomatosis (PC). Methods: On day 0, 105 SKOV3 cells transduced with a luciferase reporter gene were implanted intraperitoneally in nude mice, and tumor engraftment was verified by bioluminescent imaging (BLI). On day 15, treatment was started using 1 or 2 cycles of 3-step anti-HER2 225Ac-PRIT (37 kBq/cycle as 225Ac-Proteus DOTA), separated by a 1-wk interval. Efficacy and toxicity were monitored for up to 154 d. Results: Untreated PC-tumor-bearing nude mice showed a median survival of 112 d. We used 2 independent measures of response to evaluate the efficacy of 225Ac-PRIT. First, a greater proportion of the treated mice (9/10 1-cycle and 8/10 2-cycle; total, 17/20; 85%) survived long-term compared with controls (9/27, 33%), and significantly prolonged survival was documented (log-rank [Mantel-Cox] P = 0.0042). Second, using BLI, a significant difference in the integrated BLI signal area to 98 d was noted between controls and treated groups (P = 0.0354). Of a total of 8 mice from the 2-cycle treatment group (74 kBq total) that were evaluated by necropsy, kidney radiotoxicity was mild and did not manifest itself clinically (normal serum blood urea nitrogen and creatinine). Dosimetry estimates (relative biological effectiveness-weighted dose, where relative biological effectiveness = 5) per 37 kBq administered for tumors and kidneys were 56.9 and 16.1 Gy, respectively. One-cycle and 2-cycle treatments were equally effective. With immunohistology, mild tubular changes attributable to α-toxicity were observed in both therapeutic groups. Conclusion: Treatment of EOC PC-tumor-bearing mice with anti-HER2 225Ac-PRIT resulted in histologic cures and prolonged survival with minimal toxicity. Targeted α-therapy using the anti-HER2 225Ac-PRIT system is a potential treatment for otherwise incurable EOC.

Implant Imaging: Perspectives of Nuclear Imaging in Implant, Biomaterial, and Stem Cell Research

Until now, very few efforts have been made to specifically trace, monitor, and visualize implantations, artificial organs, and bioengineered scaffolds for tissue engineering in vivo. While mainly X-Ray, CT, and MRI methods have been used for this purpose, the applications of more sensitive, quantitative, specific, radiotracer-based nuclear imaging techniques remain a challenge. As the need for biomaterials increases, so does the need for research tools to evaluate host responses. PET (positron emission tomography) and SPECT (single photon emission computer tomography) techniques are promising tools for the clinical translation of such regenerative medicine and tissue engineering efforts. These tracer-based methods offer unique and inevitable support, providing specific, quantitative, visual, non-invasive feedback on implanted biomaterials, devices, or transplanted cells. PET and SPECT can improve and accelerate these studies through biocompatibility, inertivity, and immune-response evaluations over long investigational periods at high sensitivities with low limits of detection. The wide range of radiopharmaceuticals, the newly developed specific bacteria, and the inflammation of specific or fibrosis-specific tracers as well as labeled individual nanomaterials can represent new, valuable tools for implant research. This review aims to summarize the opportunities of nuclear-imaging-supported implant research, including bone, fibrosis, bacteria, nanoparticle, and cell imaging, as well as the latest cutting-edge pretargeting methods.

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.

Antibody theranostics in precision medicine

With the increasing use of antibody therapeutics, clinicians are faced with challenges of precisely stratifying patients and promptly assessing response to treatment. Antibody theranostics combines the advantages of radionuclides and antibodies (or antibody derivatives) to systematically integrate targeted diagnostics and therapeutics and will play important roles in precision medicine.

Radiotheranostics in oncology: Making precision medicine possible

A quintessential setting for precision medicine, theranostics refers to a rapidly evolving field of medicine in which disease is diagnosed followed by treatment of disease-positive patients using tools for the therapy identical or similar to those used for the diagnosis. Against the backdrop of only-treat-when-visualized, the goal is a high therapeutic index with efficacy markedly surpassing toxicity. Oncology leads the way in theranostics innovation, where the approach has become possible with the identification of unique proteins and other factors selectively expressed in cancer versus healthy tissue, advances in imaging technology able to report these tissue factors, and major understanding of targeting chemicals and nanodevices together with methods to attach labels or warheads for imaging and therapy. Radiotheranostics-using radiopharmaceuticals-is becoming routine in patients with prostate cancer and neuroendocrine tumors who express the proteins PSMA (prostate-specific membrane antigen) and SSTR2 (somatostatin receptor 2), respectively, on their cancer. The palpable excitement in the field stems from the finding that a proportion of patients with large metastatic burden show complete and partial responses, and this outcome is catalyzing the search for more radiotheranostics approaches. Not every patient will benefit from radiotheranostics; but, for those who cross the target-detected line, the likelihood of response is very high.

Astatine-211 based radionuclide therapy: Current clinical trial landscape

Astatine-211 (²¹¹At) has physical properties that make it one of the top candidates for use as a radiation source for alpha particle-based radionuclide therapy, also referred to as targeted alpha therapy (TAT). Here, we summarize the main results of the completed clinical trials, further describe ongoing trials, and discuss future prospects.

Immuno-PET: Design options and clinical proof-of-concept

Radioimmunoconjugates have been used for over 30 years in nuclear medicine applications. In the last few years, advances in cancer biology knowledge have led to the identification of new molecular targets specific to certain patient subgroups. The use of these targets in targeted therapies approaches has allowed the developments of specifically tailored therapeutics for patients. As consequence of the PET-imaging progresses, nuclear medicine has developed powerful imaging tools, based on monoclonal antibodies, to in vivo characterization of these tumor biomarkers. This imaging modality known as immuno-positron emission tomography (immuno-PET) is currently in fastest-growing and its medical value lies in its ability to give a non-invasive method to assess the in vivo target expression and distribution and provide key-information on the tumor targeting. Currently, immuno-PET presents promising probes for different nuclear medicine topics as staging/stratification tool, theranostic approaches or predictive/prognostic biomarkers. To develop a radiopharmaceutical drug that can be used in immuno-PET approach, it is necessary to find the best compromise between the isotope choice and the immunologic structure (full monoclonal antibody or derivatives). Through some clinical applications, this paper review aims to discuss the most important aspects of the isotope choice and the usable proteic structure that can be used to meet the clinical needs.