Recent advances in the development of 225Ac- and 211At-labeled radioligands for radiotheranostics

Development of Novel Radiotheranostic Ligand with Positively Charged Unit Targeting Prostate-Specific Membrane Antigen

Prostate-specific membrane antigen (PSMA) is an ideal target of prostate cancer (PCa) for theranostics, combining diagnosis and therapy in the field of nuclear medicine. [177Lu]Lu-PSMA-617 is a gold standard in PSMA-targeting radioligands, whereas its rapid clearance from the tumor and high uptake in the kidney may compromise the efficacy of theranostics. In this study, we developed novel PSMA-targeting radioligands, [111In]In/[225Ac]Ac-PDI2 and [111In]In/[225Ac]Ac-PDI4, by introducing a positively charged diethylenetriamine (PEI2) or tetraethylenepentamine (PEI4) structure, respectively, to PSMA-617. In the biodistribution study, higher tumor retention and lower renal uptake of [111In]In-PDI2 and [111In]In-PDI4 were observed than those of [111In]In-PSMA-617, and [111In]In-PDI2 exhibited higher tumor-residualizing properties than [111In]In-PDI4. [111In]In-PDI2 and [111In]In-PDI4 clearly visualized PSMA-expressing tumors by single photon emission computed tomography/computed tomography (SPECT/CT). The administration of [225Ac]Ac-PDI2 led to a higher antitumor effect than [225Ac]Ac-PDI4 and [225Ac]Ac-PSMA-617. These findings suggest the utility of [111In]In/[225Ac]Ac-PDI2 as theranostic ligands for PCa.

Enhancing the radionuclide theranostic concept through the radiohybrid approach

Radionuclide theranostics - a fast-growing emerging field in radiopharmaceutical sciences and nuclear medicine - offers a personalised and precised treatment approach by combining diagnosis with specific and selective targeted endoradiotherapy. This concept is based on the application of the same molecule, labelled with radionuclides possessing complementary imaging and therapeutic properties, respectively. In radionuclide theranostics, radionuclide pairs consisting of the same element, such as 61/64Cu/67Cu, 203Pb/212Pb or 123/124I/131I are of significant interest due to their identical chemical and pharmacological characteristics. However, such "true matched pairs" are seldom, necessitating the use of complementary radionuclides from different elements for diagnostics and endoradiotherapy with similar chemical characteristics, such as 99mTc/186/188Re, 68Ga/177Lu or 68Ga/225Ac. Corresponding combinations of such two radionuclides in one and the same radioconjugate is referred to as a "matched pair". Notably, the pharmacological behavior remains consistent across both diagnostic and therapeutic applications with "true matched pairs", which may differ for "matched pairs". As "true matched pairs" of theranostic radioisotopes are rare and that some relevant radionuclides do not fit with the diagnostic or therapeutic counterpart, the radionuclide theranostic concept can be expanded and improved by the introduction of the radiohybrid approach. Radiohybrid (rh) ligands represent a new class of radiopharmaceutical bearing two different positions for the introduction of a (radio)metal and (radio)halogen in one molecule, which can be then used for both therapeutic and diagnostic purposes. The following review will give an insight into recent developments of this approach.

Preclinical Characterization of Novel FAP-2286-Based Radioligand with Albumin Binder for Improved Tumor Retention

Fibroblast activation protein (FAP) is an attractive biomarker for tumor-targeting agents in cancer diagnosis and therapy. FAP-2286 shows retention in FAP-expressing tumors and is known as a promising FAP-targeting radioligand. In this study, we aimed to develop a FAP-2286 derivative that demonstrates higher tumor retention than FAP-2286. We designed DOTAGA-FAP-2286 and DOTAGA-FAP-2286-ALB by replacing DO3A with 2-(4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecan-1-yl)pentanedioic acid (DOTAGA) and introducing an albumin binder. Both compounds were successfully radiolabeled with 111In. Compared with [111In]In-DOTAGA-FAP-2286, [111In]In-DOTAGA-FAP-2286-ALB showed higher stability in murine plasma. In the cell competition binding study, In-DOTAGA-FAP-2286-ALB exhibited a higher FAP-binding affinity than In-DOTAGA-FAP-2286. In the albumin-binding assay, [111In]In-DOTAGA-FAP-2286-ALB showed a high binding rate in the solution with albumin. The biodistribution assay revealed marked tumor retention of [111In]In-DOTAGA-FAP-2286-ALB, resulting in the enhancement of predicted tumor AUC values of [225Ac]Ac-DOTAGA-FAP-2286-ALB. These results suggest advantages of the introduction of an albumin binder to FAP-2286.

Development of a Radiogallium-Labeled Heterodivalent Imaging Probe Targeting Negative Charges and Integrin on the Surface of Cancer Cell Membranes

Radiopharmaceuticals targeting tumor-specific environments are powerful tools for cancer diagnosis and treatment. We previously demonstrated the considerable high tumor uptake of the cationic amphiphilic peptide, 67Ga-NOTA-KV6, in vivo. However, because this radioligand shows a relatively rapid clearance from the tumor over time, further structural optimization is necessary. In this study, to enhance tumor accumulation and retention, we synthesized and evaluated a heterobivalent radiogallium-labeled radiotracer, [67Ga]Ga-DOTA-KV6-Mal-c(RGDyK) ([67Ga]6a), fusing the KV6 peptide targeting negatively charged sites on the cancer cell membrane and cyclic RGD peptide targeting integrin αvβ3 on the cancer cell membrane. Cellular uptake study revealed high accumulation of [67Ga]6a in integrin αvβ3-expressing U-87MG cancer cells, but uptake was significantly inhibited in the presence of an excess of the cyclic RGD peptide, c(RGDyK) (1). Peptide 6a exhibited integrin αvβ3-binding affinity comparable to those of RGD peptides 1 and DOTA-Mal-c(RGDyK) (8). In vivo biodistribution studies of U-87MG tumor-bearing mice revealed that [67Ga]6a exhibited better accumulation and retention in tumor tissues than [67Ga]Ga-DOTA-KV6-Mal-Et ([67Ga]6b; without the RGD peptide motif) and [67Ga]Ga-DOTA-Mal-c(RGDyK) ([67Ga]9; without the KV6 peptide motif). Single-photon emission computed tomography analysis also revealed high signals of [67Ga]6a in tumor tissues, which were significantly blocked in the presence of excess peptide 1. Although reducing radiotracer accumulation in nontumor tissues, such as the kidneys, remains a challenge, our developed approach exhibits potential to enhance the selectivity and retention of radiopharmaceuticals in tumor tissues.

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.

Therapeutic potential of FAPI RLT in oncology: A systematic review

Rationale: This systematic review aims to examine the safety and efficacy of fibroblast activation protein inhibitor (FAPI) radioligand therapy (RLT) for various epithelial neoplasms. Methods: PubMed, Web of Science, and Scopus databases were searched up to Jan 4, 2025, for studies involving FAPI RLT in various cancers. Data extraction focused on exploring safety and efficacy of FAPI RLT. Results: Overall, 27 studies involving a total of 144 patients who received FAPI RLT were included in this systematic review. [177Lu]Lu-FAPI was employed in 21 studies, with 225 cycles administered to 95 patients at a median dose of 6.8 GBq/cycle. Six non-randomized clinical investigations using [177Lu]Lu-FAPI reported disease control rates ranging from 18.2% to 83.3%. Only three studies documented a cumulative total of six patients who experienced grade 3 or 4 toxicity post [177Lu]Lu-FAPI RLT. Of 16 case reports utilizing [177Lu]Lu-FAPI, nine achieved disease control across various cancer types, with no reported adverse events. Four studies employed [90Y]Y-FAPI, totaling 103 cycles in 42 patients at a median dose of 6.7 GBq/cycle. Three non-randomized clinical investigations reported disease control rates of 50% to 82%, with two studies documenting eight high-grade toxicity events. Furthermore, a successful administration of [90Y]Y-FAPI was employed in a single reported case involving multiple primary neoplasms with no reported adverse events. However, the patient did not achieve disease control post [90Y]Y-FAPI. A cohort study utilized 53 [213Bi]Bi-FAPI-46 injections following a fractionated dose regimen in six cancer patients, achieving a 33.3% disease control rate without reported adverse events. One case report described dual radionuclide therapy using two cycles with a cumulative 20 GBq [153Sm]Sm-FAPI and a third 8 GBq [90Y]Y-FAPI cycle in a lung cancer patient, resulting in stable disease for eight months. Conclusion: FAPI RLT is a promising and safe therapeutic agent in oncology, with potential benefits achieved on short-term basis. However, its long-term efficacy and safety require further research with larger, controlled studies, considering the currently observed variations in patient populations, cancer types, and methodologies within reviewed studies.

In Vivo Stability Improvement of Astatobenzene Derivatives by Introducing Neighboring Substituents

211At is a promising radiohalogen for targeted α therapy. However, some astatinated compounds undergo deastatination in vivo, leading to unintended astatine accumulation in nontarget tissues. Recently, a group reported that the in vivo stability of an astato group on an alkyl group could be improved by placing specific substituents around the astato group. We hypothesized that such an approach could be applied to improve the stability of an astato group on aromatic groups. We designed and synthesized astatobenzene derivatives with neighboring substituents with different physical properties. In vitro and in vivo stabilities of these derivatives were evaluated by comparing with corresponding radioiodinated analogues. Notably, a derivative with two ortho dimethylcarbamoyl substituents significantly improved the stability of the astato group. This study supports the notion that strategic structural modification of substituents adjacent to an astato group can enhance its in vivo stability, potentially leading to the development of effective 211At-labeled radiopharmaceuticals.

Development of Radiolabeled Probes with Improved Imaging Contrast by Releasing Urinary Excretable Radiolabeled Compounds from Thermosensitive Liposomes in the Blood

In this study, thermosensitive liposomes (TSLs) encapsulating urinary excretable radiolabeled compounds were developed. We considered that the release of the radiolabeled compounds from the TSLs in the blood by heating the blood in peripheral tissues can achieve rapid clearance of radioactivity, resulting in improved imaging contrast. To demonstrate the hypothesis, classical TSLs mainly composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine with a phase transition temperature of 41 °C were used. The optimal composition of TSLs was determined by an in vitro release test using [111In]In-diethylenetriaminepentaacetic acid (DTPA)-encapsulated liposomes, which showed that the cholesterol content drastically changed the release characteristics of classical TSLs. In the biodistribution experiments, [111In]In-DTPA was significantly released from the TSLs in the blood when the tails of mice were heated at 43 °C. The tumor-to-blood ratio of the heated group was three times higher than that of the nonheated group, and accumulation in normal tissues of the heated group was lower than that of the nonheated group. These results demonstrate the usefulness of the method using TSLs to encapsulate urinary excretable radiolabeled compounds for improving imaging contrast.