Reducing the renal retention of low- to moderate-molecular-weight radiopharmaceuticals

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.

Design, Synthesis, and Evaluation of a Novel Positron Emission Tomography Tracer Targeting Fibroblast Activation Protein: From Bench to Bedside

FAPI-PET/CT has become a promising tool for cancer diagnosis. However, the pharmacokinetic properties of FAPI tracers need optimization. Here, we developed a novel FAPI tracer, [18F]AlF-NOTA-SP2A-FAPT, for cancer imaging. NOTA-SP2A-FAPT was successfully synthesized and radiolabeled with a high radiochemical purity. [18F]AlF-NOTA-SP2A-FAPT displayed satisfying stability, hydrophilicity, and affinity to FAP, as well as specific uptake in A549-FAP cells. Micro-PET/CT showed that [18F]AlF-NOTA-SP2A-FAPT is rapidly excreted through the renal system. [18F]AlF-NOTA-SP2A-FAPT exhibited high tumor uptake and excellent retention, showing better tumor delineation compared to [18F]FDG and [18F]AlF-NOTA-FAPI-42. Pilot clinical studies of [18F]AlF-NOTA-SP2A-FAPT and head-to-head comparison with [18F]FDG were performed on 13 cancer patients. Compared to [18F]FDG, [18F]AlF-NOTA-SP2A-FAPT had higher uptake in primary tumor and lymph node metastases as well as favorable distribution and good tumor retention. In conclusion, [18F]AlF-NOTA-SP2A-FAPT demonstrated high tumor accumulation, as well as improved pharmacokinetic properties. [18F]AlF-NOTA-SP2A-FAPT could emerge as a promising alternative to the currently established FAPI tracers.

Radioligand Therapy in Metastatic Breast Cancer: Harnessing Precision Oncology

Radioligand therapy (RLT) represents a promising advancement in precision oncology and enables the targeted delivery of radiation to cancer cells. This approach has shown success in other tumor types, such as prostate cancer and neuroendocrine tumors. Its potential in metastatic breast cancer (mBC) is currently under investigation. This review discusses RLT mechanism of action, therapeutic potential, and integration into the existing therapeutic landscape of mBC. While clinical trials have shown promising results, challenges remain regarding target heterogeneity, implementation, and optimizing treatment strategies. Further research is essential to integrate RLT into clinical practice and improve patient outcomes fully.

Indium-111-Labeled Single-Domain Antibody for In Vivo CXCR4 Imaging Using Single-Photon Emission Computed Tomography

C-X-C chemokine receptor type 4 (CXCR4) is highly expressed in a range of pathologies, including cancers like multiple myeloma and non-Hodgkin lymphoma, inflammatory diseases such as rheumatoid arthritis, and viral infections like HIV. Currently, the most advanced radiotracer for CXCR4 imaging in clinics is [68Ga]PentixaFor. However, its structure is prone to modifications, complicating the development of a specific CXCR4 fluorine-18-labeled tracer with good pharmacokinetic properties. This study aimed to screen multiple CXCR4-targeting variable domains of heavy-chain-only antibody (VHH or single-domain antibody (sdAb)) constructs to identify the most promising sdAb as a vector molecule for the future development of a CXCR4 fluorine-18 tracer. We have generated five CXCR4-specific sdAb constructs with a cysteine-containing C-terminal tag (C-Direct tag) (VUN400-C-Direct, VUN401-C-Direct, VUN410-C-Direct, VUN411-C-Direct, and VUN415-C-Direct) and one probe (VUN400-C) without. The reduced sdAbs were coupled to maleimide-DOTAGA for 111In-labeling. Their binding affinity against human CXCR4 (hCXCR4) was assessed by using a previously described BRET-based displacement assay. The in vivo profile was assessed using naive mice. Based on the plasma stability (60 min post injection (p.i.)), we selected VUN400-C-Direct and its derivative VUN400-C for further evaluation. These compounds ([111In]In-DOTAGA-VUN400-C-Direct and [111In]In-DOTAGA-VUN400-C) were tested in mice bearing xenografts derived from U87.CD4, U87.CXCR4, and U87.CD4.CXCR4 cells through ex vivo biodistribution studies and SPECT/CT imaging. The six sdAb constructs were labeled with a high radiochemical conversion (75-97%) and purity (>95%). In radioactive binding assays using U87.CD4.CXCR4 cells, [111In]In-DOTAGA-VUN400-C-Direct and [111In]In-DOTAGA-VUN401-C-Direct displayed the highest cellular uptake, achieving 10.4 ± 1.6% and 11.5 ± 1.1%, respectively. In naive mice, [111In]In-DOTAGA-VUN400-C-Direct showed the most favorable biodistribution profile, with low uptake across all organs except the kidneys (Standardized Uptake Value (SUV) > 50, n = 3, 60 min p.i.), but average plasma stability (40.6 ± 9.4%, n = 3, 60 min p.i.). In a xenografted tumor model, [111In]In-DOTAGA-VUN400-C-Direct showed only minor uptake (SUVU87.CXCR4 0.71 ± 0.002, n = 3, 60 min p.i.). [111In]In-DOTAGA-VUN400-C demonstrated nearly identical plasma stability (41.08 ± 5.45%, n = 4) but showed high and specific uptake in the CXCR4-expressing xenografted tumor (SUVU87.CD4.CXCR4 3.75 ± 1.08 vs SUVU87.CD4 = 0.64 ± 0.19, n = 5, 60 min p.i.), which could be blocked by coinjection of AMD3100 (5 mg/kg) (SUVU87.CD4.CXCR4 0.55 ± 0.32 vs SUVU87.CD4 = 0.39 ± 0.07, n = 2, 60 min p.i.). In conclusion, all six sdAbs exhibited high in vitro affinity against hCXCR4. Among these, [111In]In-DOTAGA-VUN400-C showed high CXCR4-specific tumor uptake and favorable pharmacokinetic properties, indicating VUN400-C's potential as a promising vector for future CXCR4 PET imaging applications with fluorine-18.

In vitro and in vivo evaluation of anti-HER2 antibody conjugates labelled with 225Ac

BACKGROUND

Overexpression of human epidermal growth factor receptor type 2 (HER2) occurs in multiple carcinomas. For example, up to 20% of breast cancer cases are classified as HER2 positive (HER2+). Treatment of this condition typically involves immunotherapy using monoclonal antibodies, such as trastuzumab or pertuzumab. The precise targeting of monoclonal antibodies to HER2+ tumour lesions can be used as well in radioimmunotherapy to deliver medical radionuclides exactly to the afflicted area and therefore minimize radiation exposure of healthy tissues. In this study, DOTA conjugates of monoclonal antibodies trastuzumab and pertuzumab were prepared and tested in vitro. One of these, 225Ac-DOTA-pertuzumab, was also the subject of an ex vivo biodistribution study with normal as well as HER2+ and HER2- tumour-xenografted mice. This radioconjugate has not been previously described.

RESULTS

Three DOTA-conjugates of HER2 targeting monoclonal antibodies, one of trastuzumab and two of pertuzumab, were prepared and radiolabelled with 225Ac in different molar ratios. This procedure led to an optimisation of the preparation and radiolabelling process. The radioconjugates were shown to be highly stable in vitro in both fetal bovine serum and phosphate buffered saline under room temperature and decreased temperature for 10 days. In vitro cell studies with HER2-overexpressing cell-line (SKOV-3) and low HER2-expressing cell line (MDA-MB-231) proved that radioconjugates of both antibodies have high binding specificity and affinity towards HER2 receptors. These findings were confirmed for a novel radioconjugate 225Ac-DOTA-pertuzumab in an ex vivo biodistribution study, where uptake in HER2+ tumour was 50 ± 14% ID/g and HER2- tumour showed uptake comparable with healthy tissues (max. 5.0 ± 1.7% ID/g). The high uptake observed in the spleen can be attributed to the elimination of the antibody, as well as the use of an immunedeficient mouse strain (SCID).

CONCLUSIONS

During this study, the optimization of preparation and radiolabelling of HER2 targeting antibodies with 225Ac was accomplished. Furthermore, the radioconjugate 225Ac-DOTA-pertuzumab was prepared and evaluated for the first time. The radioconjugates of both tested antibodies demonstrated excellent qualities in terms of stability and HER2 receptor affinity. Initial ex vivo studies indicated that especially the radioconjugate 225Ac-DOTA-pertuzumab is a very promising candidate for further more detailed in vivo studies.

Selective PET imaging of CXCR4 using the Al18F-labeled antagonist LY2510924

BACKGROUND

[68Ga]PentixaFor detects C-X-C chemokine receptor type 4 (CXCR4) overexpression in various malignancies, such as multiple myeloma and non-Hodgkin lymphomas, as well as in endocrine and inflammatory disorders. This study aimed to develop an Al18F-labeled radiotracer derived from LY2510924 for CXCR4-targeted imaging, leveraging the physical and logistical advantages of fluorine-18.

METHODS

We designed a CXCR4-specific radioprobe, [18F]AlF-NOTA-SC, based on LY2510924 by incorporating a triglutamate linker and NOTA chelator to enable Al18F-labeling. The in vitro CXCR4 affinity was assessed using cell-based binding assays. Subsequently, in vivo pharmacokinetics and tumor uptake of [18F]AlF-NOTA-SC were assessed in naïve mice and mice with xenografts derived from U87.CD4/U87.CD4.CXCR4 and MM.1 S cells. Finally, biodistribution was determined in a non-human primate using PET-MR.

RESULTS

Compared to Ga-PentixaFor, AlF-NOTA-SC demonstrated similar in vitro affinity for human CXCR4. [18F]AlF-NOTA-SC was produced with a decay-corrected radiochemical yield of 21.0 ± 7.1% and an apparent molar activity of 16.4 ± 3.6 GBq/µmol. In [18F]AlF-NOTA-SC binding assays on U87.CD4.CXCR4 cells, the total bound fraction was 7.1 ± 0.5% (58% blocking by AMD3100). In naïve mice, the radiotracer did not accumulate in any organs; however, it showed a significant CXCR4-specific uptake in xenografted tumors (SUVmeanU87.CD4 = 0.04 ± 0.00 (n = 3); SUVmeanU87.CD4.CXCR4 = 3.04 ± 0.65 (n = 3); SUVmeanMM.1 S = 1.95 ± 0.11 (n = 3)). In a non-human primate, [18F]AlF-NOTA-SC accumulated in CXCR4 expressing organs, such as the spleen and bone marrow.

CONCLUSION

[18F]AlF-NOTA-SC exhibited CXCR4-specific uptake in vitro and in vivo, with fast and persistent tumor accumulation, making it a strong candidate for clinical translation as an 18F-alternative to [68Ga]PentixaFor.

Improving Affinity while Reducing Kidney Uptake of CXCR4-Targeting Radioligands Derived from the Endogenous Antagonist EPI-X4

The C-X-C chemokine receptor 4 (CXCR4) is highly upregulated in most cancers, making it an ideal target for delivering radiation therapy to tumors. We previously demonstrated the feasibility of targeting CXCR4 in vivo using a radiolabeled derivative of EPI-X4, an endogenous CXCR4 antagonist, named DOTA-K-JM#173. However, this derivative showed undesirable accumulation in the kidneys, which would limit its clinical use. In this study, we identified that removing a positive charge from the peptide sequence significantly reduced renal uptake. We evaluated a series of optimized derivatives lacking this positive charge, in vitro and in vivo in a xenografted athymic nude mice model, after radiolabeling with 177Lu. The most promising derivatives were further assessed in vivo after 68Ga labeling. Among them, we identified DOTA-JM#173 and D-L1-DOTA-JM#173, where the D-Ile1 was replaced by D-Leu1, two optimized derivatives with a lysine residue removed. These two molecules represent the most advanced DOTA-conjugated ligands derived from EPI-X4 for CXCR4-directed theranostic applications, offering enhanced potential for targeted cancer treatment.

Evaluation of the Gly-Phe-Lys Linker to Reduce the Renal Radioactivity of a [64Cu]Cu-Labeled Multimeric cRGD Peptide

Radiometal-labeled peptide-based radiopharmaceuticals (RLPB-radiopharmaceuticals) are promising for cancer imaging and targeted radiotherapy; however, their effectiveness is often compromised by the high retention of nonspecific radioactivity in the kidneys due to renal excretion pathways. Current strategies to address this issue have limitations, highlighting the need for innovative approaches to improve targeting specificity and therapeutic efficacy. We aimed to evaluate the applicability of the Gly-Phe-Lys (GFK) tripeptide, a renal brush border (RBB) enzyme-cleavable linkage, to reduce renal radioactivity in RLPB-radiopharmaceuticals using the integrin-targeting radiopeptide [64Cu]Cu-cyclam-RAFT-c(-RGDfK-)4 ([64Cu]Cu-cyclam-RaftRGD). We designed and synthesized the model compound [64Cu]Cu-cyclam-GFK(benzoyl [Bz]), its predictive metabolites, and GFK-incorporated [64Cu]Cu-cyclam-RaftRGD derivatives [64Cu]Cu-cyclam-GFK-RaftRGD and [64Cu]Cu-cyclam-GFK(beta-alanine [βA])3-RaftRGD. In vitro studies showed that dual radiometabolites, namely, [64Cu]Cu-cyclam-G and [64Cu]Cu-cyclam-GF, were simultaneously released from [64Cu]Cu-cyclam-GFK(Bz) by different RBB enzymes, whereas both RaftRGD derivatives released only [64Cu]Cu-cyclam-GF. When injected into mice, [64Cu]Cu-cyclam-GFK(Bz) and the two RaftRGD derivatives led to the urinary excretion of [64Cu]Cu-cyclam-G and [64Cu]Cu-cyclam-GF, respectively. PET imaging and biodistribution studies showed the increased rates of reduction in renal radioactivity levels for the two RaftRGD derivatives compared to the parental [64Cu]Cu-cyclam-RaftRGD (e.g., PET: 1 to 24 h postinjection, 73.0 ± 2.3 and 75.6 ± 1.8 vs 43.0 ± 4.5%, p < 0.0001; biodistribution: 3 to 24 h, 61.1 and 74.4 vs 22.8%). Taken together, these results indicate that the designed renal cleavage occurred in vivo. We also noted the steric interference of the RaftRGD moiety on enzyme access, the spacer effect of the trimeric βA sequence (reduced steric hindrance), and the altered radiopharmacokinetics (e.g., initially increased renal accumulation) of the RaftRGD compounds upon linker incorporation. These findings provide important insights into the chemical design of RLPB-radiopharmaceuticals with reduced renal retention based on the RBB strategy.

Imaging of tumor-associated macrophage dynamics during immunotherapy using a CD163-specific nanobody-based immunotracer

Immunotherapies have emerged as an effective treatment option for immune-related diseases, such as cancer and inflammatory diseases. However, variations in patient responsiveness limit the broad applicability and success of these immunotherapies. Noninvasive whole-body imaging of the immune status of individual patients during immunotherapy could enable the prediction and monitoring of the patient's response, resulting in more personalized treatments. In this study, we developed a nanobody-based immunotracer targeting CD163, a receptor specifically expressed on macrophages. This anti-CD163 immunotracer bound to human and mouse CD163 with high affinity and specificity without competing for ligand binding. Furthermore, the tracer showed no unwanted immune cell activation and was nonimmunogenic. Upon radiolabeling of the anti-CD163 immunotracer, specific imaging of CD163+ macrophages using micro-single-photon emission computerized tomography/computed tomography or micro-positron emission tomography/CT was performed. The anti-CD163 immunotracer was able to stratify immunotherapy responders from nonresponders (NR) by visualizing differences in the intratumoral CD163+ TAM distribution in Lewis lung carcinoma-ovalbumin tumor-bearing mice receiving an anti-programmed cell death protein-1 (PD-1)/CSF1R combination treatment. Immunotherapy-responding mice showed a more homogeneous distribution of the PET signal in the middle of the tumor, while CD163+ TAMs were located at the tumor periphery in NR. As such, visualization of CD163+ TAM distribution in the tumor microenvironment could allow a prediction or follow-up of therapy response. Altogether, this study describes an immunotracer, specific for CD163+ macrophages, that allows same-day imaging and follow-up of these immune cells in the tumor microenvironment, providing a good basis for the prediction and follow-up of immunotherapy responses in cancer patients.

Future Prospect of Low-Molecular-Weight Prostate-Specific Membrane Antigen Radioisotopes Labeled as Theranostic Agents for Metastatic Castration-Resistant Prostate Cancer

Prostate cancer ranks as the fourth most common cancer among men, with approximately 1.47 million new cases reported annually. The emergence of prostate-specific membrane antigen (PSMA) as a critical biomarker has revolutionized the diagnosis and treatment of prostate cancer. Recent advancements in low-molecular-weight PSMA inhibitors, with their diverse chemical structures and binding properties, have opened new avenues for research and therapeutic applications in prostate cancer management. These novel agents exhibit enhanced tumor targeting and specificity due to their small size, facilitating rapid uptake and localization at the target site while minimizing the retention in non-target tissues. The primary aim of this study is to evaluate the potential of low-molecular-weight PSMA inhibitors labeled with radioisotopes as theranostic agents for prostate cancer. This includes assessing their efficacy in targeted imaging and therapy and understanding their pharmacokinetic properties and mechanisms of action. This study is a literature review focusing on in vitro and clinical research data. The in vitro studies utilize PSMA-targeted radioligands labeled with radioisotopes to assess their binding affinity, specificity, and internalization in prostate cancer cell lines. Additionally, the clinical studies evaluate the safety, effectiveness, and biodistribution of radiolabeled PSMA ligands in patients with advanced prostate cancer. The findings indicate promising outcomes regarding the safety and efficacy of PSMA-targeted radiopharmaceuticals in clinical settings. The specific accumulation of these agents in prostate tumor lesions suggests their potential for various applications, including imaging and therapy. This research underscores the promise of radiopharmaceuticals targeting PSMA in advancing the diagnosis and treatment of prostate cancer. These agents improve diagnostic accuracy and patients' outcomes by enhancing imaging capabilities and enabling personalized treatment strategies.

Development and evaluation of a single domain antibody targeting folate receptor alpha for radioligand therapy

BACKGROUND

Folate receptor alpha (FRα) overexpression is seen in many cancers. Radioligand therapy (RLT) has emerged as a promising tool to target FRα and has been investigated previously, but further progression was limited due to high kidney retention and, subsequently, toxicity. To circumvent this, we present here the development of a [131I]I-GMIB-conjugated anti-human FRα (hFRα) single-domain antibody (sdAb), with intrinsically fast renal clearance and concomitant low kidney retention. We report the hit-to-lead development of an anti-hFRα sdAb. We evaluated its potential in vitro and assessed its targeting ability using SPECT imaging in hFRα-knockin and tumour-bearing mice. The toxicity and therapeutic efficacy of the [131I]I-GMIB-sdAb were investigated in mouse models.

RESULTS

The lead anti-hFRα sdAb 2BD42 was developed with picomolar affinities, low koff, and radiolabelled using [131I]I with yields of > 41% and purity > 99%. [131I]I-GMIB-2BD42 retained tumour uptake (> 5%IA/g at 1 h p.i. and > 1.5%IA/g at 24 h p.i.) and fast kidney clearance (< 1%IA/g at 24 h p.i.) in athymic and hFRα-knock-in mice. Athymic mice bearing hFRα-positive xenografts treated with [131I]I-GMIB-2BD42 showed prolonged survival without toxicity compared to animals that received the vehicle solution or radioactive control.

CONCLUSION

The therapeutic lead radiopharmaceutical [131I]I-GMIB-2BD42 showed fast pharmacokinetics with specific retention in hFRα + tumours. In addition, we report therapeutic efficacy with no signs of toxicity. In this study, we successfully designed a new drug for RLT, overcoming previous limitations, such as high kidney retention, which could aid in revitalising FRα-targeted radiotherapy.

Automated radiofluorination of HER2 single domain antibody: the road towards the clinical translation of [18F]FB-HER2 sdAb

BACKGROUND

With the next generation of Human Epidermal Growth Factor Receptor 2 (HER2) -targeting therapies, such as antibody-drug conjugates, showing benefit in "HER2 low" and even "HER2 ultralow" patients, the need for novel methods to quantify HER2 expression accurately becomes even more important for clinical decision making. A HER2 PET/CT imaging assessment could evaluate HER2 positive disease locations while improving patient care, reducing the need for invasive biopsies. A single-domain antibody (sdAb)-based PET tracer could combine the high specificity of sdAbs with short-lived radionuclides such as fluorine-18 (18F) and gallium-68 (68Ga). SdAb-based PET tracers have clinically been used via a 68Ga-chelator approach. However, the distribution of 68Ga-labelled pharmaceuticals to peripheral PET centres is more challenging to organize due to the short half-life of 68Ga, most certainly when the available activity is limited by a generator. Cyclotron produced 68Ga has removed this limitation. Distribution of 18F-labelled pharmaceuticals remains less challenging due to its slightly longer half-life, and radiofluorination of sdAbs via N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB) has shown to be a promising strategy for developing sdAb-based PET tracers. Although [18F]SFB automation has been reported, automating protein conjugation proves challenging. Herein we report the fully automated, cartridge-based production of [18F]FB-HER2 sdAb on a single synthesis module.

RESULTS

[18F]FB-HER2 sdAb (> 6 GBq) was obtained after a fully automated production (95 min), with a RCP > 95%, apparent molar activity > 20 GBq/µmol and decay-corrected radiochemical yield (RCY d.c.) of 14 ± 2% (n = 4). Further upscaling amounted to production batches of 16 GBq with an apparent molar activity > 40 GBq/µmol and RCY d.c. of 8 ± 1% (n = 4). Ex vivo biodistribution and PET imaging showed specific HER2-positive tumour targeting and low kidney retention.

CONCLUSION

The [18F]FB-HER2 sdAb tracer was produced with clinically relevant activities using a fully automated production method. The automated production method was designed to ease the translation to the clinic and has the potential to be used not only in mono-centre but also multi-centre clinical trials with one central production site. [18F]FB-HER2 sdAb showed a favourable biodistribution pattern and could be a valuable alternative to 68Ga-labelled sdAb-based PET tracers in the clinic.

Heterogeneity of absorbed dose distribution in kidney tissues and dose-response modelling of nephrotoxicity in radiopharmaceutical therapy with beta-particle emitters: A review

Absorbed dose heterogeneity in kidney tissues is an important issue in radiopharmaceutical therapy. The effect of absorbed dose heterogeneity in nephrotoxicity is, however, not fully understood yet, which hampers the implementation of treatment optimization by obscuring the interpretation of clinical response data and the selection of optimal treatment options. Although some dosimetry methods have been developed for kidney dosimetry to the level of microscopic renal substructures, the clinical assessment of the microscopic distribution of radiopharmaceuticals in kidney tissues currently remains a challenge. This restricts the anatomical resolution of clinical dosimetry, which hinders a thorough clinical investigation of the impact of absorbed dose heterogeneity. The potential of absorbed dose-response modelling to support individual treatment optimization in radiopharmaceutical therapy is recognized and gaining attraction. However, biophysical modelling is currently underexplored for the kidney, where particular modelling challenges arise from the convolution of a complex functional organization of renal tissues with the function-mediated dose distribution of radiopharmaceuticals. This article reviews and discusses the heterogeneity of absorbed dose distribution in kidney tissues and the absorbed dose-response modelling of nephrotoxicity in radiopharmaceutical therapy. The review focuses mainly on the peptide receptor radionuclide therapy with beta-particle emitting somatostatin analogues, for which the scientific literature reflects over two decades of clinical experience. Additionally, detailed research perspectives are proposed to address various identified challenges to progress in this field.

Efficiency of succinylated gelatin and amino acid infusions for kidney uptake reduction of radiolabeled αvβ6-integrin targeting peptides: considerations on clinical safety profiles

PURPOSE

68Ga-Trivehexin is an investigational PET radiopharmaceutical (NCT05799274) targeting αvβ6-integrin for PET imaging of carcinomas. 177Lu-D0301 is a structurally related therapeutic peptide tetramer. However, it showed considerable kidney uptake in rodents, impeding clinical applicability. We therefore evaluated the impact of different kidney protection strategies on the biodistribution of both agents in normal and tumor-bearing mice.

METHODS

Ex-vivo biodistribution of 68Ga-Trivehexin (90 min p.i.) and 177Lu-D0301 (90 min and 24 h p.i.) was determined in healthy C57BL/6N and H2009 (human lung adenocarcinoma) xenografted CB17-SCID mice without and with co-infusion of 100 µL of solutions containing 2.5% arginine + 2.5% lysine (Arg/Lys), 4% succinylated gelatin (gelofusine, gelo), or combinations thereof. Arg/Lys was injected either i.p. 30 min before and after the radiopharmaceutical, or i.v. 2 min before the radiopharmaceutical. Gelo was administered either i.v. 2 min prior activity, or pre-mixed and injected together with the radiopharmaceutical (n = 5 per group). C57BL/6N mice were furthermore imaged by PET (90 min p.i.) and SPECT (24 h p.i.).

RESULTS

Kidney uptake of 68Ga-Trivehexin in C57BL/6N mice was reduced by 15% (Arg/Lys i.p.), 25% (Arg/Lys i.v.), and 70% (gelo i.v.), 90 min p.i., relative to control. 177Lu-D0301 kidney uptake was reduced by 2% (Arg/Lys i.p.), 41% (Arg/Lys i.v.), 61% (gelo i.v.) and 66% (gelo + Arg/Lys i.v.) 24 h p.i., compared to control. Combination of Arg/Lys and gelo provided no substantial benefit. Gelo furthermore reduced kidney uptake of 177Lu-D0301 by 76% (90 min p.i.) and 85% (24 h p.i.) in H2009 bearing SCID mice. Since tumor uptake was not (90 min p.i.) or only slightly reduced (15%, 24 h p.i.), the tumor/kidney ratio was improved by factors of 3.3 (90 min p.i.) and 2.6 (24 h p.i.). Reduction of kidney uptake was demonstrated by SPECT, which also showed that the remaining activity was located in the cortex.

CONCLUSIONS

The kidney uptake of both investigated radiopharmaceuticals was more efficiently reduced by gelofusine (61-85%) than Arg/Lys (25-41%). Gelofusine appears particularly suitable for reducing renal uptake of αvβ6-integrin targeted 177Lu-labeled peptide multimers because its application led to approximately three times higher tumor-to-kidney ratios. Since the incidence of severe adverse events (anaphylaxis) with succinylated gelatin products (reportedly 0.0062-0.038%) is comparable to that of gadolinium-based MRI or iodinated CT contrast agents (0.008% and 0.04%, respectively), clinical use of gelofusine during radioligand therapy appears feasible if similar risk management strategies as for contrast agents are applied.

Single-Domain Antibodies as Antibody-Drug Conjugates: From Promise to Practice-A Systematic Review

BACKGROUND

Antibody-drug conjugates (ADCs) represent potent cancer therapies that deliver highly toxic drugs to tumor cells precisely, thus allowing for targeted treatment and significantly reducing off-target effects. Despite their effectiveness, ADCs can face limitations due to acquired resistance and potential side effects.

OBJECTIVES

This study focuses on advances in various ADC components to improve both the efficacy and safety of these agents, and includes the analysis of several novel ADC formats. This work assesses whether the unique features of VHHs-such as their small size, enhanced tissue penetration, stability, and cost-effectiveness-make them a viable alternative to conventional antibodies for ADCs and reviews their current status in ADC development.

METHODS

Following PRISMA guidelines, this study focused on VHHs as components of ADCs, examining advancements and prospects from 1 January 2014 to 30 June 2024. Searches were conducted in PubMed, Cochrane Library, ScienceDirect and LILACS using specific terms related to ADCs and single-domain antibodies. Retrieved articles were rigorously evaluated, excluding duplicates and non-qualifying studies. The selected peer-reviewed articles were analyzed for quality and synthesized to highlight advancements, methods, payloads, and future directions in ADC research.

RESULTS

VHHs offer significant advantages for drug conjugation over conventional antibodies due to their smaller size and structure, which enhance tissue penetration and enable access to previously inaccessible epitopes. Their superior stability, solubility, and manufacturability facilitate cost-effective production and expand the range of targetable antigens. Additionally, some VHHs can naturally cross the blood-brain barrier or be easily modified to favor their penetration, making them promising for targeting brain tumors and metastases. Although no VHH-drug conjugates (nADC or nanoADC) are currently in the clinical arena, preclinical studies have explored various conjugation methods and linkers.

CONCLUSIONS

While ADCs are transforming cancer treatment, their unique mechanisms and associated toxicities challenge traditional views on bioavailability and vary with different tumor types. Severe toxicities, often linked to compound instability, off-target effects, and nonspecific blood cell interactions, highlight the need for better understanding. Conversely, the rapid distribution, tumor penetration, and clearance of VHHs could be advantageous, potentially reducing toxicity by minimizing prolonged exposure. These attributes make single-domain antibodies strong candidates for the next generation of ADCs, potentially enhancing both efficacy and safety.

Tenascin-C targeting strategies in cancer

Tenascin-C (TNC) is a matricellular and multimodular glycoprotein highly expressed under pathological conditions, especially in cancer and chronic inflammatory diseases. Since a long time TNC is considered as a promising target for diagnostic and therapeutic approaches in anti-cancer treatments and was already extensively targeted in clinical trials on cancer patients. This review provides an overview of the current most advanced strategies used for TNC detection and anti-TNC theranostic approaches including some advanced clinical strategies. We also discuss novel treatment protocols, where targeting immune modulating functions of TNC could be center stage.

Anti-Idiotypic VHHs and VHH-CAR-T Cells to Tackle Multiple Myeloma: Different Applications Call for Different Antigen-Binding Moieties

CAR-T cell therapy is at the forefront of next-generation multiple myeloma (MM) management, with two B-cell maturation antigen (BCMA)-targeted products recently approved. However, these products are incapable of breaking the infamous pattern of patient relapse. Two contributing factors are the use of BCMA as a target molecule and the artificial scFv format that is responsible for antigen recognition. Tackling both points of improvement in the present study, we used previously characterized VHHs that specifically target the idiotype of murine 5T33 MM cells. This idiotype represents one of the most promising yet challenging MM target antigens, as it is highly cancer- but also patient-specific. These VHHs were incorporated into VHH-based CAR modules, the format of which has advantages compared to scFv-based CARs. This allowed a side-by-side comparison of the influence of the targeting domain on T cell activation. Surprisingly, VHHs previously selected as lead compounds for targeted MM radiotherapy are not the best (CAR-) T cell activators. Moreover, the majority of the evaluated VHHs are incapable of inducing any T cell activation. As such, we highlight the importance of specific VHH selection, depending on its intended use, and thereby raise an important shortcoming of current common CAR development approaches.

Influence of Molecular Design on the Tumor Targeting and Biodistribution of PSMA-Binding Tracers Labeled with Technetium-99m

Previously, we designed the EuK-based PSMA ligand BQ0413 with an maE3 chelator for labeling with technetium-99m. It showed efficient tumor targeting, but our preclinical data and preliminary clinical results indicated that the renal excretion levels need to be decreased. We hypothesized that this could be achieved by a decrease in the ligand's total negative charge, achieved by substituting negatively charged glutamate residues in the chelator with glycine. The purpose of this study was to evaluate the tumor targeting and biodistribution of two new PSMA inhibitors, BQ0411 and BQ0412, compared to BQ0413. Conjugates were radiolabeled with Tc-99m and characterized in vitro, using PC3-pip cells, and in vivo, using NMRI and PC3-pip tumor-bearing mice. [99mTc]Tc-BQ0411 and [99mTc]Tc-BQ0412 demonstrated PSMA-specific binding to PC3-pip cells with picomolar affinity. The biodistribution pattern for the new conjugates was characterized by rapid excretion. The tumor uptake for [99mTc]Tc-BQ0411 was 1.6-fold higher compared to [99mTc]Tc-BQ0412 and [99mTc]Tc-BQ0413. [99mTc]Tc-BQ0413 has demonstrated predominantly renal excretion, while the new conjugates underwent both renal and hepatobiliary excretion. In this study, we have demonstrated that in such small targeting ligands as PSMA-binding EuK-based pseudopeptides, the structural blocks that do not participate in binding could have a crucial role in tumor targeting and biodistribution. The presence of a glycine-based coupling linker in BQ0411 and BQ0413 seems to optimize biodistribution. In conclusion, the substitution of amino acids in the chelating sequence is a promising method to alter the biodistribution of [99mTc]Tc-labeled small-molecule PSMA inhibitors. Further improvement of the biodistribution properties of BQ0413 is needed.

Amide-to-Triazole Switch in Somatostatin-14-Based Radioligands: Impact on Receptor Affinity and In Vivo Stability

The use of metabolically stabilized, radiolabeled somatostatin (SST) analogs ([68Ga]Ga/[177Lu]Lu-DOTA-TATE/TOC/NOC) is well established in nuclear medicine. Despite the pivotal role of these radioligands in the diagnosis and therapy of neuroendocrine tumors (NETs), their inability to interact with all five somatostatin receptors (SST1-5R) limits their clinical potential. [111In]In-AT2S is a radiolabeled DOTA-conjugate derived from the parent peptide SST-14 that exhibits high binding affinity to all SSTR subtypes, but its poor metabolic stability represents a serious disadvantage for clinical use. In order to address this issue, we have replaced strategic trans-amide bonds of [111In]In-AT2S with metabolically stable 1,4-disubstituted 1,2,3-triazole bioisosteres. From the five cyclic triazolo-peptidomimetics investigated, only [111In]In-XG1 combined a preserved nanomolar affinity for the SST1,2,3,5R subtypes in vitro and an improved stability in vivo (up to 17% of intact peptide 5 min postinjection (pi) versus 6% for [111In]In-AT2S). The involvement of neprilysin (NEP) in the metabolism of [111In]In-XG1 was confirmed by coadministration of Entresto®, a registered antihypertensive drug, in vivo releasing the selective and potent NEP-inhibitor sacubitrilat. A pilot SPECT/CT imaging study conducted in mice bearing hSST2R-positive xenografts failed to visualize the xenografts due to the pronounced kidney uptake (>200% injected activity (IA)/g at 4 h pi), likely the result of the formation of cationic metabolites. To corroborate the imaging data, the tumors and the kidneys were excised and analyzed with a γ-counter. Even if receptor-specific tumor uptake for [111In]In-XG1 could be confirmed (1.61% IA/g), further optimization is required to improve its pharmacokinetic properties for radiotracer development.

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.

Development and Biodistribution of a Nerve Growth Factor Radioactive Conjugate for PET Imaging

PURPOSE

The purpose of these studies was to develop a nerve growth factor (NGF) radiometal-chelator conjugate to determine the biodistribution and brain uptake of NGF by positron emission tomography/computerized tomography (PET-CT).

PROCEDURES

Purified NGF from llama seminal plasma was conjugated with FITC, and the chelator NOTA or DFO. NGF conjugates were evaluated for bioactivity. NOTA- and DFO-conjugated NGF were radiolabeled with gallium-68 or zirconium-89 ([68 Ga]GaCl3, half-life = 68 min; [89Zr]Zr(oxalate)4, half-life = 3.3 days). [89Zr]Zr-NGF was evaluated for biodistribution (0.5, 1, or 24 h), PET imaging (60 min), and brain autoradiography in mice.

RESULTS

Cell-based in vitro assays confirmed that the NGF conjugates maintained NGF receptor-binding and biological activity. Zirconium-89 and gallium-68 radiolabeling showed a high efficiency; however, only[89Zr]Zr-NGF was stable in vitro. Biodistribution studies showed that, as with most small proteins < 70 kDa, [89Zr]Zr-NGF uptake was predominantly in the kidney and was cleared rapidly with almost complete elimination of NGF at 24 h. Dynamic PET imaging from 0-60 min showed a similar pattern to ex vivo biodistribution with some transient liver uptake. Interestingly, although absolute brain uptake was very low, at 24 h after treatment, cerebral cortex uptake was higher than any other brain area examined and blood.

CONCLUSIONS

We conclude that conjugation of DFO to NGF through a thiourea linkage allows effective radiolabeling with zirconium-89 while maintaining NGF bioactivity. Following intravenous administration, the radiolabeled NGF targets non-neuronal tissues (e.g., kidney, liver), and although absolute brain uptake was very low, the brain uptake that was observed was restricted to the cortex.

First preclinical evaluation of [225Ac]Ac-DOTA-JR11 and comparison with [177Lu]Lu-DOTA-JR11, alpha versus beta radionuclide therapy of NETs

BACKGROUND

The [¹⁷⁷Lu]Lu-DOTA-TATE mediated peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors (NETs) is sometimes leading to treatment resistance and disease recurrence. An interesting alternative could be the somatostatin antagonist, [¹⁷⁷Lu]Lu-DOTA-JR11, that demonstrated better biodistribution profile and higher tumor uptake than [¹⁷⁷Lu]Lu-DOTA-TATE. Furthermore, treatment with alpha emitters showed improvement of the therapeutic index of PRRT due to the high LET offered by the alpha particles compared to beta emitters. Therefore, [²²⁵Ac]Ac-DOTA-JR11 can be a potential candidate to improve the treatment of NETs (Graphical abstract). DOTA-JR11 was radiolabeled with [²²⁵Ac]Ac(NO₃)₃ and [¹⁷⁷Lu]LuCl₃. Stability studies were performed in phosphate buffered saline (PBS) and mouse serum. In vitro competitive binding assay has been carried out in U2OS-SSTR2 + cells for ^natLa-DOTA-JR11, ^natLu-DOTA-JR11 and DOTA-JR11. Ex vivo biodistribution studies were performed in mice inoculated with H69 cells at 4, 24, 48 and 72 h after injection of [²²⁵Ac]Ac-DOTA-JR11. A blocking group was included to verify uptake specificity. Dosimetry of selected organs was determined for [²²⁵Ac]Ac-DOTA-JR11 and [¹⁷⁷Lu]Lu-DOTA-JR11.

RESULTS

[²²⁵Ac]Ac-DOTA-JR11 has been successfully prepared and obtained in high radiochemical yield (RCY; 95%) and radiochemical purity (RCP; 94%). [²²⁵Ac]Ac-DOTA-JR11 showed reasonably good stability in PBS (77% intact radiopeptide at 24 h after incubation) and in mouse serum (~ 81% intact radiopeptide 24 h after incubation). [¹⁷⁷Lu]Lu-DOTA-JR11 demonstrated excellent stability in both media (> 93%) up to 24 h post incubation. Competitive binding assay revealed that complexation of DOTA-JR11 with ^natLa and ^natLu did not affect its binding affinity to SSTR2. Similar biodistribution profiles were observed for both radiopeptides, however, higher uptake was noticed in the kidneys, liver and bone for [²²⁵Ac]Ac-DOTA-JR11 than [¹⁷⁷Lu]Lu-DOTA-JR11.

CONCLUSION

[²²⁵Ac]Ac-DOTA-JR11 showed a higher absorbed dose in the kidneys compared to [¹⁷⁷Lu]Lu-DOTA-JR11, which may limit further studies with this radiopeptide. However, several strategies can be explored to reduce nephrotoxicity and offer opportunities for future clinical investigations with [²²⁵Ac]Ac-DOTA-JR11.

New Long-Acting [89Zr]Zr-DFO GLP-1 PET Tracers with Increased Molar Activity and Reduced Kidney Accumulation

Positron emission tomography (PET) imaging is used in drug development to noninvasively measure biodistribution and receptor occupancy. Ideally, PET tracers retain target binding and biodistribution properties of the investigated drug. Previously, we developed a zirconium-89 PET tracer based on a long-circulating glucagon-like peptide 1 receptor agonist (GLP-1RA) using desferrioxamine (DFO) as a chelator. Here, we aimed to develop an improved zirconium-89-labeled GLP-1RA with increased molar activity to increase the uptake in low receptor density tissues, such as brain. Furthermore, we aimed at reducing tracer accumulation in the kidneys. Introducing up to four additional Zr-DFOs resulted in higher molar activity and stability, while retaining potency. Branched placement of DFOs was especially beneficial. Tracers with either two or four DFOs had similar biodistribution as the tracer with one DFO in vivo, albeit increased kidney and liver uptake. Reduced kidney accumulation was achieved by introducing an enzymatically cleavable Met-Val-Lys (MVK) linker motif between the chelator and the peptide.

Rapid nanobody-based imaging of mesothelin expressing malignancies compatible with blocking therapeutic antibodies

Introduction

Mesothelin (MSLN) is overexpressed in a wide variety of cancers with few therapeutic options and has recently emerged as an attractive target for cancer therapy, with a large number of approaches currently under preclinical and clinical investigation. In this respect, developing mesothelin specific tracers as molecular companion tools for predicting patient eligibility, monitoring then response to mesothelin-targeting therapies, and tracking the evolution of the disease or for real-time visualisation of tumours during surgery is of growing importance.

Methods

We generated by phage display a nanobody (Nb S1) and used enzymatic approaches were used to site-directed conjugate Nb S1 with either ATTO 647N fluorochrome or NODAGA chelator for fluorescence and positron emission tomography imaging (PET) respectively.

Results

We demonstrated that Nb S1 displays a high apparent affinity and specificity for human mesothelin and demonstrated that the binding, although located in the membrane distal domain of mesothelin, is not impeded by the presence of MUC16, the only known ligand of mesothelin, nor by the therapeutic antibody amatuximab. In vivo experiments showed that both ATTO 647N and [68Ga]Ga-NODAGA-S1 rapidly and specifically accumulated in mesothelin positive tumours compared to mesothelin negative tumours or irrelevant Nb with a high tumour/background ratio. The ex vivo biodistribution profile analysis also confirmed a significantly higher uptake of Nb S1 in MSLN-positive tumours than in MSLNlow tumours.

Conclusion

We demonstrated for the first time the use of an anti-MSLN nanobody as PET radiotracer for same day imaging of MSLN+ tumours, targeting an epitope compatible with the monitoring of amatuximab-based therapies and current SS1-derived-drug conjugates.

Optimizing the Safety and Efficacy of Bio-Radiopharmaceuticals for Cancer Therapy

The precise delivery of cytotoxic radiation to cancer cells through the combination of a specific targeting vector with a radionuclide for targeted radionuclide therapy (TRT) has proven valuable for cancer care. TRT is increasingly being considered a relevant treatment method in fighting micro-metastases in the case of relapsed and disseminated disease. While antibodies were the first vectors applied in TRT, increasing research data has cited antibody fragments and peptides with superior properties and thus a growing interest in application. As further studies are completed and the need for novel radiopharmaceuticals nurtures, rigorous considerations in the design, laboratory analysis, pre-clinical evaluation, and clinical translation must be considered to ensure improved safety and effectiveness. Here, we assess the status and recent development of biological-based radiopharmaceuticals, with a focus on peptides and antibody fragments. Challenges in radiopharmaceutical design range from target selection, vector design, choice of radionuclides and associated radiochemistry. Dosimetry estimation, and the assessment of mechanisms to increase tumor uptake while reducing off-target exposure are discussed.

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.

A Systematic Investigation into the Influence of Net Charge on the Biological Distribution of Radiometalated Peptides Using [68Ga]Ga-DOTA-TATE Derivatives

Recently, several radiometalated peptides have been approved for clinical imaging and/or therapy (theranostics) of several types of cancer; nonetheless, the primary challenge that most of these peptides confront is significant renal uptake and retention, which is often dose limiting and can cause nephrotoxicity. In response to this, numerous methods have been employed to reduce the uptake of radiometalated peptides in the kidneys, and among these is adding a linker to modulate polarity and/or charge. To better understand the influence of net charge on the biodistribution of radiometalated peptides, we selected the clinically popular construct DOTA-TATE (NETSPOT/LUTATHERA) as a model system. We synthesized derivatives using manual solid-phase peptide synthesis methods including mechanical and ultrasonic agitation to effectively yield the gold standard DOTA-TATE and a series of derivatives with different net charges (+2, +1, 0, -1, -2). Dynamic PET imaging from 0 to 90 min in healthy female mice (CD1) revealed high accumulation and retention of activity in the kidneys for the net-neutral (0) charged [⁶⁸Ga]Ga-DOTA-TATE and even higher for positively charged derivatives, whereas negatively charged derivatives exhibited low accumulation and fast renal excretion. Ex vivo biodistribution at 2 h post injection demonstrated a significant retention of [⁶⁸Ga]Ga-DOTA-TATE (∼74 %ID/g) in the kidneys, which increased as the net positive charge per molecule increased to +1 and +2 (∼272 %ID/g and ∼333 %ID/g, respectively), but the -1 and -2 net charged molecules exhibited lower renal uptake (∼15 %ID/g and 16 %ID/g, respectively). Interestingly, the net -2 charged [⁶⁸Ga]Ga-DOTA-(Glu)₂-PEG₄-TATE was stable in blood serum but had much higher healthy organ uptake (lungs, liver, spleen) than the net -1 compound, suggesting instability in vivo. Although the [⁶⁸Ga]Ga-DOTA-PEG₄-TATE derivative with a net charge of 0 also showed a decrease in kidney uptake, it also showed instability in blood serum and in vivo. Despite the superior pharmacokinetics of the net -1 charged [⁶⁸Ga]Ga-DOTA-Glu-PEG₄-TATE in healthy mice with respect to kidney uptake and overall profile, dynamic PET images and ex vivo biodistribution in male mice (NSG) bearing AR42J (SSTR2 overexpressing) subcutaneous tumor xenografts showed significantly diminished tumor uptake when compared to the gold standard [⁶⁸Ga]Ga-DOTA-TATE. Taken together, these findings indicate unambiguously that kidney uptake and retention are significantly influenced by the net charge of peptide-based radiotracers. In addition, it was illustrated that the negatively charged peptides had substantially decreased kidney uptake, but in this instantiation the tumor uptake was also impaired.

Enzymological Characterization of 64Cu-Labeled Neprilysin Substrates and Their Application for Modulating the Renal Clearance of Targeted Radiopharmaceuticals

The applicability of radioligands for targeted endoradionuclide therapy is limited due to radiation-induced toxicity to healthy tissues, in particular to the kidneys as primary organs of elimination. The targeting of enzymes of the renal brush border membrane by cleavable linkers that permit the formation of fast eliminating radionuclide-carrying cleavage fragments gains increasing interest. Herein, we synthesized a small library of ⁶⁴Cu-labeled cleavable linkers and quantified their substrate potentials toward neprilysin (NEP), a highly abundant peptidase at the renal brush border membrane. This allowed for the derivation of structure-activity relationships, and selected cleavable linkers were attached to the somatostatin receptor subtype 2 ligand [Tyr³]octreotate. Radiopharmacological characterization revealed that a substrate-based targeting of NEP in the kidneys with small peptides entails their premature cleavage in the blood circulation by soluble and endothelium-derived NEP. However, for a kidney-specific targeting of NEP, the additional targeting of albumin in the blood is highlighted.

[99mTc]-labeling and evaluation of a new linear peptide for imaging of glioblastoma as a αvβ3-positive tumor

PURPOSE

In this study, we designed a new linear 6-Hydrazinonicotinamide (HYNIC)-conjugated peptide (HYNIC-KRWrNM) (M-6) and labeled with technetium-99m for gamma imaging of glioblastoma as a α_vβ₃-positive tumor. We evaluated tumor targeting ability of this radio-peptide and compared with previous ^99mTc-labeled HYNIC-conjugated RGD analogue peptides.

PROCEDURES

One new linear peptide (HYNIC-KRWrNM) (M-6) was designed and labeled with technetium-99m in the presence of 2-[[1,3-dihydroxy-2-(hydroxymethyl) propan-2-yl] amino] acetic acid (Tricine)/Ethylenediamine-N,N'-diacetic acid (EDDA) as co-ligand system. Then, this ^99mTc-labeled peptide ([^99mTc]Tc-M-7) was evaluated for in vitro stability in saline and serum, specific binding assay, internalization, and binding affinity (K_d). In addition, we performed biodistribution study and planar imaging on nude mice bearing U87-MG xenograft as a α_vβ₃-positive tumor.

RESULTS

The radiochemical yield of [^99mTc]Tc-M-7 was obtained ˃95%. This ^99mTc-labeled peptide remained stable and intact in saline solution after 24 h incubation. In addition, metabolic stability of this ^99mTc-labeled peptide was obtained ˃60% after 4 h incubation in serum. The K_d value for [^99mTc]Tc-M-7 was obtained 5.2 ± 1.0 nM. Based on biodistribution results in nude mice bearing U87-MG xenograft, tumor/muscle activity ratio was 6.22 and decreased to 1.89 in blocking group at the same time point (4 h p.i.). The blocking experiment results also indicated that tumor uptake and kidney uptake were α_vβ₃-mediated. In comparison with previous HYNIC-conjugated RGD analogue peptides, kidneys had the highest uptake of this ^99mTc-labeled peptide (52.29 ± 11.48 at 1.5 h p.i. and 27.04 ± 0.66%ID/g at 4 h p.i.). Finally, similar to previous ^99mTc-labeled HYNIC-conjugated RGD analogue peptides, [^99mTc]Tc-M-7 showed acceptable tumor uptake after 4 h post-injection (based on ROI technique, target-to-background activity ratio = 3.80).

CONCLUSIONS

This small linear ^99mTc-labeled peptide, with high affinity to α_vβ₃ integrin, desirable water solubility, and cost efficient, demonstrates a potent tumor targeting ability as well as previous HYNIC-conjugated RGD analogue peptides. Hence, [^99mTc]Tc-M-7 can be of service to as a new candidate for early detection of α_vβ₃-positive tumors.

Synthesis and Evaluation of Two Long-Acting SSTR2 Antagonists for Radionuclide Therapy of Neuroendocrine Tumors

Somatostatin receptor subtype 2 (SSTR2) has become an essential target for radionuclide therapy of neuroendocrine tumors (NETs). JR11 was introduced as a promising antagonist peptide to target SSTR2. However, due to its rapid blood clearance, a better pharmacokinetic profile is necessary for more effective treatment. Therefore, two JR11 analogs (8a and 8b), each carrying an albumin binding domain, were designed to prolong the blood residence time of JR11. Both compounds were labeled with lutetium-177 and evaluated via in vitro assays, followed by in vivo SPECT/CT imaging and ex vivo biodistribution studies. [177Lu]Lu-8a and [177Lu]Lu-8b were obtained with high radiochemical purity (>97%) and demonstrated excellent stability in PBS and mouse serum (>95%). [177Lu]Lu-8a showed better affinity towards human albumin compared to [177Lu]Lu-8b. Further, 8a and 8b exhibited binding affinities 30- and 48-fold lower, respectively, than that of the parent peptide JR11, along with high cell uptake and low internalization rate. SPECT/CT imaging verified high tumor accumulation for [177Lu]Lu-8a and [177Lu]Lu-JR11 at 4, 24, 48, and 72 h post-injection, but no tumor uptake was observed for [177Lu]Lu-8b. Ex vivo biodistribution studies revealed high and increasing tumor uptake for [177Lu]Lu-8a. However, its extended blood circulation led to an unfavorable biodistribution profile for radionuclide therapy.

Application of Cleavable Linkers to Improve Therapeutic Index of Radioligand Therapies

Radioligand therapy (RLT) is an emergent drug class for cancer treatment. The dose administered to cancer patients is constrained by the radiation exposure to normal tissues to maintain an appropriate therapeutic index. When a radiopharmaceutical or its radiometabolite is retained in the kidneys, radiation dose deposition in the kidneys can become a dose-limiting factor. A good exemplar is [¹⁷⁷Lu]Lu-DOTATATE, where patients receive a co-infusion of basic amino acids for nephroprotection. Besides peptides, there are other classes of targeting vectors like antibody fragments, antibody mimetics, peptidomimetics, and small molecules that clear through the renal pathway. In this review, we will review established and emerging strategies that can be used to mitigate radiation-induced nephrotoxicity, with a focus on the development and incorporation of cleavable linkers for radiopharmaceutical designs. Finally, we offer our perspectives on cleavable linkers for RLT, highlighting future areas of research that will help advance the technology.

Evaluation of an 131I-labeled HER2-specific single domain antibody fragment for the radiopharmaceutical therapy of HER2-expressing cancers

Radiopharmaceutical therapy (RPT) is an attractive strategy for treatment of disseminated cancers including those overexpressing the HER2 receptor including breast, ovarian and gastroesophageal carcinomas. Single-domain antibody fragments (sdAbs) exemplified by the HER2-targeted VHH_1028 evaluated herein are attractive for RPT because they rapidly accumulate in tumor and clear faster from normal tissues than intact antibodies. In this study, VHH_1028 was labeled using the residualizing prosthetic agent N-succinimidyl 3-guanidinomethyl 5-[¹³¹I]iodobenzoate (iso-[¹³¹I]SGMIB) and its tissue distribution evaluated in the HER2-expressing SKOV-3 ovarian and BT474 breast carcinoma xenograft models. In head-to-head comparisons to [¹³¹I]SGMIB-2Rs15d, a HER2-targeted radiopharmaceutical currently under clinical investigation, iso-[¹³¹I]SGMIB-VHH_1028 exhibited significantly higher tumor uptake and significantly lower kidney accumulation. The results demonstrated 2.9 and 6.3 times more favorable tumor-to-kidney radiation dose ratios in the SKOV-3 and BT474 xenograft models, respectively. Iso-[¹³¹I]SGMIB-VHH_1028 was prepared using a solid-phase extraction method for purification of the prosthetic agent intermediate Boc₂-iso-[¹³¹I]SGMIB that reproducibly scaled to therapeutic-level doses and obviated the need for its HPLC purification. Single-dose (SKOV-3) and multiple-dose (BT474) treatment regimens demonstrated that iso-[¹³¹I]SGMIB-VHH_1028 was well tolerated and provided significant tumor growth delay and survival prolongation. This study suggests that iso-[¹³¹I]SGMIB-VHH_1028 is a promising candidate for RPT of HER2-expressing cancers and further development is warranted.