Development and evaluation of albumin binder-conjugated heterodimeric radiopharmaceuticals targeting integrin αvß3 and CD13 for cancer therapy.

PURPOSE: The advancement of heterodimeric tracers, renowned for their high sensitivity, marks a significant trend in the development of radiotracers for cancer diagnosis. Our prior work on [68Ga]Ga-HX01, a heterodimeric tracer targeting CD13 and integrin αvß3, led to its approval for phase I clinical trials by the China National Medical Production Administration (NMPA). However, its fast clearance and limited tumor retention pose challenges for broader clinical application in cancer treatment. This study aims to develop a new radiopharmaceutical with increased tumor uptake and prolonged retention, rendering it a potential therapeutic candidate. METHODS: New albumin binder-conjugated compounds were synthesized based on the structure of HX01. In vitro and in vivo evaluation of these new compounds were performed after labelling with 68Ga. Small-animal PET/CT imaging were conducted at different time points at 0.5-6 h post injection (p.i.) using BxPC-3 xenograft mice models. The one with the best imaging performance was further radiolabeled with 177Lu for small-animal SPECT/CT and ex vivo biodistribution investigation. RESULTS: We have synthesized novel albumin binder-conjugated compounds, building upon the structure of HX01. When radiolabeled with 68Ga, all compounds demonstrated improved pharmacokinetics (PK). Small-animal PET/CT studies revealed that these new albumin binder-conjugated compounds, particularly [68Ga]Ga-L6, exhibited significantly enhanced tumor accumulation and retention compared with [68Ga]Ga-L0 without an albumin binder. [68Ga]Ga-L6 outperformed [68Ga]Ga-L7, a compound developed using a previously reported albumin binder. Furthermore, [177Lu]Lu-L6 demonstrated rapid clearance from normal tissues, high tumor uptake, and prolonged retention in small-animal SPECT/CT and biodistribution studies, positioning it as an ideal candidate for radiotherapeutic applications. CONCLUSION: A new integrin αvß3 and CD13 targeting compound was screened out. This compound bears a novel albumin binder and exhibits increased tumor uptake and prolonged tumor retention in BxPC-3 tumors and low background in normal organs, making it a perfect candidate for radiotherapy when radiolabeled with 177Lu.

EANM guidance document: dosimetry for first-in-human studies and early phase clinical trials.

The numbers of diagnostic and therapeutic nuclear medicine agents under investigation are rapidly increasing. Both novel emitters and novel carrier molecules require careful selection of measurement procedures. This document provides guidance relevant to dosimetry for first-in human and early phase clinical trials of such novel agents. The guideline includes a short introduction to different emitters and carrier molecules, followed by recommendations on the methods for activity measurement, pharmacokinetic analyses, as well as absorbed dose calculations and uncertainty analyses. The optimal use of preclinical information and studies involving diagnostic analogues is discussed. Good practice reporting is emphasised, and relevant dosimetry parameters and method descriptions to be included are listed. Three examples of first-in-human dosimetry studies, both for diagnostic tracers and radionuclide therapies, are given.

DNA-Targeted Complexes of Tc and Re for Biomedical Applications.

Due to their favorable chemical features, Re and Tc complexes have been widely used for the development of new therapeutic agents and imaging probes to solve problems of biomedical relevance. This review provides an update of the most relevant research efforts towards the development of novel cancer theranostic agents using Re and Tc-based compounds interacting with specific DNA structures. This includes a variety of homometallic complexes, namely those containing M(CO)3 (M=Re, Tc) moieties, that exhibit different modes of interaction with DNA, such as covalent binding, intercalation, groove binding or G-quadruplex DNA binding. Additionally, heterometallic complexes, designed to potentiate synergistic effects of different metal centers to improve DNA-targeting, cytotoxicity and fluorescence properties, are also reviewed. Particular attention is also given to 99m Tc- and 188 Re-labeled oligonucleotides that have been widely explored to develop imaging and therapeutic radiopharmaceuticals through the in vivo hybridization with a specific complementary DNA or RNA target sequence to provide useful molecular tools in precision medicine for cancer diagnosis and treatment. Finally, the need for further improvement of DNA-targeted Re and Tc-based compounds as potential therapeutic and diagnostic agents is highlighted, and future directions are discussed.

Incorporation of Carboxylate Pendant Arms into 18-Membered Macrocycles: Effects on [nat/203Pb]Pb(II) Complexation.

We present a detailed investigation on the coordination chemistry of [nat/203Pb]Pb(II) with chelators H4PYTA and H4CHX-PYTA. These chelators belong to the family of ligands derived from the 18-membered macrocyclic backbone PYAN and present varying degrees of rigidity due to the presence of either ethyl or cyclohexyl spacers. A complete study of the stable Pb(II) complexes is carried out via NMR, X-Ray crystallography, stability constant determination and computational studies. While these studies indicated that Pb(II) complexation is achieved, and the thermodynamic stability of the resulting complexes is very high, a certain degree of fluxionality does exist in both cases. Nevertheless, radiolabeling studies were carried out using SPECT (single photon emission computed tomography) compatible isotope lead-203 (203Pb, t1/2=51.9 h), and while both chelators complex the radioisotope, the incorporation of carboxylate pendant arms appears to be detrimental towards the stability of the complexes when compared to the previously described amide analogues. Additionally, incorporation of a cyclohexyl spacer does not improve the kinetic inertness of the system.

Synthesis and Preclinical Evaluation of PSMA-Targeted 111In-Radioconjugates Containing a Mitochondria-Tropic Triphenylphosphonium Carrier.

Nuclear DNA is the canonical target for biological damage induced by Auger electrons (AE) in the context of targeted radionuclide therapy (TRT) of cancer, but other subcellular components might also be relevant for this purpose, such as the energized mitochondria of tumor cells. Having this in mind, we have synthesized novel DOTA-based chelators carrying a prostate-specific membrane antigen (PSMA) inhibitor and a triphenyl phosphonium (TPP) group that were used to obtain dual-targeted 111In-radioconjugates ([111In]In-TPP-DOTAGA-PSMA and [111In]In-TPP-DOTAGA-G3-PSMA), aiming to promote a selective uptake of an AE-emitter radiometal (111In) by PSMA+ prostate cancer (PCa) cells and an enhanced accumulation in the mitochondria. These dual-targeted 111In-radiocomplexes are highly stable under physiological conditions and in cell culture media. The complexes showed relatively similar binding affinities toward the PSMA compared to the reference tracer [111In]In-PSMA-617, in line with their high cellular uptake and internalization in PSMA+ PCa cells. The complexes compromised cell survival in a dose-dependent manner and in the case of [111In]In-TPP-DOTAGA-G3-PSMA to a higher extent than observed for the single-targeted congener [111In]In-PSMA-617. µSPECT imaging studies in PSMA+ PCa xenografts showed that the TPP pharmacophore did not interfere with the excellent in vivo tumor uptake of the "golden standard" [111In]In-PSMA-617, although it led to a higher kidney retention. Such kidney retention does not necessarily compromise their usefulness as radiotherapeutics due to the short tissue range of the Auger/conversion electrons emitted by 111In. Overall, our results provide valuable insights into the potential use of mitochondrial targeting by PSMA-based radiocomplexes for efficient use of AE-emitting radionuclides in TRT, giving impetus to extend the studies to other AE-emitting trivalent radiometals (e.g., 161Tb or 165Er) and to further optimize the designed dual-targeting constructs.

Radiosynthesis and Preclinical Evaluation of m-[18F]FET and [18F]FET-OMe as Novel [18F]FET Analogs for Brain Tumor Imaging.

O-([18F]Fluoroethyl)-l-tyrosine ([18F]FET) is actively transported into the brain and cancer cells by LAT1 and possibly other amino acid transporters, which enables brain tumor imaging by positron emission tomography (PET). However, tumor delivery of this probe in the presence of competing amino acids may be limited by a relatively low affinity for LAT1. The aim of the present work was to evaluate the meta-substituted [18F]FET analog m-[18F]FET and the methyl ester [18F]FET-OMe, which were designed to improve tumor delivery by altering the physicochemical, pharmacokinetic, and/or transport properties. Both tracers could be prepared with good radiochemical yields of 41-56% within 66-90 min. Preclinical evaluation with [18F]FET as a reference tracer demonstrated reduced in vitro uptake of [18F]FET-OMe by U87 glioblastoma cells and no advantage for in vivo tumor imaging. In contrast, m-[18F]FET showed significantly improved in vitro uptake and accelerated in vivo tumor accumulation in an orthotopic glioblastoma model. As such, our work identifies m-[18F]FET as a promising alternative to [18F]FET for brain tumor imaging that deserves further evaluation with regard to its transport properties and in vivo biodistribution.

Exploring the potential of radiolabeled duramycin as an infection imaging probe.

The continuous pursuit of designing an ideal infection imaging agent is a crucial and ongoing endeavor in the field of biomedical research. Duramycin, an antimicrobial peptide exerts its antimicrobial action on bacteria by specific recognition of phosphatidylethanolamine (PE) moiety present on most bacterial membranes, particularly Escherichia coli (E. coli). E. coli membranes contain more than 60% PE. Therefore, duramycin is an attractive candidate for the formulation of probes for in situ visualization of E. coli driven focal infections. The aim of the present study is to develop 99m Tc labeled duramycin as a single-photon emission computed tomography (SPECT)-based agent to image such infections. Duramycin was successfully conjugated with a bifunctional chelator, hydrazinonicotinamide (HYNIC). PE specificity of HYNIC-duramycin was confirmed by a dye release assay on PE-containing model membranes. Radiolabeling of HYNIC-duramycin with 99m Tc was performed with consistently high radiochemical yield (>90%) and radiochemical purity (>90%). [99m Tc]Tc-HYNIC-duramycin retained its specificity for E. coli, in vitro. SPECT and biodistribution studies showed that the tracer could specifically identify E. coli driven infection at 3 h post injection. While 99m Tc-labeled duramycin is employed for monitoring early response to cancer therapy and cardiotoxicity, the current studies have confirmed, for the first time, the potential of utilizing 99m Tc labeled duramycin as an imaging agent for detecting bacteria. Its application in imaging PE-positive bacteria represents a novel and promising advancement.

Towards Clinical Development of Scandium Radioisotope Complexes for Use in Nuclear Medicine: Encouraging Prospects with the Chelator 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic Acid (DOTA) and Its Analogues.

Scandium (Sc) isotopes have recently attracted significant attention in the search for new radionuclides with potential uses in personalized medicine, especially in the treatment of specific cancer patient categories. In particular, Sc-43 and Sc-44, as positron emitters with a satisfactory half-life (3.9 and 4.0 h, respectively), are ideal for cancer diagnosis via Positron Emission Tomography (PET). On the other hand, Sc-47, as an emitter of beta particles and low gamma radiation, may be used as a therapeutic radionuclide, which also allows Single-Photon Emission Computed Tomography (SPECT) imaging. As these scandium isotopes follow the same biological pathway and chemical reactivity, they appear to fit perfectly into the "theranostic pair" concept. A step-by-step description, initiating from the moment of scandium isotope production and leading up to their preclinical and clinical trial applications, is presented. Recent developments related to the nuclear reactions selected and employed to produce the radionuclides Sc-43, Sc-44, and Sc-47, the chemical processing of these isotopes and the main target recovery methods are also included. Furthermore, the radiolabeling of the leading chelator, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), and its structural analogues with scandium is also discussed and the advantages and disadvantages of scandium complexation are evaluated. Finally, a review of the preclinical studies and clinical trials involving scandium, as well as future challenges for its clinical uses and applications, are presented.

Exploring Geometrical, Electronic and Spectroscopic Properties of 2-Nitroimidazole-Based Radiopharmaceuticals via Computational Chemistry Methods.

Tumor hypoxia plays an important role in the clinical management and treatment planning of various cancers. The use of 2-nitroimidazole-based radiopharmaceuticals has been the most successful for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging probes, offering noninvasive means to assess tumor hypoxia. In this study we performed detailed computational investigations of the most used compounds for PET imaging, focusing on those derived from 2-nitroimidazole: fluoromisonidazole (FMISO), fluoroazomycin arabinoside (FAZA), fluoroetanidazole (FETA), fluoroerythronitroimidazole (FETNIM) and 2-(2-nitroimidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)acetamide (EF5). Conformational analysis, structural parameters, vibrational IR and Raman properties (within both harmonic and anharmonic approximations), as well as the NMR shielding tensors and spin-spin coupling constants were obtained by density functional theory (DFT) calculations and then correlated with experimental findings, where available. Furthermore, time-dependent DFT computations reveal insight into the excited states of the compounds. Our results predict a significant change in the conformational landscape of most of the investigated compounds when transitioning from the gas phase to aqueous solution. According to computational data, the 2-nitroimidazole moiety determines to a large extent the spectroscopic properties of its derivatives. Due to the limited structural information available in the current literature for the investigated compounds, the findings presented herein deepen the current understanding of the electronic structures of these five radiopharmaceuticals.

Synthesis of the European ALARA network 20th workshop 'ALARA for interventional radiology and nuclear medicine'.

The European as low as reasonably achievable(ALARA) network regularly organises workshops on topical issues in radiation protection (RP). The topic of the 20th workshop was: 'ALARA for interventional radiology (IR) and nuclear medicine (NM)'. The objective was to examine the challenges faced when applying the optimisation principle (ALARA) in IR and NM and to consider how ALARA could be better implemented for patient and staff exposures. This memorandum provides a synthesis of the workshop sessions, and recommendations coming from the working groups discussion. Parallels are drawn with the recommendations arising from the 13th EAN workshop on 'ALARA and the medical sector (2011)' to consider how the optimisation challenges in IR and NM have evolved over the past decade. Current levels of exposure are presented along with operational practice and the challenges and opportunities for improvement, both in monitoring and practice. Whilst RP challenges remain, the application of ALARA appears more established in IR compared with experiences reported in 2011. The application of ALARA to emerging technologies in the NM setting is in need of further development to ensure that RP is considered at all stages in the development process of new radiopharmaceuticals. Besides the obvious technical and operational aspects, the importance of education and training, human factors and broadly the RP 'culture' were deemed fundamental to the success of the application of ALARA and where further emphasis is needed. All concerned parties, medical physics experts (MPEs), radiation protection experts, clinical staff, manufacturers and regulators have a role to play in the application of ALARA and this is discussed in the memorandum. Many of the recommendations from the 13th EAN workshop remain applicable today and overlap with the recommendations arising from the 20th workshop. This should prompt attention given that the use of IR and the development of novel radiopharmaceuticals for NM is only anticipated to increase with time.

Absorbed dose coefficients for pediatric differentiated thyroid cancer patients undergoing radioiodine therapy.

The escalating incidence of differentiated thyroid cancer (DTC) in pediatric patients and the resultant growing use of radioactive iodine (RAI) reinforce the need to evaluate radiation exposure to normal tissues and radiation-induced health risks in pediatric patients undergoing RAI therapy. In the current study, we calculated absorbed dose coefficients (i.e. absorbed dose per unit activity administered, mGy MBq-1) specific for pediatric patients with localized DTC undergoing RAI therapy following total thyroidectomy for use in epidemiological studies. We first modified previously-published biokinetic models for adult thyroid cancer patients to achieve a reasonable agreement with iodine biokinetics observed in pediatric patients or design principles addressed in the International Commission on Radiological Protection (ICRP) reference age-specific biokinetic models. We then combined the biokinetic models in conjunction withSvalues derived from ICRP reference pediatric voxel phantoms. The absorbed dose coefficients for pediatric patients were overall greater than those for adults with a ratio (pediatric/adult) up to 11.6 and rapidly decreased with increasing age. The sensitivity analysis showed that the renal clearance rate andSvalues may have the greatest impact on the absorbed dose coefficients with the rank correlation coefficients ranging from -0.53 to -0.82 (negative correlations) and from 0.51 to 0.80 (positive correlations), respectively. The results of the current study may be utilized in clinical or epidemiological studies to estimate organ-specific radiation absorbed doses and radiation-associated health risks among pediatric thyroid cancer patients.

Distinguishing spheno-orbital metastatic prostate cancer mimicking a meningioma using novel 18F-PSMA PET/CT imaging.

A 78-year-old man presented with acute-onset left temporal pain, eyelid swelling, and double vision. Computed tomography (CT) demonstrated a left sphenoid wing mass with extra-osseous intra-orbital and intracranial extension, thought to be a typical sphenoid wing meningioma by the primary team. The patient was admitted for an urgent craniotomy, which was planned for the following day. However, upon consultation with ophthalmic plastic surgery, concern was raised for an alternative diagnosis given the atypical timeline, inflammatory changes, and uncharacteristic imaging findings of mixed lytic and sclerotic bony changes without hyperostosis on CT and extensive peri-lesional dural thickening and enhancement on magnetic resonance imaging. A serum prostate-specific antigen was elevated to 206 ng/mL. Subsequent positron emission tomography (PET)/CT using 18F-fluorodeoxyglucose radiotracer was negative for metastatic disease. A prostate-specific membrane antigen (PSMA) PET/CT was then obtained and demonstrated extensive metastases. An orbital biopsy revealed poorly differentiated prostatic adenocarcinoma. The significant incongruence between the standard PET/CT and PSMA PET/CT highlights the value of this novel advanced radiographic modality in narrowing the differential diagnosis and determining the extent of disease. Findings of widespread metastasis on the PSMA PET/CT ultimately helped to avoid a large, morbid neurosurgical intervention in this patient, allowing for a minimally invasive orbital biopsy to characterize the tumor for therapeutic targeting.

Expedient Access to 18F-Fluoroheteroarenes via Deaminative Radiofluorination of Aniline-Derived Pyridinium Salts.

Herein, we disclose that pyridinium salts derived from abundant (hetero)anilines represent a novel precursor class for nucleophilic aromatic substitution reactions with [18F]fluoride. The value of this new 18F-fluorodeamination is demonstrated with the synthesis of over 30 structurally diverse and complex heteroaryl 18F-fluorides, several derived from scaffolds that were yet to be labelled with fluorine-18. The protocol tolerates heteroarenes and functionalities commonly found in drug discovery libraries, and is amenable to scale-up and automation on a commercial radiosynthesiser.

The 18F-Difluoromethyl Group: Challenges, Impact and Outlook.

The difluoromethyl functionality has proven useful in drug discovery, as it can modulate the properties of bioactive molecules. For PET imaging, this structural motif has been largely underexploited in (pre)clinical radiotracers due to a lack of user-friendly radiosynthetic routes. This Minireview provides an overview of the challenges facing radiochemists and summarises the efforts made to date to access 18F-difluoromethyl-containing radiotracers. Two distinct approaches have prevailed, the first of which relies on 18F-fluorination. A second approach consists of a 18F-difluoromethylation process, which uses 18F-labelled reagents capable of releasing key reactive intermediates such as the [18F]CF2H radical or [18F]difluorocarbene. Finally, we provide an outlook for future directions in the radiosynthesis of [18F]CF2H compounds and their application in tracer radiosynthesis.

PSMA-Targeted Radiopharmaceuticals in Prostate Cancer: Current Data and New Trials.

Radiopharmaceuticals have been utilized for men with advanced prostate cancer for decades. Older agents, seldom used today, provided palliation for bone metastatic pain. In 2013, the alpha emitter radium-223 provided a catalyst for the field by prolonging survival in men with metastatic castrate-resistant prostate cancer (mCRPC). Recently radioisotopic therapies have gained further interest with the development and FDA approval of 177 lutetium (177Lu)-PSMA-617 (also known as lutetium Lu-177 vipivotide tetraxetan). This agent targets the prostate-specific membrane antigen (PSMA) expressed on the cell surface of prostate cancer cells with a beta-emitting isotope (177Lu). This clinical review summarizes key data reported from 177Lu-PSMA-617 clinical trials, including data from the phase III VISION trial which were pivotal for regulatory approval in heavily pretreated PSMA-PET-positive patients with mCRPC. The current field of radiopharmaceuticals is in a rapid state of flux. Additional phase III trials are now ongoing in patients with mCRPC and in patients with metastatic castrate-sensitive prostate cancer. The results from these potential practice-changing trials are highly anticipated. Earlier phase trials (I/II) are in progress examining combination therapies, radiolabeled monoclonal antibodies, and novel compounds. Studies of PSMA-targeted therapies using both beta emitters such as 177Lu and novel alpha emitters such 225 actinium are in progress. During the next decade, radiopharmaceuticals will likely play a central role in the management of patients with advanced prostate cancer.

The Value of Phenotypic Precision Medicine in Prostate Cancer.

Prostate cancer is the most common cancer among men and the second leading cause of cancer-related death. For patients who develop metastatic disease, tissue-based and circulating-tumor-based molecular and genomic biomarkers have emerged as a means of improving outcomes through the application of precision medicine. However, the benefit is limited to a minority of patients. An additional approach to further characterize the biology of advanced prostate cancer is through the use of phenotypic precision medicine, or the identification and targeting of phenotypic features of an individual patient's cancer. In this review article, we will discuss the background, potential clinical benefits, and limitations of genomic and phenotypic precision medicine in prostate cancer. We will also highlight how the emergence of image-based phenotypic medicine may lead to greater characterization of advanced prostate cancer disease burden and more individualized treatment approaches in patients.

Selective Chelation of the Exotic Meitner-Auger Emitter Mercury-197 m/g with Sulfur-Rich Macrocyclic Ligands: Towards the Future of Theranostic Radiopharmaceuticals.

Mercury-197 m/g are a promising pair of radioactive isomers for incorporation into a theranostic as they can be used as a diagnostic agent using SPECT imaging and a therapeutic via Meitner-Auger electron emissions. However, the current absence of ligands able to stably coordinate 197m/g Hg to a tumour-targeting vector precludes their use in vivo. To address this, we report herein a series of sulfur-rich chelators capable of incorporating 197m/g Hg into a radiopharmaceutical. 1,4,7,10-Tetrathia-13-azacyclopentadecane (NS4 ) and its derivatives, (2-(1,4,7,10-tetrathia-13-azacyclopentadecan-13-yl)acetic acid (NS4 -CA) and N-benzyl-2-(1,4,7,10-tetrathia-13-azacyclopentadecan-13-yl)acetamide (NS4 -BA), were designed, synthesized and analyzed for their ability to coordinate Hg2+ through a combination of theoretical (DFT) and experimental coordination chemistry studies (NMR and mass spectrometry) as well as 197m/g Hg radiolabeling studies and in vitro stability assays. The development of stable ligands for 197m/g Hg reported herein is extremely impactful as it would enable their use for in vivo imaging and therapy, leading to personalized treatments for cancer.

Next Generation Copper Mediators for the Efficient Production of 18 F-Labeled Aromatics.

Cu-mediated radiofluorination is a versatile tool for the preparation of 18 F-labeled (hetero)aromatics. In this work, we systematically evaluated a series of complexes and identified several generally applicable mediators for highly efficient radiofluorination of aryl boronic and stannyl substrates. Utilization of these mediators in nBuOH/DMI or DMI significantly improved 18 F-labeling yields despite use of lower precursor amounts. Impressively, application of 2.5 µmol aryl boronic acids was sufficient to achieve 18 F-labeling yields of up to 75 %. The practicality of the novel mediators was demonstrated by efficient production of five PET-tracers and transfer of the method to an automated radiosynthesis module. In addition, (S)-3-[18 F]FPhe and 6-[18 F]FDOPA were prepared in activity yields of 23±1 % and 30±3 % using only 2.5 µmol of the corresponding boronic acid or trimethylstannyl precursor.

AAZTA-Like Ligands Bearing Phenolate Arms as Efficient Chelators for 68 Ga Labelling in†vitro and in†vivo.

The introduction of a phenolate pendant arm in place of an acetate on AAZTA- and DATA-like ligands resulted in hepta- and hexadentate chelators able to form Ga(III) complexes with thermodynamic stability and kinetic inertness higher than that of other Ga(III) complexes based on the parent 6-amino-6-methylperhydro-1,4-diazepine scaffold. In particular, the heptadentate AAZ3A-endoHB with a phenolate arm on an endocyclic N-atom shows a logKGaL of 27.35 and a remarkable resistance to hydroxide coordination up to basic pH (pH>9). This behaviour allows to also improve the kinetic inertness of the complex showing a dissociation half-life (t1/2 ) at pH 7.4 of 76 h. Although also the hexadentate AAZ2A-exoHB chelator forms a stable (logKGaL =24.69) and inert (t1/2 =33 h at pH 7.4) Ga(III) complex, the 68 Ga labelling showed a better radiochemical yield with AAZ3A-endoHB, especially at room temperature. Thus, a bifunctional chelator of AAZ3A-endoHB was synthesized bearing an isothiocyanate group that was conjugated to the N-terminus of a c(RGD) peptide for integrin receptor targeting. Finally, the conjugate was successfully labelled with 68 Ga isotope, and the resulting radiotracer tested for its stability in human serum and then in vivo for targeting B16-F10 tumours with miniPET imaging.

Inductively Coupled Plasma Mass Spectrometry─A Valid Method for the Characterization of Metal Conjugates in View of the Development of Radiopharmaceuticals.

This study addresses the question whether inductively coupled plasma mass spectrometry (ICP-MS) can be used as a method for the in vitro and in vivo characterization of non-radioactive metal conjugates to predict the properties of analogous radiopharmaceuticals. In a "proof-of-concept" study, the prostate-specific membrane antigen (PSMA)-targeting [175Lu]Lu-PSMA-617 and [159Tb]Tb-PSMA-617 were compared with their respective radiolabeled analogues, [177Lu]Lu-PSMA-617 (PLUVICTO, Novartis) and [161Tb]Tb-PSMA-617. ICP-MS and conventional γ-counting of the cell samples revealed almost identical results (<6% absolute difference between the two technologies) for the in vitro uptake and internalization of the (radio)metal conjugates, irrespective of the employed methodology. In vivo, an equal uptake in PSMA-positive PC-3 PIP tumor xenografts was determined 1 h after the injection of [175Lu]Lu-/[177Lu]Lu-PSMA-617 (41 ± 6% ID/g and 44 ± 12% IA/g, respectively) and [159Tb]Tb-/[161Tb]Tb-PSMA-617 (44 ± 5% ID/g and 44 ± 5% IA/g, respectively). It was further revealed that it is crucial to use the same ratios of the (radio)metal-labeled and unlabeled ligands for both methodologies to obtain equal data in organs in which receptor saturation was reached such as the kidneys (12 ± 2% ID/g vs 10 ± 1% IA/g, 1 h after injection). The data of this study demonstrate that the use of high-sensitivity ICP-MS allows reliable and predictive quantification of compounds labeled with stable metal isotopes in cell and tissue samples obtained in preclinical studies. It can, hence, be employed as a valid alternative to the state-of-the-art γ-counting methodology to detect radioactive ligands.