In Vivo Evaluation of a Bombesin Analogue Labeled with Ga-68 and Co-55/57

Radiocobalt-Labeling of a Polypyridylamine Chelate Conjugated to GE11 for EGFR-Targeted Theranostics

The overexpression of the epidermal growth factor receptor (EGFR) in certain types of prostate cancers and glioblastoma makes it a promising target for targeted radioligand therapy. In this context, pairing an EGFR-targeting peptide with the emerging theranostic pair comprising the Auger electron emitter cobalt-58m (58mCo) and the Positron Emission Tomography-isotope cobalt-55 (55Co) would be of great interest for creating novel radiopharmaceuticals for prostate cancer and glioblastoma theranostics. In this study, GE11 (YHWYGYTPQNVI) was investigated for its EGFR-targeting potential when conjugated using click chemistry to N1-((triazol-4-yl)methyl)-N1,N2,N2-tris(pyridin-2-ylmethyl)ethane-1,2-diamine (TZTPEN). This chelator is suitable for binding Co2+ and Co3+. With cobalt-57 (57Co) serving as a surrogate radionuclide for 55/58mCo, the novel GE11-TZTPEN construct was successfully radiolabeled with a high radiochemical yield (99%) and purity (>99%). [57Co]Co-TZTPEN-GE11 showed high stability in PBS (pH 5) and specific uptake in EGFR-positive cell lines. Disappointingly, no tumor uptake was observed in EGFR-positive tumor-bearing mice, with most activity being accumulated predominantly in the liver, gall bladder, kidneys, and spleen. Some bone uptake was also observed, suggesting in vivo dissociation of 57Co from the complex. In conclusion, [57Co]Co-TZTPEN-GE11 shows poor pharmacokinetics in a mouse model and is, therefore, not deemed suitable as a targeting radiopharmaceutical for EGFR.

Design and synthesis of a new bifunctional chelating agent: Application for Al 18F/177Lu complexation

Theranostic and personalized medicine are blooming strategies to improve oncologic patients' health care and facilitate early treatment. While 18F-radiochemistry for theranostic application is attractive due to its imaging properties, combining diagnosis by positron emission tomography (PET) via aluminum-fluoride-18 and β- therapy with lutetium-177 is relevant. Nevertheless, it requires the use of two different chelating agents, which are NOTA and DOTA for aluminum-fluoride-18 and lutetium-177 radiolabeling, respectively. To overcome this issue, we propose herein the synthesis of a new hybrid chelating agent named NO2A-AHM, which can be labeled with different types of emitters (β+, β- and γ) using the mismatched Al18F/177Lu pair. NO2A-AHM, is based on a hydrazine moiety functionalized by a NOTA cycle, a chelating arm, and a linker with a maleimide function. This design is chosen to increase the flexibility and allow the formation of 5 up to 7 coordination bonds with metal ions. Moreover, this agent can be coupled to targeting moieties containing a thiol function, such as peptides, to increase selectivity towards specific cancer cells. Experimental complexation and computational chemistry studies are performed to confirm the capacity of our chelating agent to label both aluminum-fluoride and lutetium using molecular modeling approaches at Density Functional Theory (DFT) level. The proof of concept of the ability of NO2A-AHM to complex both aluminum-fluoride-18, for PET imaging applications, and lutetium-177 for radiotherapy has shown encouraging results which is prominent for the development of a fully consistent theranostic approach.

Preparation and Evaluation of [18F]AlF-NOTA-NOC for PET Imaging of Neuroendocrine Tumors: Comparison to [68Ga]Ga-DOTA/NOTA-NOC

Background: The somatostatin receptors 1–5 are overexpressed on neuroendocrine neoplasms and, as such, represent a favorable target for molecular imaging. This study investigates the potential of [18F]AlF-NOTA-[1-Nal3]-Octreotide and compares it in vivo to DOTA- and NOTA-[1-Nal3]-Octreotide radiolabeled with gallium-68. Methods: DOTA- and NOTA-NOC were radiolabeled with gallium-68 and NOTA-NOC with [18F]AlF. Biodistributions of the three radioligands were evaluated in AR42J xenografted mice at 1 h p.i and for [18F]AlF at 3 h p.i. Preclinical PET/CT was applied to confirm the general uptake pattern. Results: Gallium-68 was incorporated into DOTA- and NOTA-NOC in yields and radiochemical purities greater than 96.5%. NOTA-NOC was radiolabeled with [18F]AlF in yields of 38 ± 8% and radiochemical purity above 99% after purification. The biodistribution in tumor-bearing mice showed a high uptake in tumors of 26.4 ± 10.8 %ID/g for [68Ga]Ga-DOTA-NOC and 25.7 ± 5.8 %ID/g for [68Ga]Ga-NOTA-NOC. Additionally, [18F]AlF-NOTA-NOC exhibited a tumor uptake of 37.3 ± 10.5 %ID/g for [18F]AlF-NOTA-NOC, which further increased to 42.1 ± 5.3 %ID/g at 3 h p.i. Conclusions: The high tumor uptake of all radioligands was observed. However, [18F]AlF-NOTA-NOC surpassed the other clinically well-established radiotracers in vivo, especially at 3 h p.i. The tumor-to-blood and -liver ratios increased significantly over three hours for [18F]AlF-NOTA-NOC, making it possible to detect liver metastases. Therefore, [18F]AlF demonstrates promise as a surrogate pseudo-radiometal to gallium-68.

PET Probes for Preclinical Imaging of GRPR-Positive Prostate Cancer: Comparative Preclinical Study of [68Ga]Ga-NODAGA-AMBA and [44Sc]Sc-NODAGA-AMBA

Gastrin-releasing peptide receptors (GRPR) are overexpressed in prostate cancer (PCa). Since bombesin analogue aminobenzoic-acid (AMBA) binds to GRPR with high affinity, scandium-44 conjugated AMBA is a promising radiotracer in the PET diagnostics of GRPR positive tumors. Herein, the GRPR specificity of the newly synthetized [⁴⁴Sc]Sc-NODAGA-AMBA was investigated in vitro and in vivo applying PCa PC-3 xenograft. After the in-vitro assessment of receptor binding, PC-3 tumor-bearing mice were injected with [⁴⁴Sc]Sc/[⁶⁸Ga]Ga-NODAGA-AMBA (in blocking studies with bombesin) and in-vivo PET examinations were performed to determine the radiotracer uptake in standardized uptake values (SUV). ⁴⁴Sc/⁶⁸Ga-labelled NODAGA-AMBA was produced with high molar activity (approx. 20 GBq/µmoL) and excellent radiochemical purity. The in-vitro accumulation of [⁴⁴Sc]Sc-NODAGA-AMBA in PC-3 cells was approximately 25-fold higher than that of the control HaCaT cells. Relatively higher uptake was found in vitro, ex vivo, and in vivo in the same tumor with the ⁴⁴Sc-labelled probe compared to [⁶⁸Ga]Ga-NODAGA-AMBA. The GRPR specificity of [⁴⁴Sc]Sc-NODAGA-AMBA was confirmed by significantly (p ≤ 0.01) decreased %ID and SUV values in PC-3 tumors after bombesin pretreatment. The outstanding binding properties of the novel [⁴⁴Sc]Sc-NODAGA-AMBA to GRPR outlines its potential to be a valuable radiotracer in the imaging of GRPR-positive PCa.

Expanding the PET radioisotope universe utilizing solid targets on small medical cyclotrons

Molecular imaging with medical radioisotopes enables the minimally-invasive monitoring of aberrant biochemical, cellular and tissue-level processes in living subjects. The approach requires the administration of radiotracers composed of radioisotopes attached to bioactive molecules, the pairing of which considers several aspects of the radioisotope in addition to the biological behavior of the targeting molecule to which it is attached. With the advent of modern cellular and biochemical techniques, there has been a virtual explosion in potential disease recognition antigens as well as targeting moieties, which has subsequently opened new applications for a host of emerging radioisotopes with well-matched properties. Additionally, the global radioisotope production landscape has changed rapidly, with reactor-based production and its long-defined, large-scale centralized manufacturing and distribution paradigm shifting to include the manufacture and distribution of many radioisotopes via a worldwide fleet of cyclotrons now in operation. Cyclotron-based radioisotope production has become more prevalent given the commercial availability of instruments, coupled with the introduction of new target hardware, process automation and target manufacturing methods. These advances enable sustained, higher-power irradiation of solid targets that allow hospital-based radiopharmacies to produce a suite of radioisotopes that drive research, clinical trials, and ultimately clinical care. Over the years, several different radioisotopes have been investigated and/or selected for radiolabeling due to favorable decay characteristics (i.e. a suitable half-life, high probability of positron decay, etc.), well-elucidated chemistry, and a feasible production framework. However, longer-lived radioisotopes have surged in popularity given recent regulatory approvals and incorporation of radiopharmaceuticals into patient management within the medical community. This review focuses on the applications, nuclear properties, and production and purification methods for some of the most frequently used/emerging positron-emitting, solid-target-produced radioisotopes that can be manufactured using small-to-medium size cyclotrons (≤24 MeV).

Production, Purification, and Applications of a Potential Theranostic Pair: Cobalt-55 and Cobalt-58m

The emerging success of [⁶⁸Ga/¹⁷⁷Lu]Ga/Lu-DOTATATE as a theranostic pair has spurred interest in other isotopes as potential theranostic combinations. Here, we review cobalt-55 and cobalt-58m as a potential theranostic pair. Radionuclidically pure cobalt-55 and cobalt-58m have been produced on small cyclotrons with high molar activity. In vitro, DOTATOC labeled with cobalt has shown greater affinity for SSTR2 than DOTATOC labeled with gallium and yttrium. Similarly, [^58mCo]Co-DOTATATE has shown improved cell-killing capabilities as compared to DOTATATE labeled with either indium-111 or lutetium-177. Finally, PET imaging with an isotope such as cobalt-55 allows for image acquisition at much later timepoints than gallium, allowing for an increased degree of biological clearance of non-bound radiotracer. We discuss the accelerator targetry and radiochemistry used to produce cobalt-55,58m, emphasizing the implications of these techniques to downstream radiotracers being developed for imaging and therapy.

Prognostic and Theranostic Applications of Positron Emission Tomography for a Personalized Approach to Metastatic Castration-Resistant Prostate Cancer

Metastatic castration-resistant prostate cancer (mCRPC) represents a condition of progressive disease in spite of androgen deprivation therapy (ADT), with a broad spectrum of manifestations ranging from no symptoms to severe debilitation due to bone or visceral metastatization. The management of mCRPC has been profoundly modified by introducing novel therapeutic tools such as antiandrogen drugs (i.e., abiraterone acetate and enzalutamide), immunotherapy through sipuleucel-T, and targeted alpha therapy (TAT). This variety of approaches calls for unmet need of biomarkers suitable for patients’ pre-treatment selection and prognostic stratification. In this scenario, imaging with positron emission computed tomography (PET/CT) presents great and still unexplored potential to detect specific molecular and metabolic signatures, some of whom, such as the prostate specific membrane antigen (PSMA), can also be exploited as therapeutic targets, thus combining diagnosis and therapy in the so-called “theranostic” approach. In this review, we performed a web-based and desktop literature research to investigate the prognostic and theranostic potential of several PET imaging probes, such as ¹⁸F-FDG, ¹⁸F-choline and ⁶⁸Ga-PSMA-11, also covering the emerging tracers still in a pre-clinical phase (e.g., PARP-inhibitors’ analogs and the radioligands binding to gastrin releasing peptide receptors/GRPR), highlighting their potential for defining personalized care pathways in mCRPC.

Expanding PET-applications in life sciences with positron-emitters beyond fluorine-18

Positron-emission-tomography (PET) has become an indispensable diagnostic tool in modern nuclear medicine. Its outstanding molecular imaging features allow repetitive studies on one individual and with high sensitivity, though no interference. Rather few positron-emitters with near favourable physical properties, i.e. carbon-11 and fluorine-18, furnished most studies in the beginning, preferably if covalently bound as isotopic label of small molecules. With the advancement of PET-devices the scope of in vivo research in life sciences and especially that of medical applications expanded, and other than "standard" PET-nuclides received increasing significance, like the radiometals copper-64 and gallium-68. Especially during the last decades, positron-emitters of other chemical elements have gotten into the focus of interest, concomitant with the technical advancements in imaging and radionuclide production. With known nuclear imaging properties and main production methods of emerging positron-emitters their usefulness for medical application is promising and even proven for several ones already. Unfortunate decay properties could be corrected for, and β+-emitters, especially with a longer half-life, provided new possibilities for application where slower processes are of importance. Further on, (bio)chemical features of positron-emitters of other elements, among there many metals, not only expanded the field of classical clinical investigations, but also opened up new fields of application. Appropriately labelled peptides, proteins and nanoparticles lend itself as newer probes for PET-imaging, e.g. in theragnostic or PET/MR hybrid imaging. Furthermore, the potential of non-destructive in-vivo imaging with positron-emission-tomography directs the view on further areas of life sciences. Thus, exploiting the excellent methodology for basic research on molecular biochemical functions and processes is increasingly encouraged as well in areas outside of health, such as plant and environmental sciences.

Preclinical Evaluation of the Copper-64 Labeled GRPR-Antagonist RM26 in Comparison with the Cobalt-55 Labeled Counterpart for PET-Imaging of Prostate Cancer

Gastrin-releasing peptide receptor (GRPR) is overexpressed in the majority of prostate cancers. This study aimed to investigate the potential of ⁶⁴Cu (radionuclide for late time-point PET-imaging) for imaging of GRPR expression using NOTA-PEG₂-RM26 and NODAGA-PEG₂-RM26. Methods: NOTA/NODAGA-PEG₂-RM26 were labeled with ⁶⁴Cu and evaluated in GRPR-expressing PC-3 cells. Biodistribution of [⁶⁴Cu]Cu-NOTA/NODAGA-PEG₂-RM26 was studied in PC-3 xenografted mice and compared to the biodistribution of [⁵⁷Co]Co-NOTA/NODAGA-PEG₂-RM26 at 3 and 24 h p.i. Preclinical PET/CT imaging was performed in tumor-bearing mice. NOTA/NODAGA-PEG₂-RM26 were stably labeled with ⁶⁴Cu with quantitative yields. In vitro, binding of [⁶⁴Cu]Cu-NOTA/NODAGA-PEG₂-RM26 was rapid and GRPR-specific with slow internalization. In vivo, [⁶⁴Cu]Cu-NOTA/NODAGA-PEG₂-RM26 bound specifically to GRPR-expressing tumors with fast clearance from blood and normal organs and displayed generally comparable biodistribution profiles to [⁵⁷Co]Co-NOTA/NODAGA-PEG₂-RM26; tumor uptake exceeded normal tissue uptake 3 h p.i.. Tumor-to-organ ratios did not increase significantly with time. [⁶⁴Cu]Cu-NOTA-PEG₂-RM26 had a significantly higher liver and pancreas uptake compared to other agents. ⁵⁷Co-labeled radioconjugates showed overall higher tumor-to-non-tumor ratios, compared to the ⁶⁴Cu-labeled counterparts. [⁶⁴Cu]Cu-NOTA/NODAGA-PEG₂-RM26 was able to visualize GRPR-expression in a murine PC model using PET. However, [^55/57Co]Co-NOTA/NODAGA-PEG₂-RM26 provided better in vivo stability and overall higher tumor-to-non-tumor ratios compared with the ⁶⁴Cu-labeled conjugates.

Benefit of Later-Time-Point PET Imaging of HER3 Expression Using Optimized Radiocobalt-Labeled Affibody Molecules

HER3-binding affibody molecules are a promising format for visualization of HER3 expression. Cobalt-55, a positron-emitting isotope, with a half-life of 17.5 h, allows for next-day imaging. We investigated the influence of the charge of the radiocobalt–chelator complex on the biodistribution of anti-HER3 affibody molecule (HE)₃-Z_HER3 and compared the best radiocobalt-labeled variant with a recently optimized gallium-labeled variant. Affibody conjugates (HE)₃-Z_HER3-X (X = NOTA, NODAGA, DOTA, DOTAGA) were labeled with [⁵⁷Co]Co (surrogate for ⁵⁵Co). Affinity measurements, binding specificity and cellular processing were studied in two HER3-expressing cancer cell lines. Biodistribution was studied 3 and 24 h post-injection (pi) in mice with HER3-expressing BxPC-3 xenografts and compared to [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA. Micro-single-photon emission tomography/computed tomography (microSPECT/CT) and micro-positron emission tomography/computed tomography (microPET/CT) imaging was performed 3 and 24 h pi. Stably labeled conjugates bound to HER3 with subnanomolar affinity. [⁵⁷Co]Co-(HE)₃-Z_HER3-DOTA had the best tumor retention and a significantly lower concentration in blood than other conjugates, leading to superior tumor-to-blood and tumor-to-liver ratios 24 h pi. Compared to [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA 3 h pi, [⁵⁷Co]Co-(HE)₃-Z_HER3-DOTA provided superior imaging contrast in liver 24 h pi. Concluding, the composition and charge of the [⁵⁷Co]Co–chelator complex influenced the uptake in tumors and normal tissue. [⁵⁷Co]Co-(HE)₃-Z_HER3-DOTA provided the best imaging properties among the cobalt-labeled conjugates. Delayed imaging of HER3 expression with [⁵⁷Co]Co-(HE)₃-Z_HER3-DOTA improved imaging contrast compared to early-time-point imaging with [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA.

Selection of an optimal macrocyclic chelator improves the imaging of prostate cancer using cobalt-labeled GRPR antagonist RM26

Gastrin-releasing peptide receptors (GRPRs) are promising targets in oligometastatic prostate cancer. We have recently used 55Co (T1/2 = 17.5 h) as a label for next day PET imaging of GRPR expression obtaining high imaging contrast. The radionuclide-chelator combination can significantly influence the biodistribution of radiopeptides. Therefore, in this study, we hypothesized that the properties of 55Co-labeled PEG2-RM26 can be improved by identifying the optimal macrocyclic chelator. All analogues (X-PEG2-RM26, X = NOTA,NODAGA,DOTA,DOTAGA) were successfully labeled with radiocobalt with high yields and demonstrated high stability. The radiopeptides bound specifically and with picomolar affinity to GRPR and their cellular processing was characterized by low internalization. The best binding capacity was found for DOTA-PEG2-RM26. Ex vivo biodistribution in PC-3 xenografted mice was characterized by rapid blood clearance via renal excretion. Tumor uptake was similar for all conjugates at 3 h pi, exceeding the uptake in all other organs. Higher kidney uptake and longer retention were associated with N-terminal negative charge (DOTAGA-containing conjugate). Tumor-to-organ ratios increased over time for all constructs, although significant chelator-dependent differences were observed. Concordant with affinity measurements, DOTA-analog had the best retention of activity in tumors, resulting in the highest tumor-to-blood ratio 24 h pi, which translated into high contrast PET/CT imaging (using 55Co).

Site-specific conjugation of recognition tags to trastuzumab for peptide nucleic acid-mediated radionuclide HER2 pretargeting

Pretargeting is a promising strategy to reach high imaging contrast in a shorter time than by targeting with directly radiolabeled monoclonal antibodies (mAbs). One of problems in pretargeting is a site-specific, reproducible and uniform conjugation of recognition tags to mAbs. To solve this issue we propose a photoconjugation to covalently couple a recognition tag to a mAb via a photoactivatable Z domain. The Z-domain, a 58-amino acid protein derived from the IgG-binding B-domain of Staphylococcus aureus protein A, has a well-characterized binding site in the Fc portion of IgG. We tested the feasibility of this approach using pretargeting based on hybridization between peptide nucleic acids (PNAs). We have used photoconjugation to couple trastuzumab with the PNA-based hybridization probe, HP1. A complementary [⁵⁷Co]Co-labeled PNA hybridization probe ([⁵⁷Co]Co-HP2) was used as the secondary targeting probe. In vitro studies demonstrated that trastuzumab-ZHP1 bound specifically to human epidermal growth factor receptor 2 (HER2)-expressing cells with nanomolar affinity. The binding of the secondary [⁵⁷Co]Co-HP2 probe to trastuzumab-PNA-pretreated cells was in the picomolar affinity range. A two-fold increase in SKOV-3 tumor targeting was achieved when [⁵⁷Co]Co-HP2 (0.7 nmol) was injected 48 h after injection of trastuzumab-ZHP1 (0.5 nmol) compared with trastuzumab-ZHP1 alone (0.8 ± 0.2 vs. 0.33 ± 0.06 %ID/g). Tumor accumulation of [⁵⁷Co]Co-HP2 was significantly reduced by pre-saturation with trastuzumab or when no trastuzumab-ZHP1 was preinjected. A tumor-to-blood uptake ratio of 1.5 ± 0.3 was achieved resulting in a clear visualization of HER2-expressing xenografts as confirmed by SPECT imaging. In conclusion, the feasibility of stable site-specific coupling of a PNA-based recognition tag to trastuzumab and successful pretargeting has been demonstrated. This approach can hopefully be used for a broad range of mAbs and recognition tags.

Differences in the S value between male and female murine model for diagnostic, therapeutic and theragnostic radionuclides

The aim of this work was to calculate S values for ⁹⁹mTc, ⁶⁷Ga, ⁶⁸Ga, ¹⁸F, ²²³Ra, ¹⁶⁶Ho, ⁹⁰Y, ¹⁶¹Tb ¹³¹I and ¹⁷⁷Lu, using a mouse phantom (MOBY) standard and considering the anatomic sizes from males and females, the simulation of radiation transport was performed with GATE/Geant4 platform. This indicates that in the internal dosimetry the use of a customized geometry is relevant for each gender and a standard model is not a good choice.

Radioactive Transition Metals for Imaging and Therapy

Nuclear medicine is composed of two complementary areas, imaging and therapy. Positron emission tomography (PET) and single-photon imaging, including single-photon emission computed tomography (SPECT), comprise the imaging component of nuclear medicine. These areas are distinct in that they exploit different nuclear decay processes and also different imaging technologies. In PET, images are created from the 511 keV photons produced when the positron emitted by a radionuclide encounters an electron and is annihilated. In contrast, in single-photon imaging, images are created from the γ rays (and occasionally X-rays) directly emitted by the nucleus. Therapeutic nuclear medicine uses particulate radiation such as Auger or conversion electrons or β^- or α particles. All three of these technologies are linked by the requirement that the radionuclide must be attached to a suitable vector that can deliver it to its target. It is imperative that the radionuclide remain attached to the vector before it is delivered to its target as well as after it reaches its target or else the resulting image (or therapeutic outcome) will not reflect the biological process of interest. Radiochemistry is at the core of this process, and radiometals offer radiopharmaceutical chemists a tremendous range of options with which to accomplish these goals. They also offer a wide range of options in terms of radionuclide half-lives and emission properties, providing the ability to carefully match the decay properties with the desired outcome. This Review provides an overview of some of the ways this can be accomplished as well as several historical examples of some of the limitations of earlier metalloradiopharmaceuticals and the ways that new technologies, primarily related to radionuclide production, have provided solutions to these problems.

PET radiometals for antibody labeling

Recent advances in molecular characterization of tumors have made possible the emergence of new types of cancer therapies where traditional cytotoxic drugs and nonspecific chemotherapy can be complemented with targeted molecular therapies. One of the main revolutionary treatments is the use of monoclonal antibodies (mAbs) that selectively target the disseminated tumor cells while sparing normal tissues. mAbs and related therapeutics can be efficiently radiolabeled with a wide range of radionuclides to facilitate preclinical and clinical studies. Non-invasive molecular imaging techniques, such as Positron Emission Tomography (PET), using radiolabeled mAbs provide useful information on the whole-body distribution of the biomolecules, which may enable patient stratification, diagnosis, selection of targeted therapies, evaluation of treatment response, and prediction of dose limiting tissue and adverse effects. In addition, when mAbs are labeled with therapeutic radionuclides, the combination of immunological and radiobiological cytotoxicity may result in enhanced treatment efficacy. The pharmacokinetic profile of antibodies demands the use of long half-life isotopes for longitudinal scrutiny of mAb biodistribution and precludes the use of well-stablished short half-life isotopes. Herein, we review the most promising PET radiometals with chemical and physical characteristics that make the appealing for mAb labeling, highlighting those with theranostic radioisotopes.

Cyclotron production and radiochemical separation of 55Co and 58mCo from 54Fe, 58Ni and 57Fe targets

This work presents the production with a cyclotron of the positron emitter ⁵⁵Co via the ⁵⁴Fe(d,n) and ⁵⁸Ni(p,α) reactions and the Auger electron emitter ^58mCo via the ⁵⁷Fe(d,n) reaction after high current (40μA p and 60μA d) irradiation on electroplated targets. High specific activity radionuclides (up to 55.6 GBq/μmol ⁵⁵Co and 31.8GBq/μmol ^58mCo) with high radionuclidic purity (99.995% ⁵⁵Co from ⁵⁴Fe, 98.8% ⁵⁵Co from ⁵⁸Ni, and 98.7% ^58mCo from ⁵⁷Fe at end of bombardment, EoB), in high activity concentration (final separated radionuclide in < 0.6mL) and with almost quantitative overall activity separation yield (> 92%) were obtained after processing of the irradiated targets with novel radiochemical separation methods based on HCl dissolution and the resin N,N,N',N'-tetrakis-2-ethylhexyldiglycolamide (DGA, branched). One hour long irradiations using 38-65, 110-214 and 59-78mg of enriched ⁵⁴Fe (99.93%), ⁵⁸Ni (99.48%) and ⁵⁷Fe (95.06%), respectively, electroplated over a 1.0cm² surface, yielded 58² ± 66MBq ⁵⁵Co, 372 ± 14MBq ⁵⁵Co and 810 ± 186MBq ^58mCo, respectively, decay corrected to EoB. The separation methods allow for the recovery of the costly enriched target materials, which were reconstituted into metallic targets after novel electroplating methods, with an overall recycling efficiency of 93 ± 4% for iron. The produced radionuclides were used to radiolabel the angiogenesis marker antibody TRC105 conjugated to the chelator NOTA as a demonstration of their quality.

High Contrast PET Imaging of GRPR Expression in Prostate Cancer Using Cobalt-Labeled Bombesin Antagonist RM26

High gastrin releasing peptide receptor (GRPR) expression is associated with numerous cancers including prostate and breast cancer. The aim of the current study was to develop a ⁵⁵Co-labeled PET agent based on GRPR antagonist RM26 for visualization of GRPR-expressing tumors. Labeling with ⁵⁷Co and ⁵⁵Co, stability, binding specificity, and in vitro and in vivo characteristics of ⁵⁷Co-NOTA-PEG₂-RM26 were studied. NOTA-PEG₂-RM26 was successfully radiolabeled with ⁵⁷Co and ⁵⁵Co with high yields and demonstrated high stability. The radiopeptide showed retained binding specificity to GRPR in vitro and in vivo. ⁵⁷Co-NOTA-PEG₂-RM26 biodistribution in mice was characterized by rapid clearance of radioactivity from blood and normal non-GRPR-expressing organs and low hepatic uptake. The clearance was predominantly renal with a low degree of radioactivity reabsorption. Tumor-to-blood ratios were approximately 200 (3 h pi) and 1000 (24 h pi). The favorable biodistribution of cobalt-labeled NOTA-PEG₂-RM26 translated into high contrast preclinical PET/CT (using ⁵⁵Co) and SPECT/CT (using ⁵⁷Co) images of PC-3 xenografts. The initial biological results suggest that ⁵⁵Co-NOTA-PEG₂-RM26 is a promising tracer for PET visualization of GRPR-expressing tumors.

The use of radiocobalt as a label improves imaging of EGFR using DOTA-conjugated Affibody molecule

Several anti-cancer therapies target the epidermal growth factor receptor (EGFR). Radionuclide imaging of EGFR expression in tumours may aid in selection of optimal cancer therapy. The ¹¹¹In-labelled DOTA-conjugated Z_EGFR:2377 Affibody molecule was successfully used for imaging of EGFR-expressing xenografts in mice. An optimal combination of radionuclide, chelator and targeting protein may further improve the contrast of radionuclide imaging. The aim of this study was to evaluate the targeting properties of radiocobalt-labelled DOTA-Z_EGFR:2377. DOTA-Z_EGFR:2377 was labelled with ⁵⁷Co (T_1/2 = 271.8 d), ⁵⁵Co (T_1/2 = 17.5 h), and, for comparison, with the positron-emitting radionuclide ⁶⁸Ga (T_1/2 = 67.6 min) with preserved specificity of binding to EGFR-expressing A431 cells. The long-lived cobalt radioisotope ⁵⁷Co was used in animal studies. Both ⁵⁷Co-DOTA-Z_EGFR:2377 and ⁶⁸Ga-DOTA-Z_EGFR:2377 demonstrated EGFR-specific accumulation in A431 xenografts and EGFR-expressing tissues in mice. Tumour-to-organ ratios for the radiocobalt-labelled DOTA-Z_EGFR:2377 were significantly higher than for the gallium-labelled counterpart already at 3 h after injection. Importantly, ⁵⁷Co-DOTA-Z_EGFR:2377 demonstrated a tumour-to-liver ratio of 3, which is 7-fold higher than the tumour-to-liver ratio for ⁶⁸Ga-DOTA-Z_EGFR:2377. The results of this study suggest that the positron-emitting cobalt isotope ⁵⁵Co would be an optimal label for DOTA-Z_EGFR:2377 and further development should concentrate on this radionuclide as a label.

G. B, Sebastian P, Somananthan T, Maliekal TT. TM1-IR680 peptide for assessment of surgical margin and lymph node metastasis in murine orthotopic model of oral cancer

Treatment outcome after surgical removal in oral carcinoma is poor due to inadequate methodologies available for marking surgical margins. Even though some methodologies for intraoperative margin assessment are under clinical and preclinical trials for other solid tumours, a promising modality for oral cancer surgery is not developed. Fluorescent-based optical imaging using Near Infrared (NIR) dyes tagged to tumour specific target will be an optimal tool for this purpose. One such target, Gastrin Releasing Peptide Receptor (GRPR) was selected for the study, and its binding peptide, TM1-IR680, was tested for its efficacy for surgical margin prediction in murine orthotopic model of oral cancer, derived from primary samples. Here, for the first time in a preclinical analysis, we show that the size and margin of oral cancer can be predicted, as revealed by 3D-imaging. Interestingly, the peptide was sensitive enough to detect lymph nodes that harboured dispersed tumour cells before colonization, which was impossible to identify by conventional histopathology. We recommend the use of TM1-NIR dyes alone or in combination with other technologies to improve the clinical outcome of oral cancer surgery.