Effects of chelator modifications on (68)Ga-labeled [Tyr (3)]octreotide conjugates

From cyclotrons to chromatography and beyond: a guide to the production and purification of theranostic radiometals

Recent clinical success with metal-based radiopharmaceuticals has sparked an interest in the potential of these drugs for personalized medicine. Although often overlooked, the success and global impact of nuclear medicine is contingent upon the purity and availability of medical isotopes, commonly referred to as radiometals. For nuclear medicine to reach its true potential and change patient lives, novel production and purification techniques that increase inventory of radiometals are desperately needed. This tutorial review serves as a resource for those both new and experienced in nuclear medicine by providing a detailed explanation of the foundations for the production and purification of radiometals, stemming from nuclear physics, analytical chemistry, and so many other fields, all in one document. The fundamental science behind targetry, particle accelerators, nuclear reactors, nuclear reactions, and radiochemical separation are presented in the context of the field. Finally, a summary of the latest breakthroughs and a critical discussion of the threats and future potential of the most utilized radiometals is also included. With greater understanding of the fundamentals, fellow scientists will be able to better interpret the literature, identify knowledge gaps or problems and ultimately invent new production and purification pathways to increase the global availability of medical isotopes.

Structural modifications toward improved lead-203/lead-212 peptide-based image-guided alpha-particle radiopharmaceutical therapies for neuroendocrine tumors

PURPOSE

The lead-203 (203Pb)/lead-212 (212Pb) elementally identical radionuclide pair has gained significant interest in the field of image-guided targeted alpha-particle therapy for cancer. Emerging evidence suggests that 212Pb-labeled peptide-based radiopharmaceuticals targeting somatostatin receptor subtype 2 (SSTR2) may provide improved effectiveness compared to beta-particle-based therapies for neuroendocrine tumors (NETs). This study aims to improve the performance of SSTR2-targeted radionuclide imaging and therapy through structural modifications to Tyr3-octreotide (TOC)-based radiopharmaceuticals.

METHODS

New SSTR2-targeted peptides were designed and synthesized with the goal of optimizing the incorporation of Pb isotopes through the use of a modified cyclization technique; the introduction of a Pb-specific chelator (PSC); and the insertion of polyethylene glycol (PEG) linkers. The binding affinity of the peptides and the cellular uptake of 203Pb-labeled peptides were evaluated using pancreatic AR42J (SSTR2+) tumor cells and the biodistribution and imaging of the 203Pb-labeled peptides were assessed in an AR42J tumor xenograft mouse model. A lead peptide was identified (i.e., PSC-PEG2-TOC), which was then further evaluated for efficacy in 212Pb therapy studies.

RESULTS

The lead radiopeptide drug conjugate (RPDC) - [203Pb]Pb-PSC-PEG2-TOC - significantly improved the tumor-targeting properties, including receptor binding and tumor accumulation and retention as compared to [203Pb]Pb-DOTA0-Tyr3-octreotide (DOTATOC). Additionally, the modified RPDC exhibited faster renal clearance than the DOTATOC counterpart. These advantageous characteristics of [212Pb]Pb-PSC-PEG2-TOC resulted in a dose-dependent therapeutic effect with minimal signs of toxicity in the AR42J xenograft model. Fractionated administrations of 3.7 MBq [212Pb]Pb-PSC-PEG2-TOC over three doses further improved anti-tumor effectiveness, resulting in 80% survival (70% complete response) over 120 days in the mouse model.

CONCLUSION

Structural modifications to chelator and linker compositions improved tumor targeting and pharmacokinetics (PK) of 203/212Pb peptide-based radiopharmaceuticals for NET theranostics. These findings suggest that PSC-PEG2-TOC is a promising candidate for Pb-based targeted radionuclide therapy for NETs and other types of cancers that express SSTR2.

Towards the Magic Radioactive Bullet: Improving Targeted Radionuclide Therapy by Reducing the Renal Retention of Radioligands

Targeted radionuclide therapy (TRT) is an emerging field and has the potential to become a major pillar in effective cancer treatment. Several pharmaceuticals are already in routine use for treating cancer, and there is still a high potential for new compounds for this application. But, a major issue for many radiolabeled low-to-moderate-molecular-weight molecules is their clearance via the kidneys and their subsequent reuptake. High renal accumulation of radioactive compounds may lead to nephrotoxicity, and therefore, the kidneys are often the dose-limiting organs in TRT with these radioligands. Over the years, different strategies have been developed aiming for reduced kidney retention and enhanced therapeutic efficacy of radioligands. In this review, we will give an overview of the efforts and achievements of the used strategies, with focus on the therapeutic potential of low-to-moderate-molecular-weight molecules. Among the strategies discussed here is coadministration of compounds that compete for binding to the endocytic receptors in the proximal tubuli. In addition, the influence of altering the molecular design of radiolabeled ligands on pharmacokinetics is discussed, which includes changes in their physicochemical properties and implementation of cleavable linkers or albumin-binding moieties. Furthermore, we discuss the influence of chelator and radionuclide choice on reabsorption of radioligands by the kidneys.

Inorganic Chemistry of the Tripodal Picolinate Ligand with Gallium(III) and Radiolabeling with Gallium-68

We report here the improved synthesis of the tripodal picolinate chelator Tpaa, with an overall yield of 41% over five steps, in comparison to the previously reported 6% yield. Tpaa was investigated for its coordination chemistry with Ga(III) and radiolabeling properties with gallium-68 (68Ga). The obtained crystal structure for [Ga(Tpaa)] shows that the three picolinate arms coordinate to the Ga(III) ion, fully occupying the octahedral coordination geometry. This is supported by 1H NMR which shows that the three arms are symmetrical when coordinated to Ga(III). Assessment of the thermodynamic stability through potentiometry gives log KGa-Tpaa = 21.32, with a single species being produced across the range of pH 3.5-7.5. Tpaa achieved >99% radiochemical conversion with 68Ga under mild conditions ([Tpaa] = 6.6 μM, pH 7.4, 37 °C) with a molar activity of 3.1 GBq μmol-1. The resulting complex, [68Ga][Ga(Tpaa)], showed improved stability over the previously reported [68Ga][Ga(Dpaa)(H2O)] in a serum challenge, with 32% of [68Ga][Ga(Tpaa)] remaining intact after 30 min of incubation with fetal bovine serum.

The effects of novel macrocyclic chelates on the targeting properties of the 68Ga-labeled Gastrin releasing peptide receptor antagonist RM2

BACKGROUND

The gastrin-releasing peptide receptor (GRPr) is a molecular target for the visualization of prostate cancer. Bombesin (BN) analogs are short peptides with a high affinity for GRPr. RM2 is a bombesin-based antagonist. It has been demonstrated that RM2 have superior in vivo biodistribution and targeting properties than high-affinity receptor agonists. This study developed new RM2-like antagonists by introducing the novel bifunctional chelators AAZTA5 and DATA5m to RM2.

RESULTS

The effects of different macrocyclic chelating groups on drug targeting properties and the possibility of preparing 68Ga-radiopharmaceuticals in a kit-based protocol were investigated using 68Ga-labeled entities. Both new RM2 variants were labelled with 68Ga3+ resulting in high yields, stability, and low molarity of the ligand. DATA5m-RM2 and AAZTA5-RM2 incorporated 68Ga3+ nearly quantitatively at room temperature within 3-5 min, and the labelling yield for 68Ga-DOTA-RM2 was approximately 10% under the same conditions. 68Ga-AAZTA5-RM2 showed stronger hydrophilicity according to partition coefficient. Although the maximal cellular uptake values of the three compounds were similar, 68Ga-AAZTA5-RM2 and 68Ga-DATA5m-RM2 peaked more rapidly. Biodistribution studies showed high and specific tumor uptake, with a maximum of 9.12 ± 0.81 percentage injected activity per gram of tissue (%ID/g) for 68Ga-DATA5m-RM2 and 7.82 ± 0.61%ID/g for 68Ga-AAZTA5-RM2 at 30 min after injection.

CONCLUSIONS

The conditions for complexation of DATA5m-RM2 and AAZTA5-RM2 with gallium-68 are milder, faster and require less amount of precursors than DOTA-RM2. Chelators had an evident influence on the pharmacokinetics and targeting properties of 68Ga-X-RM2 derivatives. Positively charged 68Ga-DATA5m-RM2 provided a high tumor uptake, high image contrast and good capability of targeting GRPr.

Influence of the Position and Composition of Radiometals and Radioiodine Labels on Imaging of Epcam Expression in Prostate Cancer Model Using the DARPin Ec1

Simple Summary

Metastasis-targeting therapy might improve outcomes in oligometastatic prostate cancer. Epithelial cell adhesion molecule (EpCAM) is overexpressed in 40–60% of prostate cancer cases and might be used as a target for specific delivery of toxins and drugs. Radionuclide molecular imaging could enable non-invasive detection of EpCAM and stratification of patients for targeted therapy. Designed ankyrin repeat proteins (DARPins) are scaffold proteins, which can be selected for specific binding to different targets. The DARPin Ec1 binds strongly to EpCAM. To determine an optimal design of Ec1-based probes, we labeled Ec1 at two different positions with four different nuclides (⁶⁸Ga, ¹¹¹In, ⁵⁷Co and ¹²⁵I) and investigated the impact on Ec1 biodistribution. We found that the C-terminus is the best position for labeling and that ¹¹¹In and ¹²⁵I provide the best imaging contrast. This study might be helpful for scientists developing imaging probes based on scaffold proteins.

Abstract

The epithelial cell adhesion molecule (EpCAM) is intensively overexpressed in 40–60% of prostate cancer (PCa) cases and can be used as a target for the delivery of drugs and toxins. The designed ankyrin repeat protein (DARPin) Ec1 has a high affinity to EpCAM (68 pM) and a small size (18 kDa). Radiolabeled Ec1 might be used as a companion diagnostic for the selection of PCa patients for therapy. The study aimed to investigate the influence of radiolabel position (N- or C-terminal) and composition on the targeting and imaging properties of Ec1. Two variants, having an N- or C-terminal cysteine, were produced, site-specifically conjugated to a DOTA chelator and labeled with cobalt-57, gallium-68 or indium-111. Site-specific radioiodination was performed using ((4-hydroxyphenyl)-ethyl)maleimide (HPEM). Biodistribution of eight radiolabeled Ec1-probes was measured in nude mice bearing PCa DU145 xenografts. In all cases, positioning of a label at the C-terminus provided the best tumor-to-organ ratios. The non-residualizing [¹²⁵I]I-HPEM label provided the highest tumor-to-muscle and tumor-to-bone ratios and is more suitable for EpCAM imaging in early-stage PCa. Among the radiometals, indium-111 provided the highest tumor-to-blood, tumor-to-lung and tumor-to-liver ratios and could be used at late-stage PCa. In conclusion, label position and composition are important for the DARPin Ec1.

Positron Emission Tomography Imaging of Neurotensin Receptor-Positive Tumors with 68Ga-Labeled Antagonists: The Chelate Makes the Difference Again

Neurotensin receptor 1 (NTS₁) is involved in the development and progression of numerous cancers, which makes it an interesting target for the development of diagnostic and therapeutic agents. A small molecule NTS₁ antagonist, named [¹⁷⁷Lu]Lu-IPN01087, is currently evaluated in phase I/II clinical trials for the targeted therapy of neurotensin receptor-positive cancers. In this study, we synthesized seven compounds based on the structure of NTS₁ antagonists, bearing different chelating agents, and radiolabeled them with gallium-68 for PET imaging. These compounds were evaluated in vitro and in vivo in mice bearing a HT-29 xenograft. The compound [⁶⁸Ga]Ga-bisNODAGA-16 showed a promising biodistribution profile with mainly signal in tumor (4.917 ± 0.776%ID/g, 2 h post-injection). Its rapid clearance from healthy tissues led to high tumor-to-organ ratios, resulting in highly contrasted PET images. These results were confirmed on subcutaneous xenografts of AsPC-1 tumor cells, a model of NTS₁-positive human pancreatic adenocarcinoma.

tBu4octapa-alkyl-NHS for metalloradiopeptide preparation

The peptide is an important class of biological targeting molecule; herein, a new bifunctional octadentate non-macrocyclic H4octapa, tBu4octapa-alkyl-NHS, which is compatible with solid-phase peptide synthesis and thus useful for radiopeptide preparation, has been synthesized. To preserve denticity, the alkyl-N-hydroxylsuccinimide linker was covalently attached to the methylene-carbon on one of the acetate arms, yielding a chiral carbon center. According to density-functional theory (DFT) calculations using [Lu(octapa-alkyl-benzyl-ester)]- as a simulation model, the chirality has minimal effects on the complex geometry; regardless of the S-/R-stereochemistry, DFT calculations revealed two possible geometric isomers, distorted bicapped trigonal antiprism (DBTA) and distorted square antiprism (DSA), due to the asymmetry in the chelator. To evaluate the biological behavior of the new bifunctionalization, two well-studied PSMA (prostate-specific membrane antigen)-targeting peptidomimetics of varying hydrophobicity were chosen as proof-of-principle targeting vector molecules. Radiolabeling both bioconjugates with lutetium-177 was highly efficient at room temperature in 15 min at micromolar chelator concentration pH = 7. Both the in vitro serum challenge and the lanthanum(iii) challenge studies revealed complex lability, and notably, progressive bone accumulation was only observed with the more hydrophobic linker (i.e. H4octapa-alkyl-PSMA617). This in vivo result informs potential alterations exerted by the linker on the complex geometry and stability, with an appropriate biological targeting vector adopted for such evaluations.

Zinc and copper complexes with azacrown ethers and their comparative stability in vitro and in vivo

Copper-based radiopharmaceuticals are of high interest these days owing to the decay properties of copper radioisotopes. In contrast, labeled zinc compounds have been less studied for applications in nuclear medicine. In this study, the stability of labeled zinc and copper complexes with two azacrown ether ligands was investigated and compared. Then, the in vitro and in vivo stability of the studied zinc complexes was demonstrated, with the complexes showing promise for biomedical applications. In contrast, analogous copper complexes quickly dissociated in the presence of serum proteins. Furthermore, a simple method for the production of radiochemically pure 65Zn was proposed, and the opportunity for its use as a surrogate radionuclide for research into potential zinc-containing radiopharmaceuticals was demonstrated.

Preliminary PET/CT Imaging with Somatostatin Analogs [68Ga]DOTAGA-TATE and [68Ga]DOTAGA-TOC

PURPOSE

Somatostatin receptor positron emission tomography/X-ray computed tomography (SSTR-PET/CT) is a well-established technique for staging and detection of neuroendocrine tumors (NETs). Ga-68-labeled DOTA-conjugated octreotide analogs are the privileged radiotracers for diagnosis and therapeutic monitoring of NETs. Hence, we were interested in assessing the influence of promising, newer variant DOTAGA on the hydrophilicity, pharmacokinetics, and lesion pick-up of somatostatin analogs. Herein, the potential of ([⁶⁸Ga]DOTAGA, Tyr³, Thr⁸) octreotide ([⁶⁸Ga]DOTAGA-TATE) and ([⁶⁸Ga]DOTAGA, Tyr³) octreotide ([⁶⁸Ga]DOTAGA-TOC) as NET imaging agents has been investigated.

PROCEDURES

Amenability of [⁶⁸Ga]DOTAGA-(TATE/TOC) to kit-type formulation has been demonstrated. Biodistribution studies were carried out in normal rats at 1 h post-injection (p.i.). [⁶⁸Ga]DOTAGA-(TATE/TOC) PET/CT scans were carried out in patients (70-170 MBq, 1 h p.i.) with histologically confirmed well-differentiated NETs.

RESULTS

[⁶⁸Ga]DOTAGA-TATE exhibited hydrophilicity similar to [⁶⁸Ga]DOTA-TATE (log P = -3.51 vs -3.69) whereas [⁶⁸Ga]DOTAGA-TOC was more hydrophilic than [⁶⁸Ga]DOTA-TOC (log P = -3.27 vs -2.93). [⁶⁸Ga]DOTAGA-TATE and [⁶⁸Ga]DOTA-TATE showed almost identical blood and kidney uptake in normal rats whereas significantly fast clearance (p < 0.05) of [⁶⁸Ga]DOTAGA-TATE was observed from other non-specific organs (liver, lungs, spleen, intestine). [⁶⁸Ga]DOTAGA-TOC also demonstrated rapid clearance from blood and kidneys (p < 0.05) in comparison to [⁶⁸Ga]DOTA-TOC. The metastatic lesions in NET patients were well identified by [⁶⁸Ga]DOTAGA-TATE and [⁶⁸Ga]DOTAGA-TOC.

CONCLUSION

The phenomenal analogy was observed between [⁶⁸Ga]DOTAGA-TATE and [⁶⁸Ga]DOTA-TATE as well as between [⁶⁸Ga]DOTAGA-TOC and [⁶⁸Ga]DOTA-TOC in biodistribution studies in rats. The good lesion detection ability of the two radiotracers indicates their potential as NET imaging radiotracers.

Positron Emission Tomography Imaging of Prostate Cancer with Ga-68-Labeled Gastrin-Releasing Peptide Receptor Agonist BBN7-14 and Antagonist RM26

Radiolabeled bombesin (BBN) analogs have long been used for developing gastrin-releasing peptide receptor (GRPR) targeted imaging probes, and tracers with excellent in vivo performance including high tumor uptake, high contrast, and favorable pharmacokinetics are highly desired. In this study, we compared the ⁶⁸Ga-labeled GRPR agonist (Gln–Trp–Ala–Val–Gly–His–Leu–Met–NH₂, BBN_7–14) and antagonist (d-Phe–Gln–Trp–Ala–Val–Gly–His–Sta–Leu–NH₂, RM26) for the positron emission tomography (PET) imaging of prostate cancer. The in vitro stabilities, receptor binding, cell uptake, internalization, and efflux properties of the probes ⁶⁸Ga–1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)–Aca–BBN_7–14 and ⁶⁸Ga–NOTA–poly(ethylene glycol)₃ (PEG₃)–RM26 were studied in PC-3 cells, and the in vivo GRPR targeting abilities and kinetics were investigated using PC-3 tumor xenografted mice. BBN_7–14, PEG₃-RM26, NOTA–Aca–BBN_7–14, and NOTA–PEG₃–RM26 showed similar binding affinity to GRPR. In PC-3 tumor-bearing mice, the tumor uptake of ⁶⁸Ga–NOTA–PEG₃–RM26 remained at around 3.00 percentage of injected dose per gram of tissue within 1 h after injection, in contrast with ⁶⁸Ga–NOTA–Aca–BBN_7–14, which demonstrated rapid elimination and high background signal. Additionally, the majority of the ⁶⁸Ga–NOTA–PEG₃–RM26 remained intact in mouse serum at 5 min after injection, while almost all of the ⁶⁸Ga–NOTA–Aca–BBN_7–14 was degraded under the same conditions, demonstrating more-favorable in vivo pharmacokinetic properties and metabolic stabilities of the antagonist probe relative to its agonist counterpart. Overall, the antagonistic GRPR targeted probe ⁶⁸Ga–NOTA–PEG₃–RM26 is a more-promising candidate than the agonist ⁶⁸Ga–NOTA–Aca–BBN_7–14 for the PET imaging of prostate cancer patients.

Evaluation of Cu-64 and Ga-68 Radiolabeled Glucagon-Like Peptide-1 Receptor Agonists as PET Tracers for Pancreatic β cell Imaging

PURPOSE

Copper-64 (Cu-64) and Galium-68 (Ga-68) radiolabeled DO3A and NODA conjugates of exendin-4 were used for preclinical imaging of pancreatic β cells via targeting of glucagon-like peptide-1 receptor (GLP-1R).

PROCEDURES

DO3A-VS- and NODA-VS-tagged Cys(40)exendin-4 (DO3A-VS-Cys(40)-exendin-4 and NODA-VS-Cys(40)-exendin-4, respectively) were labeled with Cu-64 and Ga-68 using standard techniques. Biodistribution and dynamic positron emission tomography (PET) were carried out in normal Sprague-Dawley (SD) rats. Ex vivo autoradiography imaging was conducted with freshly frozen pancreatic thin sections.

RESULTS

DO3A-VS- and NODA-VS-Cys(40)-exendin-4 analogues were labeled with Cu-64 and Ga-68 to a specific activity of 518.7 ± 3.7 Ci/mmol (19.19 ± 0.14 TBq/mmol) and radiochemical yield above 98 %. Biodistribution data demonstrated pancreatic uptake of 0.11 ± 0.02 %ID/g for [(64)Cu]DO3A-VS-, 0.14 ± 0.02 %ID/g for [(64)Cu]NODA-VS-, 0.11 ± 0.03 for [(68)Ga]DO3A-VS-, and 0.26 ± 0.03 for [(68)Ga]NODA-VS-Cys(40)-exendin-4. Excess exendin-4 and exendin-(9-39)-amide displaced all four Cu-64 and Ga-68 labeled exendin-4 derivatives in blocking studies.

CONCLUSIONS

[(64)Cu]/[(68)Ga]DO3A-VS-Cys(40)- and [(64)Cu]/[(68)Ga]NODA-VS-Cys(40)-exendin-4 can be used as PET imaging agents specific for GLP-1R expressed on β cells. Here, we report the first evidence of pancreatic uptake visualized with exendin-4 derivative in a rat animal model via in vivo dynamic PET imaging.

Preclinical evaluation of [(68)Ga]NOTA-pentixafor for PET imaging of CXCR4 expression in vivo - a comparison to [(68)Ga]pentixafor

Background

Due to its overexpression in a variety of tumor types, the chemokine receptor 4 (CXCR4) represents a highly relevant diagnostic and therapeutic target in nuclear oncology. Recently, [⁶⁸Ga]pentixafor has emerged as an excellent imaging agent for positron emission tomography (PET) of CXCR4 expression in vivo. In this study, the corresponding [⁶⁸Ga]-1,4,7-triazacyclononane-triacetic acid (NOTA) analog was preclinically evaluated and compared to the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) parent compound [⁶⁸Ga]pentixafor.

Methods

NOTA-pentixafor was synthesized by combining solid and solution-phase peptide synthesis. The CXCR4 receptor affinities of [⁶⁸Ga]pentixafor and [⁶⁸Ga]NOTA-pentixafor were determined in competitive binding assays using the leukemic CXCR4-expressing Jurkat T-cell line and [¹²⁵I]FC131 as the radioligand. Internalization and cell efflux assays were performed using CXCR4-transfected Chem-1 cells. Small-animal PET and biodistribution studies were carried out using Daudi-tumor bearing SCID mice.

Results

[⁶⁸Ga]NOTA-pentixafor showed a 1.4-fold improved affinity towards CXCR4 (IC₅₀). However, internalization efficiency into CXCR4⁺-Chem-1 cells was substantially decreased compared to [⁶⁸Ga]pentixafor. Accordingly, small-animal PET imaging and biodistribution studies revealed a 9.5-fold decreased uptake of [⁶⁸Ga]NOTA-pentixafor in Daudi lymphoma xenografts (1.7 ± 0.4 % vs 16.2 ± 3.8 % ID/g at 90 min p.i.) and higher levels of non-specific accumulation, primarily in the excretory organs such as the liver, intestines, and kidneys (2.3 ± 0.9 % vs 2.0 ± 0.3 % ID/g, 1.9 ± 0.8 % vs 0.7 ± 0.2 % ID/g, and 2.7 ± 1.1 % vs 1.7 ± 0.9 % ID/g, respectively).

Conclusions

Despite enhanced CXCR4-affinity in vitro, the [⁶⁸Ga]NOTA-analog of pentixafor showed reduced CXCR4 targeting efficiency in vivo. In combination with enhanced background accumulation, this resulted in significantly inferior PET imaging contrast, and thus, [⁶⁸Ga]NOTA-pentixafor offers no advantages over [⁶⁸Ga]pentixafor.

DATATOC: a novel conjugate for kit-type 68Ga labelling of TOC at ambient temperature

Background

The widespread acceptance and application of ⁶⁸Ga-PET depends on our ability to develop radiopharmaceuticals that can be prepared in a convenient and suitable manner. A kit-type labelling protocol provides such characteristics and requires chelators that can be radiolabelled under exceptionally mild conditions. Recently the DATA chelators have been introduced that fulfil these requirements. In continuing their development, the synthesis and radiolabelling of the first DATA bifunctional chelator (BFC) and peptide conjugate are described.

Results

A BFC derived from the DATA ligand (2,2'-(6-((carboxymethyl)amino)-1,4-diazepane-1,4-diyl)diacetic acid) has been synthesised in five steps from simple building blocks, with an overall yield of 8 %. DATA^M5-3^tBu (5-[1,4-Bis-tert-butoxycarbonylmethyl-6-(tert-butoxycarbonylmethyl-methyl-amino)-[1, 4]diazepan-6-yl]-pentanoic acid) has been coupled to [DPhe¹][Tyr³]-octreotide (TOC) and the resulting peptide conjugate (DATATOC) radiolabelled with purified ⁶⁸Ga derived via four different ⁶⁸Ge/⁶⁸Ga generator post-processing (PP) methods. The stability and lipophilicity of the radiotracer have been assessed and a kit-type formulation for radiolabelling evaluated. ⁶⁸Ga-DATATOC has been prepared with a > 95 % radiochemical yield (RCY) within 1 (fractionated and acetone-PP) and 10 min (ethanol- and NaCl-PP) at 23 °C (pH 4.2-4.9, 13 nmol). The radiolabelled peptide is stable in the presence of human serum. Lipophilicity of ⁶⁸Ga-DATATOC was calculated as logP = -3.2 ± 0.3, with a HPLC retention time (t_R = 10.4 min) similar to ⁶⁸Ga-DOTATOC (logP = -2.9 ± 0.4, t_R = 10.3 min). Kit-type labelling from a lyophilised solid using acetone-PP based labelling achieves > 95 % RCY in 10 min at 23 °C.

Conclusions

The favourable labelling properties of the DATA chelators have been retained for DATATOC. High radiochemical purity can be achieved at 23 °C in less than 1 min and from a kit formulation. The speed, reliability, ease, flexibility and simplicity with which ⁶⁸Ga-DATATOC can be prepared makes it a very attractive alternative to current standards.

Positron emission tomographic imaging of copper 64- and gallium 68-labeled chelator conjugates of the somatostatin agonist tyr3-octreotate

The bifunctional chelator and radiometal have been shown to have a direct effect on the pharmacokinetics of somatostatin receptor (SSTR)-targeted imaging agents. We evaluated three Y3-TATE analogues conjugated to NOTA-based chelators for radiolabeling with 64Cu and 68Ga for small-animal positron emission tomographic/computed tomographic (PET/CT) imaging. Two commercially available NOTA analogues, p-SCN-Bn-NOTA and NODAGA, were evaluated. The p-SCN-Bn-NOTA analogues were conjugated to Y3-TATE through β-Ala and PEG8 linkages. The NODAGA chelator was directly conjugated to Y3-TATE. The analogues labeled with 64Cu or 68Ga were analyzed in vitro for binding affinity and internalization and in vivo by PET/CT imaging, biodistribution, and Cerenkov imaging (68Ga analogues). We evaluated the effects of the radiometals, chelators, and linkers on the performance of the SSTR subtype 2--targeted imaging agents and also compared them to a previously reported agent, 64Cu-CB-TE2A-Y3-TATE. We found that the method of conjugation, particularly the length of the linkage between the chelator and the peptide, significantly impacted tumor and nontarget tissue uptake and clearance. Among the 64Cu- and 68Ga-labeled NOTA analogues, NODAGA-Y3-TATE had the most optimal in vivo behavior and was comparable to 64Cu-CB-TE2A-Y3-TATE. An advantage of NODAGA-Y3-TATE is that it allows labeling with 64Cu and 68Ga, providing a versatile PET probe for imaging SSTr subtype 2-positive tumors.

Underscoring the influence of inorganic chemistry on nuclear imaging with radiometals

Over the past several decades, radionuclides have matured from largely esoteric and experimental technologies to indispensible components of medical diagnostics. Driving this transition, in part, have been mutually necessary advances in biomedical engineering, nuclear medicine, and cancer biology. Somewhat unsung has been the seminal role of inorganic chemistry in fostering the development of new radiotracers. In this regard, the purpose of this Forum Article is to more visibly highlight the significant contributions of inorganic chemistry to nuclear imaging by detailing the development of five metal-based imaging agents: (64)Cu-ATSM, (68)Ga-DOTATOC, (89)Zr-transferrin, (99m)Tc-sestamibi, and (99m)Tc-colloids. In a concluding section, several unmet needs both in and out of the laboratory will be discussed to stimulate conversation between inorganic chemists and the imaging community.