Cation exchange-based post-processing of (68)Ga-eluate: a comparison of three solvent systems for labelling of DOTATOC, NO2AP(BP) and DATA(m.)

Cyclotron Production of Gallium-68 Radiopharmaceuticals Using the 68Zn(p,n)68Ga Reaction and Their Regulatory Aspects

Designing and implementing various radionuclide production methods guarantees a sustainable supply, which is important for medical use. The use of medical cyclotrons for radiometal production can increase the availability of gallium-68 (⁶⁸Ga) radiopharmaceuticals. Although generators have greatly influenced the demand for ⁶⁸Ga radiopharmaceuticals, the use of medical cyclotrons is currently being explored. The resulting ⁶⁸Ga production is several times higher than obtained from a generator. Moreover, the use of solid targets yields end of purification and end of synthesis (EOS) of up to 194 GBq and 72 GBq, respectively. Furthermore, experiments employing liquid targets have provided promising results, with an EOS of 3 GBq for [⁶⁸Ga]Ga-PSMA-11. However, some processes can be further optimized, specifically purification, to achieve high ⁶⁸Ga recovery and apparent molar activity. In the future, ⁶⁸Ga will probably remain one of the most in-demand radionuclides; however, careful consideration is needed regarding how to reduce the production costs. Thus, this review aimed to discuss the production of ⁶⁸Ga radiopharmaceuticals using Advanced Cyclotron Systems, Inc. (ACSI, Richmond, BC, Canada) Richmond, Canada and GE Healthcare, Wisconsin, USA cyclotrons, its related factors, and regulatory concerns.

AAZTA-Derived Chelators for the Design of Innovative Radiopharmaceuticals with Theranostic Applications

With the development of 68Ga and 177Lu radiochemistry, theranostic approaches in modern nuclear medicine enabling patient-centered personalized medicine applications have been growing in the last decade. In conjunction with the search for new relevant molecular targets, the design of innovative chelating agents to easily form stable complexes with various radiometals for theranostic applications has gained evident momentum. Initially conceived for magnetic resonance imaging applications, the chelating agent AAZTA features a mesocyclic seven-membered diazepane ring, conferring some of the properties of both acyclic and macrocyclic chelating agents. Described in the early 2000s, AAZTA and its derivatives exhibited interesting properties once complexed with metals and radiometals, combining a fast kinetic of formation with a slow kinetic of dissociation. Importantly, the extremely short coordination reaction times allowed by AAZTA derivatives were particularly suitable for short half-life radioelements (i.e., 68Ga). In view of these particular characteristics, the scope of this review is to provide a survey on the design, synthesis, and applications in the nuclear medicine/radiopharmacy field of AAZTA-derived chelators.

[68Ga]Ga-THP-Pam: A Bisphosphonate PET Tracer with Facile Radiolabeling and Broad Calcium Mineral Affinity

Calcium minerals such as hydroxyapatite (HAp) can be detected noninvasively in vivo using nuclear imaging agents such as [18F]NaF (available from cyclotrons), for positron emission tomography (PET) and 99mTc-radiolabeled bisphosphonates (BP; available from 99mTc generators for single photon emission computed tomography (SPECT) or scintigraphy). These two types of imaging agents allow detection of bone metastases (based on the presence of HAp) and vascular calcification lesions (that contain HAp and other calcium minerals). With the aim of developing a cyclotron-independent PET radiotracer for these lesions, with broad calcium mineral affinity and simple one-step radiolabeling, we developed [68Ga]Ga-THP-Pam. Radiolabeling with 68Ga is achieved using a mild single-step kit (5 min, room temperature, pH 7) to high radiochemical yield and purity (>95%). NMR studies demonstrate that Ga binds via the THP chelator, leaving the BP free to bind to its biological target. [68Ga]Ga-THP-Pam shows high stability in human serum. The calcium mineral binding of [68Ga]Ga-THP-Pam was compared in vitro to two other 68Ga-BPs which have been successfully evaluated in humans, [68Ga]Ga-NO2APBP and [68Ga]Ga-BPAMD, as well as [18F]NaF. Interestingly, we found that all 68Ga-BPs have a high affinity for a broad range of calcium minerals implicated in vascular calcification disease, while [18F]NaF is selective for HAp. Using healthy young mice as a model of metabolically active growing calcium mineral in vivo, we compared the pharmacokinetics and biodistribution of [68Ga]Ga-THP-Pam with [18F]NaF as well as [68Ga]NO2APBP. These studies revealed that [68Ga]Ga-THP-Pam has high in vivo affinity for bone tissue (high bone/muscle and bone/blood ratios) and fast blood clearance (t1/2 < 10 min) comparable to both [68Ga]NO2APBP and [18F]NaF. Overall, [68Ga]Ga-THP-Pam shows high potential for clinical translation as a cyclotron-independent calcium mineral PET radiotracer, with simple and efficient radiochemistry that can be easily implemented in any radiopharmacy.

68Ga, 44Sc and 177Lu-labeled AAZTA5-PSMA-617: synthesis, radiolabeling, stability and cell binding compared to DOTA-PSMA-617 analogues

BACKGROUND

The AAZTA chelator and in particular its bifunctional derivative AAZTA⁵ was recently investigated to demonstrate unique capabilities to complex diagnostic and therapeutic trivalent radiometals under mild conditions. This study presents a comparison of ⁶⁸Ga, ⁴⁴Sc and ¹⁷⁷Lu-labeled AAZTA⁵-PSMA-617 with DOTA-PSMA-617 analogues. We evaluated the radiolabeling characteristics, in vitro stability of the radiolabeled compounds and evaluated their binding affinity and internalization behavior on LNCaP tumor cells in direct comparison to the radiolabeled DOTA-conjugated PSMA-617 analogs.

RESULTS

AAZTA⁵ was synthesized in a five-step synthesis and coupled to the PSMA-617 backbone on solid phase. Radiochemical evaluation of AAZTA⁵-PSMA-617 with ⁶⁸Ga, ⁴⁴Sc and ¹⁷⁷Lu achieved quantitative radiolabeling of > 99% after less than 5 min at room temperature. Stabilities against human serum, PBS buffer and EDTA and DTPA solutions were analyzed. While there was a small degradation of the ⁶⁸Ga complex over 2 h in human serum, PBS and EDTA/DTPA, the ⁴⁴Sc and ¹⁷⁷Lu complexes were stable at 2 h and remained stable over 8 h and 1 day. For all three compounds, i.e. [^natGa]Ga-AAZTA⁵-PSMA-617, [^natSc]Sc-AAZTA⁵-PSMA-617 and [^natLu]Lu-AAZTA⁵-PSMA-617, in vitro studies on PSMA-positive LNCaP cells were performed in direct comparison to radiolabeled DOTA-PSMA-617 yielding the corresponding inhibition constants (K_i). K_i values were in the range of 8-31 nM values which correspond with those of [^natGa]Ga-DOTA-PSMA-617, [^natSc]Sc-DOTA-PSMA-617 and [^natLu]Lu-DOTA-PSMA-617, i.e. 5-7 nM, respectively. Internalization studies demonstrated cellular membrane to internalization ratios for the radiolabeled ⁶⁸Ga, ⁴⁴Sc and ¹⁷⁷Lu-AAZTA5-PSMA-617 tracers (13-20%IA/10⁶ cells) in the same range as the ones of the three radiolabeled DOTA-PSMA-617 tracers (17-20%IA/10⁶ cells) in the same assay.

CONCLUSIONS

The AAZTA⁵-PSMA-617 structure proved fast and quantitative radiolabeling with all three radiometal complexes at room temperature, excellent stability with ⁴⁴Sc, very high stability with ¹⁷⁷Lu and medium stability with ⁶⁸Ga in human serum, PBS and EDTA/DTPA solutions. All three AAZTA⁵-PSMA-617 tracers showed binding affinities and internalization ratios in LNCaP cells comparable with that of radiolabeled DOTA-PSMA-617 analogues. Therefore, the exchange of the chelator DOTA with AAZTA⁵ within the PSMA-617 binding motif has no negative influence on in vitro LNCaP cell binding characteristics. In combination with the faster and milder radiolabeling features, AAZTA⁵-PSMA-617 thus demonstrates promising potential for in vivo application for theranostics of prostate cancer.

DOTA-ZOL: A Promising Tool in Diagnosis and Palliative Therapy of Bone Metastasis-Challenges and Critical Points in Implementation into Clinical Routine

The novel compound 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-ZOL (DOTA-conjugated zoledronic acid) is a promising candidate for the diagnosis and therapy of bone metastasis. The combination of the published methodology for this bisphosphonate with pharmaceutical and regulatory requirements turned out to be unexpectedly challenging. The scope of this work is the presentation and discussion of problems encountered during this process. Briefly, the radiolabelling process and purification, as well as the quality control published, did not meet the expectations. The constant effort setting up an automated radiolabelling procedure resulted in (a) an enhanced manual method using coated glass reactors, (b) a combination of three different reliable radio thin-layer chromatography (TLC) methods instead of the published and (c) a preliminary radio high-pressure liquid chromatography (HPLC) method for identification of the compound. Additionally, an automated radiolabelling process was developed, but it requires further improvement, e.g., in terms of a reactor vessel or purification of the crude product. The published purification method was found to be unsuitable for clinical routine, and an intense screening did not lead to a satisfactory result; here, more research is necessary. To sum up, implementation of DOTA-ZOL was possible but revealed a lot of critical points, of which not all could be resolved completely yet.

Evaluation of [68Ga]Ga-DATA-TOC for imaging of neuroendocrine tumours: comparison with [68Ga]Ga-DOTA-NOC PET/CT

PURPOSE

Recently, the new hybrid chelator DATA (6-amino-1,4-diazepine-triacetate) has been introduced, which has the advantage of high yield and radiolabelling of DATA-based octreotide derivative (TOC) at room temperature in contrast to tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA) that needs 95 °C for effective labelling. However, the diagnostic potential of DATA-TOC has not been studied with other chelators in humans. The aim of this study was to compare the diagnostic efficacy of [⁶⁸Ga]Ga-DATA-TOC with [⁶⁸Ga]Ga-DOTA-NOC (which is the current standard for imaging neuroendocrine tumours (NET)) in patients of gastroenteropancreatic neuroendocrine tumours (GEP-NETs).

METHODS

Fifty patients (thirty-one males and nineteen females) with biopsy-proven GEP-NETs were included in the study. Patients age ranged from 14 to 75 years (mean 46.11 years). All patients underwent two PET studies with [⁶⁸Ga]Ga-DATA-TOC and [⁶⁸Ga]Ga-DOTA-NOC. Images were evaluated visually and semi-quantitatively using maximum standardized uptake values (SUVmax) of tumour, mediastinum and liver. Tumour-to-liver (T/L) and tumour-to-mediastinum (T/M) SUVmax ratios were computed. For the purpose of comparison, patient-wise as well as lesion-wise analysis was carried out. The nonparametric-related samples Wilcoxon signed-rank test was used for comparison of the SUVmax values and ratios.

RESULTS

On visual evaluation, the biodistribution and image quality of [⁶⁸Ga]Ga-DATA-TOC was similar to [⁶⁸Ga]Ga-DOTA-NOC. Physiological liver uptake was lower in [⁶⁸Ga]Ga-DATA-TOC as compared with [⁶⁸Ga]Ga-DOTA-NOC, 7.65 ± 5.37 vs 8.94 ± 5.95 (p = 0.009), respectively. On a patient-wise analysis, both [⁶⁸Ga]Ga-DATA-TOC and [⁶⁸Ga]Ga-DOTA-NOC were lesion-positive in the 44 patients (88%) and were negative in the 6 patients (12%). On a lesion-based analysis, [⁶⁸Ga]Ga-DATA-TOC had 98.6% concordance with [⁶⁸Ga]Ga-DOTA-NOC (232 out of 235 lesions detected). The target tumour SUVmax on [⁶⁸Ga]Ga-DATA-TOC and [⁶⁸Ga]Ga-DOTA-NOC were 36.63 ± 32.24 and 40.82 ± 36.89, respectively (p = 0.097). The T/L SUVmax ratios were not significantly different (5.99 ± 5.52 vs 5.67 ± 4.96, p = 0.77).

CONCLUSION

[⁶⁸Ga]Ga-DATA-TOC PET/CT imaging produced results that were comparable with [⁶⁸Ga]Ga-DOTA-NOC. It, thus, has potential utility as an effective and safe alternative to ⁶⁸Ga-DOTA-NOC with the added benefit of ease, cost-effective and improved yield of instant kit-type synthesis.

From Bench to Bedside-The Bad Berka Experience With First-in-Human Studies

Precision oncology is being driven by rapid advances in novel diagnostics and therapeutic interventions, with treatments targeted to the needs of individual patients on the basis of genetic, biomarker, phenotypic, or psychosocial characteristics that distinguish a given patient from other patients with similar clinical presentations. Inherent in the theranostics paradigm is the assumption that diagnostic test results can precisely determine whether an individual is likely to benefit from a specific treatment. As part and integral in the current era of precision oncology, theranostics in the context of nuclear medicine aims to identify the appropriate molecular targets in neoplasms (diagnostic tool), so that the optimal ligands and radionuclides (therapeutic tool) with favorable labeling chemistry can be selected for personalized management of a specific disease, taking into consideration the specific patient, and subsequently monitor treatment response. Over the past two decades, the use of gallium-68 labeled peptides for somatostatin receptor (SSTR)-targeted PET/CT (or PET/MRI) imaging followed by lutetium-177 and yttrium-90 labeled SSTR-agonist for peptide receptor radionuclide therapy has demonstrated remarkable success in the management of neuroendocrine neoplasms, and paved the way to other indications of theranostics. Rapid advances are being made in the development of other peptide-based radiopharmaceuticals, small molecular-weight ligands and with newer radioisotopes with more favorable kinetics, potentially useful for theranostics strategies for the clinical application. The present review features the Bad Berka experience with first-in-human studies of new radiopharmaceuticals, for example, prostate-specific membrane antigen ligand, gastrin-releasing peptide receptor, neurotensin receptor 1 ligand, novel SSTR-targeting peptides and nonpeptide, and bone-seeking radiopharmaceuticals. Also new radioisotopes, for example, actinium (²²⁵Ac), copper (⁶⁴Cu), scandium (⁴⁴Sc), and terbium (¹⁵²Tb/¹⁶¹Tb) will be discussed briefly demonstrating the development from basic science to precision oncology in the clinical setting.

Development of a Potential Gallium-68-Labelled Radiotracer Based on DOTA-Curcumin for Colon-Rectal Carcinoma: From Synthesis to In Vivo Studies

Colorectal cancer is the third most commonly occurring cancer in men and the second most commonly occurring cancer in women worldwide. We have recently reported that curcuminoid complexes labelled with gallium-68 have demonstrated preferential uptake in HT29 colorectal cancer and K562 lymphoma cell lines compared to normal human lymphocytes. In the present study, we report a new gallium-68-labelled curcumin derivative (⁶⁸Ga-DOTA-C21) and its initial validation as marker for early detection of colorectal cancer. The precursor and non-radioactive complexes were synthesized and deeply characterized by analytical methods then the curcuminoid was radiolabelled with gallium-68. The in vitro stability, cell uptake, internalization and efflux properties of the probe were studied in HT29 cells, and the in vivo targeting ability and biodistribution were investigated in mice bearing HT29 subcutaneous tumour model. ⁶⁸Ga-DOTA-C21 exhibits decent stability (57 ± 3% after 120 min of incubation) in physiological media and a curcumin-mediated cellular accumulation in colorectal cancer cell line (121 ± 4 KBq of radiotracer per mg of protein within 60 min of incubation). In HT29 tumour-bearing mice, the tumour uptake of ⁶⁸Ga-DOTA-C21 is 3.57 ± 0.3% of the injected dose per gram of tissue after 90 min post injection with a tumour to muscle ratio of 2.2 ± 0.2. High amount of activity (12.73 ± 1.9% ID/g) is recorded in blood and significant uptake of the radiotracer occurs in the intestine (13.56 ± 3.3% ID/g), lungs (8.42 ± 0.8% ID/g), liver (5.81 ± 0.5% ID/g) and heart (4.70 ± 0.4% ID/g). Further studies are needed to understand the mechanism of accumulation and clearance; however, ⁶⁸Ga-DOTA-C21 provides a productive base-structure to develop further radiotracers for imaging of colorectal cancer.

Radiolabeling of DOTA-like conjugated peptides with generator-produced (68)Ga and using NaCl-based cationic elution method

Gallium-68 (⁶⁸Ga) is a generator-produced radionuclide with a short half-life (t_½ = 68 min) that is particularly well suited for molecular imaging by positron emission tomography (PET). Methods have been developed to synthesize ⁶⁸Ga-labeled imaging agents possessing certain drawbacks, such as longer synthesis time because of a required final purification step, the use of organic solvents or concentrated hydrochloric acid (HCl). In our manuscript, we provide a detailed protocol for the use of an advantageous sodium chloride (NaCl)-based method for radiolabeling of chelator-modified peptides for molecular imaging. By working in a lead-shielded hot-cell system, ⁶⁸Ga³⁺ of the generator eluate is trapped on a cation exchanger cartridge (100 mg, ∼8 mm long and 5 mm diameter) and then eluted with acidified 5 M NaCl solution directly into a sodium acetate-buffered solution containing a DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) or DOTA-like chelator-modified peptide. The main advantages of this procedure are the high efficiency and the absence of organic solvents. It can be applied to a variety of peptides, which are stable in 1 M NaCl solution at a pH value of 3–4 during reaction. After labeling, neutralization, sterile filtration and quality control (instant thin-layer chromatography (iTLC), HPLC and pH), the radiopharmaceutical can be directly administered to patients, without determination of organic solvents, which reduces the overall synthesis-to-release time. This procedure has been adapted easily to automated synthesis modules, which leads to a rapid preparation of ⁶⁸Ga radiopharmaceuticals (12–16 min).

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.

New (68)Ga-PhenA bisphosphonates as potential bone imaging agents

INTRODUCTION

In vivo positron emission tomography (PET) imaging of the bone using [(68)Ga]bisphosphonates may be a valuable tool for cancer diagnosis and monitoring therapeutic treatment. We have developed new [(68)Ga]bisphosphonates based on the chelating group, AAZTA (6-[bis(hydroxycarbonyl-methyl)amino]-1,4-bis(hydroxycarbonyl methyl)-6-methylperhydro-1,4-diazepine).

METHOD

Phenoxy derivative of AAZTA (2,2'-(6-(bis(carboxymethyl)amino)-6-((4-(2-carboxyethyl)phenoxy)methyl)-1,4-diazepane-1,4-diyl)diacetic acid), PhenA, 2, containing a bisphosphonate group (PhenA-BPAMD, 3, and PhenA-HBP, 4) was prepared. Labeling of these chelating agents with (68)Ga was evaluated.

RESULTS

The ligands reacted rapidly in a sodium acetate buffer with [(68)Ga]GaCl3 eluted from a commercially available (68)Ge/(68)Ga generator (pH4, >95% labeling at room temperature in 5min) to form [(68)Ga]PhenA-BPAMD, 3, and [(68)Ga]PhenA-HBP, 4. The improved labeling condition negates the need for further purification. The (68)Ga bisphosphonate biodistribution and autoradiography of bone sections in normal mice after an iv injection showed excellent bone uptake.

CONCLUSION

New (68)Ga labeled bisphosphonates may be useful as in vivo bone imaging agents in conjunction with positron emission tomography (PET).

Monolith-based 68Ga processing: a new strategy for purification to facilitate direct radiolabelling methods

The post-processing of ⁶⁸Ga generator eluate by means of a novel high capacity cation-exchange silica monolith column has been validated in this work.

Preparation of ⁶⁸Ga-labelled DOTA-peptides using a manual labelling approach for small-animal PET imaging

(68)Ga-DOTA-peptides are a promising PET radiotracers used in the detection of different tumours types due to their ability for binding specifically receptors overexpressed in these. Furthermore, (68)Ga can be produced by a (68)Ge/(68)Ga generator on site which is a very good alternative to cyclotron-based PET isotopes. Here, we describe a manual labelling approach for the synthesis of (68)Ga-labelled DOTA-peptides based on concentration and purification of the commercial (68)Ga/(68)Ga generator eluate using an anion exchange-cartridge. (68)Ga-DOTA-TATE was used to image a pheochromocytoma xenograft mouse model by a microPET/CT scanner. The method described provides satisfactory results, allowing the subsequent (68)Ga use to label DOTA-peptides. The simplicity of the method along with its implementation reduced cost, makes it useful in preclinical PET studies.

Approaching 'Kit-Type' Labelling with (68)Ga: The DATA Chelators

The DATA chelators are a novel class of tri-anionic ligands based on 6-amino-1,4-diazepine-triacetic acid, which have been introduced recently for the chelation of (68)Ga. Compared with macrocyclic chelators based on the cyclen scaffold (i.e., DOTA, DO3A, and DO2A derivatives), DATA chelators undergo quantitative radiolabelling more rapidly and under milder conditions. In this study, a systematic evaluation of the labelling of four DATA chelators--DATA(M), DATA(P), DATA(Ph), and DATA(PPh)--with (68)Ga is presented. The results highlight the extraordinary potential of this new class of chelators for application in molecular imaging using (68)Ga positron emission tomography (PET).