[111In]In/[177Lu]Lu-AAZTA5-LM4 SST2R-Antagonists in Cancer Theranostics: From Preclinical Testing to First Patient Results

Aiming to expand the application of the SST2R-antagonist LM4 (DPhe-c[DCys-4Pal-DAph(Cbm)-Lys-Thr-Cys]-DTyr-NH2) beyond [68Ga]Ga-DATA5m-LM4 PET/CT (DATA5m, (6-pentanoic acid)-6-(amino)methy-1,4-diazepinetriacetate), we now introduce AAZTA5-LM4 (AAZTA5, 1,4-bis(carboxymethyl)-6-[bis(carboxymethyl)]amino-6-[pentanoic-acid]perhydro-1,4-diazepine), allowing for the convenient coordination of trivalent radiometals of clinical interest, such as In-111 (for SPECT/CT) or Lu-177 (for radionuclide therapy). After labeling, the preclinical profiles of [111In]In-AAZTA5-LM4 and [177Lu]Lu-AAZTA5-LM4 were compared in HEK293-SST2R cells and double HEK293-SST2R/wtHEK293 tumor-bearing mice using [111In]In-DOTA-LM3 and [177Lu]Lu-DOTA-LM3 as references. The biodistribution of [177Lu]Lu-AAZTA5-LM4 was additionally studied for the first time in a NET patient. Both [111In]In-AAZTA5-LM4 and [177Lu]Lu-AAZTA5-LM4 displayed high and selective targeting of the HEK293-SST2R tumors in mice and fast background clearance via the kidneys and the urinary system. This pattern was reproduced for [177Lu]Lu-AAZTA5-LM4 in the patient according to SPECT/CT results in a monitoring time span of 4–72 h pi. In view of the above, we may conclude that [177Lu]Lu-AAZTA5-LM4 shows promise as a therapeutic radiopharmaceutical candidate for SST2R-expressing human NETs, based on previous [68Ga]Ga-DATA5m-LM4 PET/CT, but further studies are needed to fully assess its clinical value. Furthermore, [111In]In-AAZTA5-LM4 SPECT/CT may represent a legitimate alternative diagnostic option in cases where PET/CT is not available.

Modern Developments in Bifunctional Chelator Design for Gallium Radiopharmaceuticals

The positron-emitting radionuclide gallium-68 has become increasingly utilised in both preclinical and clinical settings with positron emission tomography (PET). The synthesis of radiochemically pure gallium-68 radiopharmaceuticals relies on careful consideration of the coordination chemistry. The short half-life of 68 min necessitates rapid quantitative radiolabelling (≤10 min). Desirable radiolabelling conditions include near-neutral pH, ambient temperatures, and low chelator concentrations to achieve the desired apparent molar activity. This review presents a broad overview of the requirements of an efficient bifunctional chelator in relation to the aqueous coordination chemistry of gallium. Developments in bifunctional chelator design and application are then presented and grouped according to eight categories of bifunctional chelator: the macrocyclic chelators DOTA and TACN; the acyclic HBED, pyridinecarboxylates, siderophores, tris(hydroxypyridinones), and DTPA; and the mesocyclic diazepines.

[68Ga]Ga-DATA5m-LM4, a PET Radiotracer in the Diagnosis of SST2R-Positive Tumors: Preclinical and First Clinical Results

Radiolabeled somatostatin subtype 2 receptor (SST2R)-antagonists have shown advantageous profiles for cancer theranostics compared with agonists. On the other hand, the newly introduced hybrid chelator (6-pentanoic acid)-6-(amino)methyl-1,4-diazepinetriacetate (DATA^5m) rapidly binds Ga-68 (t_1/2: 67.7 min) at much lower temperature, thus allowing for quick access to "ready-for-injection" [⁶⁸Ga]Ga-tracers in hospitals. We herein introduce [⁶⁸Ga]Ga-DATA^5m-LM4 for PET/CT imaging of SST₂R-positive human tumors. LM4 was obtained by 4Pal³/Tyr³-substitution in the known SST₂R antagonist LM3 (H-DPhe-c[DCys-Tyr-DAph(Cbm)-Lys-Thr-Cys]-DTyr-NH₂) and DATA^5m was coupled at the N-terminus for labeling with radiogallium (Ga-67/68). [⁶⁷Ga]Ga-DATA^5m-LM4 was evaluated in HEK293-SST₂R cells and mice models in a head-to-head comparison with [⁶⁷Ga]Ga-DOTA-LM3. Clinical grade [⁶⁸Ga]Ga-DATA^5m-LM4 was prepared and injected in a neuroendocrine tumor (NET) patient for PET/CT imaging. DATA^5m-LM4 displayed high SST₂R binding affinity. [⁶⁷Ga]Ga-DATA^5m-LM4 showed markedly higher uptake in HEK293-SST₂R cells versus [⁶⁷Ga]Ga-DOTA-LM3 and was stable in vivo. In HEK293-SST₂R xenograft-bearing mice, it achieved longer tumor retention and less kidney uptake than [⁶⁷Ga]Ga-DOTA-LM3. [⁶⁸Ga]Ga-DATA^5m-LM4 accurately visualized tumor lesions with high contrast on PET/CT. In short, [⁶⁸Ga]Ga-DATA^5m-LM4 has shown excellent prospects for the PET/CT diagnosis of SST₂R-positive tumors, further highlighting the benefits of Ga-68 labeling in a hospital environment via the DATA^5m-chelator route.

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.

Recent Breakthrough in 68Ga-Radiopharmaceuticals Cold Kits for Convenient PET Radiopharmacy

⁶⁸Ga-PET has emerged as an important diagnostic tool for precise detection and monitoring of oncological situations. Availability, cost, and radiosynthesis procedure are determining steps for success of a radioisotope/radiopharmaceutical in nuclear medicine. Availability of ⁶⁸Ga from a ⁶⁸Ge/⁶⁸Ga generator containing a long-lived parent radioisotope (⁶⁸Ge: t_1/2 = 271 days) and an inexpensive, simplified production of ⁶⁸Ga-radiopharmaceuticals through kit methodology has allowed smooth accommodation of ⁶⁸Ga-PET in clinics. The uncomplicated formulation of ⁶⁸Ga-radiopharmaceuticals from a lyophilized, cold kit is an impending breakthrough in clinical PET. The huge success of ⁶⁸Ga in neuroendocrine tumor and prostate cancer imaging along with the regulatory approval of respective cold kits has opened a pathway for development of kits for other evolving radiotracers. There is a definite scope for increased participation of commercial manufacturers and distributors of cold kits to spread the potential of ⁶⁸Ga worldwide across all the geographical locations and satellite centers.

Mild and efficient 64Cu labeling of perhydro-1, 4-diazepine derivatives for potential use with large peptides, proteins and antibodies

DATA (6-Amino-1,4-diazapine-triacetate) and AAZTA (6-Amino-1,4-diazapine-tetracetate) chelators represent a novel approach representing hybrid-chelates: possessing significant cyclic and acyclic character. It is believed that flexibility of the acyclic part facilitates rapid complexation, whilst the preorganized cyclic part minimizes the energy barrier to complexation and inhibits decomplexation processes. So far, these chelators have been used exclusively with 44Sc and 68Ga only. Recent results with natCu predict high stabilities for Cu-AAZTA, yet no radioactive labeling of AAZTA or DATA with 64Cu or any additional radioactive isotope has been reported. We present the one pot synthesis of the bifunctional derivatives AAZTA5OMe and DATA5mOMe and their labeling with 64Cu. In addition, in vitro stability of the respective complexes are presented.

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.

N,1,4-Tri(4-alkoxy-2-hydroxybenzyl)-DAZA: efficient one-pot synthesis and labelling with 68Ga for PET liver imaging in ovo

We report the isolation of a new type of 1,4-diazepan-6-amine (DAZA)-based ligand. Condensation of aldehydes with DAZA gives a novel class of 1,5-diazabicyclo[3.2.1]octanes in nearly quantitative yields. Subsequent reductive cleavage of these bicyclic aminal species with sodium borohydride selectively leads to N,1,4-tri(4-alkoxy-2-hydroxybenzyl)-1,4-diazepan-6-amines (alkoxy = Me: TMeOHB-DAZA; alkoxy = Et: TEtOHB-DAZA) via a unique reductive alkylation reaction in which a substituent is added to the DAZA moiety without the presence of an alkylating agent. Mass spectrometry studies of the intermediates suggest that the mechanism involves insertion of in situ released carbonyl species into an aminal bond to form hemiaminal intermediates, and subsequent reduction. TMeOHB-DAZA and TEtOHB-DAZA are hexadentate ligands suitable for effectively coordinating Ga(iii) ions. Chelation of the radionuclide 68Ga was achieved within 5 min at 100 °C. In vitro stability studies in PBS and human serum confirmed the kinetic inertness of the tracers as no 68Ga demetallation was observed over a period of 4 h. Positron emission tomography (PET)/computed tomography (CT) imaging after in ovo administration to incubated ostrich eggs showed a high uptake in the liver, namely 27% (60 min post injection), and subsequent biliary excretion. These results suggest that [68Ga]Ga-TMeOHB-DAZA and [68Ga]Ga-TEtOHB-DAZA have excellent potential as hepatobiliary PET/CT imaging agents.

Semi-automated system for concentrating 68Ga-eluate to obtain high molar and volume concentration of 68Ga-Radiopharmaca for preclinical applications

INTRODUCTION

⁶⁸Ga-radiopharmaceuticals are common in the field of Nuclear Medicine to visualize receptor-mediated processes. In contrast to straightforward labeling procedures for clinical applications, preclinical in vitro and in vivo applications are hampered for reasons like e.g. volume restriction, activity concentration, molar activity and osmolality. Therefore, we developed a semi-automatic system specifically to overcome these problems. A difficulty appeared unexpectedly, as intrinsic trace metals derived from eluate (Zn, Fe and Cu) are concentrated as well in amounts that influence radiochemical yield and thus lower molar activity.

METHODS

To purify Gallium-68 and to reduce the high elution volume of a ⁶⁸Ga-generator, a NaCl-based method using a column containing PS-H⁺ was implemented in a low volume PEEK system. Influence on reducing osmolality, acidity and the amount of PS-H⁺ resin (15-50 mg) was investigated. [⁶⁸Ga]Ga was desorbed from the PS-H⁺ resin with acidified 2-5 M NaCl (containing 0.05 M of HCl) and ⁶⁸Ga-activity was collected. DOTA-TATE was used as a peptide model. All buffers and additives used for labeling were mixed with Chelex 100 (~1 g/50 mL) for >144 h and eventually filtered using a 0.22 μm filter (Millipore). Quantification of metals was performed after labeling by HPLC (UV).

RESULTS

Gallium-68 activity could be desorbed from PS-H⁺ cation column with 3 M NaCl, and >60% (120-180 MBq) of [⁶⁸Ga]Ga was collected in <0.3 mL. Taking into account the used amount of ⁶⁸Ga-eluate, buffer and other excipients, the overall amount of trace metal per labeling was <1.5 nmol. DOTA-TATE could be labeled with [⁶⁸Ga]Ga with high radiochemical yield, >99% (ITLC), and a radiochemical purity of >95% (HPLC).

CONCLUSION

With the here described concentration system and metal purification technique, a low activity containing ⁶⁸Ga-generator can be used to label DOTA-peptide in preclinical applicable amounts >60 MBq/nmol (40-60 MBq/0.1 mL) and within 20 min.