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

A systematic review on the current status of PSMA-targeted imaging and radioligand therapy

Prostate specific membrane antigen (PSMA) has been the subject of several studies in recent decades as a promising molecular target for prostate cancer (PCa), in fact it is considered an excellent molecular target for both PCa imaging (both for staging and follow-up), by means of PET/CT and for radioligand therapy. Its interesting molecular features have enabled the development of a new diagnostic and therapeutic approach for PCa, called "theranostics." Considering the abundance of PSMA-based probes that have appeared so far in the literature, the present work focuses the attention on radiopharmaceuticals with increasing clinical application, highlighting advantages and disadvantages in terms of different metabolization and excretion processes, pharmacokinetic, binding affinity and variable internalization rate, tumor-to-background ratio, residence times and toxicity profile.

Microfluidic-based production of [68Ga]Ga-FAPI-46 and [68Ga]Ga-DOTA-TOC using the cassette-based iMiDEV™ microfluidic radiosynthesizer

BACKGROUND

The demand for 68Ga-labeled radiotracers has significantly increased in the past decade, driven by the development of diversified imaging tracers, such as FAPI derivatives, PSMA-11, DOTA-TOC, and DOTA-TATE. These tracers have exhibited promising results in theranostic applications, fueling interest in exploring them for clinical use. Among these probes, 68Ga-labeled FAPI-46 and DOTA-TOC have emerged as key players due to their ability to diagnose a broad spectrum of cancers ([68Ga]Ga-FAPI-46) in late-phase studies, whereas [68Ga]Ga-DOTA-TOC is clinically approved for neuroendocrine tumors. To facilitate their production, we leveraged a microfluidic cassette-based iMiDEV radiosynthesizer, enabling the synthesis of [68Ga]Ga-FAPI-46 and [68Ga]Ga-DOTA-TOC based on a dose-on-demand (DOD) approach.

RESULTS

Different mixing techniques were explored to influence radiochemical yield. We achieved decay-corrected yield of 44 ± 5% for [68Ga]Ga-FAPI-46 and 46 ± 7% for [68Ga]Ga-DOTA-TOC in approximately 30 min. The radiochemical purities (HPLC) of [68Ga]Ga-FAPI-46 and [68Ga]Ga-DOTA-TOC were 98.2 ± 0.2% and 98.4 ± 0.9%, respectively. All the quality control results complied with European Pharmacopoeia quality standards. We optimized various parameters, including 68Ga trapping and elution, cassette batches, passive mixing in the reactor, and solid-phase extraction (SPE) purification and formulation. The developed synthesis method reduced the amount of precursor and other chemicals required for synthesis compared to conventional radiosynthesizers.

CONCLUSIONS

The microfluidic-based approach enabled the implementation of radiosynthesis of [68Ga]Ga-FAPI-46 and [68Ga]Ga-DOTA-TOC on the iMiDEV™ microfluidic module, paving the way for their use in preclinical and clinical applications. The microfluidic synthesis approach utilized 2-3 times less precursor than cassette-based conventional synthesis. The synthesis method was also successfully validated in a similar microfluidic iMiDEV module at a different research center for the synthesis of [68Ga]Ga-FAPI-46 with limited runs. Our study demonstrated the potential of microfluidic methods for efficient and reliable radiometal-based radiopharmaceutical synthesis, contributing valuable insights for future advancements in this field and paving the way for routine clinical applications in the near future.

Scandium Radioisotopes-Toward New Targets and Imaging Modalities

The concept of theranostics uses radioisotopes of the same or chemically similar elements to label biological ligands in a way that allows the use of diagnostic and therapeutic radiation for a combined diagnosis and treatment regimen. For scandium, radioisotopes -43 and -44 can be used as diagnostic markers, while radioisotope scandium-47 can be used in the same configuration for targeted therapy. This work presents the latest achievements in the production and processing of radioisotopes and briefly characterizes solutions aimed at increasing the availability of these radioisotopes for research and clinical practice.

Molecular imaging of tumor hypoxia: Evolution of nitroimidazole radiopharmaceuticals and insights for future development

Though growing evidence has been collected in support of the concept of dose escalation based on the molecular level images indicating hypoxic tumor sub-volumes that could be radio-resistant, validation of the concept is still a work in progress. Molecular imaging of tumor hypoxia using radiopharmaceuticals is expected to provide the required input to plan dose escalation through Image Guided Radiation Therapy (IGRT) to kill/control the radio-resistant hypoxic tumor cells. The success of the IGRT, therefore, is heavily dependent on the quality of images obtained using the radiopharmaceutical and the extent to which the image represents the true hypoxic status of the tumor in spite of the heterogeneous nature of tumor hypoxia. Available literature on radiopharmaceuticals for imaging hypoxia is highly skewed in favor of nitroimidazole as the pharmacophore given their ability to undergo oxygen dependent reduction in hypoxic cells. In this context, present review on nitroimidazole radiopharmaceuticals would be immensely helpful to the researchers to obtain a birds-eye view on what has been achieved so far and what can be tried differently to obtain a better hypoxia imaging agent. The review also covers various methods of radiolabeling that could be utilized for developing radiotracers for hypoxia targeting applications.

Preclinical PET Imaging and Toxicity Study of a 68Ga-Functionalized Polymeric Cardiac Blood Pool Agent

Cardiac blood pool imaging is currently performed almost exclusively with 99mTc-based compounds and SPECT/CT imaging. Using a generator-based PET radioisotope has a few advantages, including not needing nuclear reactors to produce it, obtaining better resolution in humans, and potentially reducing the radiation dose to the patient. When the shortlived radioisotope 68Ga is used, it can be applied repeatedly on the same day—for example, for the detection of bleeding. Our objective was to prepare and evaluate a long-circulating polymer functionalized with gallium for its biodistribution, toxicity, and dosimetric properties. A 500 kDa hyperbranched polyglycerol was conjugated to the chelator NOTA and radiolabeled rapidly at room temperature with 68Ga. It was then injected intravenously into a rat, and gated imaging allowed us to easily observe wall motion and cardiac contractility, confirming the suitability of this radiopharmaceutical for cardiac blood pool imaging. Internal radiation dose calculations showed that the radiation doses that patients would receive from the PET agent would be 2.5× lower than those from the 99mTc agent. A complete 14-day toxicology study in rats concluded that there were no gross pathology findings, changes in body or organ weights, or histopathological events. This radioactive-metal-functionalized polymer might be a suitable non-toxic agent to advance for clinical application.

A robust lyophilized kit for convenient one-step formulation of [68Ga]Ga-DOTA-E-[c(RGDfK)]2 in hospital radiopharmacy for clinical PET imaging

The present article describes the development of robust lyophilized kit for convenient formulation of [⁶⁸Ga]Ga-DOTA-E-[c(RGDfK)]₂ (E = glutamic acid, R = arginine, G = glycine, D = aspartic acid, f = phenylalanine, K = lysine) radiopharmaceutical for clinical use in non-invasive monitoring of malignancies overexpressing integrin α_vβ₃ receptors. Five batches of the kit were prepared with optimized kit contents, all of which showed high ⁶⁸Ga-radiolabeling yield (>98%). Pre-clinical evaluation of the [⁶⁸Ga]Ga-radiotracer in SCID mice bearing FTC133 tumour exhibited significant accumulation in the tumor xenograft. Preliminary human clinical investigation carried out in a 60 year old male patient with metastatic lung cancer revealed high radiotracer uptake in the tumor along with satisfactory target to non-target contrast. The developed kit formulation also showed a long shelf-life of at least 12 months on storage at 0 °C. All these results point towards the promising attributes of the developed kit formulation for convenient preparation of [⁶⁸Ga]Ga-DOTA-E-[c(RGDfK)]₂ for routine clinical use.

Exploration of 68Ga-DOTA-MAL as a Versatile Vehicle for Facile Labeling of a Variety of Thiol-Containing Bioactive Molecules

Efficient and site-specific radiolabeling reactions are essential in molecular probe synthesis. Thus, selecting an effective method for radiolabeling that does not affect bioactivity of the molecule is critical. Varieties of bifunctional chelating agents provide a solution in this matter. As a chemo-specific chelator, maleimido-mono-amide-DOTA (DOTA-Mal) holds significant potential for ⁶⁸Ga labeling of bioactive molecules; it can react specifically with free sulfhydryl groups under mild conditions. Compared with amino and carboxylic acid groups, free sulfhydryl groups are relatively less common in most biomolecules and can serve as site-specific radiolabeling targets. Labeling of ⁶⁸Ga usually employs a two-step labeling strategy; first, chelators are conjugated to the biomolecules, which is followed by radiolabeling. However, the bioactivity of biomolecules may be affected by harsh labeling conditions. In this study, three ⁶⁸Ga-labeled bioactive molecules, namely, ⁶⁸Ga-DOTA-RGD, ⁶⁸Ga-DOTA-FA, and ⁶⁸Ga-DOTA-BSA, were prepared using a novel strategy under mild conditions (pH of 8.0 at room temperature). Using this strategy, DOTA-Mal was labeled by ⁶⁸Ga before it reacted with the sulfhydryl group-containing biomolecules, which avoided damage to said biomolecules caused by the harsh reaction conditions required in ⁶⁸Ga-labeling procedures. The biological and chemical properties of these three radiotracers synthesized using this strategy are well manifested. Through a series of experiments, the effectiveness of this strategy is demonstrated, and we believe that this site-specific bioactivity-friendly reaction strategy will facilitate developments and translation applications of varieties of ⁶⁸Ga-labeled positron emission tomography probes.

Synthesis of [68Ga]Ga-PSMA-11 using the iMiDEV™ microfluidic platform

Implementation of [68Ga]Ga-PSMA-11 production into the microfluidic synthesizer iMiDEV™, a proof-of-concept study opening access to the microfluidic production of various [68Ga]Ga-radiopharmaceuticals.

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.

RGD Peptide-Conjugated Dodecaborate with the Ga-DOTA Complex: A Preliminary Study for the Development of Theranostic Agents for Boron Neutron Capture Therapy and Its Companion Diagnostics

A boron neutron capture therapy (BNCT) system, using boron-10-introduced agents coupled with companion diagnostics, is anticipated as a promising cancer theranostic. Thus, this study aimed to synthesize and evaluate a probe closo-dodecaborate-(Ga-DOTA)-c(RGDfK) (16) [Ga = gallium, DOTA =1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, and c(RGDfK) = cyclo(arginine-glycine-aspartate-d-phenylalanine-lysine] containing closo-dodecaborate ([B₁₂H₁₂]^2-) as a boron cluster, a [⁶⁷Ga]Ga-DOTA derivative for nuclear medicine imaging, and an RGD peptide for tumor targeting. Moreover, we prepared a radioiodinated probe [¹²⁵I]17 in which I-125 is introduced into a closo-dodecaborate moiety of 16. [⁶⁷Ga]16 and [¹²⁵I]17 showed high stability and high uptake in cancer cells in vitro. Biodistribution experiments in tumor-bearing mice revealed similar biodistribution patterns between [⁶⁷Ga]16 and [¹²⁵I]17, such as a high uptake in the tumor and a low uptake in other non-target tissues. Meanwhile, [¹²⁵I]17 exhibited higher accumulation in most tissues, including the tumor, than [⁶⁷Ga]16, probably because of higher albumin binding. The higher the [¹²⁵I]17 accumulation in the tumor, the more desirable it is for BNCT, with the possibility that the iodo-closo-dodecaborate site may work as an albumin binder.

Good practices for 68Ga radiopharmaceutical production

Background

The radiometal gallium-68 (⁶⁸Ga) is increasingly used in diagnostic positron emission tomography (PET), with ⁶⁸Ga-labeled radiopharmaceuticals developed as potential higher-resolution imaging alternatives to traditional ^99mTc agents. In precision medicine, PET applications of ⁶⁸Ga are widespread, with ⁶⁸Ga radiolabeled to a variety of radiotracers that evaluate perfusion and organ function, and target specific biomarkers found on tumor lesions such as prostate-specific membrane antigen, somatostatin, fibroblast activation protein, bombesin, and melanocortin.

Main body

These ⁶⁸Ga radiopharmaceuticals include agents such as [⁶⁸Ga]Ga-macroaggregated albumin for myocardial perfusion evaluation, [⁶⁸Ga]Ga-PLED for assessing renal function, [⁶⁸Ga]Ga-t-butyl-HBED for assessing liver function, and [⁶⁸Ga]Ga-PSMA for tumor imaging. The short half-life, favourable nuclear decay properties, ease of radiolabeling, and convenient availability through germanium-68 (⁶⁸Ge) generators and cyclotron production routes strongly positions ⁶⁸Ga for continued growth in clinical deployment. This progress motivates the development of a set of common guidelines and standards for the ⁶⁸Ga radiopharmaceutical community, and recommendations for centers interested in establishing ⁶⁸Ga radiopharmaceutical production.

Conclusion

This review outlines important aspects of ⁶⁸Ga radiopharmacy, including ⁶⁸Ga production routes using a ⁶⁸Ge/⁶⁸Ga generator or medical cyclotron, standardized ⁶⁸Ga radiolabeling methods, quality control procedures for clinical ⁶⁸Ga radiopharmaceuticals, and suggested best practices for centers with established or upcoming ⁶⁸Ga radiopharmaceutical production. Finally, an outlook on ⁶⁸Ga radiopharmaceuticals is presented to highlight potential challenges and opportunities facing the community.

Current status and progress in using radiolabelled PARP-1 inhibitors for imaging PARP-1 expression in tumours

Poly(ADP-ribose) polymerase-1 (PARP-1) is a key enzyme in the DNA repair process, and the overexpression of PARP-1 in several tumours makes this enzyme a promising molecular target. Recently, several PARP-1 inhibitors, such as olaparib, rucaparib, niraparib and talazoparib, have been clinically approved as anticancer drugs. Several of these inhibitors have been radiolabelled for noninvasive imaging of PARP-1 expression in several types of tumours. In this review, the background and progress for using various radiolabelled PARP-1 inhibitors for cancer diagnosis are discussed and future development directions are proposed.

Cold Kit Labeling: The Future of 68Ga Radiopharmaceuticals?

Over the last couple of decades, gallium-68 (68Ga) has gained a formidable interest for PET molecular imaging of various conditions, from cancer to infection, through cardiac pathologies or neuropathies. It has gained routine use, with successful radiopharmaceuticals such as somatostatin analogs ([68Ga]Ga-DOTATOC and [68Ga]GaDOTATATE) for neuroendocrine tumors, and PSMA ligands for prostate cancer. It represents a major clinical impact, particularly in the context of theranostics, coupled with their 177Lu-labeled counterparts. Beside those, a bunch of new 68Ga-labeled molecules are in the preclinical and clinical pipelines, with some of them showing great promise for patient care. Increasing clinical demand and regulatory issues have led to the development of automated procedures for the production of 68Ga radiopharmaceuticals. However, the widespread use of these radiopharmaceuticals may rely on simple and efficient radiolabeling methods, undemanding in terms of equipment and infrastructure. To make them technically and economically accessible to the medical community and its patients, it appears mandatory to develop a procedure similar to the well-established kit-based 99mTc chemistry. Already available commercial kits for the production of 68Ga radiopharmaceuticals have demonstrated the feasibility of using such an approach, thus paving the way for more kit-based 68Ga radiopharmaceuticals to be developed. This article discusses the development of 68Ga cold kit radiopharmacy, including technical issues, and regulatory aspects.

Radiolabeled PSMA Inhibitors

PSMA has shown to be a promising target for diagnosis and therapy (theranostics) of prostate cancer. We have reviewed developments in the field of radio- and fluorescence-guided surgery and targeted photodynamic therapy as well as multitargeting PSMA inhibitors also addressing albumin, GRPr and integrin αvβ3. An overview of the regulatory status of PSMA-targeting radiopharmaceuticals in the USA and Europe is also provided. Technical and quality aspects of PSMA-targeting radiopharmaceuticals are described and new emerging radiolabeling strategies are discussed. Furthermore, insights are given into the production, application and potential of alternatives beyond the commonly used radionuclides for radiolabeling PSMA inhibitors. An additional refinement of radiopharmaceuticals is required in order to further improve dose-limiting factors, such as nephrotoxicity and salivary gland uptake during endoradiotherapy. The improvement of patient treatment achieved by the advantageous combination of radionuclide therapy with alternative therapies is also a special focus of this review.

Kit-based preparation of [68Ga]Ga-P16-093 (PSMA-093) using different commercial 68Ge/68Ga generators

PURPOSE

Prostate-specific membrane antigen (PSMA) is an important biomarker for molecular imaging and a target for radionuclide therapy of prostate cancer. Recently, U.S. Food and Drug Administration (FDA) has approved [⁶⁸Ga]Ga-PSMA-11 as a PSMA-targeted positron emission tomography (PET) imaging agent for the diagnosis of prostate cancer. As an alternative PSMA imaging agent, [⁶⁸Ga]Ga-P16-093 ([⁶⁸Ga]Ga-PSMA-093) showed excellent blood clearance and rapid tumor uptake, desirable in vivo properties for avidly detecting primary tumor and metastatic lesions in patients. To improve the availability and test the robustness of radiolabeling reaction, eluents of ⁶⁸Ga/HCl from different sources of generators were evaluated.

PROCEDURES

Commercially available ⁶⁸Ge/⁶⁸Ga generators from Eckert & Ziegler, ITG and iThemba were eluted with varying molarities of hydrochloric acid (0.05-0.6 M, as recommended by each company) and reacted with P16-093 kits. Radiolabeling yields, in vitro stabilities, in vitro cell uptakes and drug release criteria of different preparations were investigated. PET/computed tomography (CT) imaging of prostate cancer patients with [⁶⁸Ga]Ga-P16-093 produced by using different sources of ⁶⁸Ga were performed.

RESULTS

Optimized P16-093 kit containing 15 μg of P16-093 (precursor) and 68 mg of sodium acetate trihydrate (buffer), a formulation previously tested in humans, was successfully labeled with eluents from Eckert & Ziegler, ITG and iThemba's generators. In vitro cell uptake studies showed that [⁶⁸Ga]Ga-P16-093, formulated with ITG and iThemba's generators, exhibited equivalent PSMA-specific uptakes. Clinical studies in prostate cancer patients exhibited exceedingly comparable maximum standardized uptake value (SUVmax) for each lesion regardless of source of the generator used in preparation.

CONCLUSION

Using different vendors' generator and lyophilized P16-093 kits, [⁶⁸Ga]Ga-P16-093 could be conveniently and reliably prepared by a simple one-step reaction with excellent yields. Clinically useful doses of [⁶⁸Ga]Ga-P16-093 imaging tracer could be made available using different ⁶⁸Ge/⁶⁸Ga generators.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

BACKGROUND

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biyearly highlight commentary to update the readership on trends in the field of radiopharmaceutical development.

RESULTS

This commentary of highlights has resulted in 21 different topics selected by each member of the Editorial Board addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals. Also the first contribution in relation to MRI-agents is included.

CONCLUSIONS

Trends in (radio)chemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.

[68Ga]Ga-PSMA-11: The First FDA-Approved 68Ga-Radiopharmaceutical for PET Imaging of Prostate Cancer

For the positron emission tomography (PET) imaging of prostate cancer, radiotracers targeting the prostate-specific membrane antigen (PSMA) are nowadays used in clinical practice. Almost 10 years after its discovery, [⁶⁸Ga]Ga-PSMA-11 has been approved in the United States by the Food and Drug Administration (FDA) as the first ⁶⁸Ga-radiopharmaceutical for the PET imaging of PSMA-positive prostate cancer in 2020. This radiopharmaceutical combines the peptidomimetic Glu-NH-CO-NH-Lys(Ahx)-HBED-CC with the radionuclide ⁶⁸Ga, enabling specific imaging of tumor cells expressing PSMA. Such a targeting approach may also be used for therapy planning as well as potentially for the evaluation of treatment response.