The 68Ge/ 68Ga generator has high potential, but when can we use 68Ga-labelled tracers in clinical routine?

The synthesis of N,1,4-tri(alkoxy-hydroxybenzyl)-1,4-diazepane-amines: investigations on reaction characteristics and mechanism

1,4-Diazepane-6-amine (DAZA) can be alkylated with three 2-hydroxybenzyl pendant arms, resulting in hexadentate chelators suitable for coordination of radiometals like 68Ga. These chelators, N,1,4-tri(alkoxy-2-hydroxybenzyl)-DAZA, can be produced via a one-pot synthesis, with the first step being a carbonyl amine condensation of DAZA with two respective 4-alkoxy-2-hydroxybenzaldehydes, followed by reductive amination with sodium borohydride. While the first step of this reaction is predictable, the subsequent reductive amination can result in either mono-, di- or tri(alkoxy-hydroxybenzyl)-DAZA compounds. Seeking to identify dependencies that might allow a specific reaction control towards the formation of either of the three possible products, and particularly towards the favoured trialkylated DAZA compounds, a variety of synthesis trials were performed. Additionally, computational methods were employed to evaluate the underlying reaction mechanism. Synthesis trials verified that the trialkylated DAZA compounds are formed via direct reductive amination of the dialkylated DAZA compounds. Subsequently, a synthetic method was established, leading to an increase in the percentage of the trialkylated DAZA compounds, which allowed the successful isolation of those hexadentate chelators. Additionally, an alternative pathway proceeding via aminal C-N bond insertion of an attacking third carbonyl moiety was evaluated by means of quantum chemical calculations but so far remains entirely hypothetical.

Exploring Aqueous Coordination Chemistry of Highly Lewis Acidic Metals with Emerging Isotopes for Nuclear Medicine

Nuclear medicine harnesses radioisotopes for the diagnosis and treatment of disease. While the isotopes 99mTc and 111In have enabled the clinical diagnosis of millions of patients over the past 3 decades, more recent clinical translation of numerous 68Ga/177Lu-based radiopharmaceuticals for diagnostic imaging and therapy underscores the clinical utility of metal-based radiopharmaceuticals in mainstream cancer treatment. In addition to such established radionuclides, advancements in radioisotope production have enabled the production of radionuclides with a broad range of half-lives and emission properties of interest for nuclear medicine. Chemical means to form kinetically inert, in vivo-compatible species that can be modified with disease-targeting vectors is imperative. This presents a challenge for radiosiotopes of elements where the aqueous chemistry is still underdeveloped and poorly understood. Here, we discuss our efforts to date in exploring the aqueous, radioactive coordination chemistry of highly Lewis acidic metal ions and how our discoveries apply to the diagnosis and treatment of cancer in preclinical models of disease. The scope of this Account includes approaches to aqueous coordination of to-date understudied highly Lewis acidic metal ions with radioisotopes of emerging interest and the modulation of well-understood coordination environments of radio-coordination complexes to induce metal-catalyzed reactivity for separation and pro-drug applications.First, we discuss the development of seven-coordinate, small-cavity macrocyclic chelator platform mpatcn/picaga as an exemplary case study, which forms robust complexes with 44Sc/47Sc isotopes. Due to the high chemical hardness and pronounced Lewis acidity of the Sc3+ ion, the displacement of ternary ligand H2O by 18/natF- can be achieved to form an inert Sc-18/natF bond. Corresponding coordination complex natSc-18F is in vivo compatible and forms a theranostic tetrad with corresponding 44Sc/47Sc, 177Lu complexes all exhibiting homologous biodistribution profiles. Another exceptionally hard, highly Lewis acidic ion with underdeveloped aqueous chemistry and emerging interest in nuclear medicine is 45Ti4+. To develop de novo approaches to the mononuclear chelation of this ion under aqueous conditions, we employed a fragment-based bidentate ligand screening approach which identified two leads. The screen successfully predicted the formation of [45Ti][Ti(TREN-CAM)], a Ti-triscatechol complex that exhibits remarkable in vivo stability. Furthermore, the fragment-based screen also identified approaches that enabled solid-phase separation of Ti4+ and Sc3+ of interest in streamlining the isotope production of 45Ti and accessing new ways to separate 44Ti/44Sc for the development of a long-lived generator system. In addition to establishing the inert chelation of Ti4+ and Sc3+, we introduce controlled, metal-induced reactivity of corresponding coordination complexes on macroscopic and radiotracer scales. Metal-mediated autolytic amide bond cleavage (MMAAC) enables the temperature-dependent release of high-molar-activity, ready-to-inject radiopharmaceuticals; cleavage is selectively triggered by coordinated trivalent Lewis acid nat/68Ga3+ or Sc3+. Following the scope of reactivity and mechanistic studies, we validated MMAAC for the synthesis of high-molar-activity radiopharmaceuticals to image molecular targets with low expression and metal-mediated prodrug hydrolysis in vivo.This Account summarizes how developing the aqueous coordination chemistry and tuning the chemical reactivity of metal ions with high Lewis acidity at the macroscopic and tracer scales directly apply to the radiopharmaceutical synthesis with clinical potential.

In Vivo Evaluation of a Gallium-68-Labeled Tumor-Tracking Cyanine Dye for Positron Emission Tomography/Near-Infrared Fluorescence Carcinoma Imaging, Image-Guided Surgery, and Photothermal Therapy

Positron emission tomography (PET)/near-infrared fluorescence (NIRF) dual-modal imaging presents an enticing prospect for tumor diagnosis and surgical navigation. In this study, we developed a novel probe IR808-DOTA for tumor-targeted PET/NIRF imaging, image-guided surgery, and photothermal therapy. This construct had better water solubility and pharmacokinetics than IR808 and had similar photophysical properties, tumor targeting ability, and photothermal anticancer effect to IR808. By a simple labeling process, IR808-DOTA was labeled with gallium-68 and applied as a PET probe for tumor imaging in MCF-7 tumor xenografted mice. IR808-DOTA itself acted as an NIRF imaging agent in the following surgery for intraoperative navigation to aid surgeons in the delineation of tumor margins and visualizing sentinel lymph nodes to facilitate a more thorough tumor resection. Irradiation by laser, IR808-DOTA could prominently inhibit tumor growth in MCF-7 subcutaneous tumor model mice by directly ablating tumor cells, inhibiting tumor proliferation, and promoting tumor cell apoptosis. In summary, ⁶⁸Ga-DOTA-IR808 could enable a convenient and user-friendly workflow for tumor imaging and guided surgery, and therefore, it may have great prospects for clinical translation as a PET/NIRF dual-modal probe.

Production of 68Ge, 68Ga, 67Ga, 65Zn, and 64Cu important radionuclides for medical applications: theoretical model predictions for α-particles with 66Zn at ≈10–40 MeV

Theoretical predictions were made using TALYS-1.95(G) and EMPIRE 3.2 reaction-model codes for 69Ge, 67Ge, and medically used 68Ge, 67Ga, 68Ga, 65Zn, 64Cu radionuclides produced in the interaction of α-projectile with 66Zn-target at 10–40 MeV α-energies. Pearson’s statistical coefficients showed moderate to strong positive correlations between the theoretically predicted and experimentally measured production cross sections for radionuclides with practical medical applications. Furthermore, the present results indicated that a medium-sized cyclotron and a single α + 66Zn system (projectile + target system) might be an option for optimized production of 68Ge, 68Ga, 67Ga, 65Zn, and 64Cu radionuclides.

Synthesis and Characterization of Multifunctional Nanovesicles Composed of POPC Lipid Molecules for Nuclear Imaging

The integration of nuclear imaging analysis with nanomedicine has tremendously grown and represents a valid and powerful tool for the development and clinical translation of drug delivery systems. Among the various types of nanostructures used as drug carriers, nanovesicles represent intriguing platforms due to their capability to entrap both lipophilic and hydrophilic agents, and their well-known biocompatibility and biodegradability. In this respect, here we present the development of a labelling procedure of POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine)-based liposomes incorporating an ad hoc designed lipophilic NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) analogue, derivatized with an oleic acid residue, able to bind the positron emitter gallium-68(III). Based on POPC features, the optimal conditions for liposome labelling were studied with the aim of optimizing the Ga(III) incorporation and obtaining a significant radiochemical yield. The data presented in this work demonstrate the feasibility of the labelling procedure on POPC liposomes co-formulated with the ad hoc designed NOTA analogue. We thus provided a critical insight into the practical aspects of the development of vesicles for theranostic approaches, which in principle can be extended to other nanosystems exploiting a variety of bioconjugation protocols.

Towards Facile Radiolabeling and Preparation of Gallium-68-/Bismuth-213-DOTA-[Thi8, Met(O2)11]-Substance P for Future Clinical Application: First Experiences

Substance P (SP) is a small peptide commonly known as a preferential endogenous ligand for the transmembrane neurokinin-1 receptor. Nuclear Medicine procedures currently involve radiolabeled SP derivatives in peptide radioligand endotherapy of inoperable glioblastoma. Promising clinical results sparked the demand for facile production strategies for a functionalized 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-[Thi⁸, Met(O₂)¹¹]-SP to allow for rapid Gallium-68 or Bismuth-213 complexation. Therefore, we provide a simple kit-like radiotracer preparation method that caters for the gallium-68 activity eluted from a SnO₂ generator matrix as well as preliminary results on the adaptability to produce [²¹³Bi]Bi-DOTA-[Thi⁸, Met(O₂)¹¹]SP from the same vials containing the same starting material. Following a phase of radioanalysis for complexation of gallium-68 to DOTA-[Thi⁸, Met(O₂)¹¹]SP and assessing the radiolabeling parameters, the vials containing appropriate kit-prototype material were produced in freeze-dried batches. The facile radiolabeling performance was tested and parameters for future human application were calculated to meet the criteria for theranostic loco-regional co-administration of activity doses comprising [⁶⁸Ga]Ga-DOTA-[Thi⁸, Met(O₂)¹¹]SP mixed with [²¹³Bi]Bi-DOTA-[Thi⁸, Met(O₂)¹¹]SP. [⁶⁸Ga]Ga-DOTA-[Thi⁸, Met(O₂)¹¹]SP was prepared quantitatively from lyophilized starting material within 25 min providing the required molar activity (18 ± 4 GBq/µmol) and activity concentration (98 ± 24 MBq/mL), radiochemical purity (>95%) and sustained radiolabeling performance (4 months at >95% LE) as well as acceptable product quality (>95% for 120 min). Additionally, vials of the same starting materials were successfully adapted to a labeling strategy available for preparation of [²¹³Bi]Bi-DOTA-[Thi⁸, Met(O₂)¹¹]SP providing sufficient activity for 1-2 human doses. The resultant formulation of [⁶⁸Ga]Ga-/[²¹³Bi]Bi-DOTA-[Thi⁸, Met(O₂)¹¹]SP activity doses was considered of adequate radiochemical quality for administration. This investigation proposes a simple kit-like formulation of DOTA-[Thi⁸, Met(O₂)¹¹]SP-a first-line investigation into a user friendly, straightforward tracer preparation that would warrant efficient clinical investigations in the future. Quantitative radiolabeling was accomplished for [⁶⁸Ga]Ga-DOTA-[Thi⁸, Met(O₂)¹¹]SP and [²¹³Bi]Bi-DOTA-[Thi⁸, Met(O₂)¹¹]SP preparations; a key requirement when addressing the specific route of catheter-assisted co-injection directly into the intratumoral cavities.

Molecular PET/CT Profiling of ACE2 Expression In Vivo: Implications for Infection and Outcome from SARS-CoV-2

Rapid progress has been made to identify and study the causative agent leading to coronavirus disease 2019 (COVID-19) but many questions including who is most susceptible and what determines severity remain unanswered. Angiotensin-converting enzyme 2 (ACE2) is a key factor in the infection process of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). In this study, molecularly specific positron emission tomography imaging agents for targeting ACE2 are first developed, and these novel agents are evaluated in vitro, in preclinical model systems, and in a first-in-human translational ACE2 imaging of healthy volunteers and a SARS-CoV-2 recovered patient (NCT04422457). ACE2 expression levels in different organs in live subjects are quantitatively delineated and observable differences are measured in the patient recovered from COVID-19. Surprising sites of uptake in the breast, reproductive system and very low uptake in pulmonary tissues are reported. This novel method can add a unique tool to facilitate SARS-CoV-2 related research and improve understanding of this enigmatic disease. Molecular imaging provides quantitative annotation of ACE2, the SARS-CoV-2 entry receptor, to noninvasively monitor organs impacted by the COVID-19.

Simple new method for labelling of PSMA-11 with 68Ga in NaHCO3

BACKGROUND

Prostate specific membrane antigen (PSMA) is a type II membrane protein widely expressed on the surface of prostate cancer cells. One of its functions is to act as a receptor mediating the ligand internalization. This PSMA property is employed in the diagnostics and therapy of prostate cancer. Over the years, small molecules with high affinity for PSMA have been developed and labelled with positron emitters (e.g. ⁶⁸Ga, ¹⁸F, ¹¹C, ⁶⁴Cu, or ⁸⁶Y). One of these radiolabelled ligands, [⁶⁸Ga] PSMA-11, is one of the most widespread tracers for PET imaging of the prostate cancer. Many techniques have been proposed and tested for the ⁶⁸Ga labelling of PSMA-11. The aim of our work was to design a labelling method of PSMA-11 that minimizes number of the used chemicals and steps, providing quantitative labelling yield at laboratory temperature and may be easily automated.

METHODOLOGY

A⁶⁸Ge/⁶⁸Ga generator eluate in 0.1 M HCl was loaded on an activated Oasis MCX cartridge, and the cartridge was then thoroughly washed with water. The radionuclide ⁶⁸Ga was eluted from the cartridge with 0.1 M NaHCO₃ (pH = 8.5, n = 36) or with the same solution with pH adjusted to 7.2-9.0 (n = 38). Precursor PSMA-11 was mixed directly with the cartridge eluate of ⁶⁸Ga in 0.1 M NaHCO₃ of given pH. For the stability test, samples of ⁶⁸GaPSMA-11 in 0.1 M NaHCO₃ (pH 8.5) were mixed in ratio 1 : 1 with the following solutions: 0.1 M NaHCO₃ (pH 8.5), human serum, PBS and 0.9% NaCl. In order to estimate an effect of the time elapsed between ⁶⁸Ga elution from the cartridge in 0.1 M NaHCO₃ (pH 8.5) and the labelling onset of PSMA-11, the latter was initiated 0, 5, 10 and 20 min post elution and radiochemical yield was monitored. All the PSMA-11 labelled samples were subjected to radiochemical purity test using HPLC. The whole process starting from generator elution up to HPLC analysis commencement took 10-15 min.

RESULTS

Recovery of ⁶⁸Ga from cartridge Oasis MCX using 0.1 M NaHCO₃ at pH 8.5 was 71.5 ± 1.4%. Thirty six PSMA-11 samples (10 μg in reaction mixture) were labelled at pH 8.5 with total average radiochemical yield of 98 ± 2%. Recovery of ⁶⁸Ga from cartridge Oasis MCX using 0.1 M NaHCO₃ at variable pH of 7.2-9.0 was 62.5 ± 1.8% showing certain decrease with decreasing pH. A total of 138 samples of PSMA-11 were labelled with ⁶⁸ Ga at variable pH (7.2-9.0) and four different amounts of PSMA-11 (1, 2.5, 5 and 10 μg) resulting in the labelling yields of 54.0 ± 5.3%, 88.2 ± 3.2%, 99.4 ± 0.3% and 99.9 ± 0.1%, respectively. Irrespective of the pH, the radiolabelling yield was quantitative for the molar ratio PSMA-11: ⁶⁸Ga > 5000 : 1 in the reaction mixture. Stability tests in 0.1 M NaHCO₃ (pH 8.5), human serum, PBS and 0.9% NaCl revealed no observable release of ⁶⁸Ga from the ⁶⁸Ga-PSMA-11 complex within 3 h. Similarly, the delay between the ⁶⁸Ga elution from the Oasis MCX cartridge in 0.1 M NaHCO₃ (pH 8.5) and start of the labelling of PSMA-11 labelling has no effect on the radiochemical yield.

CONCLUSION

A new method of labelling PSMA-11 ligand with ⁶⁸Ga in 0.1 M NaHCO₃ using Oasis MCX cartridges was proposed, developed and tested. The results demonstrated that it is rapid, simple, reproducible and easy to automate.

Quantitative assessment of myocardial blood flow and extracellular volume fraction using 68Ga-DOTA-PET: A feasibility and validation study in large animals

BACKGROUND

Here we evaluated the feasibility of PET with Gallium-68 (68Ga)-labeled DOTA for non-invasive assessment of myocardial blood flow (MBF) and extracellular volume fraction (ECV) in a pig model of myocardial infarction. We also aimed to validate MBF measurements using microspheres as a gold standard in healthy pigs.

METHODS

8 healthy pigs underwent three sequential 68Ga-DOTA-PET/CT scans at rest and during pharmacological stress with simultaneous injection of fluorescent microspheres to validate MBF measurements. Myocardial infarction was induced in 5 additional pigs, which underwent 68Ga-DOTA-PET/CT examinations 7-days after reperfusion. Dynamic PET images were reconstructed and fitted to obtain MBF and ECV parametric maps.

RESULTS

MBF assessed with 68Ga-DOTA-PET showed good correlation (y = 0.96x + 0.11, r = 0.91) with that measured with microspheres. MBF values obtained with 68Ga-DOTA-PET in the infarcted area (LAD, left anterior descendant) were significantly reduced in comparison to remote ones LCX (left circumflex artery, P < 0.0001) and RCA (right coronary artery, P < 0.0001). ECV increased in the infarcted area (P < 0.0001).

CONCLUSION

68Ga-DOTA-PET allowed non-invasive assessment of MBF and ECV in pigs with myocardial infarction and under rest-stress conditions. This technique could provide wide access to quantitative measurement of both MBF and ECV with PET imaging.

Positron emission tomography/computed tomography guided percutaneous biopsies of Ga-68 avid lesions using an automated robotic arm

PURPOSE

The purpose of this prospective study was to evaluate the feasibility of positron emission tomography/computed tomography (PET/CT)-guided biopsy of Ga-68 avid lesions using an automated robotic arm and determine the diagnostic yield of this technique.

MATERIAL AND METHODS

Patients who underwent Ga-68 labelled tracers imaging followed by PET/CT-guided biopsies of tracer-avid lesions were prospectively included. Biopsies were performed using a dedicated automated-robotic-arm assisted PET/CT-guided biopsy device on the same-day of diagnostic PET/CT-imaging. The tissue samples were retrieved after confirming the position of needle-tip in the target lesion. Procedure-related complications and radiation exposure of the interventionist were recorded. Histopathological reports were reviewed for diagnostic yield.

RESULTS

A total of 25 patients (19 men, six women) with a mean age of 50.8±17.3 (SD) years (range: 17-83 years) were included. The biopsies were performed after PET/CT using Ga-68 DOTANOC (n=16) or Ga-68 PSMA (n=8) and Ga-68 chemokine-analogue (n=1). The biopsy samples were obtained from the liver (n=9), bone (n=8), lymph-nodes (n=3), lung (n=1), pancreas (n=1), anterior mediastinal lesion (n=1), peritoneal-deposit (n=1) and thigh-lesion (n=1). No immediate or delayed procedure-related complications were documented in any patient. PET/CT-guided molecular sampling was technically successful in all the patients. Histopathology revealed malignancies in all the biopsied specimens without the need for repeat sampling or further invasive-diagnostic workup, with a diagnostic yield of 100%. The estimated absorbed-radiation dose was 566.7μSv/year for the interventionist.

CONCLUSION

PET/CT-guided molecular biopsy using Ga-68 labelled radiotracers is feasible and can be performed safely and accurately with a high-diagnostic yield. It is helpful in accurately staging the disease when tracer-avid isolated distant lesion evident on imaging and highly practical in patients with previous inconclusive sampling.

A Comprehensive Review of Non-Covalent Radiofluorination Approaches Using Aluminum [18F]fluoride: Will [18F]AlF Replace 68Ga for Metal Chelate Labeling?

Due to its ideal physical properties, fluorine-18 turns out to be a key radionuclide for positron emission tomography (PET) imaging, for both preclinical and clinical applications. However, usual biomolecules radiofluorination procedures require the formation of covalent bonds with fluorinated prosthetic groups. This drawback makes radiofluorination impractical for routine radiolabeling, gallium-68 appearing to be much more convenient for the labeling of chelator-bearing PET probes. In response to this limitation, a recent expansion of the 18F chemical toolbox gave aluminum [18F]fluoride chemistry a real prominence since the late 2000s. This approach is based on the formation of an [18F][AlF]2+ cation, complexed with a 9-membered cyclic chelator such as NOTA, NODA or their analogs. Allowing a one-step radiofluorination in an aqueous medium, this technique combines fluorine-18 and non-covalent radiolabeling with the advantage of being very easy to implement. Since its first reports, [18F]AlF radiolabeling approach has been applied to a wide variety of potential PET imaging vectors, whether of peptidic, proteic, or small molecule structure. Most of these [18F]AlF-labeled tracers showed promising preclinical results and have reached the clinical evaluation stage for some of them. The aim of this report is to provide a comprehensive overview of [18F]AlF labeling applications through a description of the various [18F]AlF-labeled conjugates, from their radiosynthesis to their evaluation as PET imaging agents.

Gallium(iii) and iron(iii) complexes of quinolone antimicrobials

Iron is an essential nutrient for many microbes. According to the "Trojan Horse Hypothesis", biological systems have difficulties distinguishing between Fe(3+) and Ga(3+), which constitutes the antimicrobial efficacy of the gallium(iii) ion. Nine novel tris(quinolono)gallium(iii) complexes and their corresponding iron(iii) analogs have been synthesized and fully characterized. Quinolone antimicrobial agents from three drug generations were used in this study: ciprofloxacin, enoxacin, fleroxacin, levofloxacin, lomefloxacin, nalidixic acid, norfloxacin, oxolinic acid, and pipemidic acid. The antimicrobial efficacy of the tris(quinolono)gallium(iii) complexes was studied against E. faecalis and S. aureus (both Gram-positive), as well as E. coli, K. pneumonia, and P. aeruginosa (all Gram-negative) in direct comparison to the tris(quinolono)iron(iii) complexes and the corresponding free quinolone ligands at various concentrations. For the tris(quinolono)gallium(iii) complexes, no combinational antimicrobial effects between Ga(3+) and the quinolone antimicrobial agents were observed.

Re-thinking the role of radiometal isotopes: Towards a future concept for theranostic radiopharmaceuticals

The potential and future role of certain metal radionuclides, for example, ⁴⁴ Sc, ⁸⁹ Zr, ⁸⁶ Y, ⁶⁴ Cu, ⁶⁸ Ga, ¹⁷⁷ Lu, ²²⁵ Ac, and ²¹³ Bi, and several terbium isotopes has been controversially discussed in the past decades. Furthermore, the possible benefits of "matched pairs" of isotopes for tandem applications of diagnostics and therapeutics (theranostics) have been emphasized, while such approaches still have not made their way into routine clinical practice. Analysis of bibliographical data illustrates how popularity of certain nuclides has been promoted by cycles of availability and applications. We furthermore discuss the different practical requirements for diagnostic and therapeutic radiopharmaceuticals and the resulting consequences for efficient development of clinically useful pairs of radionuclide theranostics, with particular emphasis on the underlying economical factors. Based on an exemplary assessment of overall production costs for ⁶⁸ Ga and ¹⁸ F radiopharmaceuticals, we venture a look into the future of theranostics and predict that high-throughput PET applications, that is, diagnosis of frequent conditions, will ultimately rely on ¹⁸ F tracers. PET radiometals will occupy a niche in the clinical low-throughput sector (diagnosis of rare diseases), but above all, dominate preclinical research and clinical translation. Matched isotope pairs will be of lesser relevance for theranostics but may become important for future PET-based therapeutic dosimetry.

[68Ga]/[188Re] Complexed [CDTMP] Trans-1,2-Cyclohexyldinitrilotetraphosphonic Acid As a Theranostic Agent for Skeletal Metastases

Objective

Metastasis of the osseous tissue is one of the frequent and severe aggravations as a result of several neoplastic conditions, such as metabolic disorders, infections, and cancer. The objective of this study was to evaluate the pertinence of [⁶⁸Ga]-trans-1,2-cyclohexyldinitrilo tetramethylene phosphonic acid (CDTMP) as a potential bone imaging agent for positron emission tomography (PET) applications as well as to assess [¹⁸⁸Re]-CDTMP for bone pain palliation in metastatic skeletal disorders.

Methods

⁶⁸Ga complex of CDTMP was prepared at 80°C at pH 3.5, and ¹⁸⁸Re complex of CDTMP was prepared at room temperature. [⁶⁸Ga]-CDTMP complex was investigated as PET tracer while the therapeutic efficacy was assessed for [¹⁸⁸Re]-CDTMP. Labeling efficiency, biodistribution, myelotoxicity, and imaging studies were carried out for the complexes synthesized. Both PET and MicroPET imaging studies were performed for [⁶⁸Ga]-CDTMP whereas SPECT acquisitions were acquired for [¹⁸⁸Re]-CDTMP. Data were analyzed semiquantitatively for all the scintigraphic scans obtained.

Results

The radiolabeling efficiency was observed to be >70% for [⁶⁸Ga]-CDTMP. High bone uptake of [⁶⁸Ga]-CDTMP as compared to contralateral tissue was found in PET imaging in Balb/C mice and New Zealand rabbit; the similar result for bone uptake was correlated in the biodistribution study of the compound in BALB/c mice at different time intervals. Biodistribution experiments carried out in mice showed maximum uptake of 6.12 ± 1.22%ID/g at 45 min postinjection. For [¹⁸⁸Re]-CDTMP, total skeletal uptake was 8.12 ± 1.11%ID/g observed at 1 h postinjection from biodistribution data. High renal uptake confirms renal route of excretion. A good hydroxyapatite binding too was seen for both the complexes. No evidence of destruction or adverse functioning of vital organs was observed for the ¹⁸⁸Re complex.

Conclusion

[⁶⁸Ga]-CDTMP complex can be used as a promising PET bone imaging agent and [¹⁸⁸Re]-CDTMP as a surrogate moiety for therapeutic application. Owing to the short half-life of ⁶⁸Ga (68 min), cyclotron-independent radiopharmacy, fast clearance, and rapid renal excretion as evidenced in preclinical animal models. Very low myelotoxicity and highly selective bone uptake prove the potential of [¹⁸⁸Re]-CDTMP for therapeutic application.

Assessment of regional pulmonary blood flow using 68Ga-DOTA PET

Background

In vivo determination of regional pulmonary blood flow (PBF) is a valuable tool for the evaluation of many lung diseases. In this study, the use of ⁶⁸Ga-DOTA PET for the in vivo quantitative determination of regional PBF is proposed. This methodology was implemented and tested in healthy pigs and validated using fluorescent microspheres. The study was performed on young large white pigs (n = 4). To assess the reproducibility and consistency of the method, three PET scans were obtained for each animal. Each radiotracer injection was performed simultaneously to the injection of fluorescent microspheres. PBF images were generated applying a two-compartment exchange model over the dynamic PET images. PET and microspheres values were compared by regression analysis and Bland–Altman plot.

Results

The capability of the proposed technique to produce 3D regional PBF images was demonstrated. The correlation evaluation between ⁶⁸Ga-DOTA PET and microspheres showed a good and significant correlation (r = 0.74, P < 0.001).

Conclusions

Assessment of PBF with the proposed technique allows combining the high quantitative accuracy of PET imaging with the use of ⁶⁸Ga/⁶⁸Ge generators. Thus, ⁶⁸Ga-DOTA PET emerges as a potential inexpensive method for measuring PBF in clinical settings with an extended use.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-017-0259-2) contains supplementary material, which is available to authorized users.

68Ga-NOTA-Aca-BBN(7-14) PET/CT in Healthy Volunteers and Glioma Patients

This work was designed to study the safety, biodistribution, and radiation dosimetry of a gastrin-releasing peptide receptor (GRPR)-targeting, (68)Ga-labeled bombesin (BBN) peptide derivative PET tracer, NOTA-Aca-BBN(7-14) (denoted as (68)Ga-BBN) in healthy volunteers and to assess the level of receptor expression in glioma patients.

METHODS

Four healthy volunteers (2 male and 2 female) underwent whole-body PET/CT at multiple time points after a bolus injection of (68)Ga-BBN (111 ± 148 MBq). Regions of interest were drawn manually over major organs, and time-activity curves were obtained. Dosimetry was calculated using the OLINDA/EXM software. Twelve patients with glioma diagnosed by contrast-enhanced MRI underwent PET/CT at 30-45 min after (68)Ga-BBN injection. Within 1 wk afterward, the tumor was surgically removed and immunohistochemical staining of tumor samples against GRPR was performed and correlated with the PET/CT results.

RESULTS

(68)Ga-BBN was well tolerated in all healthy volunteers, with no adverse symptoms being noticed or reported. (68)Ga-BBN cleared rapidly from the circulation and was excreted mainly through the kidneys and urinary tract. The total effective dose equivalent and effective dose were 0.0335 ± 0.0079 and 0.0276 ± 0.0066 mSv/MBq, respectively. In glioma patients, all MRI-identified lesions showed high signal intensity on (68)Ga-BBN PET/CT. SUVmax and SUVmean were 2.08 ± 0.58 and 1.32 ± 0.37, respectively. With normal brain tissue as background, tumor-to-background ratios were 24.0 ± 8.85 and 13.4 ± 4.54 based on SUVmax and SUVmean, respectively. The immunohistochemical staining confirmed a positive correlation between SUV and GRPR expression level (r(2) = 0.71, P < 0.001).

CONCLUSION

(68)Ga-BBN is a PET tracer with favorable pharmacokinetics and a favorable dosimetry profile. It has the potential to evaluate GRPR expression in glioma patients and guide GRPR-targeted therapy of glioma.

Improved quantification for local regions of interest in preclinical PET imaging

In Positron Emission Tomography, there are several causes of quantitative inaccuracy, such as partial volume or spillover effects. The impact of these effects is greater when using radionuclides that have a large positron range, e.g. (68)Ga and (124)I, which have been increasingly used in the clinic. We have implemented and evaluated a local projection algorithm (LPA), originally evaluated for SPECT, to compensate for both partial-volume and spillover effects in PET. This method is based on the use of a high-resolution CT or MR image, co-registered with a PET image, which permits a high-resolution segmentation of a few tissues within a volume of interest (VOI) centered on a region within which tissue-activity values need to be estimated. The additional boundary information is used to obtain improved activity estimates for each tissue within the VOI, by solving a simple inversion problem. We implemented this algorithm for the preclinical Argus PET/CT scanner and assessed its performance using the radionuclides (18)F, (68)Ga and (124)I. We also evaluated and compared the results obtained when it was applied during the iterative reconstruction, as well as after the reconstruction as a postprocessing procedure. In addition, we studied how LPA can help to reduce the 'spillover contamination', which causes inaccurate quantification of lesions in the immediate neighborhood of large, 'hot' sources. Quantification was significantly improved by using LPA, which provided more accurate ratios of lesion-to-background activity concentration for hot and cold regions. For (18)F, the contrast was improved from 3.0 to 4.0 in hot lesions (when the true ratio was 4.0) and from 0.16 to 0.06 in cold lesions (true ratio  =  0.0), when using the LPA postprocessing. Furthermore, activity values estimated within the VOI using LPA during reconstruction were slightly more accurate than those obtained by post-processing, while also visually improving the image contrast and uniformity within the VOI.

Radiosynthesis of clinical doses of ⁶⁸Ga-DOTATATE (GalioMedix™) and validation of organic-matrix-based ⁶⁸Ge/⁶⁸Ga generators

INTRODUCTION

68Ga-DOTATATE is a radiolabeled peptide-based agonist that targets somatostatin receptors overexpressed in neuroendocrine tumors. Here, we present our results on validation of organic matrix 68Ge/68Ga generators (ITG GmbH) applied for radiosynthesis of the clinical doses of 68Ga-DOTATATE (GalioMedixTM).

METHODS

The clinical grade of DOTATATE (25 μg±5 μg) compounded in 1 M NaOAc at pH=5.5 was labeled manually with 514±218 MBq (13.89±5.9 mCi) of 68Ga eluate in 0.05 N HCl at 95°C for 10 min. The radiochemical purity of the final dose was validated using radio-TLC. The quality control of clinical doses included tests of their osmolarity, endotoxin level, radionuclide identity, filter integrity, pH, sterility and 68Ge breakthrough.

RESULTS

The final dose of 272±126 MBq (7.35±3.4 mCi) of 68Ga-DOTATATE was produced with a radiochemical yield (RCY) of 99%±1%. The total time required for completion of radiolabeling and quality control averaged approximately 35 min. This resulted in delivery of 50%±7% of 68Ga-DOTATATE at the time of calibration (not decay corrected).

CONCLUSIONS

68Ga eluted from the generator was directly applied for labeling of DOTA-peptide with no additional pre-concentration or pre-purification of isotope. The low acidity of 68Ga eluate allows for facile synthesis of clinical doses with radiochemical and radionuclide purity higher than 98% and average activity of 272±126 MBq (7.3±3 mCi). There is no need for post-labeling C18 Sep-Pak purification of final doses of radiotracer. Advances in knowledge and implications for patient care. The clinical interest in validation of 68Galabeled agents has increased in the past years due to availability of generators from different vendors (Eckert-Ziegler, ITG, iThemba), favorable approach of U.S. FDA agency to initiate clinical trials, and collaboration of U.S. centers with leading EU clinical sites. The list of 68Ga-labeled tracers evaluated in clinical studies should growth because of the sensitivity of PET technique, the simplicity of the shakebake approach for the dose preparation and reliability of 68Ge/68Ga generators. Our studies have confirmed the reproducible elution profile, and high reliability of ITG GmbH generators required for routine doses preparation according to FDA recommendations.

68Gallium- and 90Yttrium-/ 177Lutetium: "theranostic twins" for diagnosis and treatment of NETs

Abundant expression of somatostatin receptors (SSTR) is frequently identified in differentiated neuroendocrine tumors and may serve as potential target for diagnostic imaging and treatment. This article discusses the “theranostic approach” of SSTR-targeting compounds including an overview of its role for diagnosis, staging and restaging, discussing its way to being established in clinical routine, and giving an outlook about further potentially relevant developments.

Characterization of SnO2-based (68)Ge/ (68)Ga generators and (68)Ga-DOTATATE preparations: radionuclide purity, radiochemical yield and long-term constancy

Background

With the increasing utilization of ⁶⁸Ge-⁶⁸Ga radionuclide generators, ⁶⁸Ga labelled peptides like DOTATATE are receiving more attention in nuclear medicine. On the one hand, the long half-life of the parent nuclide ⁶⁸Ge is an enormous advantage for routine applications, but the question of the long-term stability of the ⁶⁸Ge breakthrough arises, which up to now has scarcely been investigated.

Method

A sum of 123 eluates from four different ⁶⁸Ge-⁶⁸Ga generators (iThemba Labs, Faure, South Africa) and 115 samples of the prepared radiopharmaceutical ⁶⁸Ga-DOTATATE were measured first with a dose calibrator and again after decay of the eluted ⁶⁸Ga via gamma-ray spectrometry. A complete decay curve was recorded for one sample eluate. A further three eluates were eluted in ten fractions of 0.5 ml in order to obtain detailed information concerning the distribution of the two nuclides within the eluates. The influences of factors such as the amount of DOTATATE, addition of Fe³⁺ salts and replacement of HEPES buffer with sodium acetate on the radiochemical synthesis were also tested.

Results

The content of long-lived ⁶⁸Ge breakthrough increases over the entire period of use to more than 100 ppm. The labelling process with the chelator DOTA removes ⁶⁸Ge efficiently. The maximum activity found in the residues of the radiopharmaceuticals investigated in this study was below 10 Bq in nearly all cases. In many cases (12% of the labelled substance), the long-lived parent nuclide could not be identified at all. The labelling process is still viable for reduced amounts of the chelator and with acetate buffer.

Conclusion

Effective doses received by the patient from ⁶⁸Ge in the injected radiopharmaceutical ⁶⁸Ga-DOTATATE are lower than 0.1 μSv and are therefore practically negligible, especially when compared with the contribution of the PET radiopharmaceutical itself. Gamma-ray spectrometry as recommended by the European Pharmacopeia is suitable for quantification of radionuclidic impurities.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-014-0036-4) contains supplementary material, which is available to authorized users.

Radiosynthesis and biological evaluation of 68Ga-labeled colchicine conjugates

OBJECTIVE

Colchicine, a plant-derived alkaloid, is a known substrate for P-glycoprotein (Pgp), which confers multidrug resistance (MDR) to cancer cells and tumors, through enhanced efflux of chemotherapeutic drugs. Hence, radiolabeled colchicine can be a suitable probe for imaging of activity of Pgp transport in vivo and early diagnosis of MDR.

METHODS

In the present study, colchicine was hydrolyzed to desacetylcolchiceine for conjugation with p-SCN-Bn-DOTA and p-SCN-Bn-NOTA. The resulting conjugates, DOTA-desacetylcolchiceine and NOTA-desacetylcolchiceine, were radiolabeled with 68Ga. The radiotracers 68Ga-DOTA-desacetylcolchiceine (68Ga-1) and 68Ga-NOTA-desacetylcolchiceine (68Ga-2) were evaluated in vitro (MCF-7 and T47D breast cancer cell lines) and in vivo (biodistribution studies, Swiss mice bearing fibrosarcoma tumors).

RESULTS

The radiotracers prepared in >97% radiochemical yield showed good in vitro binding and significant inhibition with 100-fold cold colchicine (p<0.05). In vivo the tumor uptake reached maximum at 120 minutes postinjection (68Ga-1: 2.35%±0.39% injected dose per gram [ID/g]; 68Ga-2: 1.5%±0.31% ID/g). Of the two radiotracers 68Ga-2 cleared faster from blood (p<0.05) with lower uptake in nontargeted organs as compared with 68Ga-1.

CONCLUSIONS

The radiotracer 68Ga-2 has shown improved pharmacokinetic features over 68Ga-1 and the previously reported 99mTc(CO)3-colchicine radiotracer. The preliminary studies with 68Ga-2 indicate its potential for in vivo targeting of tumor. However, the efficacy of the radiotracer for imaging of multidrug-resistant states will be ascertained in future.

Ethanol-Based Post-processing of Generator-Derived ⁶⁸Ga Toward Kit-Type Preparation of ⁶⁸Ga-Radiopharmaceuticals

Post-processing by means of a cation-exchanger-based protocol is an efficient strategy for purification and concentration of generator-derived (68)Ga. It ensures the removal of (68)Ge before (68)Ga-radiopharmaceutical preparation and high labeling yields of (68)Ga-labeled radiopharmaceuticals for routine medical application.

METHODS

In an effort to overcome the problem associated with acetone in the currently applied method, we have investigated the feasibility of replacing it with ethanol. The purification of (68)Ga from coeluted metallic impurities ((68)Ge(4+), Fe(3+), Zn(2+), and Ti(4+)) on various cation-exchange columns has been investigated with a variety of post-processing solutions. As a proof of principle, the post-processed (68)Ga was used to radiolabel DOTATOC in combination with high-purity water and various buffer solutions.

RESULTS

An effective protocol for the processing of generator-produced (68)Ga on the basis of cation-exchange chromatography using EtOH/HCl medium has been developed. Up to 95% of the initially eluted (68)Ga activity can be collected in a 1-mL fraction of 90% EtOH/0.9N HCl after removal of (68)Ge-breakthrough in a washing step. The post-processed eluate has been used to radiolabel DOTATOC in yields of approximately 97% ± 0.25% at 80°C in 5 min.

CONCLUSION

The described novel protocol improves the radiolabeling efficiency and efficacy of DOTATOC, providing yields of greater than 99% (decay-corrected). As a result, further purification to separate the desired product from uncomplexed (68)Ga is not necessary. The developed post-processing and labeling protocols permit reliable and high-yield preparation of injectable (68)Ga-DOTATOC (or other (68)Ga-labeled radiopharmaceuticals) that are suitable for routine application. It is possible to incorporate this protocol into existing automated modules.

Positron range estimations with PeneloPET

Technical advances towards high resolution PET imaging try to overcome the inherent physical limitations to spatial resolution. Positrons travel in tissue until they annihilate into the two gamma photons detected. This range is the main detector-independent contribution to PET imaging blurring. To a large extent, it can be remedied during image reconstruction if accurate estimates of positron range are available. However, the existing estimates differ, and the comparison with the scarce experimental data available is not conclusive. In this work we present positron annihilation distributions obtained from Monte Carlo simulations with the PeneloPET simulation toolkit, for several common PET isotopes ((18)F, (11)C, (13)N, (15)O, (68)Ga and (82)Rb) in different biological media (cortical bone, soft bone, skin, muscle striated, brain, water, adipose tissue and lung). We compare PeneloPET simulations against experimental data and other simulation results available in the literature. To this end the different positron range representations employed in the literature are related to each other by means of a new parameterization for positron range profiles. Our results are generally consistent with experiments and with most simulations previously reported with differences of less than 20% in the mean and maximum range values. From these results, we conclude that better experimental measurements are needed, especially to disentangle the effect of positronium formation in positron range. Finally, with the aid of PeneloPET, we confirm that scaling approaches can be used to obtain universal, material and isotope independent, positron range profiles, which would considerably simplify range correction.

Early experience with (68)Ga-DOTATATE preparation

Neuroendocrine tumors (NETs) are a rare form of cancer. NETs frequently express cell membrane-specific peptide receptors, such as somatostatin receptors (SSTRs). Radiolabeled peptides bind to SSTR and provide in vivo histopathological information for diagnostic purposes. (68)Ga-DOTATATE has higher sensitivity for low-grade tumors and greater avidity to well-differentiated NETs than (18)F-FDG, being superior to (111)In-DTPA-octreotide and (18)F-DOPA in evaluation of well-differentiated metastatic NETs. The feasibility of (68)Ga-DOTATATE application in routine clinical practice for PET imaging of NETs was determined in a limited number of known cases (n = 6). (68)Ga-DOTATATE scan could detect all known sites of NETs and in one case previously unknown peritoneal metastasis. Early regulatory consideration is important for routine clinical use.

Hypoxia imaging agents labeled with positron emitters

Imaging hypoxia using positron emission tomography (PET) is of great importance for therapy of cancer. [(18)F]Fluoromisonidazole (FMISO) was the first PET agent for hypoxia imaging, and various radiolabeled nitroimidazole derivatives such as [(18)F]fluoroerythronitroimidazole (FETNIM), [(18)F]1-α-D: -(2-deoxy-2-fluoroarabinofuranosyl)-2-nitroimidazole (FAZA), [(18)F]2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl) acetamide (EF-5), and [(18)F]fluoroetanidazole (FETA) have been developed successively. To overcome the high cost of cyclotron installation, (68)Ga-labeled nitroimidazole derivatives also have been developed. Another important hypoxia imaging agent is (64)Cu-diacetyl-bis(N (4)-methylthiosemicarbazone) ((64)Cu-ATSM), which can distribute in cancer tissue rapidly due to high lipophilicity. However, its application is limited due to high cost of radionuclide production. Although various hypoxia imaging agents have been reported and tested, hypoxia PET images still have to be improved, because of the low blood flow in hypoxic tissues and resulting low uptake of the agents.

An Increasing Role for 68Ga PET Imaging: A Perspective on the Availability of Parent 68Ge Material for Generator Manufacturing in an Expanding Market

The use of gallium-68 for molecular imaging is gaining momentum world-wide. While our understanding of 68Ga chemistry, generators, and associated synthesis modules appear to have advanced to a clinically-reliable stage, uncertainty in the supply of radiopharmaceutically-suitable parent is of significant concern. In this work, we examine the current supply of 68Ge in an effort to better understand the potential for expansion of manufacturing to meet an increasing demand for 68Ga. Although specific information on sales and demand of 68Ge is highly business sensitive and thus guarded, our examination finds no shortage in the current supply of 68Ge. On the other hand, increases in the use of 68Ge generators for clinical applications in the United States point to the need for continued support for production at DOE laboratories in the United States to ensure a reliable supply and suggests that new commercial facilities may be needed to meet the increasing demand.

How to cite this article

Schultz MK, Donahue P, Musgrave NI, Zhernosekov K, Naidoo C, Razbash A, Tworovska I, Dick DW, Watkins GL, Graham MM, Runde W, Clanton JA, Sunderland JJ. An Increasing Role for 68Ga PET Imaging: A Perspective on the Availability of Parent 68Ge Material for Generator Manufacturing in an Expanding Market. J Postgrad Med Edu Res 2013;47(1):26-30.

68Ge/68Ga Generators and 68Ga Radiopharmaceutical Chemistry on Their Way into a New Century

68Ga faces a renaissance initiated by the development of new 68Ge/68Ga radionuclide generators, sophisticated 68Ga radiopharmaceuticals, preclinical research and state-of-the-art clincial diagnoses via positron emission tomography/computed tomography (PET/CT). A new type of 68Ge/68Ga generator became commercially available in the first years of the 21st century, with eluates based on hydrochloric acid. These generators provided ‘cationic’ 68Ga instead of ‘inert’ 68Gacomplexes, and opened new pathways of MeIII radiopharmaceutical chemistry. The last decade has seen a 68Ga rush. Increasing interest in generator-based 68Ga radiopharmaceuticals in diagnostic applications has been accompanied by its potential use in the context of diease treatment planning, made possible by the inherent option expressed by theranostics. However, widespread acceptance and clinical application requires optimization of 68Ge/68Ga generators both from chemical and regulatory perspectives.

How to cite this article

Rösch F. 68Ge/68Ga Generators and 68Ga Radiopharmaceutical Chemistry on Their Way into a New Century. J Postgrad Med Edu Res 2013;47(1):18-25.