Underscoring the influence of inorganic chemistry on nuclear imaging with radiometals

Transition and Post-Transition Radiometals for PET Imaging and Radiotherapy

Radiometals are an exciting class of radionuclides because of the large number of metallic elements available that have medically useful isotopes. To properly harness radiometals, they must be securely bound by chelators, which must be carefully matched to the radiometal ion to maximize radiolabeling performance and the stability of the resulting complex. This chapter focuses on practical aspects of radiometallation chemistry including chelator selection, radiolabeling procedures and conditions, radiolysis prevention, purification, quality control, requisite equipment and reagents, and useful tips.

Sc-HOPO: A Potential Construct for Use in Radioscandium-Based Radiopharmaceuticals

Three isotopes of scandium─43Sc, 44Sc, and 47Sc─have attracted increasing attention as potential candidates for use in imaging and therapy, respectively, as well as for possible theranostic use as an elementally matched pair. Here, we present the octadentate chelator 3,4,3-(LI-1,2-HOPO) (or HOPO), an effective chelator for hard cations, as a potential ligand for use in radioscandium constructs with simple radiolabeling under mild conditions. HOPO forms a 1:1 Sc-HOPO complex that was fully characterized, both experimentally and theoretically. [47Sc]Sc-HOPO exhibited good stability in chemical and biological challenges over 7 days. In healthy mice, [43,47Sc]Sc-HOPO cleared the body rapidly with no signs of demetalation. HOPO is a strong candidate for use in radioscandium-based radiopharmaceuticals.

Equilibrium Thermodynamics of Macropa Complexes with Selected Metal Isotopes of Radiopharmaceutical Interest

To pursue the design of in vivo stable chelating systems for radiometals, a concise and straightforward method toolbox was developed combining NMR, isothermal titration calorimetry (ITC), and europium time-resolved laser-induced fluorescence spectroscopy (Eu-TRLFS). For this purpose, the macropa chelator was chosen, and Lu3+, La3+, Pb2+, Ra2+, and Ba2+ were chosen as radiopharmaceutically relevant metal ions. They differ in charge (2+ and 3+) and coordination properties (main group vs lanthanides). 1H NMR was used to determine four pKa values (±0.15; carboxylate functions, 2.40 and 3.13; amino functions, 6.80 and 7.73). Eu-TRLFS was used to validate the exclusive existence of the 1:1 Mn+/ligand complex in the chosen pH range at tracer level concentrations. ITC measurements were accomplished to determine the resulting stability constants of the desired complexes, with log K values ranging from 18.5 for the Pb-mcp complex to 7.3 for the Lu-mcp complex. Density-functional-theory-calculated structures nicely mirror the complexes' order of stabilities by bonding features. Radiolabeling with macropa using ligand concentrations from 10-3 to 10-6 M was accomplished by pointing out the complex formation and stability (212Pb > 133La > 131Ba ≈ 224Ra > 177Lu) by means of normal-phase thin-layer chromatography analyses.

Scandium-44: Diagnostic Feasibility in Tumor-Related Angiogenesis

Angiogenesis-related cell-surface molecules, including integrins, aminopeptidase N, vascular endothelial growth factor, and gastrin-releasing peptide receptor (GRPR), play a crucial role in tumour formation. Radiolabelled imaging probes targeting angiogenic biomarkers serve as valuable vectors in tumour identification. Nowadays, there is a growing interest in novel radionuclides other than gallium-68 (⁶⁸Ga) or copper-64 (⁶⁴Cu) to establish selective radiotracers for the imaging of tumour-associated neo-angiogenesis. Given its ideal decay characteristics (E_β⁺_average: 632 KeV) and a half-life (T_1/2 = 3.97 h) that is well matched to the pharmacokinetic profile of small molecules targeting angiogenesis, scandium-44 (⁴⁴Sc) has gained meaningful attention as a promising radiometal for positron emission tomography (PET) imaging. More recently, intensive research has been centered around the investigation of ⁴⁴Sc-labelled angiogenesis-directed radiopharmaceuticals. Previous studies dealt with the evaluation of ⁴⁴Sc-appended a_vb₃ integrin-affine Arg-Gly-Asp (RGD) tripeptides, GRPR-selective aminobenzoyl-bombesin analogue (AMBA), and hypoxia-associated nitroimidazole derivatives in the identification of various cancers using experimental tumour models. Given the tumour-related hypoxia- and angiogenesis-targeting capability of these PET probes, ⁴⁴Sc seems to be a strong competitor of the currently used positron emitters in radiotracer development. In this review, we summarize the preliminary preclinical achievements with ⁴⁴Sc-labelled angiogenesis-specific molecular probes.

Antibody-Based In Vivo Imaging of Central Nervous System Targets-Evaluation of a Pretargeting Approach Utilizing a TCO-Conjugated Brain Shuttle Antibody and Radiolabeled Tetrazines

Bioorthogonal pretargeted imaging using the inverse-electron-demand Diels-Alder (IEDDA) reaction between a tetrazine (Tz) and a trans-cyclooctene (TCO) represents an attractive strategy for molecular imaging via antibodies. The advantages of using a pretargeted imaging approach are on the one hand the possibility to achieve a high signal-to-noise ratio and imaging contrast; on the other hand, the method allows the uncoupling of the biological half-life of antibodies from the physical half-life of short-lived radionuclides. A brain-penetrating antibody (mAb) specific for β-amyloid (Aβ) plaques was functionalized with TCO moieties for pretargeted labeling of Aβ plaques in vitro, ex vivo, and in vivo by a tritium-labeled Tz. The overall aim was to explore the applicability of mAbs for brain imaging, using a preclinical model system. In vitro clicked mAb-TCO-Tz was able to pass the blood-brain barrier of transgenic PS2APP mice and specifically visualize Aβ plaques ex vivo. Further experiments showed that click reactivity of the mAb-TCO construct in vivo persisted up to 3 days after injection by labeling Aβ plaques ex vivo after incubation of brain sections with the Tz in vitro. An attempted in vivo click reaction between injected mAb-TCO and Tz did not lead to significant labeling of Aβ plaques, most probably due to unfavorable in vivo properties of the used Tz and a long half-life of the mAb-TCO in the blood stream. This study clearly demonstrates that pretargeted imaging of CNS targets via antibody-based click chemistry is a viable approach. Further experiments are warranted to optimize the balance between stability and reactivity of all reactants, particularly the Tz.

Chemical and Physical Characterisation of Macroaggregated Human Serum Albumin: Strength and Specificity of Bonds with 99mTc and 68Ga

Background: Macroaggregated human serum albumin (MAA) properties are widely used in nuclear medicine, labelled with ^99mTc. The aim of this study is to improve the knowledge about the morphology, size, dimension and physical–chemical characteristics of MAA and their bond with ^99mTc and ⁶⁸Ga. Methods: Commercial kits of MAA (Pulmocis^®) were used. Characterisation through experiments based on SEM, DLS and Stokes’ Law were carried out. In vitro experiments for Langmuir isotherms and pH studies on radiolabelling were performed and the stability of the radiometal complex was verified through competition reactions. Results: The study settles the MAA dimension within the range 43–51 μm. The Langmuir isotherm reveals for [^99mTc]MAA: Bmax (46.32), h (2.36); for [⁶⁸Ga]MAA: Bmax (44.54), h (0.893). Dual labelling reveals that MAA does not discriminate different radioisotopes. Experiments on pH placed the optimal pH for labelling with ^99mTc at 6. Conclusion: Radiolabelling of MAA is possible with high efficiency. The nondiscriminatory MAA bonds make this drug suitable for radiolabelling with different radioisotopes or for dual labelling. This finding illustrates the need to continue investigating MAA chemical and physical characteristics to allow for secure labelling with different isotopes.

Cyclic Analogs of Desferrioxamine E Siderophore for 68Ga Nuclear Imaging: Coordination Chemistry and Biological Activity in Staphylococcus aureus

As multidrug-resistant bacteria are an emerging problem and threat to humanity, novel strategies for treatment and diagnostics are actively sought. We aim to utilize siderophores, iron-specific strong chelating agents produced by microbes, as gallium ion carriers for diagnosis, applying that Fe(III) can be successfully replaced by Ga(III) without losing biological properties of the investigated complex, which allows molecular imaging by positron emission tomography (PET). Here, we report synthesis, full solution chemistry, thermodynamic characterization, and the preliminary biological evaluation of biomimetic derivatives (FOX) of desferrioxamine E (FOXE) siderophore, radiolabeled with ⁶⁸Ga for possible applications in PET imaging of S. aureus. From a series of six biomimetic analogs, which differ from FOXE with cycle length and position of hydroxamic and amide groups, the highest Fe(III) and Ga(III) stability was determined for the most FOXE alike compounds–FOX 2-4 and FOX 2-5; we have also established the stability constant of the Ga-FOXE complex. For this purpose, spectroscopic and potentiometric titrations, together with the Fe(III)–Ga(III) competition method, were used. [⁶⁸Ga]Ga-FOXE derivatives uptake and microbial growth promotion studies conducted on S. aureus were efficient for compounds with a larger cavity, i.e., FOX 2-5, 2-6, and 3-5. Even though showing low uptake values, Fe-FOX 2-4 seems to be also a good Fe-source to support the growth of S. aureus. Overall, proposed derivatives may hold potential as inert and stable carrier agents for radioactive Ga(III) ions for diagnostic medical applications or interesting starting compounds for further modifications.

In this work, the authors have investigated a set of novel ferrioxamine E analogs as potential Ga-68 chelators and tools for infection imaging.

89Zr-PET imaging in humans: a systematic review

Purpose

The remarkable amount of preclinical data achieved on 89Zr-PET imaging led to a significant clinical translation, concerning mainly immuno-PET applications. The aim of this systematic review is to provide a complete overview on clinical applications of 89Zr-PET imaging, using a systematic approach to identify and collect published studies performed in humans, sorted by field of application and specific disease subsections.

Methods

A systematic literature search of articles suiting the inclusion criteria was conducted on Pubmed, Scopus, Central, and Web Of Science databases, including papers published from January 1967 to November 2020. Eligible studies had to be performed on humans through PET imaging with 89Zr-labeled compounds. The methodological quality was assessed through the Quality Assessment of Diagnostic accuracy Studies-2 tool.

Results

A total of 821 articles were screened. 74 studies performed on humans were assessed for eligibility with the exclusion of further 18, thus 56 articles were ultimately selected for the qualitative analysis.

Conclusions

89Zr has shown to be a powerful PET-imaging tool, in particular for radiolabeling antibodies in order to study antigen expression, biodistribution, anticancer treatment planning and follow-up. Other than oncologic applications, 89Zr-radiolabeled antibodies have been proposed for use in inflammatory and autoimmune disorders with interesting results. 89Zr-labeled nanoparticles represent groundbreaking radiopharmaceuticals with potential huge fields of application. To evaluate the clinical usefulness of 89Zr PET-imaging in different conditions and in real-world settings, and to widen its use in clinical practice, further translation of preclinical to clinical data is needed.

Inverse electron demand Diels-Alder click chemistry for pretargeted PET imaging and radioimmunotherapy

Radiolabeled antibodies have shown promise as tools for both the nuclear imaging and endoradiotherapy of cancer, but the protracted circulation time of radioimmunoconjugates can lead to high radiation doses to healthy tissues. To circumvent this issue, we have developed an approach to positron emission tomography (PET) imaging and radioimmunotherapy (RIT) predicated on radiolabeling the antibody after it has reached its target within the body. This in vivo pretargeting strategy is based on the rapid and bio-orthogonal inverse electron demand Diels-Alder reaction between tetrazine (Tz) and trans-cyclooctene (TCO). Pretargeted PET imaging and RIT using TCO-modified antibodies in conjunction with Tz-bearing radioligands produce high activity concentrations in target tissues as well as reduced radiation doses to healthy organs compared to directly labeled radioimmunoconjugates. Herein, we describe how to prepare a TCO-modified antibody (humanized A33-TCO) as well as how to synthesize two Tz-bearing radioligands: one labeled with the positron-emitting radiometal copper-64 ([64Cu]Cu-SarAr-Tz) and one labeled with the β-emitting radiolanthanide lutetium-177 ([177Lu]Lu-DOTA-PEG7-Tz). We also provide a detailed description of pretargeted PET and pretargeted RIT experiments in a murine model of human colorectal carcinoma. Proper training in both radiation safety and the handling of laboratory mice is required for the successful execution of this protocol.

Targeted copper supplementation oriented theranostic for fluorescence and 19F NMR detection of tumors

Alteration of the levels of copper is a promising approach for cancer therapy. Herein, we develop a dual-mode copper vehicle, M985. The biotin-tailed M985 can exert tumor-directed copper supplementation and undergo self-immolative cleavage in living cancerous cells, resulting in the liberation of F542 along with the generation of excess reactive oxygen species. Thus, fluorescence and 19F NMR detection is realized to specifically discriminate cancer cells. F542 acts as a fluorescence reporter and a potent cytotoxic agent, facilitating the visualization of molecular release and distribution, as well as confirming the ER autophagy-induced apoptosis. Therefore, we present a promising dual-mode theranostic M985 for the efficient detection and therapy of cancer.

Aryl isocyanide derivative for one-pot synthesis of purification-free 99mTc-labeled hexavalent targeting probe

INTRODUCTION

^99mTc-labeled hexavalent probes can be readily synthesized by the coordination of six equivalent isocyanide ligands towards Tc^I, and alkyl isocyanide ligands have been extensively used for preparing such probes. However, high ligand concentration (>1 mM) is generally required due to their insufficient coordination ability to Tc^I.

METHODS AND RESULTS

In this study, we revealed that aryl isocyanide ligands, which have greater π-accepting ability compared with alkyl ones, provided ^99mTc-labeled hexavalent probes in high radiochemical yields (>95%) even at low ligand concentration (50 μM). We applied this finding to the synthesis of a ^99mTc-labeled hexavalent RGD probe, targeting integrin α_vβ₃. This ^99mTc-labeled probe was prepared in a 5 min reaction at ligand concentration of 50 μM, and exhibited high tumor localization in vivo without post-labeling purification.

CONCLUSION

The present findings indicate that aryl isocyanide ligands would be a useful precursor to a variety of ^99mTc-labeled hexavalent targeting probes for molecular imaging of saturable systems.

ADVANCES IN KNOWLEDGE

Aryl isocyanide is a better precursor than alkyl isocyanide for preparing ^99mTc-labeled hexavalent targeting probe.

IMPLICATION FOR PATIENT CARE

This work provides a straightforward method to prepare molecular imaging agents of high target uptake, which would facilitate nuclear medicine imaging in clinical settings.

Promising Performance of 4HMS, a New Zirconium-89 Octadendate Chelator

Over the last decade, the interest in zirconium-89 (⁸⁹Zr) as a positron-emitting radionuclide increased considerably because of its standardized production and its physical half-life (78.41 h), which matches the biological half-life of antibodies and its successful use in preclinical and clinical applications. So far, desferrioxamine (DFO), a commercially available chelator, has been mainly used as a bifunctional chelating system. However, there are some concerns regarding the in vivo stability of the [⁸⁹Zr]Zr-DFO complex. In this study, we report the synthesis of an acyclic N-hydroxy-N-methyl succinamide-based chelator (4HMS) with 8 coordination sites and our first investigations into the use of this new chelator for ⁸⁹Zr complexation. In vitro and in vivo comparative studies with [⁸⁹Zr]Zr-4HMS and [⁸⁹Zr]Zr-DFO are presented. The 4HMS chelator was synthesized in four steps starting with an excellent overall yield. Both chelators were quantitatively labeled with ⁸⁹Zr within 5-10 min at pH 7 and room temperature; the molar activity of [⁸⁹Zr]Zr-4HMS exceeded (>3 times) that of [⁸⁹Zr]Zr-DFO. [⁸⁹Zr]Zr-4HMS remained stable against transmetalation and transchelation and cleared from most tissues within 24 h. The kidney, liver, bone, and spleen uptakes were significantly low for this ⁸⁹Zr-complex. Positron emission tomography images were in accordance with the results of the biodistribution in healthy mice. Based on DFT calculations, a rationale is provided for the high stability of ⁸⁹Zr-4HMS. This makes 4HMS a promising chelator for future development of ⁸⁹Zr-radiopharmaceuticals.

Theranostic Targeting of CUB Domain Containing Protein 1 (CDCP1) in Pancreatic Cancer

PURPOSE

The recent emergence of radioligand therapies for cancer treatment has increased enthusiasm for developing new theranostic strategies coupling both imaging and cytotoxicity in the same entity. In this study, we evaluated whether CUB domain containing protein 1 (CDCP1), a single-pass transmembrane protein highly overexpressed in diverse human cancers, might be a target for cancer theranostics.

EXPERIMENTAL DESIGN

The ectodomain of CDCP1 was targeted using radiolabeled forms of 4A06, a potent and specific recombinant human antibody that we developed. Imaging and antitumor assessment studies were performed in animal models of pancreatic cancer, including two patient-derived xenograft models we developed for this study. For antitumor assessment studies, the endpoints were death due to tumor volume >3,000 mm³ or ≥20% loss in body weight. Specific tracer binding or antitumor effects were assessed with an unpaired, two-tailed Student t test and survival advantages were assessed with a log rank (Mantel-Cox) test. Differences at the 95% confidence level were interpreted to be significant.

RESULTS

⁸⁹Zr-4A06 detected a broad dynamic range of full length or cleaved CDCP1 expression on seven human pancreatic cancer tumors (n = 4/tumor). Treating mice with single or fractionated doses of ¹⁷⁷Lu-4A06 significantly reduced pancreatic cancer tumor volume compared with mice receiving vehicle or unlabeled 4A06 (n = 8; P < 0.01). A single dose of ²²⁵Ac-4A06 also inhibited tumor growth, although the effect was less profound compared with ¹⁷⁷Lu-4A06 (n = 8; P < 0.01). A significant survival advantage was imparted by ²²⁵Ac-4A06 (HR = 2.56; P < 0.05).

CONCLUSIONS

These data establish that CDCP1 can be exploited for theranostics, a finding with widespread implications given its breadth of overexpression in cancer.

A brief overview of metal complexes as nuclear imaging agents

Metallic radionuclides have been instrumental in the field of nuclear imaging for over half a century. While recent years have played witness to a dramatic rise in the use of radiometals as labels for chelator-bearing biomolecules, imaging agents based solely on coordination compounds of radiometals have long played a critical role in the discipline as well. In this work, we seek to provide a brief overview of metal complex-based radiopharmaceuticals for positron emission tomography (PET) and single photon emission computed tomography (SPECT). More specifically, we have focused on imaging agents in which the metal complex itself rather than a pendant biomolecule or targeting moiety is responsible for the in vivo behavior of the tracer. This family of compounds contains metal complexes based on an array of different nuclides as well as probes that have been used for the imaging of a variety of pathologies, including infection, inflammation, cancer, and heart disease. Indeed, two of the defining traits of transition metal complexes-modularity and redox chemistry-have both been creatively leveraged in the development of imaging agents. In light of our audience, particular attention is paid to structure and mechanism, though clinical data is addressed as well. Ultimately, it is our hope that this review will not only educate readers about some of the seminal work performed in this space over the last 30 years but also spur renewed interest in the creation of radiopharmaceuticals based on small metal complexes.

A dual-responsive probe for detecting cellular hypoxia using 19F magnetic resonance and fluorescence

We report the first dual-responsive 19F MRI and fluorescence imaging probe for cellular hypoxia. The Cu2+-based probe exhibits no 19F MR signal and reduced fluorescence signal due to paramagnetic quenching; however, the probe turns-on in both modes following reduction to Cu+. This bimodal agent can differentiate hypoxic and normoxic cells in both modalities.

Does the ATSM-Cu(II) Biomarker Integrate into the Human Cellular Copper Cycle?

Hypoxia is commonly encountered in the tumor microenvironment and drives proliferation, angiogenesis, and resistance to therapy. Imaging of hypoxia is important in many disease states in oncology, cardiology, and neurology. Finding clinically approved imaging biomarkers for hypoxia has proved challenging. Candidate biomarkers have shown low uptake into tumors and low signal to background ratios that adversely affect imaging quality. Copper complexes have been identified as potential biomarkers for hypoxia owing to their redox ability. Active uptake of copper complexes into cells could ensure selectivity and high sensitivity. We explored the reactivity and selectivity of the ATSM-Cu(II) biomarker to proteins that are involved in the copper cycle using electron paramagnetic resonance (EPR) spectroscopy and UV-vis measurements. We show that the affinity of the ATSM-Cu(II) complex to proteins in the copper cycle is low and the cell probably does not actively uptake ATSM-Cu(II).

Chelators and metal complex stability for radiopharmaceutical applications

Diagnostic and therapeutic nuclear medicine relies heavily on radiometal nuclides. The most widely used and well-known radionuclide is technetium-99m (99mTc), which has dominated diagnostic nuclear medicine since the advent of the 99Mo/99mTc generator in the 1960s. Since that time, many more radiometals have been developed and incorporated into potential radiopharmaceuticals. One critical aspect of radiometal-containing radiopharmaceuticals is their stability under in vivo conditions. The chelator that is coordinated to the radiometal is a key factor in determining radiometal complex stability. The chelators that have shown the most promise and are under investigation in the development of diagnostic and therapeutic radiopharmaceuticals over the last 5 years are discussed in this review.

The solution thermodynamic stability of desferrioxamine B (DFO) with Zr(IV)

Desferrioxamine B (DFO, [H₄L]⁺, ligand) is currently the preferred chelator for ⁸⁹Zr(IV), however the biological studies suggest that it releases the metal ion in vivo. Herein, we present the solution thermodynamics of complexes formed between Zr(IV) and this hexadentate chelating agent, the data surprisingly not yet available in the literature. Several techniques including electrospray ionization mass spectrometry (ESI-MS), potentiometry, UV-Vis spectroscopy and isothermal titration calorimetry (ITC) were used to determine the stoichiometry and thermodynamic stability of complexes formed in solution over pH range 1-11, overcoming all the difficulties with the characterisation of the aqueous solution chemistry of Zr(IV) complexes, like strong hydrolysis and lack of spectral information. A model containing only mononuclear complexes, i.e. [ZrHL]²⁺ [ZrL]⁺, [ZrLH_-1] throughout the entire measured pH range is proposed. The stability constants and pM (Zr(IV)) value determined for Zr(IV)-DFO system, place DFO among good Zr(IV) chelators, however the formation of 6-coordinate unsaturated complexes (i.e. with coordination sphere of 8-coordinate Zr(IV) completed by water molecules), together with the susceptibility of coordinated water molecule to deprotonation, are suggested to be the reason of in vivo instability of ⁸⁹Zr(IV)-DFO complexes.

Tuning the properties of tris(hydroxypyridinone) ligands: efficient 68Ga chelators for PET imaging

The prototype tris(1,6-dimethyl-3-hydroxypyridin-4-one) chelator for gallium-68, THPMe, has shown great promise for rapid and efficient kit-based 68Ga labelling of PET radiopharmaceuticals. Peptide derivatives of THPMe have been used to image expression of their target receptors in vivo in preclinical and clinical studies. Herein we describe new synthetic routes to the THP platform including replacing the 1,6-dimethyl-3-hydroxypyridin-4-one N1-CH3 group of THPMe with O (tris(6-methyl-3-hydroxypyran-4-one, THPO) and N1-H (tris(6-methyl-3-hydroxypyridin-4-one), THPH) groups. The effect of these structural modifications on lipophilicity, gallium binding and metal ion selectivity was investigated. THPH was able to bind 68Ga in extremely mild conditions (5 min, room temperature, pH 6, 1 μM ligand concentration) and, notably, in vivo, when administered to a mouse previously injected with 68Ga acetate. The 67Ga radiolabelled complex was stable in serum for more than 7 days. [68Ga(THPH)] displayed a log P value of -2.40 ± 0.02, less negative than the log P = -3.33 ± 0.02 measured for [68Ga(THPMe)], potentially due to an increase in intramolecular hydrogen bonding attributable to the N1-H pyridinone units. Spectrophotometric determination of the Ga3+/Fe3+ complex formation constants for both THPMe and THPH revealed their preference for binding Ga3+ over Fe3+, which enabled selective labelling with 68Ga3+ in the presence of a large excess of Fe3+ in both cases. Compared to THPMe, THPH showed significantly reduced affinity for Fe3+, increased affinity for Ga3+ and improved radiolabelling efficiency. THPO was inferior to both THPH and THPMe in terms of labelling efficiency, but its benzylated precursor Bn-THPO (tris(6-methyl-3-benzyloxypyran-4-one)) provides a potential platform for the synthesis of a library of THP compounds with tunable chemical properties and metal preferences.

The synthesis of a 99mTc-labeled tetravalent targeting probe upon isonitrile coordination to 99mTcI for enhanced target uptake in saturable systems

The difference in 2-proton's acidity between L_β and L_G led to dramatically different results of their reactions with [^99mTc][Tc(CO)₃(OH₂)₃]⁺.

Targeted α-therapy of prostate cancer using radiolabeled PSMA inhibitors: a game changer in nuclear medicine

Prostate cancer (PCa) is one of the most common malignancies in men and is a major contributor to cancer related deaths worldwide. Metastatic spread and disease progression under androgen deprivation therapy signify the onset of metastatic castration resistant prostate cancer (mCRPCa)-the lethal form of the disease, which severely deteriorates the quality of life of patients. Over the last decade, tremendous progress has been made toward identifying appropriate molecular targets that could enable efficient in vivo targeting for non-invasive imaging and therapy of mCPRCa. In this context, a promising enzymatic target is prostate specific membrane antigen (PSMA), which is overexpressed on PCa cells, in proportion to the stage and grade of the tumor progression. This is especially relevant for mCRPCa, which has significant overexpression of PSMA. For therapy of mCRPCa, several nuclear medicine clinics all over the world have confirmed that 177Lu-labeled-PSMA enzyme inhibitors (177Lu-PSMA-617 and 177Lu-PSMA I&T) have a favorable dosimetry and convincing therapeutic response. However, ~30% of patients were found to be short or non-responders and dose escalation was severely limited by chronic hematological toxicity. Such limitations could be better overcome by targeted alpha therapy (TAT) which has the potential to bring a paradigm shift in treatment of mCRPCa patients. This concise review presents an overview of the successes and challenges currently faced in TAT of mCRPCa using radiolabeled PSMA inhibitors. The preclinical and clinical data reported to date are quite promising, and it is expected that this therapeutic modality will play a pivotal role in advanced stage PCa management in the foreseeable future.

89 Zr-ImmunoPET companion diagnostics and their impact in clinical drug development

Therapeutic monoclonal antibodies have been used in cancer treatment for 30 years, with around 24 mAb and mAb:drug conjugates approved by the FDA to date. Despite their specificity, efficacy has remained limited, which, in part, derails nascent initiatives towards precision medicine. An image-guided approach to reinforce treatment decisions using immune positron emission tomography (immunoPET) companion diagnostic is warranted. This review provides a general overview of current translational research using Zr-89 immunoPET and opportunities for utilizing and harnessing this tool to its full potential. Patient case studies are cited to illustrate immunoPET probes as tools for profiling molecular signatures. Discussions on its utility in reinforcing clinical decisions as it relates to histopathological tumor assessment and standard diagnostic methods, and its potential as predictive biomarkers, are presented. We finally conclude with an overview of practical considerations to its utility in the clinic.

Preclinical optimization of antibody-based radiopharmaceuticals for cancer imaging and radionuclide therapy-Model, vector, and radionuclide selection

Intact antibodies and their truncated counterparts (eg, Fab, scFv fragments) are generally exquisitely specific and selective vectors, enabling recognition of individual cancer-associated molecular phenotypes against a complex and dynamic biomolecular background. Complementary alignment of these advantages with unique properties of radionuclides is a defining paradigm in both radioimmunoimaging and radioimmunotherapy, which remain some of the most adept and promising tools for cancer diagnosis and treatment. This review discusses how translational potency can be maximized through rational selection of antibody-nuclide couples for radioimmunoimaging/therapy in preclinical models.

Recent applications of a single quadrupole mass spectrometer in 11C, 18F and radiometal chemistry

Mass spectrometry (MS) has longstanding applications in radiochemistry laboratories, stemming from carbon-dating. However, research on the development of radiotracers for molecular imaging with either positron emission tomography (PET) or single photon emission computed tomography has yet to take full advantage of MS. This inertia has been attributed to the relatively low concentrations of radiopharmaceutical formulations and lack of access to the required MS equipment due to the high costs for purchase and maintenance of specialized MS systems. To date, single quadrupole (SQ)-MS coupled to liquid chromatography (LC) systems is the main form of MS that has been used in radiochemistry laboratories. These LC/MS systems are primarily used for assessing the chemical purity of radiolabeling precursor or standard molecules but also have applications in the determination of metabolites. Herein, we highlight personal experiences using a compact SQ-MS in our PET radiochemistry laboratories, to monitor the small amounts of carrier observed in most radiotracer preparations, even at high molar activities. The use of a SQ-MS in the observation of the low mass associated with non-radioactive species which are formed along with the radiotracer from the trace amounts of carrier found is demonstrated. Herein, we describe a pre-concentration system to detect dilute radiopharmaceutical formulations and metabolite analyses by SQ-MS. Selected examples where SQ-MS was critical for optimization of radiochemical reactions and for unequivocal characterization of radiotracers are showcased. We also illustrate examples where SQ-MS can be applied in identification of radiometal complexes and development of a new purification methodology for Pd-catalyzed radiofluorination reactions, shedding light on the identity of metal complexes present in the labelling solution.

Optimized synthesis and indium complex formation with the bifunctional chelator NODIA-Me

Multi-step synthetic route provides the ligand NODIA-Me in high yield. Radiolabeling with [¹¹¹In]InCl₃ yields stable complexes in high radiochemical purity and yield.

Simple, mild, one-step labelling of proteins with gallium-68 using a tris(hydroxypyridinone) bifunctional chelator: a 68Ga-THP-scFv targeting the prostate-specific membrane antigen

Background

Labelling proteins with gallium-68 using bifunctional chelators is often problematic because of unsuitably harsh labelling conditions such as low pH or high temperature and may entail post-labelling purification. To determine whether tris(hydroxypyridinone) (THP) bifunctional chelators offer a potential solution to this problem, we have evaluated the labelling and biodistribution of a THP conjugate with a new single-chain antibody against the prostate-specific membrane antigen (PSMA), an attractive target for staging prostate cancer (PCa). A single-chain variable fragment (scFv) of J591, a monoclonal antibody that recognises an external epitope of PSMA, was prepared in order to achieve biokinetics matched to the half-life of gallium-68. The scFv, J591c-scFv, was engineered with a C-terminal cysteine.

Results

J591c-scFv was produced in HEK293T cells and purified by size-exclusion chromatography. A maleimide THP derivative (THP-mal) was coupled site-specifically to the C-terminal cysteine residue. The THP-mal-J591c-scFv conjugate was labelled with ammonium acetate-buffered gallium-68 from a ⁶⁸Ge/⁶⁸Ga generator at room temperature and neutral pH. The labelled conjugate was evaluated in the PCa cell line DU145 and its PSMA-overexpressing variant in vitro and xenografted in SCID mice.

J591c-scFv was produced in yields of 4–6 mg/l culture supernatant and efficiently coupled with the THP-mal bifunctional chelator. Labelling yields > 95% were achieved at room temperature following incubation of 5 μg conjugate with gallium-68 for 5 min without post-labelling purification. ⁶⁸Ga-THP-mal-J591c-scFv was stable in serum and showed selective binding to the DU145-PSMA cell line, allowing an IC50 value of 31.5 nM to be determined for unmodified J591c-scFv. Serial PET/CT imaging showed rapid, specific tumour uptake and clearance via renal elimination. Accumulation in DU145-PSMA xenografts at 90 min post-injection was 5.4 ± 0.5%ID/g compared with 0.5 ± 0.2%ID/g in DU145 tumours (n = 4).

Conclusions

The bifunctional chelator THP-mal enabled simple, rapid, quantitative, one-step room temperature radiolabelling of a protein with gallium-68 at neutral pH without a need for post-labelling purification. The resultant gallium-68 complex shows high affinity for PSMA and favourable in vivo targeting properties in a xenograft model of PCa.

Electronic supplementary material

The online version of this article (10.1186/s13550-017-0336-6) contains supplementary material, which is available to authorized users.

89Zr-Immuno-Positron Emission Tomography in Oncology: State-of-the-Art 89Zr Radiochemistry

Immuno-positron emission tomography (immunoPET) with ⁸⁹Zr-labeled antibodies has shown great potential in cancer imaging. It can provide important information about the pharmacokinetics and tumor-targeting properties of monoclonal antibodies and may help in anticipating on toxicity. Furthermore, it allows accurate dose planning for individualized radioimmunotherapy and may aid in patient selection and early-response monitoring for targeted therapies. The most commonly used chelator for ⁸⁹Zr is desferrioxamine (DFO). Preclinical studies have shown that DFO is not an ideal chelator because the ⁸⁹Zr–DFO complex is partly unstable in vivo, which results in the release of ⁸⁹Zr from the chelator and the subsequent accumulation of ⁸⁹Zr in bone. This bone accumulation interferes with accurate interpretation and quantification of bone uptake on PET images. Therefore, there is a need for novel chelators that allow more stable complexation of ⁸⁹Zr. In this Review, we will describe the most recent developments in ⁸⁹Zr radiochemistry, including novel chelators and site-specific conjugation methods.

Clinical evaluation of the radiolanthanide terbium-152: first-in-human PET/CT with 152Tb-DOTATOC

The existence of theragnostic pairs of radionuclides allows the preparation of radiopharmaceuticals for diagnostic and therapeutic purposes.

Amino acid based gallium-68 chelators capable of radiolabeling at neutral pH

Herein we show a flexible synthesis for bifunctional chelators based on amino acids that rapidly complex ⁶⁸Ga under physiological conditions.

Nanoparticles and radiotracers: advances toward radionanomedicine

In this study, we cover the convergence of radiochemistry for imaging and therapy with advances in nanoparticle (NP) design for biomedical applications. We first explore NP properties relevant for therapy and theranostics and emphasize the need for biocompatibility. We then explore radionuclide-imaging modalities such as positron emission tomography (PET), single-photon emission computed tomography (SPECT), and Cerenkov luminescence (CL) with examples utilizing radiolabeled NP for imaging. PET and SPECT have served as diagnostic workhorses in the clinic, while preclinical NP design examples of multimodal imaging with radiotracers show promise in imaging and therapy. CL expands the types of radionuclides beyond PET and SPECT tracers to include high-energy electrons (β- ) for imaging purposes. These advances in radionanomedicine will be discussed, showing the potential for radiolabeled NPs as theranostic agents. WIREs Nanomed Nanobiotechnol 2016, 8:872-890. doi: 10.1002/wnan.1402 For further resources related to this article, please visit the WIREs website.

Stable Radiolabeling of Sulfur-Functionalized Silica Nanoparticles with Copper-64

Nanoparticles labeled with radiometals enable whole-body nuclear imaging and therapy. Though chelating agents are commonly used to radiolabel biomolecules, nanoparticles offer the advantage of attaching a radiometal directly to the nanoparticle itself without the need of such agents. We previously demonstrated that direct radiolabeling of silica nanoparticles with hard, oxophilic ions, such as the positron emitters zirconium-89 and gallium-68, is remarkably efficient. However, softer radiometals, such as the widely employed copper-64, do not stably bind to the silica matrix and quickly dissociate under physiological conditions. Here, we overcome this limitation through the use of silica nanoparticles functionalized with a soft electron-donating thiol group to allow stable attachment of copper-64. This approach significantly improves the stability of copper-64 labeled thiol-functionalized silica nanoparticles relative to native silica nanoparticles, thereby enabling in vivo PET imaging, and may be translated to other softer radiometals with affinity for sulfur. The presented approach expands the application of silica nanoparticles as a platform for facile radiolabeling with both hard and soft radiometal ions.

PET Imaging of Extracellular pH in Tumors with (64)Cu- and (18)F-Labeled pHLIP Peptides: A Structure-Activity Optimization Study

pH (low) insertion peptides (pHLIP peptides) target acidic extracellular environments in vivo due to pH-dependent cellular membrane insertion. Two variants (Var3 and Var7) and wild-type (WT) pHLIP peptides have shown promise for in vivo imaging of breast cancer. Two positron emitting radionuclides (⁶⁴Cu and ¹⁸F) were used to label the NOTA- and NO2A-derivatized Var3, Var7, and WT peptides for in vivo biodistribution studies in 4T1 orthotopic tumor-bearing BALB/c mice. All of the constructs were radiolabeled with ⁶⁴Cu or [¹⁸F]-AlF in good yield. The in vivo biodistribution of the 12 constructs in 4T1 orthotopic allografted female BALB/c mice indicated that NO2A-cysVar3, radiolabeled with either ¹⁸F (4T1 uptake; 8.9 ± 1.7%ID/g at 4 h p.i.) or ⁶⁴Cu (4T1 uptake; 8.2 ± 0.9%ID/g at 4 h p.i. and 19.2 ± 1.8% ID/g at 24 h p.i.), shows the most promise for clinical translation. Additional studies to investigate other tumor models (melanoma, prostate, and brain tumor models) indicated the universality of tumor targeting of these tracers. From this study, future clinical translation will focus on ¹⁸F- or ⁶⁴Cu-labeled NO2A-cysVar3.

Synthesis and Characterization of (89)Zr-Labeled Ultrasmall Nanoparticles

The ultrasmall nanoparticle AGuIX is a versatile platform that tolerates a range of chemical diversity for theranostic applications. Our previous work showed that AGuIX clears rapidly from normal tissues, while durably accumulating within the tumor microenvironment. On this basis, AGuIX was used to detect tumor tissue with Gd(3+) enhanced MRI and can sensitize tumors to radiation therapy. As we begin the translation of AGuIX, we appreciated that coupling AGuIX to a long-lived radioisotope would help to more completely measure the magnitude and duration of its retention within the tumor microenvironment. Therefore, we developed (89)Zr-DFO-AGuIX. AGuIX was coupled to DFO and then to (89)Zr in ∼99% radiochemical yield. Stability studies showed that (89)Zr-DFO-AGuIX did not dissociate after 72 h. In animals bearing U87MG xenografts, it was detectable at levels above background for 72 h. Lastly, (89)Zr-DFO-AGuIX did not accumulate in inflammatory abscesses in vivo, highlighting its specificity for well vascularized tumors.

Synthesis and evaluation of copper-64 labeled benzofuran derivatives targeting β-amyloid aggregates

In vivo imaging of β-amyloid (Aβ) aggregates consisting of Aβ(1-40) and Aβ(1-42) peptides by positron emission tomography (PET) contributes to the diagnosis and therapy for Alzheimer's disease (AD). Because (64)Cu (t1/2=12.7h) is a radionuclide for PET with a longer physical half-life than (11)C (t1/2=20min) and (18)F (t1/2=110min), it is an attractive radionuclide for the development of Aβ imaging probes that are suitable for routine use. In the present study, we designed and synthesized two novel (64)Cu labeled benzofuran derivatives and evaluated their utility as PET imaging probes for Aβ aggregates. In an in vitro binding assay, 6 and 8 showed binding affinity for Aβ(1-42) aggregates with a Ki value of 33 and 243nM, respectively. In addition, these probes bound to Aβ plaques deposited in the brain of an AD model mouse in vitro. In a biodistribution experiment using normal mice, these probes showed low brain uptake (0.33% and 0.36% ID/g) at 2min post-injection. Although refinement to enhance brain uptake is needed, [(64)Cu]6 and [(64)Cu]8 demonstrated the feasibility of developing novel PET probes for imaging Aβ aggregates.

New Tris(hydroxypyridinone) Bifunctional Chelators Containing Isothiocyanate Groups Provide a Versatile Platform for Rapid One-Step Labeling and PET Imaging with (68)Ga(3.)

Two new bifunctional tris(hydroxypyridinone) (THP) chelators designed specifically for rapid labeling with (68)Ga have been synthesized, each with pendant isothiocyanate groups and three 1,6-dimethyl-3-hydroxypyridin-4-one groups. Both compounds have been conjugated with the primary amine group of a cyclic integrin targeting peptide, RGD. Each conjugate can be radiolabeled and formulated by treatment with generator-produced (68)Ga(3+) in over 95% radiochemical yield under ambient conditions in less than 5 min, with specific activities of 60-80 MBq nmol(-1). Competitive binding assays and in vivo biodistribution in mice bearing U87MG tumors demonstrate that the new (68)Ga(3+)-labeled THP peptide conjugates retain affinity for the αvβ3 integrin receptor, clear within 1-2 h from circulation, and undergo receptor-mediated tumor uptake in vivo. We conclude that bifunctional THP chelators can be used for simple, efficient labeling of (68)Ga biomolecules under mild conditions suitable for peptides and proteins.

Palliative care of bone pain due to skeletal metastases: Exploring newer avenues using neutron activated (45)Ca

INTRODUCTION

With an objective to develop a cost-effective radiochemical formulation for palliation of pain due to skeletal metastases, we have demonstrated a viable method for large-scale production of (45)Ca (t½=163 days, Eβmax=0.3MeV) using moderate flux research reactor, its purification from radionuclidic impurities adopting electrochemical approach and preclinical evaluation of (45)CaCl2.

METHODS

Irradiation parameters were optimized by theoretical calculations for production of (45)Ca with highest possible specific activity along with minimum radionuclidic impurity burden. Based on this, the radioisotope was produced in reactor by irradiation of isotopically enriched (98% in (44)Ca) CaO target at a thermal neutron flux of ~1 × 10(14) n.cm(-2).s(-1) for 4 months. Scandium-46 impurity co-produced along with (45)Ca was efficiently removed adopting an electrochemical separation approach. The bone specificity of (45)CaCl2 was established by in vitro studies involving its uptake in hydroxyapatite (HA) particles and also evaluating its biodistribution pattern over a period of 2 weeks after in vivo administration in Wistar rats.

RESULTS

Thermal neutron irradiation of 100mg of enriched (98% in (44)Ca) CaO target followed by radiochemical processing and electrochemical purification procedure yielded ~37 GBq of (45)Ca with a specific activity of ~370 MBq/mg and radionuclidic purity>99.99%. The reliability and reproducibility of this approach were amply demonstrated by process demonstration in several batches. In vitro studies indicated significant uptake of (45)CaCl2 (up to 65%) in HA particles. In vivo biodistribution studies in Wistar rats showed specific skeletal accumulation (40-46%ID) with good retention over a period of 2 weeks.

CONCLUSIONS

To the best of our knowledge, this is the first study on utilization of (45)CaCl2 in the context of nuclear medicine. The results obtained in this study hold promise and warrant further investigations for future translation of (45)CaCl2 to the clinics, thereby potentially enabling a cost-effective approach for metastatic bone pain palliation especially in developing countries.