Chemical and biological evaluation of scandium(III)-polyaminopolycarboxylate complexes as potential PET agents and radiopharmaceuticals

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.

Synthesis, solid-state, solution, and theoretical characterization of an "in-cage" scandium-NOTA complex

Developing chelators that strongly and selectively bind rare-earth elements (Sc, Y, La, and lanthanides) represents a longstanding fundamental challenge in inorganic chemistry. Solving these challenges is becoming more important because of increasing use of rare-earth elements in numerous technologies, ranging from paramagnets to luminescent materials. Within this context, we interrogated the complexation chemistry of the scandium(III) (Sc³⁺) trication with the hexadentate 1,4,7-triazacyclononane-1,4,7-triacetic acid (H₃NOTA) chelator. This H₃NOTA chelator is often regarded as an underperformer for complexing Sc³⁺. A common assumption is that metalation does not fully encapsulate Sc³⁺ within the NOTA^3- macrocycle, leaving Sc³⁺ on the periphery of the chelate and susceptible to demetalation. Herein, we developed a synthetic approach that contradicted those assumptions. We confirmed that our procedure forced Sc³⁺ into the NOTA^3- binding pocket by using single crystal X-ray diffraction to determine the Na[Sc(NOTA)(OOCCH₃)] structure. Density functional theory (DFT) and ⁴⁵Sc nuclear magnetic resonance (NMR) spectroscopy showed Sc³⁺ encapsulation was retained when the crystals were dissolved. Solution-phase and DFT studies revealed that [Sc(NOTA)(OOCCH₃)]^1- could accommodate an additional H₂O capping ligand. Thermodynamic properties associated with the Sc-OOCCH₃ and Sc-H₂O capping ligand interactions demonstrated that these capping ligands occupied critical roles in stabilizing the [Sc(NOTA)] chelation complex.

Studies on production of 43,44,44mSc from 12C+natCl reactions up to 64 MeV projectile energy

For the first-time production of ^43,44,44mSc radionuclides via ¹²C + ^natCl reaction have been reported. Production yield and experimental cross sections of ^natCl(¹²C,xn)^43,44,44mSc up to 64 MeV have been reported. Experimental cross sections have been found comparable with the theoretically evaluated data using PACE4 and EMPIRE3.2.2 codes.

Production of scandium radionuclides for theranostic applications: towards standardization of quality requirements

In the frame of "precision medicine", the scandium radionuclides have recently received considerable interest, providing personalised adjustment of radiation characteristics to optimize the efficiency of medical care or therapeutic benefit for particular groups of patients. Radionuclides of scandium, namely scandium-43 and scandium-44 (43/44Sc) as positron emitters and scandium-47 (47Sc), beta-radiation emitter, seem to fit ideally into the concept of theranostic pair. This paper aims to review the work on scandium isotopes production, coordination chemistry, radiolabeling, preclinical studies and the very first clinical studies. Finally, standardized procedures for scandium-based radiopharmaceuticals have been proposed as a basis to pave the way for elaboration of the Ph.Eur. monographs for perspective scandium radionuclides.

Marine Exopolysaccharide Complexed With Scandium Aimed as Theranostic Agents

(1) Background: Exopolysaccharide (EPS) derivatives, produced by Alteromonas infernus bacterium, showed anti-metastatic properties. They may represent a new class of ligands to be combined with theranostic radionuclides, such as ⁴⁷Sc/⁴⁴Sc. The goal of this work was to investigate the feasibility of such coupling. (2) Methods: EPSs, as well as heparin used as a drug reference, were characterized in terms of molar mass and dispersity using Asymmetrical Flow Field-Flow Fractionation coupled to Multi-Angle Light Scattering (AF4-MALS). The intrinsic viscosity of EPSs at different ionic strengths were measured in order to establish the conformation. To determine the stability constants of Sc with EPS and heparin, a Free-ion selective radiotracer extraction (FISRE) method has been used. (3) Results: AF4-MALS showed that radical depolymerization produces monodisperse EPSs, suitable for therapeutic use. EPS conformation exhibited a lower hydrodynamic volume for the highest ionic strengths. The resulting random-coiled conformation could affect the complexation with metal for high concentration. The LogK of Sc-EPS complexes have been determined and showing that they are comparable to the Sc-Hep. (4) Conclusions: EPSs are very promising to be coupled with the theranostic pair of scandium for Nuclear Medicine.

Impact of prompt gamma emission of 44Sc on quantification in preclinical and clinical PET systems

⁴⁴Sc is an increasingly investigated positron emitter for use in positron emission tomography (PET) imaging. However, ⁴⁴Sc is a non-pure positron emitter, since prompt photons are co-emitted during the decay process. This study investigates coincidence energy spectra of ⁴⁴Sc and its impact on PET quantification on a preclinical and clinical PET system in comparison with ¹⁸F. The raw data of the coincidence events revealed characteristic differences comparing the photon energy distribution of ⁴⁴Sc and ¹⁸F. Due to prompt gamma emission of ⁴⁴Sc, activity recovery is underestimated on PET systems. However, clinical PET imaging of ⁴⁴Sc with acceptable quantitative accuracy appears feasible by using a single, constant correction factor.

Image quality analysis of 44Sc on two preclinical PET scanners: a comparison to 68Ga

Background

⁴⁴Sc has been increasingly investigated as a potential alternative to ⁶⁸Ga in the development of tracers for positron emission tomography (PET). The lower mean positron energy of ⁴⁴Sc (0.63 MeV) compared to ⁶⁸Ga (0.83 MeV) can result in better spatial image resolutions. However, high-energy γ-rays (1157 keV) are emitted at high rates (99.9%) during ⁴⁴Sc decay, which can reduce image quality. Therefore, we investigated the impact of these physical properties and performed an unbiased performance evaluation of ⁴⁴Sc and ⁶⁸Ga with different imaging phantoms (image quality phantom, Derenzo phantom, and three-rod phantom) on two preclinical PET scanners (Mediso nanoScan PET/MRI, Siemens microPET Focus 120).

Results

Despite the presence of high-energy γ-rays in ⁴⁴Sc decay, a higher image resolution of small structures was observed with ⁴⁴Sc when compared to ⁶⁸Ga. Structures as small as 1.3 mm using the Mediso system, and as small as 1.0 mm using the Siemens system, could be visualized and analyzed by calculating full width at half maximum. Full widths at half maxima were similar for both isotopes. For image quality comparison, we calculated recovery coefficients in 1–5 mm rods and spillover ratios in either air, water, or bone-equivalent material (Teflon). Recovery coefficients for ⁴⁴Sc were significantly higher than those for ⁶⁸Ga. Despite the lower positron energy, ⁴⁴Sc-derived spillover ratio (SOR) values were similar or slightly higher to ⁶⁸Ga-derived SOR values. This may be attributed to the higher background caused by the additional γ-rays. On the Siemens system, an overestimation of scatter correction in the central part of the phantom was observed causing a virtual disappearance of spillover inside the three-rod phantom.

Conclusion

Based on these findings, ⁴⁴Sc appears to be a suitable alternative to ⁶⁸Ga. The superior image resolution makes it an especially strong competitor in preclinical settings. The additional γ-emissions have a small impact on the imaging resolution but cause higher background noises and can effect an overestimation of scatter correction, depending on the PET system and phantom.

The First-Row Transition Metals in the Periodic Table of Medicine

In this manuscript, we describe medical applications of each first-row transition metal including nutritional, pharmaceutical, and diagnostic applications. The 10 first-row transition metals in particular are found to have many applications since there five essential elements among them. We summarize the aqueous chemistry of each element to illustrate that these fundamental properties are linked to medical applications and will dictate some of nature’s solutions to the needs of cells. The five essential trace elements—iron, copper, zinc, manganese, and cobalt—represent four redox active elements and one redox inactive element. Since electron transfer is a critical process that must happen for life, it is therefore not surprising that four of the essential trace elements are involved in such processes, whereas the one non-redox active element is found to have important roles as a secondary messenger.. Perhaps surprising is the fact that scandium, titanium, vanadium, chromium, and nickel have many applications, covering the entire range of benefits including controlling pathogen growth, pharmaceutical and diagnostic applications, including benefits such as nutritional additives and hardware production of key medical devices. Some patterns emerge in the summary of biological function andmedical roles that can be attributed to small differences in the first-row transition metals.

Promising Scandium Radionuclides for Nuclear Medicine: A Review on the Production and Chemistry up to In Vivo Proofs of Concept

Scandium radionuclides have been identified in the late 1990s as promising for nuclear medicine applications, but have been set aside for about 20 years. Among the different isotopes of scandium, ⁴³Sc and ⁴⁴Sc are interesting for positron emission tomography imaging, whereas ⁴⁷Sc is interesting for therapy. The ⁴⁴Sc/⁴⁷Sc or ⁴³Sc/⁴⁷Sc pairs could be thus envisaged as true theranostic pairs. Another interesting aspect of scandium is that its chemistry is governed by the trivalent ion, Sc³⁺. When combined with its hardness and its size, it gives this element a lanthanide-like behavior. It is then also possible to use it in a theranostic approach in combination with ¹⁷⁷Lu or other lanthanides. This article aims to review the progresses that have been made over the last decade on scandium isotope production and coordination chemistry. It also reviews the radiolabeling aspects and the first (pre) clinical studies performed.

44Sc for labeling of DOTA- and NODAGA-functionalized peptides: preclinical in vitro and in vivo investigations

Background

Recently, ⁴⁴Sc (T_1/2 = 3.97 h, Eβ⁺_av = 632 keV, I = 94.3 %) has emerged as an attractive radiometal candidate for PET imaging using DOTA-functionalized biomolecules. The aim of this study was to investigate the potential of using NODAGA for the coordination of ⁴⁴Sc. Two pairs of DOTA/NODAGA-derivatized peptides were investigated in vitro and in vivo and the results obtained with ⁴⁴Sc compared with its ⁶⁸Ga-labeled counterparts.

DOTA-RGD and NODAGA-RGD, as well as DOTA-NOC and NODAGA-NOC, were labeled with ⁴⁴Sc and ⁶⁸Ga, respectively. The radiopeptides were investigated with regard to their stability in buffer solution and under metal challenge conditions using Fe³⁺ and Cu²⁺. Time-dependent biodistribution studies and PET/CT imaging were performed in U87MG and AR42J tumor-bearing mice.

Results

Both RGD- and NOC-based peptides with a DOTA chelator were readily labeled with ⁴⁴Sc and ⁶⁸Ga, respectively, and remained stable over at least 4 half-lives of the corresponding radionuclide. In contrast, the labeling of NODAGA-functionalized peptides with ⁴⁴Sc was more challenging and the resulting radiopeptides were clearly less stable than the DOTA-derivatized matches. ⁴⁴Sc-NODAGA peptides were clearly more susceptible to metal challenge than ⁴⁴Sc-DOTA peptides under the same conditions. Instability of ⁶⁸Ga-labeled peptides was only observed if they were coordinated with a DOTA in the presence of excess Cu²⁺. Biodistribution data of the ⁴⁴Sc-labeled peptides were largely comparable with the data obtained with the ⁶⁸Ga-labeled counterparts. It was only in the liver tissue that the uptake of ⁶⁸Ga-labeled DOTA compounds was markedly higher than for the ⁴⁴Sc-labeled version and this was also visible on PET/CT images. The ⁴⁴Sc-labeled NODAGA-peptides showed a similar tissue distribution to those of the DOTA peptides without any obvious signs of in vivo instability.

Conclusions

Although DOTA revealed to be the preferred chelator for stable coordination of ⁴⁴Sc, the data presented in this work indicate the possibility of using NODAGA in combination with ⁴⁴Sc. In view of a clinical study, thorough investigations will be necessary regarding the labeling conditions and storage solutions in order to guarantee sufficient stability of ⁴⁴Sc-labeled NODAGA compounds.

Electronic supplementary material

The online version of this article (doi:10.1186/s41181-016-0013-5) contains supplementary material, which is available to authorized users.

Scandium(iii) complexes of monophosphorus acid DOTA analogues: a thermodynamic and radiolabelling study with 44Sc from cyclotron and from a 44Ti/44Sc generator

The influence of the phosphonic/phosphinic acid pendant arm in DOTA derivatives on properties of their Sc³⁺ complexes and efficiency of their ⁴⁴Sc labelling were investigated.

Cross section measurements of deuteron induced nuclear reactions on natural titanium up to 34 MeV

Experimental cross sections for deuteron induced nuclear reactions on natural titanium were measured, using the stacked-foil technique and gamma spectrometry, up to 34 MeV with beams provided by the ARRONAX cyclotron. The experimental cross section values were monitored using the (nat)Ti(d,x)(48)V reaction, recommended by the IAEA. The excitation functions for (nat)Ti(d,x)(44m,46,47,48)Sc are presented and compared with the existing ones and with the TALYS 1.6 code calculations using default models. Our experimental values are in good agreement with data found in the literature. TALYS 1.6 is not able to give a good estimation of the production cross sections investigated in this work. These production cross sections of scandium isotopes fit with the new Coordinated Research Project (CRP) launched by the International Atomic Energy Agency (IAEA) to expand the database of monitor reactions.

A comparative thermodynamic study of the formation of scandium complexes with DTPA and DOTA

The complexation of scandium(iii) by various polyaminopolycarboxylic ligands (DTPA and DOTA) was studied by capillary electrophoresis with ICP-MS detection in 0.1 mol L⁻¹ NaCl ionic strength solutions at 25 °C.

Separation of cyclotron-produced 44Sc from a natural calcium target using a dipentyl pentylphosphonate functionalized extraction resin

Significant interest in ⁴⁴Sc as a radioactive synthon to label small molecules for positron emission tomography (PET) imaging has been recently observed. Despite the efforts of several research groups, the ideal ⁴⁴Sc production and separation method remains elusive. Herein, we propose a novel separation method to obtain ⁴⁴Sc from the proton irradiation of calcium targets based on extraction chromatography, which promises to greatly simplify current production methodologies. Using the commercially available Uranium and Tetravalent Actinides (UTEVA) extraction resin we were able to rapidly (<20min) recover >80% of the activity generated at end of bombardment (EoB) in small ~1M HCl fractions (400μL). The chemical purity of the ⁴⁴Sc eluates was evaluated through chelation with DOTA and DTPA, and by trace metal analysis using microwave induced plasma atomic emission spectrometry. The distribution coefficients (K_d) of Sc(III) and Ca(II) in UTEVA were determined in HCl medium in a range of concentrations from zero to 12.1M. The ⁴⁴Sc obtained with our method proved to be suitable for the direct labeling of small biomolecules for PET imaging, with excellent specific activities and radiochemical purity.

A Bridge not too Far: Personalized Medicine with the use of Theragnostic Radiopharmaceuticals

This article deals primarily with the selection criteria, production, and the nuclear, physical, and chemical properties of certain dual-purpose radionuclides, including those that are currently being used, or studied and evaluated, and those that warrant future investigations. Various scientific and practical issues related to the production and availability of these radionuclides is briefly addressed. At brookhaven national laboratory (BNL), we have developed a paradigm that involves specific individual ‘dual-purpose’ radionuclides or radionuclide pairs with emissions suitable for both imaging and therapy, and which when molecularly (selectively) targeted using appropriate carriers, would allow pre-therapy low-dose imaging plus higher-dose therapy in the same patient. We have made an attempt to sort out and organize a number of such theragnostic radionuclides and radionuclide pairs that may thus potentially bring us closer to the age-long dream of personalized medicine for performing tailored low-dose molecular imaging (SPECT/CT or PET/CT) to provide the necessary pretherapy information on biodistribution, dosimetry, the limiting or critical organ or tissue, and the maximum tolerated dose (MTD), etc., followed by performing higher-dose targeted molecular therapy in the same patient with the same radiopharmaceutical. As an example, our preclinical and clinical studies with the theragnostic radionuclide Sn-117m are covered in somewhat greater detail.

A troublesome problem that remains yet to be fully resolved is the lack of availability, in sufficient quantities and at reasonable cost, of a majority of the best candidate theragnostic radionuclides in a no-carrier-added (NCA) form. In this regard, a summary description of recently developed new or modified methods at BNL for the production of five theragnostic radionuclide/radionuclide pair items, whose nuclear, physical, and chemical characteristics seem to show promise for therapeutic oncology and for treating other disorders that respond to radionuclide therapy, is provided.

How to cite this article

Srivastava SC. A Bridge not too Far: Personalized Medicine with the use of Theragnostic Radiopharmaceuticals. J Postgrad Med Edu Res 2013;47(1):31-46.

Stability of 47Sc-complexes with acyclic polyamino-polycarboxylate ligands

The aim of this study was to evaluate acyclic ligands which can be applied for labeling proteins such as monoclonal antibodies and their fragments with scandium radionuclides. Recently, scandium isotopes (⁴⁷Sc, ⁴⁴Sc) are more available and their properties are convenient for radiotherapy or PET imaging. They can be used together as “matched pair” in theranostic approach. Because proteins denaturize at temperature above 42 °C, ligands which efficiently form complexes at room temperature, are necessary for labelling such biomolecules. For complexation of scandium radionuclides open chain ligands DTPA, HBED, BAPTA, EGTA, TTHA and deferoxamine have been chosen. We found that the ligands studied (except HBED) form strong complexes within 10 min and that the radiolabelling yield varies between 96 and 99 %. The complexes were stable in isotonic NaCl, but stability of ⁴⁶Sc-TTHA, ⁴⁶Sc-BAPTA and ⁴⁶Sc-HBED in PBS buffer was low, due to formation by Sc³⁺stronger complexes with phosphates than with the studied ligands. From the radiolabelling studies with n.c.a. ⁴⁷Sc we can conclude that the most stable complexes are formed by the 8-dentate DTPA and EGTA ligands.