In recent years, 47 Sc has attracted attention because of its favorable decay characteristics (half-life, 3.35 d; average energy, 162 keV; Eγ, 159 keV) for therapeutic application and for SPECT imaging. The aim of the present study was to investigate the suitability of 47 Sc for radionuclide therapy in a preclinical setting. For this purpose a novel DOTA-folate conjugate (cm10) with an albumin-binding entity was used. Methods: 47 Sc was produced via the 46 Ca(n,γ) 47 Ca! b − 47 Sc nuclear reaction at the high-flux reactor at the Institut Laue-Langevin. Separation of the 47 Sc from the target material was performed by a semi-automated process using extraction chromatography and cation exchange chromatography. 47 Sc-labeled cm10 was tested on folate receptor-positive KB tumor cells in vitro. Biodistribution and SPECT imaging experiments were performed in KB tumor-bearing mice. Radionuclide therapy was conducted with two groups of mice, which received either 47 Sc-cm10 (10 MBq) or only saline. Tumor growth and survival time were compared between the two groups of mice. Results: Irradiation of 46 Ca resulted in approximately 1.8 GBq of 47 Ca, which subsequently decayed to 47 Sc. Separation of 47 Sc from 47 Ca was obtained with 80% yield in only 10 min. The 47 Sc was then available in a small volume (∼500 μL) of an ammonium acetate/HCl (pH 4.5) solution suitable for direct radiolabeling. 47 Sc-cm10 was prepared with a radiochemical yield of more than 96% at a specific activity of up to 13 MBq/nmol. In vitro 47 Sccm10 showed folate receptor-specific binding and uptake into KB tumor cells. In vivo SPECT/CT images allowed the visualization of accumulated radioactivity in KB tumors and in the kidneys. The therapy study showed a significantly delayed tumor growth in mice, which received 47 Sc-cm10 (10 MBq, 10 Gy) resulting in a more than 50% increase in survival time, compared with untreated control mice. Conclusion: With this study, we demonstrated the suitability of using 47 Sc for therapeutic purposes. On the basis of our recent results obtained with 44 Sc-folate, the present work confirms the applicability of 44 Sc/ 47 Sc as an excellent matched pair of nuclides for PET imaging and radionuclide therapy.

Tumor targeting with folic acid radioconjugates has been proposed as a promising strategy for radionuclide therapy of folate receptor α (FR)-positive cancer. Recently, it was shown that modification of radiofolates with an albumin-binding entity increased the tumor-to-kidney ratios of accumulated radioactivity in mice. The goal of this study was to evaluate the lead compound cm10 and compare it with new albumin-binding folate conjugates. Compound cm12 was designed with a long spacer consisting of a PEG-11 entity, and compound cm13 contained a short alkane chain between the albumin-binding moiety and folic acid. All of the derivatives were labeled with Lu (t = 6.65 days, E = 134 keV; E = 113 keV, 208 keV), a clinically established radionuclide for therapeutic purposes. The evaluation revealed that all of the albumin-binding radiofolates exhibited increased in vitro stability compared with the reference compound (Lu-cm14) without albumin binder. Serum protein binding, determined with an ultrafiltration assay, was high (>88%) for the derivatives with albumin-binding entities. The FR-binding affinity was in the same range (K = 4.0-7.5 nM) for all of the radiofolates, independent of the albumin-binding entity and spacer length. FR-specific uptake was proven in vitro using FR-positive KB tumor cells. In vivo studies with KB-tumor-bearing mice were performed in order to assess the tissue distribution profile of the novel radiofolates. Lu-cm13 showed high tumor uptake at late time points (13.3 ± 2.94% IA/g, 48 h p.i.) and tumor-to-kidney ratios (0.59 ± 0.03, 48 h p.i.) in the same range asLu-cm10 (0.55 ± 0.07, 48 h p.i.). However, the tumor-to-kidney ratio of Lu-cm12 (0.28 ± 0.07, 48 h p.i.) was reduced compared withLu-cm10 and Lu-cm13. The results of this study indicate that the spacer entity between folic acid and the albumin binder is of critical importance with regard to the tissue distribution profile of the radiofolate. The PEG spacer compromised the beneficial effects of the lead compound, but the design with a short alkane spacer appeared to be promising. Future studies will focus on the design of radiofolates with lipophilic and more rigid spacer entities, which may allow a further improvement of their tissue distribution profiles.

Compared to (177)Lu-cm09 we demonstrated equal imaging features for (161)Tb-cm09 but an increased therapeutic efficacy for (161)Tb-cm09 in both tumour cell lines in vitro and in vivo. Further preclinical studies using other tumour-targeting radioconjugates are clearly necessary to draw final conclusions about the future clinical perspectives of (161)Tb.

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BackgroundRadiotheragnostics makes use of the same molecular targeting vectors, labeled either with a diagnostic or therapeutic radionuclide, ideally of the same chemical element. The matched pair of scandium radionuclides, 44Sc and 47Sc, satisfies the desired physical aspects for PET imaging and radionuclide therapy, respectively. While the production and application of 44Sc was extensively studied, 47Sc is still in its infancy. The aim of the present study was, therefore, to investigate and compare two different methods of 47Sc production, based on the neutron irradiation of enriched 46Ca and 47Ti targets, respectively.Methods 47Sc was produced by thermal neutron irradiation of enriched 46Ca targets via the 46Ca(n,γ)47Ca → 47Sc nuclear reaction and by fast neutron irradiation of 47Ti targets via the 47Ti(n,p)47Sc nuclear reaction, respectively. The product was compared with regard to yield and radionuclidic purity. The chemical separation of 47Sc was optimized in order to obtain a product of sufficient quality determined by labeling experiments using DOTANOC. Finally, preclinical SPECT/CT experiments were performed in tumor-bearing mice and compared with the PET image of the 44Sc labeled counterpart.ResultsUp to 2 GBq 47Sc was produced by thermal neutron irradiation of enriched 46Ca targets. The optimized chemical isolation of 47Sc from the target material allowed formulation of up to 1.5 GBq 47Sc with high radionuclidic purity (>99.99%) in a small volume (~700 μL) useful for labeling purposes. Three consecutive separations were possible by isolating the in-grown 47Sc from the 46/47Ca-containing fraction. 47Sc produced by fast neutron irradiated 47Ti targets resulted in a reduced radionuclidic purity (99.95–88.5%). The chemical purity of the separated 47Sc was determined by radiolabeling experiments using DOTANOC achievable at specific activities of 10 MBq/nmol. In vivo the 47Sc-DOTANOC performed equal to 44Sc-DOTANOC as determined by nuclear imaging.ConclusionThe production of 47Sc via the 46Ca(n,γ)47Ca nuclear reaction demonstrated significant advantages over the 47Ti production route, as it provided higher quantities of a radionuclidically pure product. The subsequent decay of 47Ca enabled the repeated separation of the 47Sc daughter nuclide from the 47Ca parent nuclide. Based on the results obtained from this work, 47Sc shows potential to be produced in suitable quality for clinical application.Electronic supplementary materialThe online version of this article (doi:10.1186/s41181-017-0024-x) contains supplementary material, which is available to authorized users.

BackgroundThe radiolanthanide 161Tb has, in recent years, attracted increasing interest due to its favorable characteristics for medical application. 161Tb exhibits similar properties to the widely-used therapeutic radionuclide 177Lu. In contrast to 177Lu, 161Tb yields a significant number of short-ranging Auger/conversion electrons (≤50 keV) during its decay process. 161Tb has been shown to be more effective for tumor therapy than 177Lu if applied using the same activity. The purpose of this study was to investigate long-term damage to the kidneys after application of 161Tb-folate and compare it to the renal effects caused by 177Lu-folate.MethodsRenal side effects were investigated in nude mice after the application of different activities of 161Tb-folate (10, 20, and 30 MBq per mouse) over a period of 8 months. Renal function was monitored by the determination of 99mTc-DMSA uptake in the kidneys and by measuring blood urea nitrogen and creatinine levels in the plasma. Histopathological analysis was performed by scoring of the tissue damage observed in HE-stained kidney sections from euthanized mice.ResultsDue to the co-emitted Auger/conversion electrons, the mean absorbed renal dose of 161Tb-folate (3.0 Gy/MBq) was about 24 % higher than that of 177Lu-folate (2.3 Gy/MBq). After application of 161Tb-folate, kidney function was reduced in a dose- and time-dependent manner, as indicated by the decreased renal uptake of 99mTc-DMSA and the increased levels of blood urea nitrogen and creatinine. Similar results were obtained when 177Lu-folate was applied at the same activity. Histopathological investigations confirmed comparable renal cortical damage after application of the same activities of 161Tb-folate and 177Lu-folate. This was characterized by collapsed tubules and enlarged glomeruli with fibrin deposition in moderately injured kidneys and glomerulosclerosis in severely damaged kidneys.ConclusionsTb-folate induced dose-dependent radionephropathy over time, but did not result in more severe damage than 177Lu-folate when applied at the same activity. These data are an indication that Auger/conversion electrons do not exacerbate overall renal damage after application with 161Tb-folate as compared to 177Lu-folate, even though they result in an increased dose deposition in the renal tissue. Global toxicity affecting other tissues than kidneys remains to be investigated after 161Tb-based therapy, however.

Terbium-149 is among the most interesting therapeutic nuclides for medical applications. It decays by emission of short-range α-particles (Eα = 3.967 MeV) with a half-life of 4.12 h. The goal of this study was to investigate the anticancer efficacy of a 149Tb-labeled DOTA-folate conjugate (cm09) using folate receptor (FR)-positive cancer cells in vitro and in tumor-bearing mice. 149Tb was produced at the ISOLDE facility at CERN. Radiolabeling of cm09 with purified 149Tb resulted in a specific activity of ~1.2 MBq/nmol. In vitro assays performed with 149Tb-cm09 revealed a reduced KB cell viability in a FR-specific and activity concentration-dependent manner. Tumor-bearing mice were injected with saline only (group A) or with 149Tb-cm09 (group B: 2.2 MBq; group C: 3.0 MBq). A significant tumor growth delay was found in treated animals resulting in an increased average survival time of mice which received 149Tb-cm09 (B: 30.5 d; C: 43 d) compared to untreated controls (A: 21 d). Analysis of blood parameters revealed no signs of acute toxicity to the kidneys or liver in treated mice over the time of investigation. These results demonstrated the potential of folate-based α-radionuclide therapy in tumor-bearing mice.

The folate receptor (FR) is upregulated in various epithelial cancer types (FR α-isoform), while healthy tissues show only restricted expression. FR-targeted imaging using folate radiopharmaceuticals is therefore a promising approach for the detection of FR-positive cancer tissue. Almost all folate-based radiopharmaceuticals have been prepared by conjugation at the γ-carboxylic functionality of the glutamate moiety of folic acid. In this work, three pairs of fluorinated α- and γ-conjugated folate derivatives were synthesized and their in vitro and in vivo properties compared. The syntheses of all six regioisomers were obtained in good chemical yields using a multistep synthetic approach including the highly selective Cu(I)-catalyzed 1,3-dipolar cycloaddition. The radiosyntheses of the α- and γ-conjugated (18)F-labeled folate derivatives were accomplished in moderate to good radiochemical yields, high radiochemical purities (>95%), and specific activities ranging from 25 to 196 GBq/μmol. In vitro, all folate derivatives showed high binding affinity to the FR-α (IC50 = 1.4-2.2 nM). In vivo PET imaging and biodistribution studies in FR-positive KB tumor-bearing mice demonstrated similar FR-specific tumor uptake for both regioisomers of each pair of compounds. However, FR-unspecific liver uptake was significantly lower for the α-regioisomers compared to the corresponding γ-regioisomers. In contrast, kidney uptake was up to 50% lower for the γ-regioisomers than for the α-regioisomers. These results show that the site of conjugation in the glutamyl moiety of folic acid has a significant impact on the in vivo behavior of (18)F-based radiofolates, but not on their in vitro FR-binding affinity. These findings may potentially stimulate new directions for the design of novel (18)F-labeled folate-based radiotracers.