155Tb production by cyclotrons: what level of 155Gd enrichment allows clinical applications?

BACKGROUND: 155Tb represents a potentially useful radionuclide for diagnostic medical applications, but its production remains a challenging problem, in spite of the fact that many production routes have been already investigated and tested. A recent experimental campaign, conducted with low-energy proton beams impinging on a 155Gd target with 91.9% enrichment, demonstrated a significant co-production of 156gTb, a contaminant of great concern since its half-life is comparable to that of 155Tb and its high-energy γ emissions severely impact on the dose released and on the quality of the SPECT images. In the present investigation, the isotopic purity of the enriched 155Gd target necessary to minimize the co-production of contaminant radioisotopes, in particular 156gTb, was explored using various computational simulations. RESULTS: Starting from the recent experimental data obtained with a 91.9% 155Gd-enriched target, the co-production of other Tb radioisotopes besides 155Tb has been theoretically evaluated using the Talys code. It was found that 156Gd, with an isotopic content of 5.87%, was the principal contributor to the co-production of 156gTb. The analysis also demonstrated that the maximum amount of 156Gd admissible for 155Tb production with a radionuclidic purity higher than 99% was 1%. A less stringent condition was obtained through computational dosimetry analysis, suggesting that a 2% content of 156Gd in the target can be tolerated to limit the dose increase to the patient below the 10% limit. Moreover, it has been demonstrated that the imaging properties of the produced 155Tb are not severely affected by this level of impurity in the target. CONCLUSIONS: 155Tb can be produced with a quality suitable for medical applications using low-energy proton beams and 155Gd-enriched targets, if the 156Gd impurity content does not exceed 2%. Under these conditions, the dose increase due to the presence of contaminant radioisotopes remains below the 10% limit and good quality images, comparable to those of 111In, are guaranteed.

The innovative 52g Mn for positron emission tomography (PET) imaging: Production cross section modeling and dosimetric evaluation.

BACKGROUND: Manganese is a paramagnetic element suitable for magnetic resonance imaging (MRI) of neuronal function. However, high concentrations of Mn2 + can be neurotoxic. 52g Mn may be a valid alternative as positron emission tomography (PET) imaging agent, to obtain information similar to that delivered by MRI but using trace levels of Mn2 + , thus reducing its toxicity. Recently, the reaction n a t $^{nat}$ V(α,x)52g Mn has been proposed as a possible alternative to the standard n a t $^{nat}$ Cr(p,x)52g Mn one, but improvements in the modeling were needed to better compare the two production routes. PURPOSE: This work focuses on the development of precise simulations and models to compare the 52g Mn production from both reactions in terms of amount of activity and radionuclidic purity (RNP), as well as in terms of dose increase (DI) due to the co-produced radioactive contaminants, versus pure 52g MnCl2 . METHODS: The nuclear code Talys has been employed to optimize the n a t $^{nat}$ V(α,x)52g Mn cross section by tuning the parameters of the microscopic level densities. Thick-target yields have been calculated from the expression of the rates as energy convolution of cross sections and stopping powers, and finally integrating the time evolution of the relevant decay chains. Dosimetric assessments of [ x x $^{xx}$ Mn]Cl2 have been accomplished with OLINDA software 2.2.0 using female and male adult phantoms and biodistribution data for 52g MnCl2 in normal mice. At the end, the yield of x x $^{xx}$ Mn radioisotopes estimated for the two production routes have been combined with the dosimetric results, to assess the DI at different times after the end of the irradiation. RESULTS: Good agreement was obtained between cross-section calculations and measurements. The comparison of the two reaction channels suggests that n a t $^{nat}$ V(α,x)52g Mn leads to higher yield and higher purity, resulting in more favorable radiation dosimetry for patients. CONCLUSIONS: Both n a t $^{nat}$ V(α,x) and n a t $^{nat}$ Cr(p,x) production routes provide clinically acceptable 52g MnCl2 for PET imaging. However, the n a t $^{nat}$ V(α,x)52g Mn reaction provides a DI systematically lower than the one obtainable with n a t $^{nat}$ Cr(p,x)52g Mn and a longer time window in which it can be used clinically (RNP ≥ 99%).

Nuclear Cross-Section of Proton-Induced Reactions on Enriched 48Ti Targets for the Production of Theranostic 47Sc Radionuclide, 46cSc, 44mSc, 44gSc, 43Sc, and 48V.

The cross-sections of the 48Ti(p,x)47Sc, 46cSc, 44mSc, 44gSc, 43Sc, and 48V nuclear reactions were measured from 18 to 70 MeV, with particular attention to 47Sc production. Enriched 48Ti powder was deposited on an aluminum backing and the obtained targets were characterized via elastic backscattering spectroscopy at the INFN-LNL. Targets were exposed to low-intensity proton irradiation using the stacked-foils technique at the ARRONAX facility. Activated samples were measured using γ-spectrometry; the results were compared with the data int he literature and the theoretical TALYS-based values. A regular trend in the new values obtained from the different irradiation runs was noted, as well as a good agreement with the literature data, for all the radionuclides of interest: 47Sc, 46cSc, 44mSc, 44gSc, 43Sc, and 48V. 47Sc production was also discussed, considering yield and radionuclidic purity, for different 47Sc production scenarios.

Impact of Different [Tc(N)PNP]-Scaffolds on the Biological Properties of the Small cRGDfK Peptide: Synthesis, In Vitro and In Vivo Evaluations.

Background: The [99mTc][Tc(N)(PNP)] system, where PNP is a bisphosphinoamine, is an interesting platform for the development of tumor 'receptor-specific' agents. Here, we compared the reactivity and impact of three [Tc(N)(PNP)] frameworks on the stability, receptor targeting properties, biodistribution, and metabolism of the corresponding [99mTc][Tc(N)(PNP)]-tagged cRGDfK peptide to determine the best performing agent and to select the framework useful for the preparation of [99mTc][Tc(N)(PNP)]-housing molecular targeting agents. Methods: cRGDfK pentapeptide was conjugated to Cys and labeled with each [Tc(N)(PNP)] framework. Radioconjugates were assessed for their lipophilicity, stability, in vitro and in vivo targeting properties, and performance. Results: All compounds were equally synthetically accessible and easy to purify (RCY ≥ 95%). The main influences of the synthon on the targeting peptide were observed in in vitro cell binding and in vivo. Conclusions: The variation in the substituents on the phosphorus atoms of the PNP enables a fine tuning of the biological features of the radioconjugates. ws[99mTc][Tc(N)(PNP3OH)]- and [99mTc][Tc(N)(PNP3)]- are better performing synthons in terms of labeling efficiency and in vivo performance than the [99mTc][Tc(N)(PNP43)] framework and are therefore more suitable for further radiopharmaceutical purposes. Furthermore, the good labeling properties of the ws[99mTc][Tc(N)(PNP3OH)]- framework can be exploited to extend this technology to the labeling of temperature-sensitive biomolecules suitable for SPECT imaging.

225Ac-rHDL Nanoparticles: A Potential Agent for Targeted Alpha-Particle Therapy of Tumors Overexpressing SR-BI Proteins.

Actinium-225 and other alpha-particle-emitting radionuclides have shown high potential for cancer treatment. Reconstituted high-density lipoproteins (rHDL) specifically recognize the scavenger receptor B type I (SR-BI) overexpressed in several types of cancer cells. Furthermore, after rHDL-SR-BI recognition, the rHDL content is injected into the cell cytoplasm. This research aimed to prepare a targeted 225Ac-delivering nanosystem by encapsulating the radionuclide into rHDL nanoparticles. The synthesis of rHDL was performed in two steps using the microfluidic synthesis method for the subsequent encapsulation of 225Ac, previously complexed to a lipophilic molecule (225Ac-DOTA-benzene-p-SCN, CLog P = 3.42). The nanosystem (13 nm particle size) showed a radiochemical purity higher than 99% and stability in human serum. In vitro studies in HEP-G2 and PC-3 cancer cells (SR-BI positive) demonstrated that 225Ac was successfully internalized into the cytoplasm of cells, delivering high radiation doses to cell nuclei (107 Gy to PC-3 and 161 Gy to HEP-G2 nuclei at 24 h), resulting in a significant decrease in cell viability down to 3.22 ± 0.72% for the PC-3 and to 1.79 ± 0.23% for HEP-G2 at 192 h after 225Ac-rHDL treatment. After intratumoral 225Ac-rHDL administration in mice bearing HEP-G2 tumors, the biokinetic profile showed significant retention of radioactivity in the tumor masses (90.16 ± 2.52% of the injected activity), which generated ablative radiation doses (649 Gy/MBq). The results demonstrated adequate properties of rHDL as a stable carrier for selective deposition of 225Ac within cancer cells overexpressing SR-BI. The results obtained in this research justify further preclinical studies, designed to evaluate the therapeutic efficacy of the 225Ac-rHDL system for targeted alpha-particle therapy of tumors that overexpress the SR-BI receptor.

A feasibility study of the therapeutic application of a mixture of 67/64 Cu radioisotopes produced by cyclotrons with proton irradiation.

PURPOSE: 64 Cu and 67 Cu radioisotopes have nuclear characteristics suitable for nuclear medicine applications. The production of 64 Cu is already well established. However, the production of 67 Cu in quantities suitable to conduct clinical trials is more challenging as it leads to the coproduction of other Cu isotopes, in particular 64 Cu. The aim of this study is to investigate the possibility of using a CuCl2 solution with a mixture of 67/64 Cu radioisotopes for therapeutic purposes, providing an alternative solution for the cyclotron production problem. METHODS: Copper radioisotopes activities were calculated by considering proton beam irradiation of the following targets: (i) 70 Zn in the energy range 70-45 MeV; (ii) 68 Zn in the energy range 70-35 MeV; (iii) a combination of 70 Zn (70-55 MeV) and 68 Zn (55-35 MeV). The contribution of each copper radioisotope to the human-absorbed dose was estimated with OLINDA/EXM software using the biokinetic model for CuCl2 published by ICRP 53. The total absorbed dose generated by the 67/64 CuCl2 mixture, obtained through different production routes, was calculated at different times after the end of the bombardment (EOB). A simple spherical model was used to simulate tumors of different sizes containing uniformly distributed 67/64 Cu mixture and to calculate the absorbed dose of self-irradiation. The biological damage produced by 67 Cu and 64 Cu was also evaluated through cellular dosimetry and cell surviving fraction assessment using the MIRDcell code, considering two prostate cancer cell lines with different radiosensitivity. RESULTS: The absorbed dose to healthy organs and the effective dose (ED) per unit of administered activity of 67 CuCl2 are higher than those of 64 CuCl2 . Absorbed dose values per unit of administered activity of 67/64 CuCl2 mixture increase with time after the EOB because the amount of 67 Cu in the mixture increases. Survival data showed that the biological damage caused per each decay of 67 Cu is greater than that of 64 Cu, assuming that radionuclides remain accumulated in the cell cytoplasm. Sphere model calculations demonstrated that 64 Cu administered activity must be about five times higher than that of 67 Cu to obtain the same absorbed dose for tumor mass between 0.01 and 10 g and about 10 times higher for very small spheres. Consequently, the 64 CuCl2 -absorbed dose to healthy organs will reach higher values than those of 67 CuCl2 . The supplemental activity of the 67/64 CuCl2 mixture, required to get the same tumor-absorbed dose produced by 67 CuCl2 , triggers a dose increment (DI) in healthy organs. The waiting time post-EOB necessary to keep this DI below 10% (t10% ) depends on the irradiation methods employed for the production of the 67/64 CuCl2 mixture. CONCLUSIONS: A mixture of cyclotron produced 67/64 Cu radioisotopes proved to be an alternative solution for the therapeutic use of CuCl2 with minimal DI to healthy organs compared with pure 67 Cu. Irradiation of a 70 Zn+68 Zn target in the 70-35 MeV proton energy range for 185 h appears to be the best option from among all the production routes investigated, as it gives the maximum amount of activity, the shortest t10% (10 h), and less than 1% of 61 Cu and 60 Cu impurities.

Preclinical dosimetric studies of 177 Lu-scFvD2B and comparison with 177 Lu-PSMA-617 and 177 Lu-iPSMA endoradiotherapeutic agents.

PURPOSE: Internal dosimetry has become a very important tool to evaluate the risks and benefits of new endoradiotherapeutic agents. Nowadays, some of the most successful targeted radionuclide therapy (TRT) agents are 177 Lu-DOTA conjugates based on low molecular weight (LMW) Glu-ureido PSMA inhibitors. It has, however, been demonstrated that the DOTA chelating moiety reduces the internalization of the LMW-PSMA agent and its radiation dose to the tumor. Previously, we reported that 177 Lu-scFvD2B, an antibody-based construct, demonstrated statistically significant higher cell uptake and internalization in LNCaP prostate cancer (PCa) cells (PSMA-positive) when compared to the LMW-PSMA agents, 177 Lu-PSMA-617 and 177 Lu-iPSMA, two of the endoradiotherapeutic agents which currently are the most used in PCa therapy. The aim of this study is to estimate the preclinical 177 Lu-scFvD2B organ and tumor-absorbed doses, and to compare the values with those of 177 Lu-PSMA-617 and 177 Lu-iPSMA. METHODS: 177 Lu-scFvD2B, 177 Lu-PSMA-617, and 177 Lu-iPSMA were prepared and their radiochemical purity determined. Biodistribution studies of each radiopharmaceutical were then carried out in healthy mice to define the main source organs (SO) and to calculate the number of disintegrations in each source organs per unit of administered activity (NSO ). Absorbed dose in the main organs were then calculated for each 177 Lu-conjugate by means of OLINDA/EXM 2.1.1 software, using the calculated NSO for both the adult male and the mouse phantoms as program inputs. Images of mice bearing micropulmonary tumors injected with 177 Lu-conjugates were also obtained. Tumor standardized uptake values (SUV) for the different conjugates, obtained from the 3D SPECT image reconstruction of these mice, were used as the number of disintegrations in a tumor site per unit of administered activity (NT ). The tumor-absorbed dose was calculated using the published electron dose S-values for sphere models with diameters ranging from 10 µm to 10 mm and considering a uniform activity distribution and tumor density equivalent to water density. RESULTS: All 177 Lu-labeled agents were obtained in high yield (98%). Dosimetric studies carried out using mouse phantoms demonstrated that organ absorbed doses of 177 Lu-scFvD2B were from 1.4 to 2.3 times higher than those for 177 Lu-iPSMA and from 1.5 to 2.6 times higher than those for 177 Lu-PSMA-617. However, the 177 Lu-scFvD2B values of tumor-absorbed doses for all investigated tumor sizes were from 2.8 to 3.0 times greater than those calculated for 177 Lu-iPSMA and 177 Lu-PSMA-617, respectively. Moreover, 177 Lu-scFvD2B showed the highest tumor/kidney ratio when compared to those reported for 177 Lu-albumin conjugates. CONCLUSIONS: In this preclinical study, we demonstrated the potential of 177 Lu-scFvD2B as a therapeutic agent for PSMA-expressing tumors, due to its higher tumor-absorbed dose when compared with 177 Lu-LMW agents.

Development of 177Lu-scFvD2B as a Potential Immunotheranostic Agent for Tumors Overexpressing the Prostate Specific Membrane Antigen.

The clinical translation of theranostic 177Lu-radiopharmaceuticals based on inhibitors of the prostate-specific membrane antigen (PSMA) has demonstrated positive clinical responses in patients with advanced prostate cancer (PCa). However, challenges still remain, particularly regarding their pharmacokinetic and dosimetric properties. We developed a potential PSMA-immunotheranostic agent by conjugation of a single-chain variable fragment of the IgGD2B antibody (scFvD2B) to DOTA, to obtain a 177Lu-labelled agent with a better pharmacokinetic profile than those previously reported. The labelled conjugated 177Lu-scFvD2B was obtained in high yield and stability. In vitro, 177Lu-scFvD2B disclosed a higher binding and internalization in LNCaP (PSMA-positive) compared to PC3 (negative control) human PCa cells. In vivo studies in healthy nude mice revealed that 177Lu-scFvD2B present a favorable biokinetic profile, characterized by a rapid clearance from non-target tissues and minimal liver accumulation, but a slow wash-out from kidneys. Micro-SPECT/CT imaging of mice bearing pulmonary microtumors evidenced a slow uptake by LNCaP tumors, which steadily rose up to a maximum value of 3.6 SUV at 192 h. This high and prolonged tumor uptake suggests that 177Lu-scFvD2B has great potential in delivering ablative radiation doses to PSMA-expressing tumors, and warrants further studies to evaluate its preclinical therapeutic efficacy.

Radiation effective dose assessment of [51Mn]- and [52Mn]-chloride.

In order to establish the potential of [51/52Mn]Cl2 as safe PET brain tracers, the radiation effective dose (ED) of [51Mn]- and [52Mn]-chloride has been assessed by using biokinetic models in anthropomorphic phantoms. Results showed that [52Mn]-chloride releases one hundred thirty times more radiation dose (ED = 1.35 mSv/MBq) than [51Mn]-chloride (ED = 1.02E-02 mSv/MBq). Although the maximum positron energy of 52Mn allows a PET image resolution similar to that of 18F, activities below 15 MBq should be administered.

Internal radiation dose assessment of radiopharmaceuticals prepared with cyclotron-produced 99m Tc.

PURPOSE: Technetium-99m (99m Tc) is the radioisotope most widely used in diagnostic nuclear medicine. It is readily available from 99 Mo/99m Tc generators as the ß- decay product of the 99 Mo (T½  = 66 h) parent nuclide. This latter is obtained as a fission product in nuclear reactors by neutron-induced reactions on highly enriched uranium. Alternative production routes, such as direct reactions using proton beams on specific target materials [100 Mo(p,2n)99m Tc], have the potential to be both reliable and relatively cost-effective. However, results showed that the 99m Tc extracted from proton-bombarded 100 Mo-enriched targets contains small quantities of several Tc radioisotopes (93m Tc, 93 Tc, 94 Tc, 94m Tc, 95 Tc, 95m Tc, 96 Tc, and 97m Tc). The aim of this work was to estimate the dose increase (DI) due to the contribution of Tc radioisotopes generated as impurities, after the intravenous injection of four radiopharmaceuticals prepared with cyclotron-produced 99m Tc (CP-99m Tc) using 99.05% 100 Mo-enriched metallic targets. METHODS: Four 99m Tc radiopharmaceuticals (pertechnetate, sestamibi (MIBI), hexamethylpropylene-amine oxime (HMPAO) and disodium etidronate (HEDP)) were considered in this study. The biokinetic models reported by the International Commission on Radiological Protection (ICRP) for each radiopharmaceutical were used to define the main source organs and to calculate the number of disintegrations per MBq that occurred in each source organ (Nsource ) for each Tc radioisotope present in the CP-99m Tc solution. Then, target organ equivalent doses and effective dose were calculated for each Tc radioisotope with the OLINDA/EXM software versions 1.1 and 2.0, using the calculated Nsource values and the adult male phantom as program inputs. Total effective dose produced by all Tc isotopes impurities present in the CP-99m Tc solution was calculated using the fraction of total activity corresponding to each radioisotope and compared with the effective dose delivered by the generator-produced 99m Tc. RESULTS: In all cases, the total effective DI of CP-99m Tc radiopharmaceuticals calculated with either versions of the OLINDA software was less than 10% from 6 up to 12 h after EOB. 94m Tc and 93m Tc are the Tc radioisotopes with the highest concentration in the CP-99m Tc solution at EOB. However, their contribution to DI 6 h after EOB is minimal, due to their short half-lives. The radioisotopes with the largest contribution to the effective DI are 96 Tc, followed by 95 Tc and 94 Tc. This is due to the types of their emissions and relatively long half-lives, although their concentration in the CP-99m Tc solution is five times lower than that of 94m Tc and 93m Tc at the EOB. CONCLUSIONS: The increase in the radiation dose caused by other Tc radioisotopes contained in CP-99m Tc produced as described here is quite low. Even though the concentrations of the 94 Tc and 95 Tc radioisotopes in the CP-99m Tc solution exceed the limits established by the European Pharmacopoeia, CP-99m Tc radiopharmaceuticals could be used in routine nuclear medicine diagnostic studies if administered from 6 to 12 h after the EOB, thus maintaining the effective DI within the 10% limit.

A novel 10B-enriched carboranyl-containing phthalocyanine as a radio- and photo-sensitising agent for boron neutron capture therapy and photodynamic therapy of tumours: in vitro and in vivo studies.

The synthesis of a Zn(ii)-phthalocyanine derivative bearing four 10B-enriched o-carboranyl units (10B-ZnB4Pc) and its natural isotopic abundance analogue (ZnB4Pc) in the peripheral positions of the tetraazaisoindole macrocycle is presented. The photophysical properties of ZnB4Pc, as tested against model biological systems, were found to be similar with those typical of other photodynamically active porphyrin-type photosensitisers, including a singlet oxygen quantum yield of 0.67. The carboranyl-carrying phthalocyanine was efficiently accumulated by B16F1 melanotic melanoma cells in vitro, appeared to be partitioned in at least some subcellular organelles and, upon red light irradiation, induced extensive cell mortality. Moreover, ZnB4Pc, once i.v.-injected to C57BL/6 mice bearing a subcutaneously transplanted pigmented melanoma, photosensitised an important tumour response, provided that the irradiation at 600-700 nm was performed 3 h after the phthalocyanine administration, when appreciable concentrations of ZnB4Pc were still present in the serum. Analogously, irradiation of the 10B-ZnB4Pc-loaded pigmented melanoma with thermal neutrons 24 h after injection led to a 4 day delay in tumour growth as compared with control untreated mice. These results open the possibility to use one chemical compound as both a photosensitising and a radiosensitising agent for the treatment of tumours by the combined application of photodynamic therapy and boron neutron capture therapy.