Bulk production and evaluation of high specific activity 186gRe for cancer therapy using enriched 186WO3 targets in a proton beam

Rhenium Radioisotopes for Medicine, a Focus on Production and Applications

In recent decades, the use of alpha; pure beta; or beta/gamma emitters in oncology, endocrinology, and interventional cardiology rheumatology, has proved to be an important alternative to the most common therapeutic regimens. Among radionuclides used for therapy in nuclear medicine, two rhenium radioisotopes are of particular relevance: rhenium-186 and rhenium-188. The first is routinely produced in nuclear reactors by direct neutron activation of rhenium-186 via 185Re(n,γ)186Re nuclear reaction. Rhenium-188 is produced by the decay of the parent tungsten-188. Separation of rhenium-188 is mainly performed using a chromatographic 188W/188Re generator in which tungsten-188 is adsorbed on the alumina column, similar to the 99Mo/99mTc generator system, and the radionuclide eluted in saline solution. The application of rhenium-186 and rhenium-188 depends on their specific activity. Rhenium-186 is produced in low specific activity and is mainly used for labeling particles or diphosphonates for bone pain palliation. Whereas, rhenium-188 of high specific activity can be used for labeling peptides or bioactive molecules. One of the advantages of rhenium is its chemical similarity with technetium. So, diagnostic technetium analogs labeled with radiorhenium can be developed for therapeutic applications. Clinical trials promoting the use of 186/188Re-radiopharmaceuticals is, in particular, are discussed.

The role of chemistry in accelerator-based production and separation of radionuclides as basis for radiolabelled compounds for medical applications

Radiochemical separations used in large scale routine production of diagnostic and therapeutic radionuclides at a particle accelerator for patient care are briefly outlined. The role of chemistry at various stages of development of a production route of a novel radionuclide, namely nuclear data measurement, high-current targetry, chemical processing and quality control of the product, is discussed in detail. Special attention is paid to production of non-standard positron emitters (e.g. 44gSc, 64Cu, 68Ga, etc.) at a cyclotron and novel therapeutic radionuclides (e.g. 67Cu, 225Ac, etc.) at an accelerator. Some typical examples of radiochemical methods involved are presented.

Cross sections of 3He-particle induced reactions on 186W

Irradiation of ¹⁸⁶W targets by ³He particles was carried out. For the first time the cross sections for the reactions of production of ^{183, 184, 186, 188}Re, ^{183, 185}Os, ¹⁸⁷W were measured by the stack foil technique in the ³He energy range of 15-45 MeV. The results were compared to the data from the TENDL-2019 library. Using the experimental excitation functions, the thick target yields of medically relevant rhenium radioisotopes were calculated.

A facile method for electrochemical separation of 181-186Re from proton irradiated natural tungsten oxide target

Routine availability of high specific activity ¹⁸⁶Re would provide a significant boost to the development of potent theranostic radiopharmaceuticals. In the present study, ^181-186Re was produced by proton bombardment (12 MeV, average beam intensity 180 nΑ) for 60 h on natural tungsten oxide target. A facile electrochemical method has been developed for radiochemical separation of Re radioisotopes from irradiated target material. The overall yield of the process was >80% and Re radioisotopes could be separated in a form suitable for preparation of radiopharmaceuticals.

Steric influence of salicylaldehyde-based Schiff base ligands on the formation of trans-[Re(PR3)2(Schiff base)]+ complexes

Complexes of the type trans-[Re(PR₃)₂(Schiff base)]⁺ (R = ethyl and/or phenyl) 2-7 were prepared by the reaction of (nBu₄N)[ReOCl₄] with H₂sal₂en or H₂sal₂ibn followed by addition of a tertiary phosphine. The trans-[Re(PR₃)₂(sal₂en)]⁺ complexes 2-4 were stable in solution, whereas the trans-[Re(PR₃)₂(sal₂ibn)]⁺ complexes 6-7 were observed to convert to their corresponding cis-[ReO(PR₃)(sal₂ibn)]⁺ products through a process involving ligand dissociation, metal oxidation, and Schiff base ligand rearrangement. The conversion of the trans-[Re(PR₃)₂(sal₂ibn)]⁺ complexes is likely driven by steric interactions between the bulky backbone gem-dimethyl groups of the sal₂ibn ligand and the phosphine ligands. These complexes were isolated and characterized by ¹H and ¹³C NMR, FT-IR spectroscopy, cyclic voltammetry, and single crystal X-ray diffraction. The results reported herein provide insight into the factors that drive trans-[Re(PR₃)₂(Schiff base)]⁺ complex formation. This will aid in the development of novel ^186/188Re therapeutic agents and the design of novel bifunctional N₂O₂ Schiff base ligands.

NOTA and NODAGA [99mTc]Tc- and [186Re]Re-Tricarbonyl Complexes: Radiochemistry and First Example of a [99mTc]Tc-NODAGA Somatostatin Receptor-Targeting Bioconjugate

With the long-term goal of developing theranostic agents for applications in nuclear medicine, in this work we evaluated the well-known NOTA and NODAGA chelators as bifunctional chelators (BFCs) for the [^99mTc/¹⁸⁶Re]Tc/Re-tricarbonyl core. In particular, we report model complexes of the general formula fac-[M(L)(CO)₃]⁺ (M = Re, ^99mTc, ¹⁸⁶Re) where L denotes NOTA-Pyr (1) or NODAGA-Pyr (2), which are derived from conjugation of NOTA/NODAGA with pyrrolidine (Pyr). Further, as proof-of-principle, we synthesized the peptide bioconjugate NODAGA-sst₂-ANT (3) and explored its complexation with the fac-[Re(CO)₃]⁺ and fac-[^99mTc][Tc(CO)₃]⁺ cores; sst₂-ANT denotes the somatostatin receptor (SSTR) antagonist 4-NO₂-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH₂. Rhenium complexes Re-1 through Re-3 were synthesized and characterized spectroscopically, and receptor binding affinity was demonstrated for Re-3 in SSTR-expressing cells (AR42J, IC₅₀ = 91 nM). Radiolabeled complexes [^99mTc]Tc/[¹⁸⁶Re]Re-1/2 and [^99mTc]Tc-3 were prepared in high radiochemical yield (>90%, determined by radio-HPLC) by reacting [^99mTc]/[¹⁸⁶Re][Tc/Re(OH₂)₃(CO)₃]⁺ with 1-3 and correlated well with the respective Re-1 through Re-3 standards in comparative HPLC studies. All radiotracers remained intact through 24 h (^99mTc-labeled complexes) or 48 h (¹⁸⁶Re-labeled complexes) against 1 mM l-histidine and 1 mM l-cysteine (pH 7.4, 37 °C). Similarly, rat serum stability studies displayed no decomposition and low nonspecific binding of 9-24% through 4 h. Biodistribution of [^99mTc]Tc-3 in healthy CF-1 mice demonstrated a favorable pharmacokinetic profile. Rapid clearance was observed within 1 h post-injection, predominantly via the renal system (82% of the injected dose was excreted in urine by 1 h), with low kidney retention (% ID/g: 11 at 1 h, 5 at 4 h, and 1 at 24 h) and low nonspecific uptake in other organs/tissues. Our findings establish NOTA and NODAGA as outstanding BFCs for the fac-[M(CO)₃]⁺ core in the design and development of organometallic radiopharmaceuticals. Future in vivo studies of [^99mTc]Tc- and [¹⁸⁶Re]Re-tricarbonyl complexes of NODAGA/NOTA-biomolecule conjugates will further probe the potential of these chelates for nuclear medicine applications in diagnostic imaging and targeted radiotherapy, respectively.

Technetium and Rhenium Schiff Base Compounds for Nuclear Medicine: Syntheses of Rhenium Analogues to 99mTc-Furifosmin

Rhenium, the third-row congener of technetium, is often used to develop the macroscopic chemistry of potential ^99mTc diagnostic radiopharmaceuticals. The rhenium analogues to ^99mTc-furifosmin are being developed for potential radiotherapy of multidrug-resistant tumors. Complexes of the form trans-[M^III(PR₃)₂(N₂O₂-Schiff base)]⁺ are of interest for the potential imaging and treatment of multidrug-resistant tumors. Reaction of the tetradentate Schiff ligand 4,4'-[(1 E,1' E)-[ethane-1,2-diylbis(azanylylidene)]bis(methanylylidene)]bis(2,2,5,5-tetramethyl-2,5-dihydrofuran-3-ol) (tmf₂enH₂) with the M(V) starting materials ( nBu₄N)[TcOCl₄] and ( nBu₄N)[ReOCl₄] gave the monomeric products trans-[TcOCl(tmf₂en)] and trans-[ReOCl(tmf₂en)], respectively. Reduction of in situ formed trans-[ReOCl(tmf₂en)] by various tertiary phosphines yielded disubstitued Re(III) products of the general type trans-[Re^III(PR₃)₂(tmf₂en)]⁺. The rhenium(III) compounds were found to be water-soluble and stable in aqueous solution. Reversible Re^III/Re^IV and Re^III/Re^II redox processes were observed at about 0.8-0.9 and -0.65 to -0.8 V, respectively, for each of the rhenium(III) species. Reaction of in situ formed trans-TcOCl(tmf₂en) with triethylphosphine yielded the reduced, disubstituted trans-[Tc(PEt₃)₂(tmf₂en)]PF₆. A reversible Tc^III/Tc^II redox couple was observed for the technetium(III) species, about 200 mV less negative than their rhenium(III) analogues, in addition to an irreversible Tc^III/Tc^IV process. All compounds were characterized using conventional spectroscopic techniques, single-crystal X-ray crystallography, and cyclic voltammetry.

Evaluation of N-Succinimidyl S-Acetylthioacetate Ligand for Radiolabeling of Humanized Antibodies with 188Rhenium

Radioimmunotherapy offers an effective way to direct ionizing radiation to cancer cells through attachment of radionuclides to antibodies while limiting negative effects of off-target irradiation. This, however, requires effective facile methods for attachment of therapeutic radionuclides onto antibodies. Herein, the authors report their efforts in evaluating N-succinimidyl S-acetylthioacetate (SATA), a commercially available reagent, for use as a bifunctional chelating agent (BCA) to attach ¹⁸⁸Rhenium (¹⁸⁸Re) onto h8C3, a humanized IgG antibody that can effectively target extracellular melanin present in malignant melanoma. Micro single photon emission computer tomography/computer tomography was used to determine an effective timeline for antibody uptake in B16-F10 tumor bearing C57BL6 mice guiding the selection of ¹⁸⁸Re with its 16.9 h physical half-life. Radio instant thin layer chromatography coupled with radio high-performance liquid chromatography was used to assess radioisotope incorporation, as well as stability during the labeling process for SATA conjugated h8C3. It was determined that despite the relatively mild conditions used, incorporation of the SATA conjugate resulted in antibody instability during labeling requiring a different BCA to facilitate rhenium incorporation onto the antibodies.

Development of novel radionuclides for medical applications

Medical radionuclide production technology is well established. There is, however, a constant need for further development of radionuclides. The present efforts are mainly devoted to nonstandard positron emitters (eg, ⁶⁴ Cu, ⁸⁶ Y, ¹²⁴ I, and ⁷³ Se) and novel therapeutic radionuclides emitting low-range β^- particles (eg, ⁶⁷ Cu and ¹⁸⁶ Re), conversion or Auger electrons (eg, ^117m Sn and ⁷⁷ Br), and α particles (eg, ²²⁵ Ac). A brief account of various aspects of development work (ie, nuclear data, targetry, chemical processing, and quality control) is given. For each radionuclide under consideration, the status of technology for clinical scale production is discussed. The increasing need of intermediate-energy multiple-particle accelerating cyclotrons is pointed out.

Scale-up of high specific activity 186gRe production using graphite-encased thick 186W targets and demonstration of an efficient target recycling process

Production of high specific activity 186gRe is of interest for development of theranostic radiopharmaceuticals. Previous studies have shown that high specific activity 186gRe can be obtained by cyclotron irradiation of enriched 186W via the 186W(d,2n)186gRe reaction, but most irradiations were conducted at low beam currents and for short durations. In this investigation, enriched 186W metal targets were irradiated at high incident deuteron beam currents to demonstrate production rates and contaminants produced when using thick targets. Full-stopping thick targets, as determined using SRIM, were prepared by uniaxial pressing of powdered natural abundance W metal or 96.86% enriched 186W metal encased between two layers of graphite flakes for target material stabilization. An assessment of structural integrity was made on each target preparation. To assess the performance of graphite-encased thick 186W metal targets, along with the impact of encasing on the separation chemistry, targets were first irradiated using a 22 MeV deuteron beam for 10 min at 10, 20, and 27 μA, with an estimated nominal deuteron energy of 18.7 MeV on the 186W target material (after energy degradation correction from top graphite layer). Gamma-ray spectrometry was performed post EOB on all targets to assess production yields and radionuclidic byproducts. The investigation also evaluated a method to recover and recycle enriched target material from a column isolation procedure. Material composition analyses of target materials, pass-through/wash solutions and recycling process isolates were conducted with SEM, FTIR, XRD, EDS and ICP-MS spectrometry. To demonstrate scaled-up production, a graphite-encased 186W target made from recycled 186W was irradiated for ~2 h with 18.7 MeV deuterons at a beam current of 27 μA to provide 0.90 GBq (24.3 mCi) of 186gRe, decay-corrected to the end of bombardment. ICP-MS analysis of the isolated 186gRe solution provided data that indicated the specific activity of 186gRe in this scaled-up production run was 2.6±0.5 GBq/μg (70±10 Ci/mg).

Chemistry and radiochemistry of As, Re and Rh isotopes relevant to radiopharmaceutical applications: high specific activity radionuclides for imaging and treatment

Advances in production, separation, target recovery, and chelation chemistry of high specific activity radionuclides will promote new theranostic agent development.