Novel and Efficient Preparation of Precursor [188Re(OH2)3(CO)3]+ for the Labeling of Biomolecules

Bifunctional chelators for radiorhenium: past, present and future outlook

Targeted radionuclide therapy (TRNT) is an ever-expanding field of nuclear medicine that provides a personalised approach to cancer treatment while limiting toxicity to normal tissues. It involves the radiolabelling of a biological targeting vector with an appropriate therapeutic radionuclide, often facilitated by the use of a bifunctional chelator (BFC) to stably link the two entities. The radioisotopes of rhenium, 186Re (t 1/2 = 90 h, 1.07 MeV β-, 137 keV γ (9%)) and 188Re (t 1/2 = 16.9 h, 2.12 MeV β-, 155 keV γ (15%)), are particularly attractive for radiotherapy because of their convenient and high-abundance β--particle emissions as well as their imageable γ-emissions and chemical similarity to technetium. As a transition metal element with multiple oxidation states and coordination numbers accessible for complexation, there is great opportunity available when it comes to developing novel BFCs for rhenium. The purpose of this review is to provide a recap on some of the past successes and failings, as well as show some more current efforts in the design of BFCs for 186/188Re. Future use of these radionuclides for radiotherapy depends on their cost-effective availability and this will also be discussed. Finally, bioconjugation strategies for radiolabelling biomolecules with 186/188Re will be touched upon.

APPLICATION OF TECHNETIUM AND RHENIUM IN NUCLEAR MEDICINE

Technetium and Rhenium are the two lower elements in the manganese triad. Whereas rhenium is known as an important part of high resistance alloys, technetium is mostly known as a cumbersome product of nuclear fission. It is less known that its metastable isotope 99mTc is of utmost importance in nuclear medicine diagnosis. The technical application of elemental rhenium is currently complemented by investigations of its isotope 188Re , which could play a central role in the future for internal, targeted radiotherapy. This article will briefly describe the basic principles behind diagnostic methods with radionuclides for molecular imaging, review the 99mTc -based radiopharmaceuticals currently in clinical routine and focus on the chemical challenges and current developments towards improved, radiolabeled compounds for diagnosis and therapy in nuclear medicine.

A study of the radiosynthesis of fac-[¹⁸⁸ReCO₃(H₂O)₃]⁺ and its application in labeling 1,2,3-triazole analogs obtained by click chemistry

OBJECTIVES

To optimize the conditions for the preparation of the organometallic precursor fac-[¹⁸⁸ReCO₃(H₂O)₃]⁺ and to synthesize the radiolabeling compounds of tricarbonyl rhenium. 1,2,3-Triazole analogs were synthesized by click chemistry and labeled with fac-[ReCO₃(H₂O)₃]Br and fac-[¹⁸⁸ReCO₃(H₂O)₃]⁺. The aim was to improve the methods for the synthesis of ¹⁸⁸Re-labeled radiopharmaceuticals for therapy.

METHODS

With potassium boranocarbonate as the CO source and ammonia borane as the reducing agent, fac-[¹⁸⁸ReCO₃(H₂O)₃]⁺ was synthesized, and the click chemistry method was used to prepare the tricarbonyl rhenium complex.

RESULTS

At the optimal reaction condition (the amounts of K₂[H₃BCO₂] and BH₃·NH₃ are 5 and 5 mg, respectively; reaction temperature is 75°C; and reaction time is 15 min), the radiochemical yields were 90%, and the labeling yield of bis(pyridin-2-ylmethyl) amine with fac-[¹⁸⁸ReCO₃(H₂O)₃]⁺ was more than 99% in 1 h at 75°C; the conjugation yields of triazole analog obtained by click chemistry with 'cold' and 'radio' tricarbonyl rhenium were more than 80%.

CONCLUSION

The organometallic precursor fac-[¹⁸⁸ReCO₃(H₂O)₃]⁺ was prepared under optimal reaction conditions with a yield of 90%, and the triazole analogs synthesized by click chemistry were suitable ligands for tricarbonyl rhenium.

Preparation and evaluation of 186/188Re-labeled antibody (A7) for radioimmunotherapy with rhenium(I) tricarbonyl core as a chelate site

OBJECTIVE

Rhenium is one of the most valuable elements for internal radiotherapy because (186)Re and (188)Re have favorable physical characteristics. However, there are problems when proteins such as antibodies are used as carriers of (186/188)Re. Labeling methods that use bifunctional chelating agents such as MAG3 require the conjugation of the (186/188)Re complex to protein after radiolabeling with the bifunctional chelating agent. These processes are complicated. Therefore, we planned the preparation by a simple method and evaluation of a stable (186/188)Re-labeled antibody. For this purpose, we selected (186/188)Re(I) tricarbonyl complex as a chelating site. In this study, A7 (an IgG1 murine monoclonal antibody) was used as a model protein. (186/188)Re-labeled A7 was prepared by directly reacting a (186/188)Re(I) tricarbonyl precursor, [(186/188)Re(CO)(3)(H(2)O)(3)](+), with A7. We then compared the biodistribution of (186/188)Re-labeled A7 in tumor-bearing mice with (125)I-labeled A7.

METHODS

For labeling A7, [(186/188)Re(CO)(3)(H(2)O)(3)](+) was prepared according to a published procedure. (186/188)Re-labeled A7 ((186/188)Re-(CO)(3)-A7) was prepared by reacting [(186/188)Re(CO)(3)(H(2)O)(3)](+) with A7 at 43 degrees C for 2 h. Biodistribution experiments were performed by the intravenous administration of (186/188)Re-(CO)(3)-A7 solution into tumor-bearing mice.

RESULTS

(186)Re-(CO)(3)-A7 and (188)Re-(CO)(3)-A7 were prepared with radiochemical yields of 23 and 28%, respectively. After purification with a PD-10 column, (186/188)Re-(CO)(3)-A7 showed a radiochemical purity of over 95%. In biodistribution experiments, 13.1 and 13.2% of the injected dose/g of (186)Re-(CO)(3)-A7 and (188)Re-(CO)(3)-A7, respectively, accumulated in the tumor at 24-h postinjection, and the tumor-to-blood ratios were over 2.0 at the same time point. Meanwhile, uptake of (125)I-A7 in the tumor was almost the same as that of (186/188)Re-(CO)(3)-A7 at 24-h postinjection. Blood clearances of (186/188)Re-(CO)(3)-A7 were faster than those of (125)I-A7.

CONCLUSION

(186/188)Re-labeled A7 showed high uptakes in the tumor. However, further modification of the labeling method would be necessary to improve radiochemical yields and their biodistribution.

Carbohydrate-Appended 3-Hydroxy-4-pyridinone Complexes of the [M(CO)3]+ Core (M = Re, 99mTc, 186Re)

This work describes the use of 3-hydroxy-4-pyridinone ligands for binding the [M(CO)(3)](+) core (M = Re, Tc) in the context of preparing novel Tc(I) and Re(I) glucose conjugates. Five pyridinone ligands bearing pendent carbohydrate moieties, HL(1-5), were coordinated to the [M(CO)(3)](+) core on the macroscopic scale (M = Re) and on the tracer scale (M = (99m)Tc, (186)Re). On the macroscopic scale the complexes, ReL(1-5)(CO)(3)(H(2)O), were thoroughly characterized by mass spectrometry, IR spectroscopy, UV-visible spectroscopy, elemental analysis, and 1D/2D NMR spectroscopy. Characterization confirmed the bidentate coordination of the pyridinone and the pendent nature of the carbohydrate and suggests the presence of a water molecule in the sixth coordination site. In preliminary biological evaluation, both the ligands and complexes were assessed as potential substrates or inhibitors of hexokinase, but showed no activity. Labeling via the [(99m)Tc(CO)(3)(H(2)O)(3)](+) precursor gave the tracer species (99m)TcL(1-5)(CO)(3)(H(2)O) in high radiochemical yields. Similar high radiochemical yields when labeling with (186)Re were facilitated by in situ preparation of the [(186)Re(CO)(3)(H(2)O)(3)](+) species in the presence of HL(1-5) to give (186)ReL(1-5)(CO)(3)(H(2)O). Stability challenges, incubating (99m)TcL(1-5)(CO)(3)(H(2)O) in the presence of excess cysteine and histidine, confirmed complex stability up to 24 h.