Preclinical evaluation of MC1R targeting theranostic pair [155Tb]Tb-crown-αMSH and [161Tb]Tb-crown-αMSH.

BACKGROUND: Targeted radionuclide therapy is established as a highly effective strategy for the treatment of metastatic tumors; however, the co-development of suitable imaging companions to therapy remains significant challenge. Theranostic isotopes of terbium (149Tb, 152Tb, 155Tb, 161Tb) have the potential to provide chemically identical radionuclidic pairs, which collectively encompass all modes of nuclear decay relevant to nuclear medicine. Herein, we report the first radiochemistry and preclinical studies involving 155Tb- and 161Tb-labeled crown-αMSH, a small peptide-based bioconjugate suitable for targeting melanoma. METHODS: 155Tb was produced via proton induced spallation of Ta targets using the isotope separation and acceleration facility at TRIUMF with isotope separation on-line (ISAC/ISOL). The radiolabeling characteristics of crown-αMSH with 155Tb and/or 161Tb were evaluated by concentration-dependence radiolabeling studies, and radio-HPLC stability studies. LogD7.4 measurements were obtained for [161Tb]Tb-crown-αMSH. Competitive binding assays were undertaken to determine the inhibition constant for [natTb]Tb-crown-αMSH in B16-F10 cells. Pre-clinical biodistribution and SPECT/CT imaging studies of 155Tb and 161Tb labeled crown-αMSH were undertaken in male C57Bl/6 J mice bearing B16-F10 melanoma tumors to evaluate tumor specific uptake and imaging potential for each radionuclide. RESULTS: Quantitative radiolabeling of crown-αMSH with [155Tb]Tb3+ and [161Tb]Tb3+ was demonstrated under mild conditions (RT, 10 min) and low chelator concentrations; achieving high molar activities (23-29 MBq/nmol). Radio-HPLC studies showed [161Tb]Tb-crown-αMSH maintains excellent radiochemical purity in human serum, while gradual metabolic degradation is observed in mouse serum. Competitive binding assays showed the high affinity of [natTb]Tb-crown-αMSH toward MC1R. Two different methods for preparation of the [155Tb]Tb-crown-αMSH radiotracer were investigated and the impacts on the biodistribution profile in tumor bearing mice is compared. Preclinical in vivo studies of 155Tb- and 161Tb- labeled crown-αMSH were performed in parallel, in mice bearing B16-F10 tumors; where the biodistribution results showed similar tumor specific uptake (6.06-7.44 %IA/g at 2 h pi) and very low uptake in nontarget organs. These results were further corroborated through a series of single-photon emission computed tomography (SPECT) studies, with [155Tb]Tb-crown-αMSH and [161Tb]Tb-crown-αMSH showing comparable uptake profiles and excellent image contrast. CONCLUSIONS: Collectively, our studies highlight the promising characteristics of [155Tb]Tb-crown-αMSH and [161Tb]Tb-crown-αMSH as theranostic pair for nuclear imaging (155Tb) and radionuclide therapy (161Tb).

Optimized production, purification, and radiolabeling of the 203Pb/212Pb theranostic pair for nuclear medicine.

TRIUMF is one of the only laboratories in the world able to produce both lead-203 (203Pb, t1/2 = 51.9 h) and 212Pb (t1/2 = 10.6 h) onsite via its 13 and 500 MeV cyclotrons, respectively. Together, 203Pb and 212Pb form an element-equivalent theranostic pair that potentiate image-guided, personalized cancer treatment, using 203Pb as a single-photon emission computed tomography (SPECT) source, and 212Pb for targeted alpha therapy. In this study, improvements to 203Pb production were accomplished by manufacturing electroplated, silver-backed thallium (Tl) targets to improve target thermal stability, which allow for higher currents during irradiation. We implemented a novel, two-column purification method that employs selective Tl precipitation (203Pb only) alongside extraction and anion exchange chromatography to elute high specific activity and chemical purity 203/212Pb in a minimal volume of dilute acid, without the need for evaporation. Optimization of the purification method translated to improvements in radiolabeling yields and apparent molar activity of lead chelators TCMC (S-2-(4-Isothiocyanatobenzyl)-1,4,7,10-tetraaza-1,4,7,10-tetra(2-carbamoylmethyl)cyclododecane) and Crypt-OH, a derivative of a [2.2.2]-cryptand.

Preclinical Evaluation of [155/161Tb]Tb-Crown-TATE-A Novel SPECT Imaging Theranostic Agent Targeting Neuroendocrine Tumours.

Terbium radioisotopes (149Tb, 152Tb, 155Tb, 161Tb) offer a unique class of radionuclides which encompass all four medicinally relevant nuclear decay modalities (α, ß+, γ, ß-/e-), and show high potential for the development of element-matched theranostic radiopharmaceuticals. The goal of this study was to design, synthesise, and evaluate the suitability of crown-TATE as a new peptide-conjugate for radiolabelling of [155Tb]Tb3+ and [161Tb]Tb3+, and to assess the imaging and pharmacokinetic properties of each radiotracer in tumour-bearing mice. [155Tb]Tb-crown-TATE and [161Tb]Tb-crown-TATE were prepared efficiently under mild conditions, and exhibited excellent stability in human serum (>99.5% RCP over 7 days). Longitudinal SPECT/CT images were acquired for 155Tb- and 161Tb- labelled crown-TATE in male NRG mice bearing AR42J tumours. The radiotracers, [155Tb]Tb-crown-TATE and [161Tb]Tb-crown-TATE, showed high tumour targeting (32.6 and 30.0 %ID/g, respectively) and minimal retention in non-target organs at 2.5 h post-administration. Biodistribution studies confirmed the SPECT/CT results, showing high tumour uptake (38.7 ± 8.0 %ID/g and 38.5 ± 3.5 %ID/g, respectively) and favourable tumour-to-background ratios. Blocking studies further confirmed SSTR2-specific tumour accumulation. Overall, these findings suggest that crown-TATE has great potential for element-matched molecular imaging and radionuclide therapy using 155Tb and 161Tb.

A simple and automated method for 161Tb purification and ICP-MS analysis of 161Tb.

BACKGROUND: 161Tb is a radiolanthanide with the potential to replace 177Lu in targeted radionuclide therapy. 161Tb is produced via the neutron irradiation of [160Gd]Gd2O3 targets, and must be purified from 160Gd and the decay product 161Dy prior to use. Established purification methods require complex conditions or high-pressure ion chromatography (HPIC) which are inconvenient to introduce in a broad user community. This study aims to find a simpler small solid-phase extraction (SPE) column method for 161Tb purification that is more suitable for automation with commercially available systems like TRASIS. RESULTS: We first tested the distribution coefficients on TK211 and TK212 resins for the separation of Gd, Tb, and Dy, and subsequently developed a method to separate these metal ions, with an additional TK221 resin to concentrate the final product. A side-by-side comparison of the products purified using this new method with the HPIC method was undertaken, assessing the radionuclidic purity, chemical purity regarding Gd and Dy, and labeling efficiency with a standard chelate (DOTA) and a novel chelate (crown). The two methods have comparable radionuclidic purity and labeling efficiency. The small SPE column method reduced Gd content to nanogram level, although still higher than the HPIC method. An ICP-MS method to quantify 161Tb, 159Tb, 160Gd, and 161Dy was developed with the application of mass-shift by ammonia gas. Last, 161Tb produced from the small SPE column method was used to assess the biodistribution of [161Tb]Tb-crown-αMSH, and the results were comparable to the HPIC produced 161Tb. CONCLUSIONS: 161Tb was successfully purified by a semi-automated TRASIS system using a combination of TrisKem extraction resins. The resulting product performed well in radiolabelling and in vivo experiments. However, improvement can be made in the form of further reduction of 160Gd target material in the final product. An ICP-MS method to analyze the radioactive product was developed. Combined with gamma spectroscopy, this method allows the purity of 161Tb being assessed before the decay of the product, providing a useful tool for quality control.