Evaluation of the inverse electron demand Diels-Alder reaction in rats using a scandium-44-labelled tetrazine for pretargeted PET imaging

Abstract

Background Pretargeted imaging allows the use of short-lived radionuclides when imaging the accumulation of slow clearing targeting agents such as antibodies. The biotin-(strept)avidin and the bispecific antibody-hapten interactions have been applied in clinical pretargeting studies; unfortunately, these systems led to immunogenic responses in patients. The inverse electron demand Diels-Alder (IEDDA) reaction between a radiolabelled tetrazine (Tz) and a trans -cyclooctene (TCO)-functionalized targeting vector is a promising alternative for clinical pretargeted imaging due to its fast reaction kinetics. This strategy was first applied in nuclear medicine using an 111 In-labelled Tz to image TCO-functionalized antibodies in tumour-bearing mice. Since then, the IEDDA has been used extensively in pretargeted nuclear imaging and radiotherapy; however, these studies have only been performed in mice. Herein, we report the 44 Sc labelling of a Tz and evaluate it in pretargeted imaging in Wistar rats. Results 44 Sc was obtained from an in house 44 Ti/ 44 Sc generator. A 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-functionalized tetrazine was radiolabelled with 44 Sc resulting in radiochemical yields of 85–95%, a radiochemical purity > 99% at an apparent molar activity of 1 GBq/mmol. The 44 Sc-labelled Tz maintained stability in solution for up to 24 h. A TCO-functionalized bisphosphonate, which accumulates in skeletal tissue, was used as a targeting vector to evaluate the 44 Sc-labelled Tz. Biodistribution data of the 44 Sc-labelled Tz showed specific uptake (0.9 ± 0.3% ID/g) in the bones (humerus and femur) of rats pre-treated with the TCO-functionalized bisphosphonate. This uptake was not present in rats not receiving pre-treatment (< 0.03% ID/g). Conclusions We have prepared a 44 Sc-labelled Tz and used it in pretargeted PET imaging with rats treated with TCO-functionalized bisphosponates. This allowed for the evaluation of the IEDDA reaction in animals larger than a typical mouse. Non-target accumulation was low, and there was a 30-fold higher bone uptake in the pre-treated rats compared to the non-treated controls. Given its convenient half-life and the ability to perform positron emission tomography with a previously studied DOTA-functionalized Tz, scandium-44 ( t 1/2 = 3.97 h) proved to be a suitable radioisotope for this study. Electronic supplementary material The online version of this article (10.1186/s13550-019-0520-y) contains supplementary material, which is available to authorized users.

Evaluation of Organo [18F]Fluorosilicon Tetrazine as a Prosthetic Group for the Synthesis of PET Radiotracers

“…The MALDI-MS analysis indicates that 65% of the total 62 lysine residues were functionalized with a TCO in one albumin molecule. The IEDDA cycloaddition product of SiFA-Tz (6) and albumin-TCO (9), fluoroalbumin (10), was synthesized as a reference for the radiolabeling studies by mixing the two reagents together at room temperature. Bovine serum albumin (7) was functionalized with N-hydroxysuccinimide (NHS) ester of transcyclooctene (TCO) (200 eq), from the lysine residues available in the protein structure (Scheme 2).…”

Evaluation of [64Cu]Cu-NOTA-PEG7-H-Tz for Pretargeted Imaging in LS174T Xenografts—Comparison to [111In]In-DOTA-PEG11-BisPy-Tz

“…For example, we have recently used the same experimental setup to compare the performance of [ 111 In]In-DOTA-PEG 11 -BisPy-Tz ( 2 ) to a 68 Ga-labeled variant of the tracer [ 11 ]. In general, it is usually difficult to compare the performance of different tetrazine-based molecular imaging probes, because factors such as animal strain, tumor models, pretargeting vectors, and evaluation time points are varied [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. This is also the case for [ 64 Cu]Cu-NOTA-PEG 7 -H-Tz ( 1 ), another radiolabeled tetrazine, which is reported to be highly effective for pretargeted imaging, however, using a different TCO-conjugated pretargeting vector and mouse tumor model [ 3 ].…”

Evaluation of a 68Ga-Labeled DOTA-Tetrazine as a PET Alternative to 111In-SPECT Pretargeted Imaging

“…The indium labeled variant, [ 111 In]2, was used alongside a TCO modified CC49 mAb that targets the tumor associated glycoprotein 72 (TAG-72) antigen in human colorectal cancer cells to image tumors in mice [6]. Following this seminal work, radiolabeled variants of 1 have been used in combination with different TCO functionalized primary agents such as antibodies [6,[12][13][14]17,18], affibodies [19], diabodies [15], antibody drug conjugates [20] and small molecules [21,22]. These studies often incorporated long-lived radionuclides such as indium-111 [12][13][14][15]19,20], lutetium-177 (t 1/2 = 6.6 d) [12][13][14][15][18][19][20], and lead-212 (t 1/2 = 10.6 h) [17].…”

“…TCO functionalized alendronate (4) was selected as the first targeting vector ( Figure 2). It binds to exposed hydroxyapatite surfaces in bone [23] and has been used in pretargeting experiments to evaluate tetrazines bearing the short-lived radioisotopes, technetium-99m, (t 1/2 = 6 h) and scandium-44 (t 1/2 = 4 h) [21][22][23][24][25][26]. The second agent was based on the mAb CC49, which targets the TAG-72 antigen in human colorectal cancer cells.…”