Harnessing the Bioorthogonal Inverse Electron Demand Diels-Alder Cycloaddition for Pretargeted PET Imaging

Summary

The bioorthogonal inverse electron demand Diels-Alder cycloaddition has been harnessed to create an effective and modular pretargeted PET imaging strategy for cancer. In this protocol, the steps of this methodology are described in the context of a model system employing the colorectal cancer targeted antibody huA33 and a ⁶⁴Cu-labeled radioligand.

Abstract

Due to their exquisite affinity and specificity, antibodies have become extremely promising vectors for the delivery of radioisotopes to cancer cells for PET imaging. However, the necessity of labeling antibodies with radionuclides with long physical half-lives often results in high background radiation dose rates to non-target tissues. In order to circumvent this issue, we have employed a pretargeted PET imaging strategy based on the inverse electron demand Diels-Alder cycloaddition reaction. The methodology decouples the antibody from the radioactivity and thus exploits the positive characteristics of antibodies, while eschewing their pharmacokinetic drawbacks. The system is composed of four steps: (1) the injection of a mAb-trans-cyclooctene (TCO) conjugate; (2) a localization time period during which the antibody accumulates in the tumor and clears from the blood; (3) the injection of the radiolabeled tetrazine; and (4) the in vivo click ligation of the components followed by the clearance of excess radioligand. In the example presented in the work at hand, a ⁶⁴Cu-NOTA-labeled tetrazine radioligand and a trans-cyclooctene-conjugated humanized antibody (huA33) were successfully used to delineate SW1222 colorectal cancer tumors with high tumor-to-background contrast. Further, the pretargeting methodology produces high quality images at only a fraction of the radiation dose to non-target tissue created by radioimmunoconjugates directly labeled with ⁶⁴Cu or ⁸⁹Zr. Ultimately, the modularity of this protocol is one of its greatest assets, as the trans-cyclooctene moiety can be appended to any non-internalizing antibody, and the tetrazine can be attached to a wide variety of radioisotopes.

Introduction

Over the last thirty years, positron emission tomography (PET) has become an indispensable clinical tool in the diagnosis and management of cancer. Antibodies have long been considered promising vectors for the delivery of positron-emitting radioisotopes to tumors due to their exquisite affinity and specificity for cancer biomarkers.^1,2 However, the relatively slow in vivo pharmacokinetics of antibodies mandates the use of radioisotopes with multi-day physical half-lives. This combination can yield high radiation doses to the non-target organs of patients, an important complication that is of particular clinical significance since radioimmunoconjug....

Protocol

ETHICS STATEMENT: All of the in vivo animal experiments described were performed according to an approved protocol and under the ethical guidelines of the Memorial Sloan Kettering Cancer Center Institutional Animal Care and Use Committee (IACUC).

1. Synthesis of Tz-Bn-NOTA

1. In a small reaction vessel, dissolve 7 mg NH₂-Bn-NOTA (1.25 x 10^-2 mmol) in 600 µl NaHCO₃ buffer (0.1 M, pH 8.1). Check the pH of the solution. If needed, adjust the pH of the solution to 8.1 using small aliquots of 0.1 M Na₂CO₃.
2. Add the NH₂-Bn-NOTA solut....

Results

The initial three steps of the experiment — the synthesis of Tz-Bn-NOTA, the conjugation of TCO to huA33, and the radiolabeling of the Tz-Bn-NOTA construct (Figures 3 and 4) — are highly reliable. In the case of the procedure above, the Tz-Bn-NOTA construct was synthesized in high yield and purity. The huA33 antibody was modified with 4.2 ± 0.6 TCO/mAb, and Tz-Bn-NOTA was radiolabeled with ⁶⁴Cu to yield the purified radioligand in >99% radiochemical purity, >85% decay-co.......

Discussion

The principal advantage of this pretargeted PET imaging strategy is that it is capable of delineating tumors with target-to-background image contrast at only a fraction of the background radiation dose produced by directly labeled antibodies. For example, in the colorectal cancer imaging system described here, data from acute biodistribution experiments were employed to perform dosimetry calculations for the ⁶⁴Cu-based pretargeting strategy along with directly-labeled ⁶⁴Cu-NOTA-huA33 and ⁸⁹

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Tetrazine NHS Ester	Sigma-Aldrich	764701	Store at -80 °C
Trans-cyclooctene NHS Ester	Sigma-Aldrich	764523	Store at -80 °C
p-NH2-Bn-NOTA	Macrocyclics	B-601	Store at -80 °C