(18)F-Based Pretargeted PET Imaging Based on Bioorthogonal Diels-Alder Click Chemistry

Abstract

A first-of-its-kind (18)F pretargeted PET imaging approach based on the bioorthogonal inverse electron demand Diels-Alder (IEDDA) reaction between tetrazine (Tz) and trans-cyclooctene (TCO) is presented. As proof-of-principle, a TCO-bearing immunoconjugate of the anti-CA19.9 antibody 5B1 and an Al[(18)F]NOTA-labeled tetrazine radioligand were harnessed for the visualization of CA19.9-expressing BxPC3 pancreatic cancer xenografts. Biodistribution and (18)F-PET imaging data clearly demonstrate that this methodology effectively delineates tumor mass with activity concentrations up to 6.4 %ID/g at 4 h after injection of the radioligand.

Scheme 1. Radiochemical Synthesis of the Radioligand Tz-PEG₁₁-Al[¹⁸F]-NOTA ([¹⁸F]2)

[¹⁸F]2 was obtained in 54–56% RCY (d.c.) and high SAs (21.4–26.7 GBq/μmol) after a total synthesis time of 108 min. Purification of the crude reaction mixture using a C18-cartridge gave [¹⁸F]2 in purities >96%.

Figure 1

Biodistribution of the radioligand [¹⁸F]2 in healthy athymic nude mice. The ¹⁸F-labeled tracer (1.33 nmol, 1.8–2.0 MBq) was injected via the tail vein before the mice were euthanized, and the organs collected at the appropriate time points.

Figure 2

Results of the biodistribution pretargeting CA19.9 with [¹⁸F]2/5B1-TCO. Subcutaneous BxPC3 xenograft bearing mice were administered 5B1-TCO (1.33 nmol) 72 h prior to the injection of the ¹⁸F-labeled tracer (1.33 nmol, 1.8–2.0 MBq) via the tail vein before the mice were euthanized, and the organs collected at the appropriate time points.

Figure 3

PET images of Tz-PEG₁₁-Al[¹⁸F]-NOTA/5B1-TCO pretargeting strategy. Subcutaneous BxPC3 xenograft bearing mice were administered 5B1-TCO (1.33 nmol) 72 h prior to the injection of the ¹⁸F-labeled tracer (1.33 nmol, 18–20 MBq) via the tail vein. Transverse (top) and coronal (middle) planar images intersect the center of the tumors. The maximum intensity projections (MIPs, bottom) clearly illustrate tumor uptake after 1 h with increasing tumor-to-background ratios over the course of the experiment.