[68Ga]Ga-THP-tetrazine for bioorthogonal click radiolabelling: pretargeted PET imaging of liposomal nanomedicines

Pretargeted PET imaging using bioorthogonal chemistry is a leading strategy for the tracking of long-circulating agents such as antibodies and nanoparticle-drug delivery systems with short-lived isotopes. Here, we report the synthesis, characterisation and in vitro/vivo evaluation of a new 68Ga-based radiotracer [68Ga]Ga-THP-Tetrazine ([68Ga]Ga-THP-Tz) for bioorthogonal click radiochemistry and in vivo labelling of agents with slow pharmacokinetics. THP-tetrazine (THP-Tz) can be radiolabelled to give [68/67Ga]Ga-THP-Tz at room temperature in less than 15 minutes with excellent radiochemical stability in vitro and in vivo. [68Ga]Ga-THP-Tz was tested in vitro and in vivo for pretargeted imaging of stealth PEGylated liposomes, chosen as a leading clinically-approved platform of nanoparticle-based drug delivery, and for their known long-circulating properties. To achieve this, PEGylated liposomes were functionalised with a synthesised transcyclooctene (TCO) modified phospholipid. Radiolabelling of TCO-PEG-liposomes with [68/67Ga]Ga-THP-Tz was demonstrated in vitro in human serum, and in vivo using both healthy mice and in a syngeneic cancer murine model (WEHI-164 fibrosarcoma). Interestingly in vivo data revealed that [68Ga]Ga-THP-Tz was able to in vivo radiolabel liposomes present in the liver and spleen, and not those in the blood pool or in the tumour. Overall, these results demonstrate the potential of [68Ga]Ga-THP-Tz for pretargeted imaging/therapy but also some unexpected limitations of this system.

Recent Advances in Bioorthogonal Click Chemistry for Enhanced PET and SPECT Radiochemistry

Due to their high reaction rate and reliable selectivity, bioorthogonal click reactions have been extensively investigated in numerous research fields, such as nanotechnology, drug delivery, molecular imaging, and targeted therapy. Previous reviews on bioorthogonal click chemistry for radiochemistry mainly focus on ¹⁸F-labeling protocols employed to produce radiotracers and radiopharmaceuticals. In fact, besides fluorine-18, other radionuclides such as gallium-68, iodine-125, and technetium-99m are also used in the field of bioorthogonal click chemistry. Herein, to provide a more comprehensive perspective, we provide a summary of recent advances in radiotracers prepared using bioorthogonal click reactions, including small molecules, peptides, proteins, antibodies, and nucleic acids as well as nanoparticles based on these radionuclides. The combination of pretargeting with imaging modalities or nanoparticles, as well as the clinical translations study, are also discussed to illustrate the effects and potential of bioorthogonal click chemistry for radiopharmaceuticals.

Towards Complete Tumor Resection: Novel Dual-Modality Probes for Improved Image-Guided Surgery of GRPR-Expressing Prostate Cancer

Nuclear and optical dual-modality probes can be of great assistance in prostate cancer localization, providing the means for both preoperative nuclear imaging and intraoperative surgical guidance. We developed a series of probes based on the backbone of the established GRPR-targeting radiotracer NeoB. The inverse electron demand of the Diels–Alder reaction was used to integrate the sulfo-cyanine 5 dye. Indium-111 radiolabeling, stability studies and a competition binding assay were carried out. Pilot biodistribution and imaging studies were performed in PC-3 tumor-bearing mice, using the best two dual-labeled probes. The dual-modality probes were radiolabeled with a high yield (>92%), were proven to be hydrophilic and demonstrated high stability in mouse serum (>94% intact labeled ligand at 4 h). The binding affinity for the GRPR was in the nanomolar range (21.9–118.7 nM). SPECT/CT images at 2 h p.i. clearly visualized the tumor xenograft and biodistribution studies, after scanning confirmed the high tumor uptake (8.47 ± 0.46%ID/g and 6.90 ± 0.81%ID/g for probe [111In]In-12 and [111In]In-15, respectively). Receptor specificity was illustrated with blocking studies, and co-localization of the radioactive and fluorescent signal was verified by ex vivo fluorescent imaging. Although optimal tumor-to-blood and tumor-to-kidney ratios might not yet have been reached due to the prolonged blood circulation, our probes are promising candidates for the preoperative and intraoperative visualization of GRPR-positive prostate cancer.

Targeting Infiltrating Myeloid Cells in Gastric Cancer Using a Pretargeted Imaging Strategy Based on Bio-Orthogonal Diels-Alder Click Chemistry and Comparison with 89Zr-Labeled Anti-CD11b Positron Emission Tomography Imaging

Gastric cancer (GC) is a common cancer worldwide, with high incidence and mortality rates. Therefore, early and precise diagnosis is critical to improving GC prognosis. Tumor-associated myeloid cells infiltrate the tumor microenvironment (TME) and can produce immunosuppressive effects in the early stage of the tumor. The surface integrin receptor CD11b is widely expressed in the specific subsets of myeloid cells, and it has the characteristics of high abundance, high specificity, and high potential for targeted immunotherapy. In this study, two strategies for labeling anti-CD11b, including ⁸⁹Zr-DFO-anti-CD11b and pretargeted imaging (⁶⁸Ga-NOTA-polypeptide-PEG₁₁-Tz/anti-CD11b-TCO), were used to evaluate the value of early diagnosis of GC and confirm the advantages of the pretargeted strategy for the diagnosis of GC. Pretargeted molecular probe ⁶⁸Ga-NOTA-polypeptide-PEG₁₁-Tz was synthesized. The binding affinity of the Tz-radioligand to CD11b was evaluated in vitro, and its blood pharmacokinetic test was performed in vivo. Moreover, the anti-CD11b antibody was conjugated with a p-isothiocyanatobenzyl-desferrioxamine (SCN-DFO) chelator and radiolabeled with zirconium-89. Biodistribution and positron-emission computed tomography imaging experiments were performed in MGC-803 tumor-bearing model mice to evaluate the value of the early diagnosis of GC. Histological evaluation of MGC-803 tumors was conducted to confirm the infiltration of the GC TME with CD11b⁺ myeloid cells. ⁶⁸Ga-NOTA-polypeptide-PEG₁₁-Tz was successfully radiosynthesized, with the radiochemical purity above 95%, as confirmed by reversed-phase high-performance liquid chromatography. The radioligand showed favorable stability in normal saline and phosphate-buffered saline, good affinity to RAW264.7 cells, and rapid blood clearance in mice. The results of biodistribution and imaging experiments using the pretargeted method showed that the tumor/muscle ratios were 5.17 ± 2.98, 5.94 ± 1.46, and 4.46 ± 2.73 at the pretargeting intervals of 24, 48, and 72 h, respectively. The experimental results using the method of the directly labeling antibody (⁸⁹Zr-DFO-anti-CD11b) showed that, despite radioactive accumulation in the tumor, there was a higher level of radioactive accumulation in normal tissues. The tumor/muscle ratios were 1.09 ± 0.67, 1.66 ± 0.95, 2.94 ± 1.24, 3.64 ± 1.21, and 3.55 ± 1.64 at 1, 24, 48, 72, and 120 h. The current research proved the value of ⁶⁸Ga-NOTA-polypeptide-PEG₁₁-Tz/anti-CD11b-TCO in the diagnosis of GC using the pretargeted strategy. Compared to ⁸⁹Zr-DFO-anti-CD11b, the image contrast achieved by the pretargeted strategy was relatively improved, and the background accumulation of the probe was relatively low. These advantages can improve the diagnostic efficiency for GC and provide supporting evidence for radioimmunotherapy targeting CD11b receptors.

Covalent Immobilization of Antibodies through Tetrazine-TCO Reaction to Improve Sensitivity of ELISA Technique

Enzyme-linked immunosorbent assay (ELISA) is routinely used to detect biomolecules related to several diseases facilitating diagnosis and monitoring of these, as well as the possibility of decreasing their mortality rate. Several methods have been carried out to improve the ELISA sensitivity through antibodies immobilization on the microtiter plates. Here, we have developed a strategy of antibodies immobilization to improve the ELISA sensitivity increasing the antibody density surface through the tetrazine (Tz)-trans-cyclooctene (TCO) reaction. For this, we prepared surfaces with tetrazine groups while the captured antibody was conjugated with TCO. The tetrazine surfaces were prepared in two different ways: (1) from aminated plates and (2) from Tz-BSA-coated plates. The surfaces were evaluated using two sandwich ELISA models, one of them using the low-affinity antibody anti-c-myc as a capture antibody to detect the c-myc-GST-IL8h recombinant protein, and the other one to detect the carcinoembryonic human protein (CEA). The sensitivity increased in both surfaces treated with tetrazine in comparison with the standard unmodified surface. The c-myc-GST-IL8h detection was around 10-fold more sensible on both tetrazine surfaces, while CEA ELISA detection increased 12-fold on surfaces coated with Tz-BSA. In conclusion, we show that it is possible to improve the ELISA sensitivity using this immobilization system, where capture antibodies bond covalently to surfaces.

Site-Specific and Residualizing Linker for 18F Labeling with Enhanced Renal Clearance: Application to an Anti-HER2 Single-Domain Antibody Fragment

Single-domain antibody fragments (sdAbs) are promising vectors for immuno-PET; however, better methods for labeling sdAbs with 18F are needed. Herein, we evaluate a site-specific strategy using an 18F residualizing motif and the anti-epidermal growth factor receptor 2 (HER2) sdAb 5F7 bearing an engineered C-terminal GGC tail (5F7GGC). Methods: 5F7GGC was site-specifically attached with a tetrazine-bearing agent via thiol-maleimide reaction. The resultant conjugate was labeled with 18F by inverse electron demand Diels-Alder cycloaddition with a trans-cyclooctene attached to 6-18F-fluoronicotinoyl moiety via a renal brush border enzyme-cleavable linker and a PEG4 chain (18F-5F7GGC). For comparisons, 5F7 sdAb was labeled using the prototypical residualizing agent, N-succinimidyl 3-(guanidinomethyl)-5-125I-iodobenzoate (iso-125I-SGMIB). The 2 labeled sdAbs were compared in paired-label studies performed in the HER2-expressing BT474M1 breast carcinoma cell line and athymic mice bearing BT474M1 subcutaneous xenografts. Small-animal PET/CT imaging after administration of 18F-5F7GGC in the above mouse model was also performed. Results:18F-5F7GGC was synthesized in an overall radiochemical yield of 8.9% ± 3.2% with retention of HER2 binding affinity and immunoreactivity. The total cell-associated and intracellular activity for 18F-5F7GGC was similar to that for coincubated iso-125I-SGMIB-5F7. Likewise, the uptake of 18F-5F7GGC in BT474M1 xenografts in mice was similar to that for iso-125I-SGMIB-5F7; however, 18F-5F7GGC exhibited significantly more rapid clearance from the kidney. Small-animal PET/CT imaging confirmed high uptake and retention in the tumor with very little background activity at 3 h except in the bladder. Conclusion: This site-specific and residualizing 18F-labeling strategy could facilitate clinical translation of 5F7 anti-HER2 sdAb as well as other sdAbs for immuno-PET.

A Pretargeted Imaging Strategy for EGFR-Positive Colorectal Carcinoma via Modulation of Tz-Radioligand Pharmacokinetics

PURPOSE

Previously, we successfully developed a pretargeted imaging strategy (atezolizumab-TCO/[^99mTc]HYNIC-PEG₁₁-Tz) for evaluating programmed cell death ligand-1 (PD-L1) expression in xenograft mice. However, the surplus unclicked [^99mTc]HYNIC-PEG₁₁-Tz is cleared somewhat sluggishly through the intestines, which is not ideal for colorectal cancer (CRC) imaging. To shift the excretion of the Tz-radioligand to the renal system, we developed a novel Tz-radioligand by adding a polypeptide linker between HYNIC and PEG₁₁.

PROCEDURES

Pretargeted molecular probes [^99mTc]HYNIC-polypeptide-PEG₁₁-Tz and cetuximab-TCO were synthesized. [^99mTc]HYNIC-polypeptide-PEG₁₁-Tz was evaluated for in vitro stability and in vivo blood pharmacokinetics. In vitro ligation reactivity of [^99mTc]HYNIC-polypeptide-PEG₁₁-Tz towards cetuximab-TCO was also tested. Biodistribution assay and imaging of [^99mTc]HYNIC-polypeptide-PEG₁₁-Tz were performed to observe its excretion pathway. Pretargeted biodistribution was measured at three different accumulation intervals to determine the optimal pretargeted interval time. Pretargeted (cetuximab-TCO 48 h/[^99mTc]HYNIC-PEG₁₁-Tz 6 h) and (cetuximab-TCO 48 h/[^99mTc]HYNIC-Polypeptide-PEG₁₁-Tz 6 h) imagings were compared to examine the effect of the excretion pathway on tumor imaging.

RESULTS

[^99mTc]HYNIC-polypeptide-PEG₁₁-Tz showed favorable in vitro stability and rapid blood clearance in mice. SEC-HPLC revealed almost complete reaction between cetuximab-TCO and [^99mTc]HYNIC-polypeptide-PEG₁₁-Tz in vitro, with the 8:1 Tz-to-mAb reaction providing a conversion yield of 87.83 ± 3.27 %. Biodistribution and imaging analyses showed that the Tz-radioligand was cleared through the kidneys. After 24, 48, and 72 h of accumulation in HCT116 tumor, the tumor-to-blood ratio of cetuximab-TCO was 0.83 ± 0.13, 1.40 ± 0.31, and 1.15 ± 0.21, respectively. Both pretargeted (cetuximab-TCO 48 h/[^99mTc]HYNIC-PEG₁₁-Tz 6 h) and (cetuximab-TCO 48 h/[^99mTc]HYNIC-polypeptide-PEG₁₁-Tz 6 h) clearly delineated HCT116 tumor. Pretargeted imaging strategy using cetuximab-TCO/[^99mTc]HYNIC-polypeptide-PEG₁₁-Tz could be used for diagnosing CRC, as the surplus unclicked [^99mTc]HYNIC-polypeptide-PEG₁₁-Tz was cleared through the urinary system, leading to low abdominal uptake background.

CONCLUSION

Our novel pretargeted imaging strategy (cetuximab-TCO/[^99mTc]HYNIC-polypeptide-PEG₁₁-Tz) was useful for imaging CRC, broadening the application scope of pretargeted imaging strategy. The pretargeted imaging strategy clearly delineated HCT116 tumor, showing that its use could be extended to selection of internalizing antibodies.

In Vivo Pretargeting Based on Cysteine-Selective Antibody Modification with IEDDA Bioorthogonal Handles for Click Chemistry

Pretargeted imaging has emerged as an effective multistep strategy aiming to improve imaging contrast and reduce patient radiation exposure through decoupling of the radioactivity from the targeting vector. The inverse electron-demand Diels-Alder (IEDDA) reaction between a trans-cyclooctene (TCO)-conjugated antibody and a labeled tetrazine holds great promise for pretargeted imaging applications due to its bioorthogonality, rapid kinetics under mild conditions, and formation of stable products. Herein, we describe the use of functionalized carbonylacrylic reagents for site-specific incorporation of TCO onto a human epidermal growth factor receptor 2 (HER2) antibody (THIOMAB) containing an engineered unpaired cysteine residue, generating homogeneous conjugates. Precise labeling of THIOMAB-TCO with a fluorescent or radiolabeled tetrazine revealed the potential of the TCO-functionalized antibody for imaging the HER2 after pretargeting in a cellular context in a HER2 positive breast cancer cell line. Control studies with MDA-MD-231 cells, which do not express HER2, further confirmed the target specificity of the modified antibody. THIOMAB-TCO was also evaluated in vivo after pretargeting and subsequent administration of an ¹¹¹In-labeled tetrazine. Biodistribution studies in breast cancer tumor-bearing mice showed a significant activity accumulation on HER2⁺ tumors, which was 2.6-fold higher than in HER2^- tumors. Additionally, biodistribution studies with THIOMAB without the TCO handle also resulted in a decreased uptake of ¹¹¹In-DOTA-Tz on HER2⁺ tumors. Altogether, these results clearly indicate the occurrence of the click reaction at the tumor site, i.e., pretargeting of SK-BR-3 HER2-expressing cells with THIOMAB-TCO and reaction through the TCO moiety present in the antibody. The combined advantages of site-selectivity and stability of TCO tagged-antibodies could allow application of biorthogonal chemistry strategies for pretargeting imaging with minimal side-reactions and background.

Recent Advances in Bioorthogonal Click Chemistry for Efficient Synthesis of Radiotracers and Radiopharmaceuticals

In recent years, several catalyst-free site-specific reactions have been investigated for the efficient conjugation of biomolecules, nanomaterials, and living cells. Representative functional group pairs for these reactions include the following: (1) azide and cyclooctyne for strain-promoted cycloaddition reaction, (2) tetrazine and trans-alkene for inverse-electron-demand-Diels–Alder reaction, and (3) electrophilic heterocycles and cysteine for rapid condensation/addition reaction. Due to their excellent specificities and high reaction rates, these conjugation methods have been utilized for the labeling of radioisotopes (e.g., radiohalogens, radiometals) to various target molecules. The radiolabeled products prepared by these methods have been applied to preclinical research, such as in vivo molecular imaging, pharmacokinetic studies, and radiation therapy of cancer cells. In this review, we explain the basics of these chemical reactions and introduce their recent applications in the field of radiopharmacy and chemical biology. In addition, we discuss the significance, current challenges, and prospects of using bioorthogonal conjugation reactions.

Pretargeted Nuclear Imaging and Radioimmunotherapy Based on the Inverse Electron-Demand Diels-Alder Reaction and Key Factors in the Pretargeted Synthetic Design

The exceptional speed and biorthogonality of the inverse electron-demand Diels-Alder (IEDDA) click chemistry between 1,2,4,5-tetrazines and strained alkene dienophiles have made it promising in the realm of pretargeted imaging and therapy. During the past 10 years, the IEDDA-pretargeted strategies have been tested and have already proven capable of producing images with high tumor-to-background ratios and improving therapeutic effect. This review will focus on recent applications of click chemistry ligations in the pretargeted imaging studies of single photon emission computed tomography (SPECT), positron emission tomography (PET), and pretargeted radioimmunotherapy investigations. Additionally, the influence factors of stability, reactivity, and pharmacokinetic properties of TCO tag modified immunoconjugates and radiolabeled Tz derivatives were also summarized in this article, which should be carefully considered in the system design in order to develop a successful pretargeted methodology. We hope that this review will not only equip readers with a knowledge of pretargeted methodology based on IEDDA click chemistry but also inspire synthetic chemists and radiochemists to develop pretargeted radiopharmaceutical components in a more innovative way with various influence factors considered.