Bioorthogonal Reactions in Radionuclide Conjugates

Bioorthogonal reactions are a series of chemical reactions that occur in biological environments with high yields, selectivity, efficiency, and no side reactions. These reactions include copper-catalyzed alkyne-azide cycloaddition (CuAAC), strain-promoted alkyne-azide cycloaddition, inverse electron demand Diels-Alder (IEDDA) reaction, and more. These bioorthogonal reactions find extensive applications in various fields, including radionuclide conjugate, metabolic engineering, drug target identification, medicinal chemistry, and more. With continuous advancements in radiochemistry, bioorthogonal reactions have played an increasingly significant role in the development of radionuclide conjugates.

CuAAC in Radionuclide Conjugates

The first reported application of CuAAC in the field of radionuclide conjugates is illustrated in Figure 1. Initially, [¹⁸F]alkynes are prepared using [¹⁸F]KFK222, followed by their coupling with azide-functionalized peptides using CuAAC, resulting in peptide-radionuclide conjugates.

Figure 1. The first reported CuAAC in radionuclide conjugates. (Marik, 2006)

RGD peptide (Arg-Gly-Asp) is a cell adhesion sequence for integrins such as α_vβ₁, α_vβ₃, α_vβ₅. In 2020, Zhang's group used CuAAC to incorporate [¹⁸F] fragments into cyclic RGD peptide (c-RGD), creating a near-infrared probe, ¹⁸F-NIR-cRGD. By optimizing the probe structure, they found that it effectively tracked the distribution of U-87 MG glioblastoma xenografts in mice, demonstrating its potential for PET and intraoperative NIRF imaging tools (Figure 2).

Figure 2. Preparation of RGD peptide-radionuclide conjugates. (Zhang, 2020)

Another important application of CuAAC is enhancing the molar activity of radiolabeling. Molar activity refers to the radioactivity per mole of the radionuclide conjugate. Low molar activity can affect targeted uptake and signal-to-noise ratio. In 2017, Millward et al. developed a method to increase molar activity. Excess alkynylated peptides were coupled with [¹⁸F]azides, and a carrier modified with an azide ligand was introduced to remove unconjugated peptides, thereby increasing the purity and molar activity of the radionuclide conjugate (Figure 3).

Figure 3. CuAAC enhances the molar activity of radionuclide conjugate. (Zhang, 2020)

Beyond ¹⁸F, radiometal ions can be conjugated to radionuclides using a click-to-chelate strategy after CuAAC. The resulting triazoles and ligand cavities created by heteroatoms in the molecule form stable radionuclide conjugates. This strategy is effective for developing ^99mTc and ^186/188Re radionuclides. Kim et al. utilized this method to prepare ^99mTc-labeled cMBP radionuclide conjugates and investigated the effect of linker length on conjugate stability in plasma and coordination environments with coordinating amino acids (His, Cys).

Radionuclide conjugates prepared using CuAAC have found successful clinical applications. In 2016, [¹⁸F]FET-βAG-TOCA was first used in clinical trials for neuroendocrine tumor patients. Recently, [⁶⁸Ga]Ga-Trivehexin, synthesized via CuAAC, was used for PET-CT imaging in patients with pancreatic ductal adenocarcinoma (Figure 4).

Figure 4. [⁶⁸Ga]Ga-Trivehexin

SPAAC in Radionuclide Conjugates

In CuAACs, the copper catalyst itself competes with radiometal ions for ligand binding, and copper residues can introduce biological toxicity. Hence, researchers prefer copper-free SPAAC reactions. In 2011, Campbell-Verduyn et al. reported the first application of SPAAC in radionuclide conjugates, preparing a series of ¹⁸F-labeled peptides targeting GRPR. SPAAC reactions were also used to label azide-functionalized nanoparticles (SCK-NP) resulting in [⁶⁴Cu]Cu-DOTA-SCK-NP with high molar activity (~36 TBq/μmol) (Figure 5).

Figure 5. Preparation of radionuclide conjugate by SPAAC. (Zeng, 2012)

SPAAC has become crucial in antibody-radionuclide conjugate preparation, often requiring genetically engineered antibodies with non-natural amino acids modified for SPAAC with DBCO ligands. As shown in Figure 6, a non-natural amino acid NEAK with a terminal azide group was introduced into the monoclonal antibody through genetic engineering. Subsequently, it underwent a SPAAC with the DBCO-DOTA fragment, and finally, DOTA coordinated with ⁶⁴Cu to prepare the antibody-radionuclide conjugate. Additionally, enzyme-catalyzed conjugation methods have been reported to avoid the complexity of engineered antibody production. This involves introducing azide-modified sugars into the antibody's glycan chain under enzyme-catalyzed conditions, followed by a similar approach to generate antibody-radionuclide conjugates. Recently, Sarrett et al. reported a site-specific conjugation method based on lysine, which avoids both genetically engineered antibodies and the complexity of enzyme-catalyzed methods, resulting in the preparation of ⁸⁹Zr antibody-radionuclide conjugates.

Figure 6. Preparation of antibody-radionuclide conjugates by SPAAC. (Bauer, 2023)

IEDDA in Radionuclide Conjugates

When preparing radionuclide conjugates, it's important to consider the radioactive decay of the radionuclide. Therefore, the IEDDA reaction based on tetrazine (Tz) and trans-cyclooctene (TCO) with the fastest reaction rate (k > 105 M^-1 s^-1) offers significant advantages. For instance, ¹⁸F-labeled nanobody-deoxyglucose conjugates can be rapidly synthesized using this strategy. Additionally, researchers have developed a pre-targeting technique for in vivo imaging. In this approach, TCO-modified antibodies are initially injected. The modified small molecule part of the antibody binds to tumor cells without being internalized. After a certain period, radioactive Tz fragments are injected to bind to the antibody on the tumor cell surface via the IEDDA reaction, completing the labeling process. This distribution method allows precise delivery of the radioactive nuclide and reduces the decay of the radioactive nuclide before detection (Figure 7).

Figure 7. Pre-targeting technique. (Bauer, 2023)