Site-Specifically Labeled Immunoconjugates for Molecular Imaging--Part 2: Peptide Tags and Unnatural Amino Acids

Abstract

Molecular imaging using radioisotope- or fluorophore-labeled antibodies is increasingly becoming a critical component of modern precision medicine. Yet despite this promise, the vast majority of these immunoconjugates are synthesized via the random coupling of amine-reactive bifunctional probes to lysines within the antibody, a process that can result in heterogeneous and poorly defined constructs with suboptimal pharmacological properties. In an effort to circumvent these issues, the last 5 years have played witness to a great deal of research focused on the creation of effective strategies for the site-specific attachment of payloads to antibodies. These chemoselective modification methods yield immunoconjugates that are more homogenous and better defined than constructs created using traditional synthetic approaches. Moreover, site-specifically labeled immunoconjugates have also been shown to exhibit superior in vivo behavior compared to their randomly modified cousins. The over-arching goal of this two-part review is to provide a broad yet detailed account of the various site-specific bioconjugation approaches that have been used to create immunoconjugates for positron emission tomography (PET), single photon emission computed tomography (SPECT), and fluorescence imaging. In Part 1, we covered site-specific bioconjugation techniques based on the modification of cysteine residues and the chemoenzymatic manipulation of glycans. In Part 2, we will detail two families of bioconjugation approaches that leverage biochemical tools to achieve site-specificity. First, we will discuss modification methods that employ peptide tags either as sites for enzyme-catalyzed ligations or as radiometal coordination architectures. And second, we will examine bioconjugation strategies predicated on the incorporation of unnatural or non-canonical amino acids into antibodies via genetic engineering. Finally, we will compare the advantages and disadvantages of the modification strategies covered in both parts of the review and offer a brief discussion of the overall direction of the field.

Keywords: Antibody fragment; Bioconjugation; Bioorthogonal chemistry; Click chemistry; Fluorescence imaging; Glycoengineering; Immunoglobulins; Near-infrared fluorescence imaging; Non-canonical amino acids; Optical imaging; PET; Peptide tags; Positron emission tomography; Protein engineering, antibody; SPECT; Single photon emission tomography; Site-selective conjugation; Site-specific conjugation; Unnatural amino acids.

Fig. 1

Detailed structural schematic of a full length IgG as well as an assortment of antibody fragments.

Fig. 2

The basic chemical reactions underpinning the bioconjugation strategies discussed in this work.

Fig. 3

Selected chelators and cargoes used in the site-specifically labeled immunoconjugates discussed in this work.

Fig. 4

Site-specific bioconjugation strategies based on the use of peptide tags.

Fig. 5

Schematics of transglutaminase-based strategies for the site-specific modification of a deglycosylated antibodies, b aglycosylated antibodies, and c immunoglobulins bearing an LLQG transglutaminase recognition sequence.

Fig. 6

Small animal PET/CT images collected 60 min after the injection of a site-specifically labeled scFv_anti-LIBS-LPET-[⁶⁴Cu]L² (left) and a control fragment (scFv_mut-LPET-[⁶⁴Cu]L²; right) into a mouse model of carotid artery thrombosis. Figure adapted and reprinted with permission of [28].

Fig. 7

Diagram of the DNA_E intein-based site-specific modification procedure developed by Möhlmann, et al. Figure adapted and reprinted with permission of [26].

Fig. 8

Structures of selected unnatural and non-canonical amino acids.

Fig. 9

The advantages and limitations of selected approaches to site-specific bioconjugation.