Emerging site-specific bioconjugation strategies for radioimmunotracer development

ABSTRACT

Introduction: Radioimmunotracers are a promising class of companion diagnostics for precision medicine. They are composed of an antibody-based targeting agent and a radiolabeled imaging probe. Together with the tendency towards the use of small antibody-derived fragments, the employed conjugation method is gaining increasing attention. Conventional bioconjugation methods result in heterogeneous tracer populations of which the single elements can differ in immunoreactivity, pharmacokinetic behavior and stability. Site-specific conjugation strategies try to overcome these shortcomings and facilitate radioimmunotracer delivery, characterization and manufacturing.

Areas covered: An overview is provided of site-specific conjugation strategies for use in radioimmunotracer development. Currently applied strategies are discussed, together with other emerging site-specific conjugation methods that are applicable to diabodies, single-chain variable fragments (scFvs) and camelid single-domain antibody-fragments (sdAbs or nanobodies).

Expert opinion: The ultimate goal of site-specific bioconjugation strategies is to allow precise control over the conjugation site, to result in homogenous tracer populations, and to be versatile in use with different imaging probes. Chemoenzymatic methods appear to be promising in this respect.

KEYWORDS:

• Chemoenzymatic conjugation
• diabody
• molecular imaging
• nanobody
• scFv
• sdAb
• site-specific labeling

Article highlights

• Small Ab-derived fragments like sdAbs, scFvs and Dbs have beneficial characteristics for use as in vivo targeting agents in radioimmunotracer development.

• Conventional random bioconjugation to lysines or interchain disulfides leads to heterogeneity and Ag-binding interference.

• Defining the attachment site and obtaining homogeneous tracer populations are key goals of site-specific conjugation.

• Chemoenzymatic methods take the lead in current developments of site-specific conjugation methods.

• The site-specific installation of bioorthogonal functional groups in the targeting agent is a versatile approach that allows conjugation to a wide range of imaging probes.

This box summarizes key points contained in the article.

Declaration of interest

S. Massa is supported by a PhD fellowship of the Research Foundation – Flanders (FWO) and the Wetenschappelijk Fonds Willy Gepts of UZ Brussel. This work is supported by Research Foundation - Flanders (FWO) [grant number G066615N]. N. Devoogdt is co-founder and consultant of Camel-IDS nv. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

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