Site-specific labeling of cysteine-tagged camelid single-domain antibody-fragments for use in molecular imaging.

TitleSite-specific labeling of cysteine-tagged camelid single-domain antibody-fragments for use in molecular imaging.
Publication TypeJournal Article
Year of Publication2014
AuthorsMassa, S., C. Xavier, J. De Vos, V. Caveliers, T. Lahoutte, S. Muyldermans, and N. Devoogdt
JournalBioconjug Chem
Volume25
Issue5
Pagination979-88
Date Published2014 May 21
ISSN1520-4812
Abstract

Site-specific labeling of molecular imaging probes allows the development of a homogeneous tracer population. The resulting batch-to-batch reproducible pharmacokinetic and pharmacodynamic properties are of great importance for clinical translation. Camelid single-domain antibody-fragments (sdAbs)-the recombinantly produced antigen-binding domains of heavy-chain antibodies, also called Nanobodies-are proficient probes for molecular imaging. To safeguard their intrinsically high binding specificity and affinity and to ensure the tracer's homogeneity, we developed a generic strategy for the site-specific labeling of sdAbs via a thio-ether bond. The unpaired cysteine was introduced at the carboxyl-terminal end of the sdAb to eliminate the risk of antigen binding interference. The spontaneous dimerization and capping of the unpaired cysteine required a reduction step prior to conjugation. This was optimized with the mild reducing agent 2-mercaptoethylamine in order to preserve the domain's stability. As a proof-of-concept the reduced probe was subsequently conjugated to maleimide-DTPA, for labeling with indium-111. A single conjugated tracer was obtained and confirmed via mass spectrometry. The specificity and affinity of the new sdAb-based imaging probe was validated in a mouse xenograft tumor model using a modified clinical lead compound targeting the human epidermal growth factor receptor 2 (HER2) cancer biomarker. These data provide a versatile and standardized strategy for the site-specific labeling of sdAbs. The conjugation to the unpaired cysteine results in the production of a homogeneous group of tracers and is a multimodal alternative to the technetium-99m labeling of sdAbs.

DOI10.1021/bc500111t
Alternate JournalBioconjug. Chem.
PubMed ID24815083
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