Single-domain antibodies: Next-generation targeting vectors for molecular imaging

Single-domain antibodies (sdAbs) provide significant benefits over conventional antibodies and fragments thereof in terms of size, stability, solubility as well as tumour uptake and blood clearance. Thus, sdAbs have been identified as valuable next-generation targeting moieties for molecular imaging and drug delivery in the past years. Since these probes are much less complex than conventional antibody fragments, bacterial expression represents a facile method for production of sdAbs in large amounts as soluble and functional proteins. Herein we report on heterologous high-yield expression of substantial amounts of soluble and functional sdAbs, which have an antagonistic effect on their molecular target, the epidermal growth factor receptor (EGFR). Upon radiolabeling with Tc-99m using the tricarbonyl method, we evaluated binding specificity and affinity to human EGFR-expressing tumor cells. Furthermore, we describe bioconjugation of sdAbs to fluorescent nanoparticles and characterization of sdAb-nanoparticles conjugates covering in vitro cancer cell imaging, cell proliferation as well as EGFR phosphorylation and signaling. The herein highlighted valuable properties of radiolabeled sdAbs combined with tailored effector functions may result in innovative next-generation theranostics.