Alternative non-antibody protein scaffolds for molecular imaging of cancer

Abstract

The development of improved methods for early detection and characterization of cancer presents a major clinical challenge. One approach that has shown excellent potential in preclinical and clinical evaluation is molecular imaging with small-scaffold, non-antibody based, engineered proteins. These novel diagnostic agents produce high contrast images due to their fast clearance from the bloodstream and healthy tissues, can be evolved to bind a multitude of cancer biomarkers, and are easily functionalized by site-specific bioconjugation methods. Several small protein scaffolds have been verified for in vivo molecular imaging including affibodies and their two-helix variants, knottins, fibronectins, DARPins, and several natural ligands. Further, the biodistribution of these engineered ligands can be optimized through rational mutation of the conserved regions, careful selection and placement of chelator, and modification of molecular size.

Quantitative Impact of Plasma Clearance and Down-regulation on GLP-1 Receptor Molecular Imaging

“…Uptake of the dimer and trimer in the proximal tubule by scavenging receptors appears to be more efficient than the monomer and highlights a drawback of multimerization. Our results here are consistent with other reports[70–71] showing higher renal uptake for multimers, motivating the development of high affinity monovalent constructs[68, 72–73] rather than using avidity to compensate for low affinity. Although not statistically significant in all cases, the dimer and trimer generally have higher signal in other tissues as well.…”

Peptide Aptamers: Development and Applications

“…The first is derived from the cystine knot trypsin inhibitor from Ecballium elaterium , and the second, based on the agouti-related protein Knottin, exhibited corresponding affinities of <5 nM and 44 nM against cultured cancer cells [96, 231]. More applications of Adnectins and DARPins designs in molecular imaging are reviewed in [232]. …”

A 45-Amino-Acid Scaffold Mined from the PDB for High-Affinity Ligand Engineering

“…Antibodies have been successfully used for these applications in many cases, but their drawbacks have instigated a search for alternative protein scaffolds from which improved binding molecules can be developed (Banta et al, 2013; Stern et al, 2013). Biodistribution mechanisms such as extravasation (Schmidt and Wittrup, 2009; Yuan et al, 1995) and tissue penetration (Thurber et al, 2008a, 2008b) are limited by large size (150 kDa for immunoglobulin G, 50 kDa for antigen-binding fragments, and even 27 kDa for single-chain variable fragments) thereby reducing delivery to numerous locales including many solid tumors.…”

“…Protein scaffolds, frameworks upon which numerous functionalities can be independently engineered, offer a consistent source of binding reagents for the multitude of biomarkers and applications thereof (Banta et al, 2013; Sidhu, 2012; Stern et al, 2013). A successful protein scaffold should be efficiently evolvable to contain all of the following properties.…”