Tumor-Targeting Antibody–Anticalin Fusion Proteins for in Vivo Pretargeting Applications

Anticalin® proteins: from bench to bedside

INTRODUCTION

Anticalin proteins are engineered versions of lipocalins that constitute a novel class of clinical-stage biopharmaceuticals. The lipocalins exhibit a central β-barrel with eight antiparallel β-strands and an α-helix attached to its side. Four structurally variable loops at the open end of the β-barrel form a pronounced binding pocket, which can be reshaped to generate specificities toward diverse disease-relevant molecular targets.

AREAS COVERED

This article reviews the current status of Anticalin engineering, from the basic principles to the development of Anticalins with high target affinity and specificity via combinatorial protein design and directed evolution, including examples of Anticalin-based drug candidates under preclinical and clinical development.

EXPERT OPINION

Combinatorial gene libraries together with powerful molecular selection techniques have enabled the expansion of the natural ligand specificities of lipocalins from small molecules to peptides and proteins. This biomolecular concept has been validated by structural analyses of a series of Anticalin•target complexes. Promising Anticalin lead candidates have reached different preclinical and clinical development stages in the areas of (immuno)oncology, metabolic, and respiratory diseases, as antidotes to treat intoxications and as novel antibiotics. Thus, Anticalins offer an alternative to antibodies with promising and potentially superior features as next-generation biologics.

RPtag as an Orally Bioavailable, Hyperstable Epitope Tag and Generalizable Protein Binding Scaffold

Antibodies are the most prolific biologics in research and clinical environments because of their ability to bind targets with high affinity and specificity. However, antibodies also carry liabilities. A significant portion of the life-science reproducibility crisis is driven by inconsistent performance of research-grade antibodies, and clinical antibodies are often unstable and require costly cold-chain management to reach their destinations in active form. In biotechnology, antibodies are also limited by difficulty integrating them in many recombinant systems due to their size and structural complexity. A switch to small, stable, sequence-verified binding scaffolds may overcome these barriers. Here we present such a scaffold, RPtag, based on a ribose-binding protein (RBP) from extremophile Caldanaerobacter subterraneus. RPtag binds an optimized peptide with pM affinity, is stable to extreme temperature, pH, and protease treatment, readily refolds after denaturation, is effective in common laboratory applications, was rationally engineered to bind bioactive PDGF-β, and was formulated as a gut-stable orally bioavailable preparation.

Beyond Antibodies as Binding Partners: The Role of Antibody Mimetics in Bioanalysis

The emergence of novel binding proteins or antibody mimetics capable of binding to ligand analytes in a manner analogous to that of the antigen-antibody interaction has spurred increased interest in the biotechnology and bioanalytical communities. The goal is to produce antibody mimetics designed to outperform antibodies with regard to binding affinities, cellular and tumor penetration, large-scale production, and temperature and pH stability. The generation of antibody mimetics with tailored characteristics involves the identification of a naturally occurring protein scaffold as a template that binds to a desired ligand. This scaffold is then engineered to create a superior binder by first creating a library that is then subjected to a series of selection steps. Antibody mimetics have been successfully used in the development of binding assays for the detection of analytes in biological samples, as well as in separation methods, cancer therapy, targeted drug delivery, and in vivo imaging. This review describes recent advances in the field of antibody mimetics and their applications in bioanalytical chemistry, specifically in diagnostics and other analytical methods.

Addressing challenges of heterogeneous tumor treatment through bispecific protein-mediated pretargeted drug delivery

Tumors are frequently characterized by genomically and phenotypically distinct cancer cell subpopulations within the same tumor or between tumor lesions, a phenomenon termed tumor heterogeneity. These diverse cancer cell populations pose a major challenge to targeted delivery of diagnostic and/or therapeutic agents, as the conventional approach of conjugating individual ligands to nanoparticles is often unable to facilitate intracellular delivery to the full spectrum of cancer cells present in a given tumor lesion or patient. As a result, many cancers are only partially suppressed, leading to eventual tumor regrowth and/or the development of drug-resistant tumors. Pretargeting (multistep targeting) approaches involving the administration of 1) a cocktail of bispecific proteins that can collectively bind to the entirety of a mixed tumor population followed by 2) nanoparticles containing therapeutic and/or diagnostic agents that can bind to the bispecific proteins accumulated on the surface of target cells offer the potential to overcome many of the challenges associated with drug delivery to heterogeneous tumors. Despite its considerable success in improving the efficacy of radioimmunotherapy, the pretargeting strategy remains underexplored for a majority of nanoparticle therapeutic applications, especially for targeted delivery to heterogeneous tumors. In this review, we will present concepts in tumor heterogeneity, the shortcomings of conventional targeted systems, lessons learned from pretargeted radioimmunotherapy, and important considerations for harnessing the pretargeting strategy to improve nanoparticle delivery to heterogeneous tumors.

Challenges and opportunities for non-antibody scaffold drugs

The first candidates from the promising class of small non-antibody protein scaffolds are now moving into clinical development and practice. Challenges remain, and scaffolds will need to be further tailored toward applications where they provide real advantages over established therapeutics to succeed in a rapidly evolving drug development landscape.

Vectors for the delivery of radiopharmaceuticals in cancer therapeutics

Internal radiation using radiopharmaceuticals promises efficient cancer therapeutics. The specificity and selectivity required for screening and pinpointing tumor cells for cell-kill has been made possible by targeted ligands based on 'magic bullet' and tracer principle- theories nearing a century. Overexpression of certain receptors has been exploited using biomolecules for targeting. The pragmatic analysis, however, is not as promising compared with the theoretical knowledge of available gamut of vectors and targets. The complex interplay of in vitro and in vivo parameters, and the effect of radionuclides involve a systematic assessment of radiopharmaceuticals as diagnostic and therapeutic agent. This review presents different vectors with their pros and cons, present status and recent design variations followed by a future perspective based on novel approaches.

Evaluation of near infrared fluorescent labeling of monoclonal antibodies as a tool for tissue distribution

The pharmacokinetic (PK) behavior of monoclonal antibodies (mAbs) is influenced by target-mediated drug disposition, off-target effects, antidrug antibody-mediated clearance, and interaction with fragment-crystallizable domain (Fc) receptors such as neonatal Fc receptor. All of these interactions hold the potential to impact mAb biodistribution. Near infrared (NIR) fluorescent probes offer an approach complementary to radionuclides to characterize drug disposition. Notably, the use of FDA-approved IRDye800 (IR800; LI-COR, Lincoln, NE) as a protein-labeling agent in preclinical work holds the potential for quantitative tissue analysis. Here, we tested the utility of the IR800 dye as a quantitative mAb tracer during pharmacokinetic analysis in both plasma and tissues using a model mouse monoclonal IgG1 (8C2) labeled with ≤1.5 molecules of IR800. The plasma PK parameters derived from a mixture of IR800-8C2 and 8C2 dosed intravenously to C57BL/6 mice at 8 mg/kg exhibited a large discrepancy in exposure depending on the method of quantitation [CLplasma = 8.4 ml/d per kilogram (NIR fluorescence detection) versus 2.5 ml/d per kilogram (enzyme-linked immunosorbent assay)]. The disagreement between measurements suggests that the PK of 8C2 is altered by addition of the IR800 dye. Additionally, direct fluorescence analysis of homogenized tissues revealed several large differences in IR800-8C2 tissue uptake when compared with a previously published study using [(125)I]8C2, most notably an over 4-fold increase in liver concentration. Finally, the utility of IR800 in combination with whole body imaging was examined by comparison of IR800-8C2 levels observed in animal sagittal cross-sections to those measured in homogenized tissues. Our results represent the first PK analysis in both mouse plasma and tissues of an IR800-mAb conjugate and suggest that mAb disposition is significantly altered by IR800 conjugation to 8C2.

Discovery of hapten-specific scFv from a phage display library and applications for HER2-positive tumor imaging

In this study, an anti-hapten antibody (single chain Fv, scFv) against a hapten probe was developed as a unique reporter system for molecular imaging or therapy. The hapten peptide (histamine-succinyl-GSYK, Him) was synthesized for phage displayed scFv affinity selection and for conjugation with cypate (Cy-Him) for in vivo near-infrared (NIR) optical imaging. Hapten-specific scFvs were affinity selected from the human single fold phage display scFv libraries (Tomlinson I + J) with high specificity and affinity. Utilizing HER2 targeting as a model system, the highest affinity scFv (clone J42) was recombinantly fused to an anti-HER2 affibody (scFv-L-Aff) with no loss of affinity of either protein. The functionality of the hapten-scFv reporter system was tested in vitro with a HER2-positive human breast cancer cell line, SK-BR3, and in vivo with SK-BR3 xenografts. ScFv-L-Aff mediated the binding of the hapten to HER2 on SK-BR3 cells and from tissue from the SK-BR3 xenograft; however, scFv-L-Aff did not mediate uptake of the hapten in the SK-BR3 xenografted tumors, presumably due to rapid internalization of the HER2/scFv-L-Aff complex. Our results suggest that this hapten-peptide and anti-hapten scFv can be a universal reporter system in a wide range of imaging and therapeutic applications.