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Despite great progress in the diagnosis and treatment of localized prostate cancer (PCa), there remains a need for new diagnostic markers that can accurately distinguish indolent and aggressive variants. One promising approach is the antibody-based targeting of prostate stem cell antigen (PSCA), which is frequently overexpressed in PCa. Here, we show the construction of a molecular imaging probe comprising a humanized scFv fragment recognizing PSCA genetically fused to an engineered version of the human DNA repair enzyme O6-alkylguanine-DNA alkyltransferase (AGT), the SNAP-tag, enabling specific covalent coupling to various fluorophores for diagnosis of PCa. Furthermore, the recombinant immunotoxin (IT) PSCA(scFv)-ETA′ comprising the PSCA(scFv) and a truncated version of Pseudomonas exotoxin A (PE, ETA′) was generated. We analyzed the specific binding and internalization behavior of the molecular imaging probe PSCA(scFv)-SNAP in vitro by flow cytometry and live cell imaging, compared to the corresponding IT PSCA(scFv)-ETA′. The cytotoxic activity of PSCA(scFv)-ETA′ was tested using cell viability assays. Specific binding was confirmed on formalin-fixed paraffin-embedded tissue specimen of early and advanced PCa. Alexa Fluor® 647 labeling of PSCA(scFv)-SNAP confirmed selective binding to PSCA, leading to rapid internalization into the target cells. The recombinant IT PSCA(scFv)-ETA′ showed selective binding leading to internalization and efficient elimination of target cells. Our data demonstrate, for the first time, the specific binding, internalization, and cytotoxicity of a scFv-based fusion protein targeting PSCA. Immunohistochemical staining confirmed the specific ex vivo binding to primary PCa material.

Background Targeted imaging and therapy (theranostics) is a promising approach for the simultaneous improvement of cancer diagnosis, prognosis and management. Therapeutic and imaging reagents are coupled to tumor-targeting molecules such as antibodies, providing a basis for truly personalized medicine. However, the development of antibody-drug conjugates with acceptable pharmaceutical properties is a complex process and several parameters must be optimized, such as the controlled conjugation method and the drug-to-antibody ratio. Objective The major aim of this work is to address fundamental key challenges for the development of versatile technology platform for generating homogenous immunotheranostic reagent. Method We conjugated the theranostics reagent IRDye700dx to a recombinant antibody fusion protein containing a self-labeling protein (SNAP-tag) which provides a unique reaction site. Results The resulting conjugate was suitable for the imaging of cancer cells expressing the epidermal growth factor receptor and demonstrated potent phototherapeutic and imaging activities against them. Conclusion Here, we describe a simple, rapid and robust site-directed labeling method that can be used to generate homogeneous immunoconjugate with defined pharmacological properties.

Over the past few years, the SNAP-tag technology has become a methodology with great potential in a variety of applications, e.g. the (specific) visualization of individual proteins and studies of protein interaction in living cells. Furthermore, the tag can be used for immunopurification and detection of recombinant proteins or site-specific coupling of recombinant proteins to surfaces. Next to the in vitro applications, it also enables detection of tagged proteins in vivo. This review gives an overview of the SNAP-tag technology in different fields of research and its potential for future developments.

SNAP/CLIP-tag technology is a novel approach that allows tagged proteins to be covalently coupled to diverse labels, such as fluorochromes and particles, using a convenient and specific enzymatic reaction. A monoclonal antibody (mAb) that binds to the SNAP/CLIP-tag would be useful to determine labeling efficiency, and to achieve reproducible detection in a variety of experimental formats. We therefore generated the murine mAb M2D11 by standard immunization and hybridoma technology. M2D11 binds to both the SNAP- and the CLIP-tag in either the coupled or uncoupled configurations and can be detected in the context of ELISA, flow cytometry, immunohistochemistry and western blot. The new antibody increases the versatility of the SNAP-tag technology by enabling the detection of tagged proteins using conventional immunological methods and widely available secondary antibodies.

Antibody-drug conjugates (ADCs) combine the potency of cytotoxic drugs with the specificity of monoclonal antibodies (mAbs). Most ADCs are currently generated by the nonspecific conjugation of drug-linker reagents to certain amino acid residues in mAbs, resulting in a heterogeneous product. To overcome this limitation and prepare ADCs with a defined stoichiometry, we use SNAP-tag technology as an alternative conjugation strategy. This allows the site-specific conjugation of O(6)-benzylguanine (BG)-modified small molecules to SNAP-tag fusion proteins. To demonstrate the suitability of this system for the preparation of novel recombinant ADCs, here we conjugated SNAP-tagged single chain antibody fragments (scFvs) to a BG-modified version of auristatin F (AURIF). We used two scFv-SNAP fusion proteins targeting members of the epidermal growth factor receptor (EGFR) family that are frequently overexpressed in breast cancer. The conjugation of BG-AURIF to EGFR-specific 425(scFv)-SNAP and HER2-specific αHER2(scFv)-SNAP resulted in two potent recombinant ADCs that specifically killed breast cancer cell lines by inducing apoptosis when applied at nanomolar concentrations. These data confirm that SNAP-tag technology is a promising tool for the generation of novel recombinant ADCs.

Triple-negative breast cancer (TNBC) is associated with poor prognosis and high prevalence among young premenopausal women. Unlike in other breast cancer subtypes, no targeted therapy is currently available. Overexpression of epithelial cell adhesion molecule (EpCAM) in 60% of TNBC tumors correlates with poorer prognosis and is associated with cancer stem cell phenotype. Thus, selective elimination of EpCAM(+) TNBC tumor cells is of clinical importance.Therefore, we constructed a fully human targeted cytolytic fusion protein, designated GbR201K-alpha EpCAM(scFv), in which an EpCAM-selective single-chain antibody fragment (scFv) is genetically fused to a granzyme B (Gb) mutant with reduced sensitivity to its natural inhibitor serpin B9. In vitro studies confirmed its specific binding, internalization and cytotoxicity toward a panel of EpCAM-expressing TNBC cells. Biodistribution kinetics and tumor-targeting efficacy using MDA-MB-468 cells in a human TNBC xenograft model in mice revealed selective accumulation of GbR201K-aEpCAM(scFv) in the tumors after i.v. injection. Moreover, treatment of tumor-bearing mice demonstrated a prominent inhibition of tumor growth of up to 50 % in this proof-of-concept study. Taken together, our results indicate that GbR201K-alpha EpCAM(scFv) is a promising novel targeted therapeutic for the treatment of TNBC. (C) 2016 Elsevier Ireland Ltd. All rights reserved.

Chondroitin sulfate proteoglycan 4 (CSPG4) has been identified as a highly promising target antigen for immunotherapy of triple-negative breast cancer (TNBC). TNBC represents a highly aggressive heterogeneous group of tumors lacking expression of estrogen, progesterone and human epidermal growth factor receptor 2. TNBC is particularly prevalent among young premenopausal women. No suitable targeted therapies are currently available and therefore, novel agents for the targeted elimination of TNBC are urgently needed. Here, we present a novel cytolytic fusion protein (CFP), designated αCSPG4(scFv)-MAP, that consists of a high affinity CSPG4-specific single-chain antibody fragment (scFv) genetically fused to a functionally enhanced form of the human microtubule-associated protein (MAP) tau. Our data indicate that αCSPG4(scFv)-MAP efficiently targets CSPG4(+) TNBC-derived cell lines MDA-MB-231 and Hs 578T and potently inhibits their growth with IC50 values of ∼200 nM. Treatment with αCSPG(scFv)-MAP resulted in induction of the mitochondrial stress pathway by activation of caspase-9 as well as endonuclease G translocation to the nucleus, while induction of the caspase-3 apoptosis pathway was not detectable. Importantly, in vivo studies in mice bearing human breast cancer xenografts revealed efficient targeting to and accumulation of αCSPG4(scFv)-MAP at tumor sites resulting in prominent tumor regression. Taken together, this preclinical proof of concept study confirms the potential clinical value of αCSPG4(scFv)-MAP as a novel targeted approach for the elimination of CSPG4-positive TNBC.