Development of novel bioconjugation strategies for creating cancer-targeting nanomaterials

Abstract

Nanomaterials used in therapy and in vivo imaging diagnostics have the potential to be highly specific through the use of monoclonal antibody targeting, and there are several promising antibodytargeted nanomaterial candidates being tested in clinical trials. Numerous bioconjugation approaches for the attachment of antibody and antibody fragments to the surface of nanoparticles have been investigated. nThe challenge is to create stable and reliable bio-nanomaterials, as the conjugation approaches are complex, require optimization, and have low conjugation efficiency. n Indeed, the development of an optimal and efficient method of bioconjugation is crucial for routinely producing hybrid bio-nanomaterials suitable for therapeutic and in vivo diagnostic applications. nIn this study, we focused on the development of bio-nanomaterial conjugates targeting prostate cancer as a model disease. For receptor targeting, we focused on prostate specific membrane antigen (PSMA), since this transmembrane protein is over-expressed in all prostate cancer stages. As such, PSMA is an ideal target in prostate cancer diagnosis and therapy. Therefore, in this investigation, we describe an active-targeting ligand for prostate cancer diagnosis and therapy, based on the anti-PSMA J591 antibody. n We have developed two novel methodologies for bioconjugation of the anti-PSMA antibody fragment to nanomaterials to create novel targeting bio-nanomaterials. Both methods were applied in novel applications and provide a simplistic and efficient way to create targeted nanomaterials for in vivo imaging and therapy. nIn the first method, we developed a bispecific antibody (BsAb) consisting of two antibody single-chain variable fragments (scFvs) linked together in a tandem scFv format by a simple glycineserine peptide linker. nOne scFv is specific for PSMA (J591 anti-PSMA antibody) and the other scFv is specific for methoxy polyethylene glycol (mPEG) epitopes, which are present on the surface of many PEG based nanomaterials, including the hyperbranched PEG polymer (HBP) used in this study. The BsAb was expressed in CHO cells, purified and characterized as a stable monomeric species based on chromatography and dynamic light scattering size analysis methods. nThe BsAb could specifically bind to recombinant PSMA and mPEG HBP targets as determined by ELISA, as well as to native PSMA expressed in cancer cell lines as determined by immunoblot assays. The simple, onestep mixing of BsAb with HBP resulted in the non-covalent formation of bio-nano conjugates through BsAb-mPEG binding, evidenced by a size shift of the mPEG complex as determined by dynamic light scattering. This study highlighted the simple and rapid BsAb conjugation strategy, demonstrating formation of BsAb-HBP conjugates which could target cancer cells effectively in in vitro assays. Furthermore, the BsAb-HBP complex, loaded with cytotoxic drugs showed an enhanced in vitro accumulation and cytotoxic effect compared to non-targeted HBP with drug attached.nnHowever, development of this hybrid bionanomaterial proved to be complicated, as it exhibited different biological behaviors between in vitro and in vivo assays. The enhanced active-targeting of BsAb-HBP complexes vs untargeted HBP to tumors in xenograft cancer models was not as pronounced as in vitro assays. The BsAb-HBP also demonstrated increased localization within reticuloendothelial system (RES) organs, such as spleen, lungs and liver, compared to the unconjugated HBP. Therefore, it would suggest the need for further optimizations of ligand density in this antibody nanomaterial hybrid. n The second method utilized chemoselective conjugation via the introduction of an azidebearing unnatural amino acid (UAA) into an anti-PSMA J591 scFv, expressed in a periplasmic E. coli production system. We have evaluated the incorporation of azide-bearing UAA into J591 scFv and investigated the ability to bind to dibenzocyclo-octyne (DBCO) containing fluorescent dye or polymer through copper-free click chemistry reactions. In vitro analyses showed that this azidemodified scFv can specifically bind and internalize into PSMA-overexpressing PC3-PIP prostate cancer cells both independently and while bound to DBCO containing fluorescent dye or polymer. In vivo studies demonstrated specific binding of azide-modified J591 scFv, conjugated to fluorescently labelled DBCO, in PSMA positive xenograft tumor with rapid renal clearance and shorter circulation time, which is more ideal for the application of imaging tracers. n nThe novel bioconjugation methodologies described here have contributed to the bionanomaterial research field, and have facilitated the creation of versatile antibody-nanoparticle conjugates, for the active targeting of bionanomaterials, as well as complex imaging contrast agents for in vivo diagnostic purposes. Furthermore, the utilizing of BsAbs presents a generic methodology for targeting drug-loaded nanomaterials to potentially any tumor-associated antigen. nCurrently, our group are utilizing this BsAbs for targeting HBP to other prostate cancer cell biomarkers, such as glypican-1 (GPC-1) antigen. Our group is also demonstrating the potential benefit of incorporating azide-bearing UAA into anti-PSMA J591 scFv for bioconjugation into larger structures such as a DBCO-containing thermo-responsive PEG-based polymer in micellar systems. The conjugation strategies developed here show much promise for simplifying the production of antibody nanomaterial hybrids, for use in targeted in vivo cancer diagnosis and for therapeutic applications, advancing the field of precision nanomedicine. nn