XTEN-Annexin A5: XTEN Allows Complete Expression of Long-Circulating Protein-Based Imaging Probes as Recombinant Alternative to PEGylation

Abstract

The coupling of polyethylene glycol (PEG) to proteins (PEGylation) has become a standard method to prolong blood circulation of imaging probes and other proteins, liposomes, and nanoparticles. However, concerns have arisen about the safety of PEG, especially with respect to its poor biodegradability and antibody formation, including new evidence about preformed anti-PEG antibodies in a quarter of healthy blood donors. Here, we apply a new hydrophilic polypeptide XTEN to extend the blood half-life of an imaging probe. As an example, we chose annexin A5 (AnxA5), a recombinant 35-kD protein extensively used for the in vitro and in vivo detection of apoptosis, that has a blood half-life of less than 7 min in mice, limiting its accumulation in target tissues and therefore limiting its utility as an imaging reagent. Methods: The sequence of XTEN was developed by Volker Schellenberger and colleagues by evolutionary in vitro optimization to yield PEG-like properties but provides several key advantages in comparison to PEG. The DNA of a 288-amino-acid version of XTEN with an additional Nterminal cysteine for site-directed coupling was fused to AnxA5 (XTENAnxA5). The fusion protein could be highly expressed in Escherichia coli and efficiently purified using XTEN conveniently as a purification tag. It was labeled with a thiol-reactive fluorescent dye and via a chelator with a radionuclide. Results: SPECT/CT imaging revealed a blood half-life of about 1 h in mice, markedly longer than the 7-min blood half-life for unmodified AnxA5, which should allow improved imaging of target tissues with low perfusion. In comparison to AnxA5, XTENAnxA5 demonstrated a substantially higher accumulation in tumors under chemotherapy in near-infrared fluorescence imaging. Conclusion: The presented method allows the expression and production of high amounts of long-circulating XTEN-AnxA5 without the necessity of PEGylation, thereby simplifying the synthesis while avoiding labelinginduced inactivation of AnxA5 and potential adverse effects of PEG. It is readily applicable to other recombinant protein or peptide-based imaging probes and allows fine-tuning of the desired blood half-life, because longer XTEN variants yield longer blood half-lives.

A Dual-Labeled Annexin A5 is not Suited for SPECT Imaging of Brain Cell Death in Experimental Murine Stroke

“…We recently developed a longcirculating version of AnxA5 with a blood half-life of B1 hour, which needs to be further investigated. 16 In addition, it has been described that continuous tracer infusion or step-down infusion compared with single-bolus infusion may enhance tracer delivery. [27][28][29] Also, intraventricular infusion of AnxA5 might be a possibility to overcome the BBB.…”

Anti-PEG antibodies in the clinic: Current issues and beyond PEGylation

“…Several XTEN-modified peptides and small proteins have been tested in pre-clinical or clinical studies, including glucagon, [86] glucagon-like peptide 2,[87] human growth hormone (hGH),[88] and annexin A5. [89]…”

Protein–polymer conjugation — moving beyond PEGylation

“…XTEN generated no substantial immune response, toxicity or kidney vacuoles in several animal models [77●●]. This technology is being commercialized by Amunix Operating Inc. and has been applied to a variety of therapeutic biomolecules [78-80] and an imaging agent [81]. Exendin-XTEN (VRS-859) is currently in a Phase I clinical trial for biweekly s.c. administration for treatment of type 2 diabetes and an XTEN fusion of human growth hormone (hGH-XTEN, VRS-317) is undergoing a Phase II clinical trial as monthly administration for treament of hGH deficiency [80].…”