(640f) Peg-Conjugated Extracellular Mega-Hemoglobin for Diverse Oxygen Therapeutic Applications

Motivation: The newest generation of hemoglobin-based oxygen carriers (HBOCs) has found generic applications as oxygen therapeutics apart from their traditional use as resuscitative fluids. Recently, HBOCs have been used in organ and graft preservation, ex-vivo machine perfusion, cell growth media supplements, hypoxic tumor models, and wound healing. These diverse applications have necessitated the development of oxygen therapeutics that can be used for multiple applications. Steering away from traditional mammalian hemoglobins, this work focuses on the mega-hemoglobin (LtEc) of the annelid Lumbricus terrestris for use as an oxygen therapeutic. Its large size (~30 nm) reduces the possibility of in vivo extravasation and scavenging of endothelial-derived nitric oxide (NO). The large molecular diameter of LtEc along with its’ high structural stability, low rate of auto-oxidation, moderate colloid osmotic pressure (COP), and human blood-like oxygen binding properties make it an ideal candidate for use as a resuscitative fluid in trauma treatment. Subsequently, LtEc is also a promising candidate for use in graft preservation and as an oxygen supplement in cell culture media because of its ability to stay in the reduced ferrous form for long periods of time. However, to mask the exterior protein surface and reduce potential immunogenic activity, surface conjugation is necessary. In this study, we conjugated polyethylene glycol (PEG) to the surface of LtEc to camouflage it and performed comprehensive in vitro characterization and preliminary in vivo studies to investigate the potential of PEG modified-LtEc as an oxygen therapeutic.

Methods: Purification of LtEc from Lumbricus terrestris worms was performed by modifying previously devised protocols from our lab with a focus on improving the purity of the resulting product. In short, Canadian nightcrawlers (Lumbricus terrestris) were rinsed to remove dirt and mucus prior to homogenization in a modified Tris buffer. The homogenate was centrifuged via multiple cycles to eliminate worm debris and dirt and obtain a cloudy red supernatant. Post vacuum filtration, multi-step tangential flow filtration through 0.65 µm and 0.22 µm hollow fiber filters was performed to eliminate large aggregates and bacterial components. Continuous volume diafiltrations were carried out over a 500 kDa filter to eliminate a majority of the unwanted impurities. Retentate and permeate samples were analyzed using analytical size exclusion chromatography (SEC) to track loss of proteins and the purification was concluded when the retentate reached >99% purity. In vitro biophysical characterization included dynamic light scattering (DLS), oxygen equilibrium analysis, rapid deoxygenation kinetics, gel electrophoresis (SDS-PAGE), and auto-oxidation kinetics. Surface modification of purified LtEc was carried out using thiol-maleimide chemistry by converting surface lysine residues to thiols and attaching PEG chains with monofunctional ends. Unreacted components were removed, and the conjugated products was analyzed using amino acid residue assays, DLS, SEC, and SDS-PAGE to confirm surface modification. Oxygen equilibrium properties and other in vitro properties were tested and compared to unmodified material. The degree of surface camouflage was engineered by exploiting the numerous lysine residues on the LtEc surface and desirable in vitro properties were obtained.

Results: The modified LtEc purification protocol was successful in obtaining an LtEc product with >99% purity, which was verified using SEC and SDS-PAGE analyses. This was an important result as the removal of non-mammalian proteins and impurities is crucial for a pharmaceutical product to be used in transfusion medicine. Biophysical characterization was performed to determine advantageous biophysical properties of LtEc such as its’ large hydrodynamic diameter, moderate oxygen affinity, high oxygen cooperativity, low auto-oxidation rate, and fast deoxygenation kinetics. In vivo analysis in a golden Syrian hamster model corroborated that the ultra-pure product performed better in animals than product purified using older protocols with significantly better flow in arterioles and higher functional capillary density. Various degrees of surface modification with PEG resulted in PEG-LtEc products with varying biophysical properties. Even though the hydrodynamic size varied little, we observed changes in molecular masses of subunits using the SEC, SDS-PAGE, and matrix-assisted laser desorption ionization (MALDI) mass spectral analyses, further confirming successful surface camouflage. The attachment of PEG resulted in a product with higher oxygen affinity, lower oxygen cooperativity, higher auto-oxidation rates, and slower kinetic rate of oxygen offloading than LtEc. In spite of the changes to the biophysical properties, it is expected that modified products will yield higher circulatory times and lesser immunogenic responses, which will be tested next.

Takeaways: The streamlined purification protocol devised in the study allows for the production of an extracellular hemoglobin with >99% purity. This purity is essential to enable a wide variety of applications of LtEc as an oxygen therapeutic. We show varying extents of camouflage of the LtEc surface with PEG and their effect on biophysical properties of the product. The products are expected to have longer circulation time in vivo owing to its surface masking. Future animal tests will test this hypothesis. However, the use of LtEc can be expanded to cell media supplement and graft preservation, making it a diverse oxygen therapeutic.