Evaluation of Phenomena that Determine the Performance of Immunoaffinity and Peptide-Based Affinity Sorbents

Insufficient supply and pathogen safety concerns regarding plasmaderived therapeutic proteins, such as fibrinogen and immunoglobulins, have been the impetus for the development of genetic engineering techniques and separations methods for the economical and safe production of these proteins. This study is concerned with the isolation of these important therapeutics from complex media. Immunoaffinity chromatography has been an important method in the isolation of these products, typically being implemented as the final cleanup step yielding an extremely pure, homogenous final product. However, the use of immunoaffinity chromatography in large-scale purification processes have been precluded due to high capital costs and the inherent lability of immunosorbents. Peptide-based affinity sorbents are being developed in order to surmount the inherent limitations posed by monoclonal antibodies that are used as ligands in immunosorbents. The objective of this research was to quantitatively assess the impact of affinity ligand orientation, local density and transport phenomena on peptidebased affnity sorbent performance. The peptides under study herein can form high-affinity complexes with protein targets, thus these ligands are one of the newest technologies arising from combinatorial chemistry with applications to the difficult problem of purifying high-molecular weight proteins from complex mixtures. Two types of structural motifs which are common to small peptide affinity ligands derived from combinatorial chemistry are studied here: a linear peptide which is comprised of the affinity recognition sequence in its entirety and a chain structure which displays multiple branches of the recognition sequence emanating from a central lysinic core structure. Two recognition sequences are studied here which bind plasma proteins. One peptide recognition sequence forms a high affinity complex with fibrinogen. Another peptide recognition sequence binds the Fc region of immunoglobulins. Immunglobulins are plasma proteins which range in molecular weight from 155 to 900-kDa and are valuable for therapeutic uses for imparting passive immunity. This study seeks to identify factors analogous to those manifested in immunosorbent performance that may also be important in the optimal design of peptide-based affinity sorbents. In general, previous research with the design of immunosorbents have found that immunosorbent performance, i.e., targetbinding efficiency or activity, is substantially dependent upon several factors which include effects associated with ligand orientation, and local density as related to steric incumbrance of target binding sites, and transport phenomena as related to under utilization of intra matrix volume. In summary, this study asks the questions: (1) What factors regarding ligand orientation, local ligand density, and intraparticle transport phenomena, are important in the optimal design of peptide affinity sorbents?; and (2) Are these effects analogous to those manifested in immunosorbent performance? This study seeks to investigate the use of techniques used to mitigate the effects associated with these negative factors upon immunosorbent performance in order to elucidate the nature of these same effects upon peptidebased affinity sorbents. For example, oriented ligand immobilization can be facilitated through selective coupling chemistries and the premasking of ligand binding domains prior to immobilization. In addition, the manipulation of local ligand density using novel spatially controlled matrix activation and ligand immobilization methods can be assessed and implemented for the optimization of the performance and design of peptide-based affinity sorbents. This study has found that enhanced transport phenomena into the matrix interior volume can be achieved by using low solids content cellulose matrices having a low extent of crosslinking. This study demonstrates the effective use of these largeparticle diameter, low-solids content cellulose hydrogel matrices in immunoaffinity, peptide-based affinity and ion exchange chromatography applications for the separation of high-molecular weight therapeutic proteins.