Process Implications for Metal‐Dependent Immunoaffinity Interactions

Immunosorbents have been developed which utilize metal‐dependent interactions between monoclonal antibody(s) (Mab) and human plasma Protein C and Factor IX, members of the vitamin K‐dependent plasma protein family. In this report, we describe the potential process advantages associated with two different types of divalent metal‐dependent systems: 1) a Mab‐Protein C (antigen) complex which is stabilized in a divalent metal‐free environment and 2) a Mab‐Factor IX (antigen) complex which is stabilized in the presence of divalent metal ions. Chelating agents such as citrate or EDTA are effective agents for sequestering divalent metals. Thus, Protein C binds to the immunosorbent in the presence of EDTA or sodium citrate and can be eluted with buffer containing 25 mM calcium chloride or magnesium chloride. In the second case, Factor IX binds to the immunosorbent in the presence of magnesium chloride and can be eluted with sodium citrate or EDTA. These gentle elution conditions are desirable in order to retain the native structure as well as functional activity of the protein to be immunopurified. Furthermore, unwanted plasma proteins, which may interact nonspecifically with either the agarose matrix or immunoglobulin G (IgG) ligand are less likely to be eluted under these conditions. The calcium‐free citrate buffers used for immunosorption by either of these processes preclude the formation of fibrin solids and eliminate eluents which are difficult to process and which interfere with the biological activity of the product (i.e. chaotropes). The immunoaffinity process we describe for Protein C and Factor IX provides high functionality, purity and yield and demonstrates the feasibility of large‐scale processes utilizing metal‐dependent immunosorbents.