The synthesis of a peanut agglutinin-ricin A chain conjugate: potential as an in vitro purging agent for autologous bone marrow in multiple myeloma.

The lectin peanut agglutinin (PNA) and CD19 monoclonal antibody have been covalently linked to magnetic beads and utilized in an in vitro purging system for autologous bone marrow in multiple myeloma (MM). An alternative to mechanical purging involves the use of immunotoxins to provide specifically targeted cellular toxicity; however, no studies to date have examined the utility of a lectin-ricin A chain (RCA) combination as a purging agent in MM. Initially, we studied the internalization of PNA by target cells (Raji) using flow cytometry. The surface fluorescence intensity of PNA-treated Raji cells was reduced upon incubation at 37 degrees C, and subsequent studies with fixed cells detected the endocytosed PNA. Complete internalization occurred within 120 minutes, indicating the potential of PNA as a purging agent. We manufactured a novel PNA-RCA conjugate and demonstrated its strong and specific binding to PNA reactive cell targets. Subsequent experiments assessed the toxicity of the conjugate to Raji cells and normal bone marrow progenitor cells. 3H-leucine uptake assays showed that PNA-RCA was capable of reducing protein synthesis in Raji cells and that the toxic effects were specific. In addition, at concentrations of conjugate achieving greater than 99% selective cytotoxicity for Raji cells, adequate CFU-GM were preserved in normal marrow. These studies suggest that PNA-RCA may be of value as an in vitro purging agent for MM.