Selenocysteine Enables a New Platform for Immunoconjugates

The Experimental Transplantation and Immunology Branch of the National Cancer Institute is seeking statements of capability or interest from parties interested in forming a collaborative research alliance to further develop, evaluate, or commercialize a new platform technology of immunoconjugates. This technology avoids previous challenges that have limited anti-tumor activity and the broad application of antibody conjugates as therapeutics. An application of interest is therapeutic agents for hematologic malignancies, such as leukemia, lymphoma, and myeloma.

A new class of molecularly defined immunoconjugates exploits selenocysteine as an engineered interface between biological and chemical entities. The nucleophilic selenol group of selenocysteine enables regiospecific, covalent conjugation in the presence of the other natural amino acids. Selenocysteine, a rare amino acid with unique chemical reactivity, is inserted into whole antibodies (e.g., IgG) or antibody fragments (e.g., Fc, Fab, and scFv) by recoding a stop codon from termination to selenocysteine insertion in yields comparable to conventional monoclonal antibodies. A derivative of a small synthetic molecule subsequently can be regiospecifically and covalently conjugated to the selenocysteine interface. Through this conjugation, both biological and chemical components acquire pharmacological advantages. Conventional immunoconjugates, which are usually based on covalent conjugations to amine or thiol groups, are not regiospecific due to the abundance of lysines, cysteines, and other reactive amino acids, and can cause substantial batch-to-batch variability. In contrast to other technologies that utilize unique chemical reactivities of natural or unnatural amino acids in proteins for regiospecific covalent conjugation of small synthetic molecules, antibody variable domains, enzymatic modification, or artificial translational machinery are not required.

• Pre-clinical research with focus on immunoconjugates that utilize the Fc antibody fragment. In vitro proof-of-concept; in vivo (mouse) toxicity and pharmacokinetics.
• Hofer T., Thomas J., Burke T. R. Jr., and Rader C. "An engineered selenocysteine defines a new class of pharmaceuticals," IBC Drug Discovery & Development Conference (Boston), 08/08/2007.
• Hofer T., Thomas J., Burke T. R. Jr., and Rader C. "An engineered selenocysteine defines a new class of pharmaceuticals," submitted for publication.
  

U.S. Provisional Application No. 60/909,665 filed April 2, 2007.

• A generic human Fc antibody fragment with a C-terminal selenocysteine can be expressed and purified in yields comparable to conventional monoclonal antibodies.
• Small synthetic molecules can be conjugated regiospecifically and covalently to the Fc antibody fragment through the selenocysteine interface
• The small synthetic molecule acquires Fcy and FcRn receptor binding capability, increased circulatory half-life, capability of aerosol delivery through the lung, increased solubility, and increased capability to interfere with protein/protein interactions.
• The Fc antibody fragment acquires access to unlimited chemical diversity resulting in high affinity, high specificity, and cross-species reactivity.
• The same technology can be used to generate immunoconjugates that utilize whole antibodies (e.g., IgG) or other antibody fragments (e.g., Fab and scFv).
• An economic advantage compared to conventional monoclonal antibodies is the defined combination of a generic biological component with a variable chemical component.
• The technology has broad therapeutic, diagnostic, and preventive applicability.