Enzyme Reactors for the Removal of Amino Acids from Plasma

This chapter deals with the preparation of enzyme reactors for the removal of amino acids from plasma as a potential form of tumor treatment. Certain amino acids such as L-asparagine, L-glutamine, and L-tryptophan have been recognized as important cancer nutrients, the removal of which can lead to remissions in tumors. Since the so-called antitumor enzymes are derived from bacterial or fungal sources, immunological responses are observed after parenteral administration. In an attempt to prevent immunological sensitization, extracorporeal enzyme reactors were developed containing antitumor enzymes covalently bound to a water-insoluble, polymeric surface. However, with the successful preparation of enzyme reactors, new problems arose, notably a decrease in the stability of the immobilized enzyme, an appreciable change in the substrate affinity, and difficult in sterilization of the devices. The development of an enzyme reactor system for cancer treatment is still in its infancy, although considerable progress has been made toward stable, nonhydrolyzable, and highly active systems. The main problem in the future to be dealt with is the rapid rebound phenomenon of L-asparagine and L-glutamine which greatly reduces the value of extracorporeal tumor treatment as a sole method. It will be necessary to combine extracorporeal enzyme treatment with the administration of amino acid analogs, which either decrease the utilization of their respective amino acid or decrease its synthesis rate. The combination of a tryptophan-degrading enzyme administered parenterally along with a tryptophan analog has shown great promise in cancer therapy.