Characterization of polyethylene glycol-modified L-asparaginase from Escherichia coli and its application to therapy of leukemia.

For the purpose of clinical application to the therapy of human leukemia and lymphosarcoma, L-asparaginase from Escherichia coli was modified with 2,4-bis(O-methoxypolyethylene glycol)-6-chloro-s-triazine (activated PEG2) by an improved method, which involves a purification step of activated PEG2 by gel filtration. The PEG2-modified asparaginase retained approximately 30% (73 IU/mg of protein) of the enzymic activity of the native enzyme, while it had almost wholly lost the immunoreactivity towards anti-asparaginase antibodies. The modified enzyme retained the characteristics of the native enzyme in terms of the pH- and temperature-dependencies of activity and stability, and the Km value for L-asparagine. Administration of the modified enzyme to a dog with spontaneous lymphosarcoma induced complete remission without any toxic side effects. Seven children with multiple relapses of acute leukemia were treated with a regimen of cycles of methotrexate and native asparaginase. Three of these children developed anaphylactic shock. In contrast to the native enzyme, the successive administration of PEG2-modified asparaginase to those three patients was therapeutically effective without causing any allergic reaction.