[Preparation of repeatedly and effectively usable L-asparaginase by a chemical modification]

The anti-tumor activity of L-asparaginase (EC. 3.5.1.1.) has been conclusively demonstrated. Its therapeutic application, however, has been hampered by its short clearance time in the circulation and high immunogenicity. In order to solve these problems, polyethylene glycol, a linear synthetic and non-immunogenic polymer, was introduced covalently into the amino groups in the enzyme molecule. Monomethoxypolyethylene glycol with molecular weight of 5000 was activated with cyanuric chloride to obtain activated PEGs [2, 4-bis (O-methoxypolyethyleneglycol)-6-chloro-s-triazine]. L-Asparaginase lost its immunoreactivity against its antibodies after the modification of 52 out of 92 amino groups in the molecule. This modified asparaginase retained 11% of its enzymic activity under the physiological condition. When the modified asparaginase was administered in rodents, it diminished the serum asparagine level and its enzymic activity persisted in the circulation 10 to 20 times longer than that of native enzyme. Furthermore, repeated injections of modified asparaginase did not induce any significant anti-asparaginase antibody production. Modified asparaginase showed a superior anti-tumor activity to the native counterpart irrespective of the presence of anti-asparaginase antibodies, when it was tested with murine Gardner lymphoma. This chemical modification should make it possible for the first time to repeatedly and effectively use L-asparaginase obtained from a bacterium.