Gel entrapped L-asparaginase: kinetic behavior and antitumor activity

L-Asparaginase from Escherichia coli was immobilized by entrapment in a gel based on poly(2-hydroxyethyl methacrylate) with an activity as high as 730 I.U./g of dry gel. The apparent Michaelis constant for these gels was similar to that of the free enzyme. At 37 degrees C the immobilized enzyme had a half-life of more than 40 days, in vitro. The gel was freeze-dried, crushed and sieved to pass a 38 mum screen, giving a median particle size of 12 mum. C3H mice were injected intraperitoneally with 40 I.U. of L-asparaginase; the peak plasma activity after 4 hours was only 0.9 I.U. for the gel entrapped enzyme compared to a peak activity of 5.0 I.U. after 2 hours for the native L-asparaginase. Ninety percent of the plasma enzyme activity for the gel entrapped case was sedimentable at 21,000 X g, indicating a small leakage of the enzyme from the gel; the clearance for the enzyme activity in plasma had an initial half-life of 13 hours in contrast to a half-life of 2 hours for the native preparation. After intraperitineal injection of 5.0 I.U. into C3H mice, plasma L-asparagine fell to undetectable levels for 4 days and reappeared by day 8 for both the native and immobilized enzymes. Subcutaneously transplanted 6C3HED murine lymphoma was inhibited by 35, 78 and 100% after single intraperitoneal injections of immobilized L-asparaginase of 2, 4 and 8 I.U., respectively, as compared to 36, 53 and 86% for the native enzyme by the 14th day. Body weight changes after receiving immobilized L-asparaginase were essentially similar to those of animals receiving a comparable dose of native enzyme. These results indicate that while most of the immobilized L-asparaginase remains at the injection site, it produces a significant plasma L-asparagine depression and antitumor acitivity comparable to that of the native preparation without major toxicity.