Role of the LDH-elevating virus in leukemia therapy by asparaginase

Therapeutic efficacy of asparaginase encapsulated in intact erythrocytes

Asparaginase has been encapsulated in intact erythrocytes by a gentle loading technique. The loaded cells were found to survive removal by the recticuloendothelial system when returned to the circulation of mice. In addition the enzyme removed all detectable asparagine from the plasma in vivo for at least two weeks after the injection of the loaded cells. In vitro evidence suggested that the asparagine entered the cell and was metabolised by the loaded enzyme in situ. No evidence was found to suggest that the enzyme left the cell. When the encapsulated asparaginase was tested against the 6C3HED tumour in C3H mice the encapsulated preparation was superior to the free enzyme in treating the tumour and was the only treatment to produce 'cured' mice. Encapsulated asparaginase also lowered glutamine levels both in vivo and in vitro. The possibility that LDH virus was responsible for the excellent results obtained with the encapsulated enzyme was investigated and eliminated.

Theoretical and practical considerations in the use of hemodialysis for the therapeutic removal of asparagine and other amino acids

The endogenous asparagine influx in mice undergoing asparaginase therapy can be neutralized by asparaginase activity equivalent to less than one IU/ml of plasma, as determined with a new, highly sensitive micro assay for asparaginase. It appears feasible that an asparagine influx of this magnitude may be cleared from the blood plasma by hemodialysis employing an artificial kidney apparatus with a clearance capacity somewhat greater than that routinely employed by renal patients. Application of this method for removing asparagine or other amino acids from the blood as an approach to cancer therapy ultimately depends upon the specific amino acid influx rates of the subject and upon the ability of the dialyzer to deplete the free amino acids in the blood plasma at a sufficiently high rate to lower specific amino acid concentrations in malignant tissues, or in adjoining interstitial spaces, to therapeutic levels. Based upon the data available, hemodialysis has potential therapeutic application as a means to control the removal of various free amino acids or other metabolic substrates from blood plasma.

Antibody-producing cells: virus-induced alteration of response to antigen

Spleen antibody-forming cells of mice yield a 3- to 10-fold increase in their response to sheep erythrocyte antigen if they are acutely infected by lactate dehydrogenase-elevating virus. This early stimulation is replaced by a long-term inhibition of the antibody-forming cells as the viremia goes into its persisting chronic stage. These contrasting immunological phenomena are examined as contributing factors responsible for the enhancement by this virus of asparaginase (EC 3.5.1.1; L-asparagine amidolydrolase) therapy against leukemia in mice, and for the alteration of the susceptibility of mice to various neoplastic processes.

Purification and properties of L-asparaginase from Serratia marcescens

The purification and properties of a tumor inhibitory l-asparaginase from Serratia marcescens are described. The following properties of the enzyme were examined: kinetics of the enzyme reaction, catalytic activity as a function of pH, boundary sedimentation velocity, electrophoresis on polyacrylamide gel, immuno-electrophoresis against homologous and heterologous antisera, immunodiffusion, blood clearance rate in mice, and inhibition of the 6C3HED lymphoma in C3H mice. Complete regression of this tumor was obtained with a smaller dose of the enzyme from S. marcescens than with enzyme from Escherichia coli. The reason for this difference was not evident from a comparison of several properties of the two enzymes.