The effect of polysialylation on the immunogenicity and antigenicity of asparaginase: implication in its pharmacokinetics

Erwinia carotovora L-asparaginase was conjugated via the epsilon-amino groups of its lysine residues with colominic acid (CA) (polysialic acid) of average molecular mass of 10 kDa by reductive amination in the presence of NaCNBH3. Polysialylation using 50-, 100- and 250-fold molar excess CA relative to the enzyme led to an increasing proportion of the enzyme's in-amino groups (5.8, 7.6 and 11.3%, respectively) being conjugated to CA. Polysialylated and native (intact) asparaginase were used to immunize mice intravenously. Results (total IgG immune responses) indicate that all preparations elicited antibody production against the enzyme moiety but not against the CA of the conjugates. Moreover, antibody titres appeared highest for the native enzyme and were generally reduced as the degree of polysialylation increased. In other experiments mice pre-immunized with native or polysialylated asparaginase, with anti-asparaginase antibodies in their blood, were injected intravenously with the corresponding enzyme preparations. Results revealed that polysialylation reduces the antigenicity of asparaginase thus leading to circulatory half-lives (t 1/2 beta) that were 3-4-fold greater than that of the native enzyme, and similar to those observed in naive, non-immunized mice. Our data suggest that polysialylation of therapeutic enzymes and other proteins may be useful in maintaining their pharmacokinetics in individuals with antibodies to the therapeutic proteins as a result of chronic treatment.