Fernandes, Ana Isabel Henriques Dias; (1996) Polysialylation of proteins: an approach to improve their therapeutic efficacy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Use of proteins in intravenous therapy can be severely hampered by their proteolytic degradation, short circulating half-lives and the development of adverse immune responses. Circumvention of such problems has been achieved with various degrees of success by grafting on to proteins hydrophilic macromolecules. Work on related potential use in protein delivery of the highly hydrophilic, biodegradable polymers of N-acetylneuraminic acid (polysialic acids), namely the low molecular weight, poorly immunogenic and antigenic colominic acid (CA), is reported here. The first stage of the work involved the covalent coupling of CA to poly-L-lysine (a model protein) by reductive amination. Polysialylated catalase and asparaginase (two therapeutically useful enzymes) were subsequently prepared by the same method and in some experiments, the enzyme conjugates were simultaneously tritiated. These neoglycoproteins retained considerable enzymatic activity, apparently independently of the degree of polysialylation achieved. Methods for the characterization of the polysialylated enzymes included electrophoresis, gel filtration chromatography, enzyme kinetics and stability in the presence of proteases, mouse blood plasma and on freeze-drying. Polysialylated catalase and asparaginase were tested in vivo in terms of clearance rates from the circulation of intravenously injected mice. Both enzymes exhibited improved pharmacokinetics in comparison with their native counterparts. The increased blood residence of the polysialylated enzymes was apparently related to their degree of polysialylation. The immunogenicity of the polysialylated catalase and asparaginase following intravenous and intramuscular injection of mice was compared with that of the native enzymes. There was significant supression of immune responses against polysialylated catalase but not against polysialylated asparaginase. The pharmacokinetics of polysialylated asparaginase in pre-immunized mice was also investigated in order to assess whether the presence of specific antibodies against the enzyme would modify its blood clearance. Although asparaginase was cleared from the blood circulation of immunized mice more rapidly than in intact animals, clearance of the polysialylated enzyme was slower than that of the unmodified one.

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