The covalent attachment of poly-dl-alanine peptides to lysyl residues on the surface of Erwinia carotovoral-asparaginase has produced a modified enzyme which is much less immunogenic in mice and demonstrates 100-fold longer plasma half-life in the rhesus monkey. Immunogenic responses towards both the immunoglobulin G (IgG) and immunoglobulin E (IgE) antibody subclasses were evaluated in c57bl × DBA/2 F1 mice exposed to 250 rads of whole-body irradiation 4 hr prior to immunization with 5-diazo-4-oxynorvaline-inactivated native and modified l-asparaginase in complete Freund's adjuvant. Under these immunologically stressful conditions, the native enzyme evoked an IgE and IgG response which could be further amplified by a secondary immunization, whereas the modified enzyme evoked no IgE or IgG response even after a tertiary immunization. In experiments mimicking an intensive therapeutic schedule, whereby two groups of mice were given weekly injections of 5 to 10 units of either native or modified asparaginase for up to 14 weeks, neither enzyme form evoked a significant IgE response, and only the mice given injections of the native enzyme produced an IgG response. In a preliminary patient study, skin testing of a child who had shown an allergic reaction to the native enzyme resulted in a negative response after an intradermal injection of the modified enzyme, whereas a wheal and flare reaction was observed to both the native enzyme and a histamine control. All of these results suggest that the modified enzyme should show a definite reduction in immunological reactions associated with l-asparaginase treatment of childhood leukemia.


Supported by Research Grant CA 26358 from NIH, Bethesda, MD.

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