"Improved protection from immune recognition is enabled by modifying each uricase subunit with 2-10 strands of high molecular weight PEG (>5 kD-120 kD) Saifer, et al. (U.S. Pat. No. 6,576,235; (1994) Adv Exp Med Biol 366:377-387, each incorporated herein by reference in its entirety). This strategy enabled retention of >75% enzymatic activity of uricase from various species, following PEGylation, enhanced the circulating life of uricase, and enabled repeated injection of the enzyme without eliciting antibodies in mice and rabbits.
"Hershfield and Kelly (International Patent Publication WO 00/08196; U.S. Application No. 60/095,489, incorporated herein by reference in its entirety) developed means for providing recombinant uricase proteins of mammalian species with optimal numbers of PEGylation sites. They used PCR techniques to increase the number of available lysine residues at selected points on the enzyme which is designed to enable reduced recognition by the immune system, after subsequent PEGylation, while substantially retaining the enzyme's uricolytic activity. Some of their uricase proteins are truncated at the carboxy and/or amino termini. They do not provide for directing other specific genetically-induced alterations in the protein.
"In this application, the term 'immunogenicity' refers to the induction of an immune response by an injected preparation of PEG-modified or unmodified uricase (the antigen), while 'antigenicity' refers to the reaction of an antigen with preexisting antibodies. Collectively, antigenicity and immunogenicity are referred to as 'immunoreactivity.' In previous studies of PEG-uricase, immunoreactivity is assessed by a variety of methods, including: 1) the reaction in vitro of PEG-uricase with preformed antibodies; 2) measurements of induced antibody synthesis; and 3) accelerated clearance rates after repeated injections.
"Previous attempts to eliminate the immunogenicity of uricases from several sources by coupling various numbers of strands of PEG through various linkers have met with limited success. PEG-uricases were first disclosed by F F Davis and by Y Inada and their colleagues. Davis, et al., (1978); U.S. Pat. No. 4,179,337; Nishimura, et al., (1979); Japanese Patents 55-99189 and 62-55079, each incorporated herein by reference in its entirety. The conjugate disclosed in U.S. Pat. No. 4,179,337 is synthesized by reacting uricase of unspecified origin with a 2,000-fold molar excess of 750 dalton PEG, indicating that a large number of polymer molecules is likely to have been attached to each uricase subunit. U.S. Pat. No. 4,179,337 discloses the coupling of either PEG or polypropylene glycol) with molecular weights of 500 to 20,000 daltons, preferably about 500 to 5,000 daltons, to provide active, water-soluble, non-immunogenic conjugates of various polypeptide hormones and enzymes including oxidoreductases, of which uricase is one of three examples. In addition, U.S. Pat. No. 4,179,337 emphasizes the coupling of 10 to 100 polymer strands per molecule of enzyme, and the retention of at least 40% of enzymatic activity. No test results were reported for the extent of coupling of PEG to the available amino groups of uricase, the residual specific uricolytic activity, or the immunoreactivity of the conjugate.
"In previous publications, significant decreases in uricolytic activity measured in vitro were caused by coupling various numbers of strands of PEG to uricase from Candida utilis. Coupling a large number of strands of 5 kDa PEG to porcine liver uricase gave similar results, as described in both the Chen publication and a symposium report by the same group. Chen, et al., (1981); Davis, et al., (1978).
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