Due to recent advances in biotechnology, therapeutic proteins and other biomolecules, e.g. antibodies and antibody fragments, can now be prepared on a large scale, making such biomolecules more widely available. Unfortunately, the clinical usefulness of potential therapeutic biomolecules in unmodified form is often hampered by their rapid proteolytic degradation, instability upon manufacture, storage or administration, or by their immunogenicity. These deficiencies can often be overcome by covalent attachment of a water-soluble polymer, such as polyethylene glycol (PEG). See, for example, Abuchowski, A. et al., J. Biol. Chem. 252(11):3579 (1977); Davis, S. et al., Clin. Exp Immunol. 46:649-652 (1981). The biological properties of PEG-modified proteins, also referred to as PEG-conjugates or PEGylated proteins, have been shown, in many cases, to be considerably improved over those of their non-PEGylated counterparts (Herman et al., Macromol. Chem. Phys. 195:203-209 (1994)). Polyethylene glycol-modified proteins have been shown to possess longer circulatory times in the body, due to increased resistance to proteolytic degradation, and also to possess increased thermostability (Abuchowski, A. et al., J. Biol. Chem. 252:3582-3586 (1977). A similar increase in bioefficacy is observed with other biomolecules, e.g. antibodies and antibody fragments (Chapman, A., Adv. Drug Del. Rev. 54:531-545 (2002)).
Polyethylene glycol having activated end groups suitable for reaction with amino groups are commonly used for modification of proteins. Such activated PEGs or “polymeric reagents” include PEG-aldehydes (Harris, J. M. and Herati, R. S., Polym Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 32(1):154-155 (1991)), mixed anhydrides, N-hydroxysuccinimide esters, carbonylimidazolides, and chlorocyanurates (Herman, S. et al., Macromol. Chem. Phys. 195:203-209 (1994)). In some cases, however, polymer attachment through protein amino groups can be undesirable, such as when derivatization of specific lysine residues inactivates the protein (Suzuki, T. et al., Biochimica et Biophysica Acta 788:248-255 (1984)). Therefore, it would be advantageous to have additional methods for the modification of a protein by PEG using another target amino acid for attachment, such as cysteine. Attachment to protein thiol groups on cysteine offers an advantage in that cysteines are typically less abundant in proteins than lysines, thus reducing the likelihood of protein deactivation upon conjugation to these thiol-containing amino acids. Thiol-selective activated polymers are described, for example, in commonly owned PCT publication no. WO 2004/063250.
Polymeric thiol derivatives, and specifically PEG thiols, are one type of thiol-selective activated polymer. However, many prior art polymeric thiols suffer from being highly susceptible to oxidative coupling to form disulfides, a degradative process that reduces the active component and adds difficult-to-remove impurities. The latter can be particularly problematic in the preparation of bioconjugates from these materials. Therefore, it would be advantageous to provide polymeric thiol reagents having enhanced stability over prior art reagents.