title: Thiolytically cleavable dithiobenzyl urethane-linked polymer-protein conjugates as macromolecular prodrugs: reversible PEGylation of proteins.
abstract: New thiolytically cleavable dithiobenzyl (DTB) urethane-linked conjugates of methoxypoly(ethylene glycol) (mPEG) and a model protein, lysozyme, were prepared and thoroughly characterized. In contrast to our earlier communication [Zalipsky, et al. (1999) Bioconjugate Chem. 10, 703], in the current study we used a more sterically hindered form of para-DTB urethane linkage containing a methyl group on the alpha-carbon to the disulfide moiety. The new reagent for covalent attachment of mPEG-DTB to amino groups of proteins was synthesized via a seven-step process. As a result of PEG conjugation, the lysozyme was shown to completely lose its bacterial cell wall-lysing activity. However, activity was almost fully restored upon cysteine-mediated cleavage of the PEG component. The conjugate decomposition process was monitored by RP-HPLC and by ion spray LC-MS, which showed the formation of the p-mercaptobenzyl urethane-lysozyme intermediate, and ultimately its conversion to the unmodified lysozyme as the sole protein component. Pharmacokinetic evaluation of (125)I-labeled cleavable and noncleavable PEG-lysozyme given intravenously in rats revealed similar clearance patterns; both cleared in a significantly slower manner compared to that of the native protein. However, subcutaneous administration of the same conjugates showed a significantly larger AUC of the cleavable conjugate, indicating that some cleavage of the DTB urethane may have occurred. Although the DTB-linked PEG-lysozyme exhibited almost the same plasma clearance as the noncleavable counterpart, hinting that methyl-DTB linkage might be stable in the bloodstream, SDS-PAGE examination of the conjugate incubated in plasma showed decomposition at least partially mediated by albumin. These results suggest the potential of PEG-DTB-proteins as macromolecular prodrugs capable of generating fully active native proteins under in vivo conditions.
authors.affiliation: ALZA Corporation, Mountain View, CA 94043, USA. szalipsky@intradigm.com
authors.forename: Samuel
authors.initials: S
authors.lastname: Zalipsky
authors.forename: Nasreen
authors.initials: N
authors.lastname: Mullah
authors.forename: Charles
authors.initials: C
authors.lastname: Engbers
authors.forename: Maria U
authors.initials: MU
authors.lastname: Hutchins
authors.forename: Radwan
authors.initials: R
authors.lastname: Kiwan
mesh_terms: D000818:Animals; D001554:Benzene / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D003432:Cross-Linking Reagents / Q000737:chemistry; D008297:Male; D008968:Molecular Conformation; D009113:Muramidase / Q000737:chemistry* / Q000493:pharmacokinetics; D011092:Polyethylene Glycols / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry*; D011355:Prodrugs / Q000737:chemistry* / Q000493:pharmacokinetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D014520:Urethane / Q000737:chemistry*
chemical_list: D003432:Cross-Linking Reagents; D011108:Polymers; D011355:Prodrugs; D013438:Sulfhydryl Compounds; D014520:Urethane; D011092:Polyethylene Glycols; D009113:Muramidase; D001554:Benzene