Convincing UV and NMR spectrophotometric evidence is presented which demonstrates that at physiological pH, 7.4, 20-O-acyl derivatives of camptothecin (CPT) are substantially more stable in the lactone form than the 20-OH parent. Additionally, it was determined by HPLC analysis that the lactone ring of a 20-O-ether derivative of CPT underwent endocyclic ring opening at pH > or =8.5, while the lactone ring of 20-O-acyl CPT derivatives remained unaffected. PEG (and other smaller alkyl) 20-O-acyl-CPT derivatives released native CPT at pH > 9.5, which arises from exocyclic cleavage, thus precluding isolation of any open CPT acyl PEG (or alkyl) carboxylate forms.
The unique properties of poly(ethylene glycol) (or PEG) and its general compatibility with polypeptide materials facilitated development of a variety of different applications of this polymer. 1 -ll A marked proportion of these applications involve the use of covalently linked polypeptide-PEG adducts (reviewed else-where5-11 ). For example, a number of PEG-enzymes were shown to be useful as catalysts, soluble and active in organic solvents. 7 Due to the affinity to the upper phase of PEG/Dextran and PEG/salt two-phase systems, PEG-modified proteins were proven useful both as diagnostic tools 8 and in preparative separations of biological cells. 9 Unquestionably, the properties of polymer-polypeptide conjugates in vivo were a significant reason for the substantial amount of research reported in the area during the last two decades. It has been repeatedly demonstrated that covalent attachment of multiple strands of PEG to proteins produces conjugates with dramatically reduced immunogenicity and antigenicity. IO Such preparations also show great resistance to proteolytic digestion, and remain present in the bloodstream a considerably longer time than the parent polypeptides. These beneficial properties of modified polypeptides allowed development of a number of PEG-modified therapeutic proteins. IO Slight to moderate modification of potent allergenic proteins with PEG can often be SAMUEL ZALIPSKY and CHYI LEE· Enzon, Inc., South Plainfield, New Jersey 07080. Present address for s. Z.:
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