Biodistribution and excretion of radiolabeled 40 kDa polyethylene glycol following intravenous administration in mice.

The pharmacokinetics, excretion, and tissue distribution of [(14)C]-labeled polyethylene glycol-alanine (PEG-Ala) were determined after slow bolus administration into the femoral vein of male CD-1 mice. The pharmacokinetics of PEG-Ala in blood and plasma revealed a biphasic elimination with a terminal half-life of 20 h. Eighty-five percent of the excreted material was voided in the urine and the remaining amount was detected in the feces. PEG-Ala-derived radioactivity was widely distributed with detectable levels of radioactivity observed in all tissues examined. The highest concentration was observed in the kidneys followed by lungs, heart, and liver. Six hours after administration, PEG-Ala levels were significantly reduced in all tissues. Despite a slow prolonged decrease, radioactivity was still detectable after 28 days.

PEG prodrugs of 6-mercaptopurine for parenteral administration using benzyl elimination of thiols.

6-Mercaptopurine (6-MP) is an orally administered, water-insoluble purine analog that is effective against acute lymphatic leukemia. Oral absorption of 6-MP, however, is quite erratic, with only 16-50% of the administered dose reaching the blood. In this report, water-soluble parenterally administered poly(ethylene glycol) (PEG) prodrugs of 6-MP were synthesized using several chemical approaches that enabled the protection of the thiol group through a modification of the benzyl elimination (BE) system. In our earlier work on antimetabolites, it was found that branching of the PEG allowed greater loading of the active drug. This approach was also utilized within this work to give multiloaded systems. The resulting conjugates were stable in pH 7.4 PBS buffer as well as in rat plasma for extended periods. However, these conjugates did act as prodrugs in vivo and a number of PEG-6-MP constructs had significant (P < 0.05) activity in murine leukemia, as well as certain solid tumors, compared with unconjugated 6-MP in a solubilizing vehicle. The fact that some PEG-6-MP conjugates were stable during in vitro plasma dissociation assays, but demonstrated in vivo anticancer activity, suggests extravascular cleavage of the linking group. This work demonstrates that PEG conjugation is an effective means of solubilizing 6-MP for parenteral administration.

Utility of poly(ethylene glycol) conjugation to create prodrugs of amphotericin B.

This paper reports on the synthesis, safety, and efficacy of a series of water-soluble derivatives of poly(ethylene glycol) (PEG)-conjugated amphotericin B (AmB). PEG 40 000 attached to the sugar amino group of AmB via labile carbamate and carbonate linkages was examined. The synthetic program conducted for this investigation provided a series of disubstituted PEG-AmB derivatives which had in vitro PEG half-life of hydrolyses rates in rat plasma varying between 1 and 3 h. Importantly, all conjugates demonstrated less than 6% hydrolysis following 24 h incubation in pH 7.4 phosphate buffer at 25 degrees C and showed solubilities greater than 46 mg/mL in aqueous solutions. The solubility of AmB in the conjugates increased up to approximately 200 times compared to unmodified AmB in saline. As a major finding, this investigation demonstrated that conjugation of PEG to AmB could produce conjugates that were significantly (6x) less toxic than AmB-deoxycholate and maintained, or even had enhanced, in vivo antifungal activity.

Controlled release of proteins from their poly(ethylene glycol) conjugates: drug delivery systems employing 1,6-elimination.

Several tripartate releasable PEG linkers (rPEG) that can provide anchimeric assistance to hydrolysis (cyclization prodrugs) were prepared and, after conjugation to lysozyme demonstrated rapid cleavage in rat plasma compared to nonassisted, permanently bound PEG. By varying the chemical structure and adding steric hindrance, the half-life of the protein conjugates can be adjusted from slow to very fast. The pharmacokinetics (PK) of regeneration of native protein, from various rPEG conjugates can, for the first time, be easily followed in the rat using green fluorescent protein. The PK in mice was also determined for rPEG-Interleukin 2 (rPEG-IL-2) conjugates in vivo using an ELISA assay. Thus, a systematic study of rPEGylated proteins, either in vivo or in vitro during processing, has been investigated based on regeneration of native protein. The employment of releasable PEG polymers substantially broadens the applications of PEGylation drug delivery technology by introducing the benefits of controlled release of native protein therapeutics.

Effective drug delivery by PEGylated drug conjugates.

The current review presents an update of drug delivery using poly(ethylene glycol) (PEG), that focuses on recent developments in both protein and organic drugs. Certainly the past 10 years has resulted in a renaissance of the field of PEG drug conjugates, initiated by the use of higher molecular weight PEGs (M(w)>20,000), especially 40,000 which is estimated to have a plasma circulating t(1/2) of approximately 10 h in mice. This recent resuscitation of small organic molecule delivery by high molecular weight PEG conjugates was founded on meaningful in vivo testing using established tumor models, and has led to a clinical candidate, PEG-camptothecin (PROTHECAN), an ester based prodrug currently in phase II trials. Additional applications of high molecular weight PEG prodrug strategies to amino containing drugs are presented: similar tripartate systems based on lower M(w) PEG and their use with proteins is expounded on. The modification of a benzyl elimination tripartate prodrug specific for mercaptans is presented, and its successful application to 6-mercaptopurine giving a water soluble formulation is discussed. Recent novel PEG oligonucleotides and immunoconjugates are also covered. Clinical results of FDA approved PEGylated proteins are also presented.

Anticancer drug delivery systems: N4-acyl poly(ethyleneglycol) prodrugs of ara-C. I. Efficacy in solid tumors.

A systematic study of N(4) amino PEG-prodrugs of ara-C (1) was conducted and provided a series of disubstituted amides, as well as a carbamate derivative. These conjugates showed hydrolysis half lives in rat plasma from about 1 h to 3 days, but were stable for >24 h in phosphate buffer, pH 7.4. In an LX-1 solid lung tumor model some of the PEG prodrugs exhibited superior activity to ara-C when compared on a molar basis. One problematic issue that was identified in this investigation was the need to increase the loading of ara-C onto PEG in order to avoid highly viscous solutions.

Anticancer drug delivery systems: multi-loaded N4-acyl poly(ethylene glycol) prodrugs of ara-C. II. Efficacy in ascites and solid tumors.

The synthesis of branched PEG (40,000) acids has been achieved using aspartic acid (Asp) and AspAsp dendrons. Complete conjugation of these dendritic acids with cytosine arabinoside (ara-C) was achieved by the use of spacers that allowed a greater separation of the branches to accommodate several large ara-C molecules in proximity to each other. The tetrameric and octameric PEG-ara-C amide prodrugs were much more effective in the treatment of solid and ascites tumors compared to the native drug. The greater loading of the PEG backbone appears to have achieved a minimum threshold concentration for the therapeutic delivery of ara-C.