Parenteral Delivery of Peptides and Proteins - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 578

ent drug t 1/2 is 2.5 h); t 1/2 of PEG-calcitonin was found to be 15.4 h (parent drug t 1/2 is
3.31 h). Thus, conjugated proteins and peptide drugs may have a plasma half-life as
much as 3- to 486-fold higher than the native protein along with increased therapeutic
activity and reduced immunogenicity [32,393,394] .
Site-specific conjugation chemistries are important because undesirable isomers
can result from nonspecific chemistries. The molecular weight of the PEG used with
the extent of pegylation gives control over the biophysical properties of PEG-pro-
tein produced, such as size, hydrophobicity, and charge, because heavily a hydrated
PEG molecule excludes the protein from immediate uptake by organs, and similarly,
the PEG molecules shield the antigenic sites, thereby reducing the immunogenic-
ity of the protein [395] . Monosubstituted PEGs (mPEG) have been found as good
candidates for PEGylation because they contain only one hydroxyl group per chain,
thus limiting activation and coupling to one site and preventing the crosslinking and
polymerization of modified molecules, as found in the study of mPEG conjugated
insulin. On the other hand, di- and tri-PEGylated insulin showed a great loss in bio-
logical activity [396] . Table 11.14 lists many PEGylated proteins and peptides cur-
rently in market or in clinical trials.
Some other PEGylated products available on the market or are in clinical trials are
PEGylated synthetic thrombopoietin (TPO) for the treatment of acute lymphoblastic
leukemia, PEGylated omniferon, PEGylated arginine deaminase, PEGylated anti-GFR
(growth factor receptor), antibody fragment, PEGylated anti-IL-1b antibody fragment,
PEGylated anti-PDGF -receptor antibody fragment, and PEG-ADA (polyethylene
glycol-modified adenosine deaminase), which is used to trigger immune defense
mechanisms in severe combined immunodeficiency syndrome (SCIDS) [398-400] .
The main drawback of PEGylation is usually reduced biological activity in vitro ,
which is compensated in vivo by significantly improved PK behavior [391,413,414] .
PEGylation of proteins and peptides has proved to be the most established tech-
nology for extension of drug residence in the body and will play an important role in
the next generation of therapeutics, such as peptides, protein nanobodies, and scaf-
folds, which due to their inherent PK/PD characteristics and diminished molecular
charecteristics need half-life extension. However, for full-fledged utilization of this
technology, researchers should focus on several important factors in the preparation
and purification of PEGylated biopharmaceuticals to meet stringent regulatory crite-
ria for their use in human therapy. Areas that should be addressed are PEG proper-
ties, the specificity of PEGylation reactions, separation and large-scale purification,
the availability and analysis of PEG reagents, analysis of PEG-protein conjugates,
the consistency of products and processes, and approaches used for rapid screening
of pharmacokinetic properties of PEG-protein conjugates.
11.6.2 Protein Crystals
The development of various delivery systems for P/P drugs involves organic phase.
Harsh manufacturing conditions like temperature may deteriorate proteins and ruin
them prior to their administration. In recent years, many attempts have been made
to develop either new formulation strategies or, with the help of protein engineering,