Pharmaceutical Manufacturing Handbook: Production and

410 BIODEGRADABLE POLYMER-BASED MICROSPHERES
Excipients used
Solubilization Preservation Stabilization
Moisture-induced instability prevention Microclimate pH-induced instability prevention
FIGURE 2 Flow chart of excipients used to prevent/minimize protein instability
problems.
Another review by Bilati et al. [130] also envisioned the strategic approaches for
overcoming peptide and protein instability within biodegradable nano - and microparticles.
The reader is also referred to related publications edited by Sanders and
Hendren [131] and Senior and Radomsky [132] for information on excipients used
in injectable peptide/protein - loaded formulations including microspheres. This
section will not cover all excipients used in parenteral protein formulations because
the aforementioned publications already do so. Rather this section highlights examples
of synergestic and antagonistic interactions that have been reported mainly
between the excipients and the peptide/protein drugs, especially before microsphere
preparation, followed by a brief discussion of a major instability problem of proteins/peptides
inside the microspheres. The published research paper is being organized
according to major functions of parenteral excipients, namely, solubilization,
stabilization, and preservation (see Figure 2 ) [133] .
5.2.5.1 Solubility - and Stability - Increasing Excipients
The traditional approach is to solubilize directly the peptide/protein in organic solvents.
This can be achieved by different means. Cleland and Jones [61] assumed that
native protein conformation could be maintained by precipitating the protein at its
pI. The molecule is then free of charge and can be readily solubilized in organic
solvents. Conversely, an alternative concept is based on the freeze drying of the
protein at a pH away from its pI value before formulating it. It was thought that
this strategy could increase protein solubility and stability in various polar and
water - miscible organic solvents such as DMSO [134, 135] . It should be noted that
a preformulation procedure consisting of using spray freeze drying with a suitable
excipient was able to stabilize BSA before encapsulation by a nonaqueous method
[35] . Using the dissolution approach, lysozyme was successfully formulated but
incomplete lysozyme release from microspheres was observed and ascribed to
aggregation [136] . Protein solubility can also be increased via an ion - pairing mechanism.
The protein is modifi ed by adding an oppositely charged surfactant that binds
to the protein, so as to obtain a neutral hydrophobic entity and thus reduce direct
contact between the protein and the organic solvent. Positively charged proteins
and negatively charged surfactants should be employed, since cationic surfactants
might have toxic side effects. This technique was shown to improve lysozyme conformational
stability after a hydrophobic complex between lysozyme and oleic acid
[137, 138] . A new interesting concept is to encapsulate an aggregated protein in a
reversibly dissociable form in order to avoid the formation of irreversible aggregates
during processing and to promote the sustained release of the native monomeric
form. Growth hormone was successfully formulated with this approach [139] .