Development of novel formulations for mucosal delivery of protein ...

with pH-dependent swelling and drug release characteristics. At low pH, chitosan containing
HTCC/GP hydrogel displayed increased hydrophilicity leading to polymer dissolution and
consequent release of the entrapped drug. Furthermore, it has been shown that polymers (e.g.
chitosan) with thiol groups (thiomers) exhibit much higher adhesive properties than other
known mucoadhesive polymers. Sulfhydryl bearing agents can be covalently attached to their
primary amino group via the formation of amide bonds where the carboxylic acid group of the
ligand, such as thioglycolic acid (TGA) is conjugated with chitosan via the primary amino
group, mediated by a water soluble carbodiimide (Kast & Bernkop-Schnürch, 2001). The
improved mucoadhesive properties of thiolated chitosans are explained by the formation of
stronger covalent bonds between thiol groups of the polymer and cysteine rich sub-domains
of glycoproteins present in the mucus layer (Snyder et al., 1983; Leitner et al., 2003). Various
factors that can alter the interaction between the polymer and the mucosal layer can affect the
mucoadhesive property of a polymer. These include polymer molecular weight (Andrew et
al., 2009; Tiwari et al., 1999), polymer concentration (Solomonidou et al., 2001) and method
of drying.
Lyophilisation (freeze-drying) as a method of drying is preferred over others as it
overcomes most of the limitations associated with formulation of protein-based products.
Freeze-drying of formulations offers stable products, extends shelf-life, allows storage of
products at room temperature instead of - 20 °C, avoids the complications of cold chain
supply management, facilitates transportation of the products and increases patient
compliance (Bunte et al., 2010). During freezing, or even storage however, a number of small
molecule drugs and excipients tend to undergo incomplete or delayed crystallization resulting
in the formation of mixtures of different polymorphic forms or vial breakage as in the case of
the cryoprotectant mannitol (Kim et al., 1998; Liao et al., 2007). Such processes may cause
reproducibility and product characterization problems. It is therefore critically important that
the selection of conditions during the freezing step in a lyophilisation cycle be informed by
knowledge from investigating potential for freeze induced damage, product characteristics
and the attainment of product stability. Annealing as a process step may be employed to
maintain samples at a particular temperature below the equilibrium freezing point but above
the glass transition temperature for a specified period of time. The goal is to facilitate
crystallization of the active ingredients and the cryoprotectant and to improve the pore size
distribution of ice crystals as in Ostwald ripening (Schwegman, 2009a). Jiang et al., (2007)
observed that mannitol based formulations sometimes caused vial breakage during early
primary drying due mainly to moderate to fast freezing rates (when the mannitol remains
amorphous), high fill volume and high total solid content of mannitol in the formulation. The