Parenteral Delivery of Peptides and Proteins - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 580

to change the protein's inherent characteristics in order to achieve a stable protein
formulation and high protein loading into various drug delivery vehicles. One of the
most effective options is formulations based on protein crystals [415-417] . Naturally
occurring proteins are amorphous in nature, and with the help of protein engineer-
ing, it is now possible to convert amorphous proteins into fragile crystals that are
held together by weak intermolecular force, remain hydrated, and in a native folded
state.
Protein crystals are energetically more stable and less prone to physical and chem-
ical degradation than amorphous forms; this results in increased integrity/stability of
the protein drug in crystalline lattice. Protein crystals technology offers advantages in
the form of effective remedy for protein aggregation; protein crystals can withstand
organic solvents and water-organic solvent interfaces, are stable in storage for years
without significant degradation, can potentially achieve highest loading efficiency,
higher bioavailability, are easy to handle, and provide sustained release due to slower
dissolution of protein crystals than in amorphous form (usual size range of protein
crystal is 0.1-100 m) [418,419] . Methods like crosslinking and coprecipitation with
metal ions or some other stabilizing agents can further increase the stability of frag-
ile crystals. Controlled release depends upon characteristics of proteins such as size,
shape, degree of crosslinking, and presence of excipients [163,420-422] .
A specific advantage of protein crystallization can be achieved in therapeutic
monoclonal antibodies as they typically need high doses for administration. Their
crystalline form makes it possible to inject a high concentration of monoclonal anti-
bodies subcutaneously for controlled release rather than intravenous infusion, at
low viscosity, with good syringeability. MAb products such as herceptin, remicade,
and rituxan have been crystallized and explored for advantages of protein crystals.
Crystalline monoclonal antibodies have shown similar pharmacokinetic profiles as
soluble monoclonal antibodies following intravenous injection, and it was found that
crystallization had not affected the physicochemical properties, secondary structure,
or bioactivity of the antibody [423] .
Altus Pharmaceuticals has developed various technologies to crystallize many
proteins, including therapeutic monoclonal antibodies and crosslinked enzymes
crystals (CLEC) [423] . Protein crystals can also be useful for oral or other deliv-
ery means, and clinical trials are underway for parenteral and oral delivery [423] .
Technologies related to protein crystals have a wide range of application, from con-
trolled release to cell culture, fermentation, and purification.
A few examples of successful protein crystals are recombinant insulin [422] , crys-
talline hGH [424] , aprotinin, Apo2L, and amylase [418] ,which has also been used to
produce long-acting formulations.
However, full fledge application of crystal formulations has not yet been achieved
to a significant extent as crystallization methods include various steps such as evapo-
ration, cooling, precipitation, melts, and supercritical crystallization, and all P/P are
not able to withstand many of these process conditions due to their labile physico-
chemical properties. However, with recent advances in the area of protein crystalliza-
tion and stabilization of crystal products, such obstacles are being reduced to a great
extent [68,425] .