Biomedical Engineering Reference
In-Depth Information
Figure 11.5 Three-dimensional structure of the engineered fast-acting insulin, 'Insulin lispro'. Structural
details courtesy of the Protein Data Bank, http://www.rcsb.org/pdb/
which it is least soluble) from 5.4 to a value approaching 7.0. The engineered insulin is expressed
in a recombinant E. coli K12 host strain and is produced via the 'proinsulin route' as described
previously. The purifi ed product is formulated at pH 4.0, a pH value at which it is fully solu-
ble. Upon s.c. injection, the insulin experiences an increase in pH towards more neutral values
and, consequently, appears to precipitate in the subcutaneous tissue. It resolubilizes very slowly
and, hence, a greatly prolonged duration of release into the bloodstream is noted. Consequently,
a single daily injection supports the maintenance of acceptable basal blood insulin levels, and
insulin molecules are still detected at the site of injection in excess of 24 h after administration.
Levemir (tradename) is an alternative engineered long-acting insulin product that gained ap-
proval for general medical use in 2004 (Table 11.3). This differs from native insulin in that it is
devoid of the threonine B30 residue and (more importantly from a pharmacokinetic perspective)
contains a 14-carbon fatty acid residue covalently attached to the side chain of lysine residue B29.
This allows the insulin to bind reversibly to albumin, both at the site of injection and in plasma
(albumin contains three high-affi nity fatty acid binding sites). This, in turn, ensures constant and
prolonged release of free insulin, bestowing upon it a prolonged duration of action of up to 24 h.
Product manufacture entails initial expression of insulin in engineered Saccharomyces cerevisiae ,
purifi cation and acylation (attachment of the fatty acid group).
The generation of engineered insulin analogues raises several important issues relating to prod-
uct safety and effi cacy. Alteration of a native protein's amino acid sequence could render the
engineered product immunogenic. Such an effect would be particularly signifi cant in the case of
insulin, as the product is generally administered daily for life. In addition, alteration in structure
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