Biomedical Engineering Reference
In-Depth Information
FIGURE 6.1
Factors influencing the bioavailability of a therapeutic protein. Proteins disappear
from circulation by proteolysis or metabolism. In the kidney, they are excreted if their size permits
passage through the glomeruli. Absorption or transfer into specific organs or compartments can
influence the bioavailability as well as receptor-mediated recycling.
demonstrated by a rational design of granulocyte colony-
stimulating factor (G-CSF). Optimizing the receptor binding
for the different pH conditions in the endosome and on
the surface by histidine mutants improved the half-life by
1 order of magnitude [4]. The most important recycling
system is the neonatal receptor for the fraction crystallizable
(FcRn). This receptor handles both, Fc-containing mole-
cules such as antibodies and serum albumin (HSA), but
binding occurs at different positions of the FcRn, hence there
is no competition between both molecules.
A good example for the different strategies of half-life
prolongation is glucagon-like peptide (GLP-1). The first
optimization was mutagenesis to introduce resistance to
dipeptidyl peptidase-4 (DPP IV). With GLP-1, a plethora
of methods has been tested; therefore, it can serve as
benchmarking tool [5]. Some of the currently published
data is summarized in Table 6.1. Exenedin-4 acts similar to
GLP-1 but has naturally a longer half-life. Nevertheless this
peptide has been optimized with fusion partners as well
(Table 6.2).
Recently, a global view on half-life extension strategies
summarized several nonfusion protein based techniques [6].
In Figure 6.2, molecular mass of many therapeutic and
natural proteins versus their half-life in humans is plotted.
Most fusion proteins are positioned in the upper right
quadrant, indicating that they have been optimized with
regard to half-life. The following paragraphs aim to give an
overview on the various fusion concepts, but also serve as
brief description of strategies that could not be included as
full chapters in this topic.
6.2 HALF-LIFE EXTENSION THROUGH SIZE
AND RECYCLING
6.2.1 Direct Genetic Fusions
6.2.1.1 Albumin Fusions Human serum albumin, the
most abundant serum protein with a size of 66 kDa and a
diameter of 90A
, is above the limit of fast kidney clear-
ance. Therefore it remains in circulation with a half-life of
up to 19 days [7]. Both, albumin and gamma globulins are
recycled through the neonatal Fc receptor. This receptor is
present on many epithelia. The primary function of the
FcRn during infancy is the absorption of maternal protec-
tive antibodies from the milk through the intestinal epi-
thelium. The FcRn in the intestine is downregulated for
adults; however, FcRn remains present on vascular cells.
The critical regions both on the FcRn and albumin contain
several conserved histidines that stabilize flexible loops
[8]. Antibodies or albumin are taken up through pino-
cytosis. When the pH in the endosome is lowered, these
Search WWH ::
Custom Search