RECOMBINANT ALBUMIN FUSION PROTEINS - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 158

10

RECOMBINANT ALBUMIN FUSION PROTEINS

T HOMAS W EIMER ,H UBERT J. M ETZNER , AND S TEFAN S CHULTE

Preclinical Research and Development, CSL Behring GmbH, Marburg, Germany

10.1 Concept
10.2 Technological aspects
10.3 Typical applications and indications
10.4 Successes and failures in preclinical and clinical research
10.5 Challenges
10.6 Future perspectives
10.7 Conclusion
Acknowledgment
References

Human serum albumin (HSA) is a naturally occurring
and widely distributed carrier protein. It is the most abundant
plasma protein and, together with immunoglobulin G (IgG),
accounts for 80-90% of all plasma proteins, with a serum
concentration of 35-50mg/mL [3,4]. HSA is produced in the
liver, is highly stable, and is found mainly in the vascular
compartment. Structurally, HSA is composed of 585 amino
acids and has a molecular weight of

67 kDa [5]. It is
composed of three structurally homologous domains (DI,
DII, and DIII), each of which contains two subdomains (A
and B) connected by flexible loops [3]. Functionally, HSA
helps to regulate osmotic pressure and pH balance [4,6], and
acts as a transporter of many insoluble and hydrophobic
ligands, including fatty acids and amino acids [3]. However,
HSA has no enzymatic activity and is not

10.1 CONCEPT

involved in

immune functioning.
The half-life of albumin in humans is 19-22 days, while
most circulating proteins have a half-life, that is measured in
a few days or hours [3,6]. The remarkably long half-life of
albumin is due in part to pH-dependent recycling mediated
by the neonatal Fc receptor (FcRn) [7,8]. FcRn resides in
cells and on the surface of cells that have direct contact with
circulating blood, such as vascular endothelial cells. Similar
to IgG, circulating albumin is pinocytosed by these cells and
binds to the FcRn. The complexes IgG-FcRn-albumin,
FcRn-IgG, and FcRn-albumin avoid lysosomal degradation
and instead are redirected to the cell membrane. Exposure to
the physiological pH of circulating blood releases IgG and
albumin back into the bloodstream.
In the case of large therapeutic proteins, it is thought
that this recycling mechanism contributes to the prolonged
half-life of the fusion protein compared to the therapeutic
protein by itself. For smaller peptides, however, both the
recycling mechanism and the increase in the molecular

Many peptides and small proteins identified in drug develop-
ment have failed to produce the desired effect in vivo because
of a short half-life, often measured in minutes or hours [1].
The relatively small size of these peptides and molecules
places them below the threshold for renal clearance. While
several therapeutic proteins with a short half-life have made it
to the clinic, their application is often limited by the need for
frequent administration, which precludes long-term use, hin-
ders compliance, and potentially increases treatment costs
and the risk of adverse events. Fusion of these proteins with
well-characterized proteins with a longer half-life, such as
recombinant albumin, may prolong their time in circulation
and thus, increase their efficacy. Recombinant albumin fusion
proteins are novel proteins created by fusing the complemen-
tary DNA (cDNA) sequence encoding albumin with a
sequence encoding a therapeutic protein. The resulting hybrid
polypeptide retains the biological activity of the therapeutic
protein but has enhanced pharmacokinetic properties due to
the inherent long half-life of albumin [2].