MONOMERIC Fc-FUSION PROTEINS - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 103

MONOMERIC Fc-FUSION PROTEINS

B AISONG M EI ,S USAN C. L OW , y S NEJANA K RASSOVA ,R OBERT T. P ETERS ,G LENN F. P IERCE ,
AND J ENNIFER A. D UMONT

Biogen Idec Hemophilia, Weston, MA, USA

7.1 Introduction
7.2 FcRn and monomeric Fc-fusion proteins
7.3 Typical applications
7.4 Alternative applications
7.5 Expression and purification of monomeric Fc-fusion proteins
7.6 Conclusions and future perspectives
References

Further proof came when it was observed that short serum
IgG half-life, along with low serum IgG levels occurred in
mice deficient in the FcRn heavy chain [8]. The only report
of a defect in FcRn in humans is for two family members
with familial hypercatabolic hypoproteinaemia [9]. The
clinical manifestation was that of low serum IgG levels and
the genetic defect was a single nucleotide transversion in
b 2m [10]. The half-life of IgG was 3daysinthese

siblings and although IgG synthesis was determined to be
normal, they each showed hypercatabolism of IgG. There-
fore, the critical role of FcRn in IgG homeostasis has been
demonstrated in what is essentially a naturally occurring
knockout for FcRn.
It is now known that FcRn plays various roles and is
present in many cell types and tissues [11-18]. The regions
of IgG that are critical for the control of IgG catabolism are
the CH2-CH3 domains of the Fc fragment [19]. FcRn has
been leveraged to prolong the half-life of therapeutic pep-
tides and proteins through fusion of the Fc domain (hinge-
CH2-CH3) of IgG with the therapeutic molecule of interest.
Because of the disulfide bond formation in the hinge region
of Fc, the traditional Fc fusion molecules were dimeric with
respect to both the Fc domain and also the therapeutic
moiety. Many effector molecules have been coupled to Fc
with the aim to extend half-life [20,21]. For example, soluble
receptors for cytokines, such as tumor necrosis factor (TNF)
and vascular endothelial growth factor (VEGF), have been
designed to function as inhibitors of their cognate cytokines
as an approach to regulate excessive levels of cytokines.
However, these soluble cytokine receptors have a relatively
short half-life and are cleared from the body rapidly. As an
approach to extend the half-life of these soluble receptors,
they were linked to the Fc domain of IgG1. Although fusion

7.1 INTRODUCTION

Immunoglobulin G (IgG) is one of the most abundant
proteins in the blood, present at 10-12mg/mL in the

circulation. Moreover, IgG has a uniquely long half-life
of 21 days in humans [1,2]. The persistence of IgG in

the circulation is attributed to the neonatal Fc receptor
(FcRn), which salvages IgG from intracellular lysosomal
degradation and recycles it from the sorting endosome back
to the cell surface and back into the circulation.
FcRn is a heterodimer comprising a transmembrane heavy
chain with structural homology to the extracellular domain of
the major histocompatibility complex (MHC) class I mole-
cules and a soluble light chain consisting of b 2-microglobulin
( b 2m) [3]. It was originally identified as the receptor that
transports IgG from mother's milk to the neonate in rodents
during the suckling period and was later found to also mediate
IgG homeostasis [4,5]. Evidence that FcRn was responsible
for IgG homeostasis came from mice that had genetically
deleted b 2m, which resulted in low serum IgG levels [6,7].

y Present address: Bind Biosciences, Cambridge, MA, USA