STRUCTURAL ASPECTS OF FUSION PROTEINS DETERMINING THE LEVEL OF COMMERCIAL SUCCESS - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 39

3.1.3 Half-L ife Extensio n Represe nts a Domina nt
Theme (1c-7c)

While a longe r half-lif e can bring significa nt comme rcial
advantages, it shoul d be noted that fusion to a large partner,
such as an Fc doma in, can o bscure and impai r the FP's
activity and thi s is pote ntially worsened by the dimer ization
of mos t Fc fusions [4]. An FP may also exhibit a shorter half-
life than expected due to confo rmationa l change s o r steric
hindranc e effects resulting from fusio n to the partner com -
ponent [6]. Strategies undertak en to improve half-life may
not theref ore yield the benefit s hoped for, although reduc -
tions in pote ncy can be offset by superior pharmac okinetic
propert ies [4]. Biogen Idec acquired Synton ix in 2007,
gaining its monome ric Fc technol ogy that aims to avoid
the steric hindranc e typical of Fc dimer s.
Endogenous IgG Fc domains have effector functions,
inducing antibody-dependent cell-mediated cytotoxicity
(ADCC) and complement-dependent cytotoxicity (CDC)
that can be either beneficial for efficacy or a safety concern.
These functions are generally achieved via interaction with Fc
receptors on effector cells or via complement binding. Devel-
opers of Fc fusions seek to modify sequences and block
effector function where resulting reactions such as cell killing,
cytokine release, or inflammation are undesirable. Strategies
to improve the immunological profile of Fc fusions include
humanization, hybridization (e.g., Genexine's HyFc), and
removal of sugar chains (deglycosylation).
Despite the challe nges, Fc-additio n represe nts the mos t
widely u sed FP half-lif e extensi on approach. Indee d, Fc
fusions account for almost half of the FPs in Phase II or more
advanced . We consi der this to have been driven in part by the
experience with Fc doma ins gain ed thr ough mAb develop-
ment. It also suppor ted by the fact that there are six marketed
or approved Fc fusions: Pfiz er/Amge n's Enbrel 1 (etaner-
cept), Amgen' s Nplate 1 (romi plostim) , Astella s' Ame-
vive 1 (alefacept), Bristol-Myers Squibb's Orencia 1
(abatacept), Regeneron's Arcalyst 1 (rilonacept), and Bris-
tol-Myers Squibb's Nulojix 1 (belatacept). Enbrel is the
most successful FP to date, generating over $7 billion in
2010 to account for the bulk of total FP sales (see Chapter 2).
As such, Fc fusions are well validated in both clinically and
commercially compared to other novel or less-tested half-
life extension technologies.

Of the 43 FPs identified, 20 invol ve partners that have a
primary function of convey ing an ext ended half-life to the
a c t ivi t y c o m p o n e n t . R evol u t i o n s i n t h e a r e a s o f g e n e t i c s
and biotechnology have give n rise to t he discovery of
pr oteins with highly s pecific activity on targets that a re
either nove l, or were not previously druggabl e with small
m o l e c u l e a p p r o a c h e s . H owever, t h e s e b i o l o g i c s t r u c t u r e s
can oft en be afflicted by unfavorable pha rmacokinetics.
Natural m et abolic pr ocesses resul t i n the activity of a
protei n d im inishi ng and in vivo half-lives can be less
than 3 min [1]. Increasi ng the p lasma h alf-li fe of a p rotein
ca n ei ther a llow it to b ec ome a viable t he ra p eut ic opti o n or
w h e r e ex t e n d e d f u r t h e r, g ive a c o m m e r c i a l a d van t a g e ove r
competitors requi ring more c umbersome admi ni st ration
reg ime s. A qua rt er of the h al f-life ex tensi on FPs relate
to next -generati on ve rsions of ex isting m arket ed t herapeu-
tic proteins that are designe d to r equire less-frequent
dosing. In addition to dosing bene fits, greater efficacy
can pot entially result from the active component remaining
a t t h e r a p e u t i c a l l y r e l eva n t l evel s f o r l onger, while the
avoidance of dosing spikes associated with short-live d
molecules can reduce associated side effects.
The half-life of a protein at sites of action in the body is
determined by a balance of factors, such as tissue biodistri-
bution, receptor interactions, enzymatic cleavage sites,
immune system recognition, susceptibility to processing in
the liver, and rate of renal clearance. Available strategies to
increase half-life, therefore, seek to address a number of these
susceptibilities. The technologies employed can offer differ-
ent advantages or disadvantages on a case-by-case basis.

3.1.3.1 Fc Fusion is the Half- Life Extens ion Standar d
Aside from seque nce modifi cations to reduc e degradation,
the FPs iden tified with dedi cated half-lif e extending part ners
are designed to exploit a dual strat egy. First, addition of a
larger partner protein (fus ion or conjugated) can reduce
clearance in the kidney [2]. Almo st all of thes e FPs use
addition of an immun oglobulin (IgG) Fc doma in (Fc
fusions), which conveys further half-life extension by bind-
ing to the neonatal Fc recep tor (FcRn) , and as a result,
protects the partner protei n from bein g broke n down. Plas ma
proteins are ordi narily intern alize d by endothel ial cel ls and
targeted for degradation by the lysosom e; however, those
bound to FcRn are instead recycled back to the cell surface
and protected [3]. Pfizer's BeneFIX (rFactorIX) typicall y
requires 2-3 inject ions per week to be given for the p rophy-
lactic treat ment for hemophi lia B, due to its h alf-life of

3.1.3.2 Albumin Fusion is the Most Advanced non-FcFP
Half-Life Approach Albumin is the most abundant human
plasma protein, with a physiological role as a carrier of
blood substances. 1 Like IgG Fc, it has a long half-life of
around 19 days and interacts with FcRn, albeit at distinct and
separate sites. However, human albumin does not convey the
effector functions associated with unmodified IgG Fc
regions. Furthermore, dependent on the fused partner com-
ponent, albumin fusions can be produced in yeast (as well as

18 h. By comparison , the half-lif e of Biogen Idec's Phase
II/III candidate rFIX-Fc is approxim ately thr ee to four times
longer [4]. Th e immu noglobulins IgG1 , IgG2, and IgG4
have seru m half-lives of 21 days [5].

1 http://www.biospace.com/News/novozymes-biopharma-presents-
research-on/206971