FUSION PROTEINS WITH A TARGETING FUNCTION - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 374

25.2.4 Bone

Bones are h ighly special ized parts of the body that constitute
the endosk eleton. Th erefore, it is very relevant to develop
specific drug delivery techniq ues to treat b one-relat ed dis-
eases, such as hypophos phatasia (H PP). In HPP, a critica l
enzyme, the tissue -nons pecific isozym e of alka line phos-
phatase (TNALP) is no nfunctional , which leads to extrac-
ellular accumul ation of phospha te cont aining substra tes that
cause ricket s or osteom alacia. A fusi on prot ein was designed
consisting o f a truncated TNALP wi thout its hy drophobic C-
terminus connec ted to an Fc-part and a deca-asp artate
sequence (D 10) to target miner alizing tissue . This construc t
could rescue mice wi th a TNALP deletion from all skel etal
and dental dise ase or epilepsy. Fu rthermor e, a posi tive
correlation betwee n dosi s and survival was dem onstrated
[43]. Th e bone-targeting sequence of asparta te repe ats was
first identified when analyzi ng several noncolla genous bone
proteins, whi ch cont ained all repe titive acidic amino acid
repeats o f aspar tate or glutam ate that could b ind to hydrox y-
apatite (HA) which is the main const ituent of hard tissue.
Fluoresc ein conju gated to hexameric asparta te had a bone
half-life of 14 days in contras t to 1 h for fluores cein alone
[44]. Several acidic oligopep tide-tagg ed drugs have b een
tested in the meantim e for their ability of bone targeting
[45]. More details on the alka line p hosphatas e fusi on protein
are explained in Cha pter 28.

FIGURE 25.2 Transport to and from the lung. The lung is an
entry point for therapeutic proteins as well as a target. (A) The
neonatal (N) Fc receptor binds antibodies, Fc-fusion proteins (D),
or albumin containing formulations. This transcytosis can transfer
proteins bidirectionally and relies on pH-dependent binding to N.
(B) In cases where pulmonary delivery is impossible, transport to
the lung can be facilitated through the polymeric immunoglobulin
receptor (P). It transfers polymeric antibodies such as IgA from the
basal to the apical side. This passage is essentially irreversible
because the secretory component (SC) to which IgA is attached to
is cleaved from the receptor.

25.2.5 Thr ombus Target ing

In complex organisms, not only organs, tissue s, and cells
exist, but also aggregates of large molecu les can be found.
Some of these aggregates fulfi ll a n atural functi on such as
blood clott ing, whi ch is an essential functional ity that allows
survival of injuries that otherwis e woul d lead to lethal blood
losses. However, vascular blood clots cause serious diseases
such as myoc ardial infarction, pulmona ry emboli sm, or deep
vein thromb osis. Treatment of these various forms of
thromb i is always accompa nied b y a hem orrhagi c risk.
This dilemma betwee n maximal thromb olysis and minimal
bleedin g risk has led to the development of novel fusion
proteins. They com bine the fibrinoly tic pote ntial of stap h-
ylokinas e (SA K) with the anticoa gula nt activity of hir udin
(HIR) and recog nition seque nces that target coagulatio n
factors.
SAK binds to plasminoge n; this complex then converts
plasminoge n to plasmin, which dissolves fib rin clots. Treat-
ment with SAK shoul d always be combine d with an anti-
coagulant to prevent the formation of secon dary clots. One
suitable agent is hirudin that bloc ks thromb in, thus prohi b-
iting the conversion of fibri nogen to fibrin. However, HIR
can cause systemic blee ding. Based on the experience that
blocking the N- terminus of HIR reduces activity and rapid
degradation of SAK as C-te rminal fusion partner the n ovel

domain has a high affinity to heparin and glycosaminoglycans
that can be found on surface of the respiratory epithelial [40].
Although the enzyme is not of human origin, a low level of
immunogenicity can be expected, because A. viscosus is a
normal inhabitant of human oral and gastrointestinal epithe-
lia. The concept of this drug avoids the generation of resist-
ances because it alters the structure of the human receptor and
does not directly address a viral protein. This fusion protein,
also called Fludase 1 , has shown both safety and efficacy in
animal studies [41,42]. Currently, Phase II clinical trials are
ongoing 1 .

1 http://clinicaltrials.gov/ct2/show/NCT01037205