BONE-TARGETED ALKALINE PHOSPHATASE - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 413

28

BONE-TARGETED ALKALINE PHOSPHATASE

J OS E L UIS M ILL AN

Sanford Children's Health Research Center, Sanford-Burnham Medical Research Institute, La Jolla, CA, USA

28.1 Detailed description of the concept
28.2 Technical aspects
28.3 Applications and indications
28.4 Preclinical and clinical research
28.5 Alternatives/variants of this approach
28.6 Challenges in production and development
28.7 Conclusions and future perspectives
Acknowledgments
References

levels of calcium and phosphorus are generally normal or even
elevated [3]. The natural substrates of TNAP that accumulate
endogenously in HPP include inorganic pyrophosphate (PP i )
[4], an inhibitor of mineralization [5], and pyridoxal 5 0 -phos-
phate (PLP) [6,7], the major circulating form of vitamin B6 [8].
Rickets or osteomalacia develops in HPP because high extrac-
ellular levels of PP i block hydroxyapatite crystal growth.
Consequently, severely affected patients can acquire hypercal-
cemia and hyperphosphatemia [3]. High PP i concentrations
also lead to cementum, dentin, and alveolar bone phenotypes in
HPP [9]. Diminished hydrolysis of PLP by TNAP explains the
pyridoxine-responsive seizures observed in some severely
affected HPP patients [7,10] and invariably noted in untreated
Akp2 / (TNAP-knockout) mice [11-14], a murine model
that faithfully mimics infantile HPP.
HPP severity is inversely related to age of onset [3]. In its
most extreme form (the perinatal variety) HPP causes death in
utero or in early infancy. In these cases, there is profound
hypomineralization of the skeleton, frequently with foreshort-
ened and deformed limbs. Death generally results from
respiratory insufficiency due to pulmonary hypoplasia and
biomechanical compromise due to rachitic disease of the
chest. Radiographic examination reveals hypomineralization,
which in some cases may appear as almost total absence of
bony structures. Infants diagnosed with HPP generally appear
normal at birth but present with signs and symptoms within
the first 6 months of postnatal life, most often failure to thrive
or respiratory failure. Other clinical features may include
functional craniosynostosis, increased intracranial pressure
and papilledema, as well as the occurrence of nontraumatic
fractures. Radiographic examination reveals skeletal hypo-
mineralization and rickets. Hypercalcemia and hypercalciuria
are not uncommon and nephrocalcinosis may occur. Mortal-
ity, usually due to pulmonary complications caused by rib

28.1 DETA ILED DESC RIPTI ON OF THE
CONCEPT

Bone-targeted alkaline p hosphatas e (a.k.a . sALP- Fc-D 10 ,
ENB-0040 or asfo tase alfa) is a prot ein therape utic designed
to prevent and treat the clini cal man ifestations of hypo-
phosphata sia (HP P), a rar e inherited form of rickets and
osteomala cia cause d by inactivating mutat ions in the ALPL
gene, whi ch encodes the tissue-nons pecific isoz yme of

alkaline phospha tase (TNAP).
Hypophosphatasia is a highly variable disease. The severity
and type of inheritance depend on the nature of the ALPL
mutation and to date, 239 missense or splice site mutations
having been catalogued. 1 Th e p reva le nc e o f t he mo st s eve re
forms of the disease is 1:100,000, although it is markedly

higher in a small Canadian Mennonite population [1,2]. Inher-
itance can be autosomal recessive or dominant, and penetrance
is variable, resulting in wide clinical expressivity. HPP differs
from other forms of rickets and osteomalacia in that, that serum

1 http://www.sesep.uvsq.fr/03_hypo_mutations.php