Biology Reference
In-Depth Information
are simultaneously present. Simultaneous administration by fusing two drugs
can simplify administration, especially when the two proteins have different
pharmacokinetic parameters. The ability to independently adjust dosing of the
fusion partners is lost, however. Furthermore, the difficulty in retaining a non-
immunogenic structure has been a challenge and neither EPO/IL-3 nor
EPO/GM-CSF fusion molecules have been approved for use in humans.
EPO dimer has been generated as a potential therapeutic [46, 47]. In gener-
al,the increased size can reduce clearance because of slowed transport out of
the serum compartment [46]. EPO dimer may also have increased
in vitro
activity due to altered avidity to the receptor [48].
The two protein partners are typically joined by a linker peptide that includ-
ed Gly, Ala, and Ser. These three amino acids are thought to result in linkers
that are flexible and relatively inert, allowing independent folding of the two
proteins into their appropriate three-dimensional structures. Full
in vitro
activ-
ity of both proteins in fusion proteins does not always occur [44]. The fusion
proteins have been reported to have increased risk of immunogenicity [49],
presumably because of altered folding or stability.
EPO mimetics
rHuEPO is currently administered by either subcutaneous or intravenous injec-
tion. Because of its large size and peptidic nature, delivery by other routes such
as oral or transdermal or by inhalation can be quite challenging. The potential
use of rHuEPO for treatment of neuronal trauma by promoting neuronal sur-
vival [50] is limited by its poor transport across the blood brain barrier [51].
One possible solution is a small molecule, an EPO mimetic, capable of stimu-
lating EPOR. EPO mimetics are compounds that mimic the activity of EPO
but bear no structural homology. EPO mimetics can have new biological or
biophysical properties not present in EPO. Designed appropriately,such a
compound has the potential to be delivered by routes that are more convenient
than currently in use for rHuEPO.
Significant challenges are associated with the identification and develop-
ment of a useful small molecule EPO mimetic. First, the need to be small to
be delivered orally is confounded by the need to be large enough to have suf-
ficient affinity for EPOR to be effective. The compound must have appropri-
ate pharmacokinetic parameters so that it persists sufficiently long in the serum
to be efficacious. Finally, the compound should not have unwanted side-effects
due to either toxicities of the compound itself or breakdown products of it. In
spite of these challenges, work has proceeded and progress has been made in
attempts to identify lead compounds that may be amenable to oral delivery.
Several strategies have been used to identify EPO mimetics. The first is to
screen peptide and small molecule libraries for those compounds that can stim-
ulate erythropoiesis using
in vitro
bioassays as screens. According to this strat-
egy, an understanding of the mechanism of activation is not necessary, and
