Biomedical Engineering Reference
In-Depth Information
dose [55]. However, the AUC
INF
of rFVIIIFc was 1.48 and
1.56-fold greater than that of rFVIII at 25 IU/kg (p
the rFIXFc monomer construct has markedly improved PKs
after intravenous administration in rodents in comparison to
the dimer configuration.
Additionally, the PKs of rFIXFcmonomer clotting activity
was compared with recombinant FIX (BeneFIX, rFIX) in
FIX-deficient mice. Activity was measured by both a one-
stage clotting assay (aPTT) and by WBCT. Hemophilia B
mice were administered a single equivalent intravenous dose
of rFIXFc monomer or rFIX. The plasma FIX activity
declined in a bi-exponential manner following injection of
both rFIXFc and rFIX (Figure 7.4a). The systemic exposure
was markedly greater for rFIXFc compared to rFIX, with a
27% increase in C
max
and 3.4-fold increase in AUC, and the
half-life was also increased. Thus, the PK parameters are
greatly improved for the rFIXFc compared to rFIX.
The functional activity of rFIXFc was also evaluated
ex vivo by measuring the WBCT. rFIXFc monomer and rFIX
were administered as a single intravenous dose to hemo-
philia B mice and WBCT measured as described [61].
Fifteen minutes after dosing, normal clotting was observed
in all mice treated with either rFIXFc or rFIX. WBCT
returned to baseline by 72 h in all mice treated with rFIX,
whereas blood from rFIXFc mice still clotted normally. At
144 h, all mice treated with rFIXFc returned to baseline. The
percentage of animals able to clot blood at each time point is
shown in Figure 7.4b.
The PKs of rFIXFc and rFIX have also been studied in
FIX-deficient dogs and normal cynomolgus monkeys. In
FIX-deficient dogs and cynomolgus monkeys, the half-life
for rFIXFc was approximately 3- to 4-fold greater compared
to rFIX (Table 7.1). The dog model was one of severe
hemophilia B for which the mutation is similar to that found
in humans [62]. As a result, the concentration of rFIXFc in
the circulation could be readily measured by both activity
and antigen levels since there was no endogenous FIX
activity in these animals. The half-life for rFIXFc (38.3 h)
was markedly longer compared to that reported for rFIX
(17-18 h) in FIX-deficient dogs based on activity data
[62,63]. In the same way, in cynomolgus monkeys, the
half-life for rFIXFc was 47 h compared to 12.7 h for rFIX
reported earlier [63].
To prove the mechanism of action for the prolonged half-
life, the PK of rFIXFc was assessed in a murine FcRn
knockout model. As expected, the elimination half-life of
rFIXFcwas similar to rFIX in the FcRn knockout mice (16.9 h
for rFIXFc and 16.5 h for rFIX). However in mice that lacked
murine FcRn (mFcRn
¼
0.002)
and 65 IU/kg (p
0.001), respectively [55]. There were no
significant differences in incremental recovery between
rFVIII and rFVIIIFc. Therefore, within each patient, rFVIIIFc
demonstrated an improved PK profile compared with rFVIII.
The results from the Phase I/IIa study supported the develop-
ment of the Phase III study to evaluate the safety, PKs, and
efficacy of rFVIIIFc in previously treated patients with
severe hemophilia A (www.clinicaltrials.org identifier
NCT01181128). In conclusion, rFVIIIFc has been shown
to have improved PKs in humans and animal models such
as hemophiliaAmice and dogs. Treatment with this new long-
lasting product has the potential to result in less frequent
dosing and improved quality of life for hemophilia A patients.
<
7.3.4 Factor IX-Fc Monomer
Factor IX (FIX) is a 55 kDa serine protease that is an
essential component of the coagulation cascade [56-58].
Deficiency of FIX results from a X-linked inherited disorder
known as hemophilia B or Christmas disease [59] that
affects 1 in 25,000-30,000 males [45]. In severe form of
hemophilia B, levels of functional FIX are at
1
IU/dL), moderate form at 1-5% and mild at 5-30%. Plasma
derived FIX products (AlphaNine
1
and Mononine
1
) and
the recombinant FIX product (BeneFIX
1
[nonacog alfa])
are used in the on-demand and prophylactic treatment (2-3
times weekly) of hemophilia B. The coupling of recombi-
nant FIXwith recombinant Fc was pursued as an approach to
extend the half-life of FIX which could lead to a reduction in
the frequency of administration required for the treatment of
hemophilia B.
rFIXFc monomer and dimer were constructed and
expressed in a way similar to that described in Section
7.5. The PKs of rFIXFc monomer and dimer were evaluated
in adult rats and in adult FIX-deficient mice [60] after a
single intravenous dose [41]. The systemic exposure, meas-
ured by C
max
and AUC, of rFIXFc monomer was consider-
ably greater (three to fivefold) compared to that of the
rFIXFc dimer in rats and mice (Table 7.1). The PKs of
the rFIXFc dimer and rFIXFc monomer were also compared
in neonatal rats after oral administration where the uptake of
the rFIXFc monomer was much greater compared to the
rFIXFc dimer after a similar molar dose, with a 10-fold
greater C
max
and greater than 12-fold increase in AUC
(Table 7.1).
In rats (neonatal and adult), the terminal half-life of the
monomer was also greater compared to the rFIXFc dimer.
However, in FIX-deficient mice, the half-lives for rFIXFc
monomer and dimer were similar. In both models, the
recovery of the monomer appeared to be greater compared
to the dimer. Moreover, the half-life for both rFIXFc fusion
proteins was markedly longer compared to commercially
available rFIX (BeneFIX) (22 and 33 h vs. 5.8 h). Overall,
1% (
<
<
), but were transgenic
for hFcRn (hFcRn
þ
/
þ
,h
b
2m
þ
/
þ
), the half-life of FIXFc
was 53.0 h while rFIX remained unchanged (14.2 h) com-
pared to the FcRn knockout mice. These data confirm that
FcRn mediates the prolonged half-life of rFIXFc [61].
rFIXFc was evaluated in an open-label Phase I/IIa dose
escalation study in previously treated patients (n
/
,m
b
2m
/
14) with
severe hemophilia B at intravenous doses ranging from 1 to
100 IU/kg [64,65]. The lowest
¼
two rFIXFc dose levels
Search WWH ::
Custom Search