CLINICAL PHARMACOLOGY
Mechanism Of Action
Hereditary angioedema (HAE) is a rare genetic disorder
caused by mutations to C1-esterase-inhibitor (C1-INH) located on Chromosome 11q
and inherited as an autosomal dominant trait. HAE is characterized by low
levels of C1-INH activity and low levels of C4. C1-INH functions to regulate
the activation of the complement and intrinsic coagulation (contact system
pathway) and is a major endogenous inhibitor of plasma kallikrein. The
kallikrein-kinin system is a complex proteolytic cascade involved in the
initiation of both inflammatory and coagulation pathways. One critical aspect
of this pathway is the conversion of High Molecular Weight (HMW) kininogen to
bradykinin by the protease plasma kallikrein. In HAE, normal regulation of
plasma kallikrein activity and the classical complement cascade is therefore
not present. During attacks, unregulated activity of plasma kallikrein results
in excessive bradykinin generation. Bradykinin is a vasodilator which is
thought by some to be responsible for the characteristic HAE symptoms of
localized swelling, inflammation, and pain.
KALBITOR is a potent (Ki = 25 pM), selective, reversible
inhibitor of plasma kallikrein. KALBITOR binds to plasma kallikrein and blocks
its binding site, inhibiting the conversion of HMW kininogen to bradykinin. By
directly inhibiting plasma kallikrein, KALBITOR reduces the conversion of HMW
kininogen to bradykinin and thereby treats symptoms of the disease during acute
episodic attacks of HAE.
Pharmacodynamics
No exposure-response relationships for KALBITOR to
components of the complement or kallikrein-kinin pathways have been
established.
The effect of KALBITOR on activated partial
thromboplastin time (aPTT) was measured because of potential effect on the
intrinsic coagulation pathway. Prolongation of aPTT has been observed following
intravenous dosing of KALBITOR at doses ≥ 20 mg/m² . At 80 mg
administered intravenously in healthy subjects, aPTT values were prolonged
approximately two-fold over baseline values and returned to normal by 4 hours
post-dose.
For patients taking KALBITOR, no significant QT
prolongation has been seen. In a randomized, placebo-controlled trial (EDEMA4)
studying the 30 mg subcutaneous dose versus placebo, 12-lead ECGs were obtained
at baseline, 2 hours and 4 hours post-dose (covering the time of expected Cmax),
and at follow-up (day 7). ECGs were evaluated for PR interval, QRS complex, and
QTc interval. KALBITOR had no significant effect on the QTc interval, heart
rate, or any other components of the ECG.
Pharmacokinetics
Following the administration of a single 30 mg subcutaneous
dose of KALBITOR to healthy subjects, a mean (± standard deviation) maximum
plasma concentration of 586 ± 106 ng/mL was observed approximately 2 to 3 hours
post-dose. The mean area under the concentration-time curve was 3017 ± 402
ng*hr/mL. Following administration, plasma concentration declined with a mean
elimination half-life of 2.0 ± 0.5 hours. Plasma clearance was 153 ± 20 mL/min
and the volume of distribution was 26.4 ± 7.8 L. Based on a population
pharmacokinetic analysis, body weight, age, and gender were not found to affect
KALBITOR exposure significantly. Ecallantide is a small protein (7054 Da) and
renal elimination in the urine of treated subjects has been demonstrated.
No pharmacokinetic data are available in patients or
subjects with hepatic or renal impairment.
Clinical Studies
The safety and efficacy of KALBITOR to treat acute
attacks of hereditary angioedema in adolescents and adults were evaluated in 2
randomized, double-blind, placebo-controlled trials (EDEMA4 and EDEMA3) in 168
patients with HAE. Patients having an attack of hereditary angioedema, at any
anatomic location, with at least 1 moderate or severe symptom, were treated
with 30 mg subcutaneous KALBITOR or placebo. Because patients could participate
in both trials, a total of 143 unique patients participated. Of the 143
patients, 94 were female, 123 were Caucasian, and the mean age was 36 years
(range 11-77). There were 64 patients with abdominal attacks, 55 with
peripheral attacks, and 24 with laryngeal attacks.
In both trials, the effects of KALBITOR were evaluated
using the Mean Symptom Complex Severity (MSCS) score and the Treatment Outcome
Score (TOS). These endpoints evaluated attack severity (MSCS) and patient
response to treatment (TOS) for an acute HAE attack.
MSCS score is a point-in-time measure of symptom
severity. At baseline, and post-dosing at 4 hours and 24 hours, patients rated
the severity of each affected symptom on a categorical scale (0 = normal, 1 = mild,
2 = moderate, 3 = severe). Patient-reported severity was based on each
patient's assessment of symptom impact on their ability to perform routine
activities. Ratings were averaged to obtain the MSCS score. The endpoint was
reported as the change in MSCS score from baseline. A decrease in MSCS score
reflected an improvement in symptom severity; the maximum possible change
toward improvement was -3.
TOS is a measure of symptom response to treatment. At 4
hours and 24 hours post-dosing, patient assessment of response for each
anatomic site of attack involvement was recorded on a categorical scale
(significant improvement [100], improvement [50], same [0], worsening [-50],
significant worsening [-100]). The response at each anatomic site was weighted
by baseline severity and then the weighted scores across all involved sites
were averaged to calculate the TOS. A TOS value > 0 reflected an improvement
in symptoms from baseline. The maximum possible score was +100.
EDEMA4
EDEMA4 was a randomized, double-blind, placebo-controlled
trial in which 96 patients were randomized 1:1 to receive KALBITOR 30 mg
subcutaneous or placebo for acute attacks of HAE. The primary endpoint was the
change from baseline in MSCS score at 4 hours, and the TOS at 4 hours was a key
secondary endpoint. Patients treated with KALBITOR demonstrated a greater
decrease from baseline in the MSCS than placebo and a greater TOS than patients
with placebo and the results were statistically significant (Table 2). At 24
hours, patients treated with KALBITOR also demonstrated a greater decrease from
baseline in the MSCS than placebo (-1.5 vs. -1.1; p = 0.04) and a greater TOS
(89 vs. 55, p = 0.03).
Table 2: Change in MSCS Score and TOS at 4 Hours
|
EDEMA4 |
EDEMA3 |
KALBITOR
(N=48) |
Placebo
(N=48) |
KALBITOR
(N=36) |
Placebo
(N=36) |
Change in MSCS Score at 4 Hours |
n |
47 |
42 |
34 |
35 |
Mean |
-0.8 |
-0.4 |
-1.1 |
-0.6 |
95% CI |
0. - .0, - |
-0.6, -0.1 |
-1.4, -0.8, |
-0.8, -0.4 |
P-value |
0.010 |
0.041 |
TOS at 4 Hours |
n |
47 |
42 |
34 |
35 |
Mean |
53 |
8 |
63 |
36 |
95% CI |
39, 68 |
-12, 28 |
49, 76 |
17, 54 |
P-value |
0.003 |
0.045 |
MSCS: Mean Symptom Complex Severity
TOS: Treatment Outcome Score
CI: confidence interval |
More patients in the placebo group (24/48, 50%) required
medical intervention to treat unresolved symptoms within 24 hours compared to
the KALBITOR-treated group (16/48, 33%).
Some patients reported improvement following a second 30
mg subcutaneous dose of KALBITOR, administered within 24 hours following the
initial dose for symptom persistence or relapse, but efficacy was not
systematically assessed for the second dose.
EDEMA3
EDEMA3 was a randomized, double-blind, placebo-controlled
trial in which 72 patients were randomized 1:1 to receive KALBITOR or placebo
for acute attacks of HAE. EDEMA3 was similar in design to EDEMA4 with the
exception of the order of the prespecified efficacy endpoints. In EDEMA3, the
primary endpoint was the TOS at 4 hours, and the key secondary efficacy
endpoint was the change from baseline in MSCS at 4 hours. As in EDEMA4,
patients treated with KALBITOR demonstrated a greater decrease from baseline in
the MSCS than placebo and a greater TOS than patients treated with placebo and
the results were statistically significant (Table 2).
In addition, more patients in the placebo group (13/36,
36%) required medical intervention to treat unresolved symptoms within 24 hours
compared to the KALBITOR-treated group (5/36, 14%).