CLINICAL PHARMACOLOGY
Mechanism Of Action
Carglumic acid is a synthetic
structural analogue of N-acetylglutamate (NAG) which is produced from glutamate
and acetyl-CoA in a reaction catalyzed by N-acetylglutamate synthase (NAGS), a
mitochondrial liver enzyme. NAG acts as an essential allosteric activator of
Carbamoyl Phosphate Synthetase 1 (CPS 1), a mitochondrial liver enzyme which
catalyzes the first reaction of the urea cycle. The urea cycle, whose role is
the disposition of ammonia, includes a series of biochemical reactions in the
liver resulting in the conversion of ammonia into urea, which is then excreted through
the urine. Carglumic acid acts as a CPS1 activator in patients with NAGS
deficiency, thereby removing the block in the urea cycle and facilitating
ammonia detoxification and urea production.
Pharmacodynamics
In a retrospective review of
the clinical course in 23 patients with NAGS deficiency, carglumic acid reduced
plasma ammonia levels within 24 hours when administered with and without
concomitant ammonia lowering therapies. No dose response relationship has been
identified.
Pharmacokinetics
The pharmacokinetics of carglumic acid have been studied
in healthy male subjects using both radiolabeled and non-radiolabeled carglumic
acid.
Absorption
The median Tmax of CARBAGLU was 3 hours (range: 2 to 4
hours). Absolute bioavailability has not been determined.
Distribution
The apparent volume of distribution was 2657 L (range:
1616-5797). Protein binding has not been determined.
Elimination
Metabolism
A proportion of carglumic acid may be metabolized by the
intestinal bacterial flora. The likely end product of carglumic acid metabolism
is carbon dioxide, eliminated through the lungs.
Excretion
Median value for the terminal half-life was 5.6 hours
(range 4.3 to 9.5 hours), the apparent total clearance was 5.7 L/min (range 3.0
to 9.7 L/min), the renal clearance was 290 mL/min (range 204 to 445 mL/min),
and the 24-hour urinary excretion was 4.5% of the dose (range 3.5 to 7.5%).
Following administration of a single radiolabeled oral dose of 100 mg/kg of
body weight, 9% of the dose was excreted unchanged in the urine and up to 60%
of the dose was excreted unchanged in the feces.
Drug Interaction Studies
No drug interaction studies have been performed. Based on
in vitro studies, CARBAGLU is not an inducer of CYP1A1/2, CYP2B6, CYP2C, and
CYP3A4/5 enzymes, and not an inhibitor of CYP1A2, CYP2A6, CYP2B6, CYP2C8,
CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4/5 enzymes.
Clinical Studies
Responses Of Patients With NAGS Deficiency To Acute And Chronic
Treatment
The efficacy of CARBAGLU in the treatment of
hyperammonemia due to NAGS deficiency was evaluated in a retrospective review
of the clinical course of 23 NAGS deficiency patients who received CARBAGLU
treatment for a median of 7.9 years (range 0.6 to 20.8 years). Treatment with
CARBAGLU was divided in two regimens. For acute treatment, patients received a
total daily dose of 100 to 250 mg/kg per day primarily administered in 2 to 4
divided doses for the first few days of treatment. For maintenance treatment,
the dosage was reduced over time based upon biochemical and clinical responses.
The demographics characteristics of the patient
population are shown in Table 2.
Table 2: Baseline characteristics of the 23 NAGS
deficiency patients
|
|
Patients
N=23 |
Gender |
Male |
14 (61%) |
Female |
9 (39%) |
Age at initiation of CARBAGLU therapy (years) |
Mean (SD) |
2 (4) |
Min-Max |
0-13 |
Age groups at initiation of CARBAGLU therapy |
<30 days |
9 (39%) |
>30 days - 11 months |
9 (39%) |
≥1 - 13 years |
5 (22%) |
NAGS gene mutations by DNA testing |
homozygous |
14 (61%) |
heterozygous |
4 (17%) |
Not available |
5 (22%) |
Patients current treatment status |
On-going |
18 (78%) |
Discontinued |
5 (22%) |
The clinical observations in the 23 patient case series were retrospective, unblinded and uncontrolled and
preclude any meaningful formal statistical analyses of the data. However,
short-term efficacy was evaluated using mean and median change in plasma
ammonia levels from baseline to days 1 to 3. Persistence of efficacy was
evaluated using long-term mean and median change in plasma ammonia level. Table
3 summarizes the plasma ammonia levels at baseline, days 1 to 3 post-CARBAGLU
treatment, and long-term CARBAGLU treatment for 13 evaluable patients. Of the
23 NAGS deficiency patients who received treatment with CARBAGLU, a subset of
13 patients who had both well documented plasma ammonia levels prior to
CARBAGLU treatment and after long-term treatment with CARBAGLU were selected
for analysis.
All 13 patients had abnormal
ammonia levels at baseline. The overall mean baseline plasma ammonia level was
271 micromol/L. By day 3, normal plasma ammonia levels were attained in
patients for whom data were available. Long-term efficacy was measured using
the last reported plasma ammonia level for each of the 13 patients analyzed
(median length of treatment was 6 years; range 1 to 16 years). The mean and
median ammonia levels were 23 micromol/L and 24 micromol/L, respectively, after
a mean treatment duration of 8 years.
Table 3: Plasma ammonia
levels at baseline and after treatment with CARBAGLU
Timepoint |
Statistics
(N = 13*) |
Ammonia** (micromol/L) |
Baseline (prior to first treatment with CARBAGLU) |
N |
13 |
Mean (SD) |
271 (359) |
Median |
157 |
Range |
72-1428 |
Missing Data |
0 |
Day 1 |
N |
10 |
Mean (SD) |
181 (358) |
Median |
65 |
Range |
25-1190 |
Missing Data |
3 |
Day 2 |
N |
8 |
Mean (SD) |
69 (78) |
Median |
44 |
Range |
11-255 |
Missing Data |
5 |
Day 3 |
N |
5 |
Mean (SD) |
27 (11) |
Median |
25 |
Range |
12-42 |
Missing Data |
8 |
Long-term Mean: 8 years Median: 6 years 1 to 16 years (last available value on CARBAGLU treatment) |
N |
13 |
Mean (SD) |
23 (7) |
Median |
24 |
Range |
9-34 |
Missing Data |
0 |
* 13/23 patients with complete
short-term and long-term plasma ammonia documentation
** Mean ammonia normal range: 5 to 50 micromol/L |
The mean plasma ammonia level at baseline and the decline that is observed after treatment with CARBAGLU in
13 evaluable patients with NAGS deficiency is illustrated in Figure 1.
Figure 1: Ammonia response
for 13 evaluable NAGS deficiency patients at baseline and after treatment with
CARBAGLU