CLINICAL PHARMACOLOGY
Mechanism Of Action
The mechanism by which amifampridine exerts its
therapeutic effect in LEMS patients has not been fully elucidated.
Amifampridine is a broad spectrum potassium channel blocker.
Pharmacodynamics
Cardiac Electrophysiology
The effect of RUZURGI on QTc interval prolongation was
studied in a double-blind, randomized, placebo-and positive-controlled study in
52 healthy volunteers (including 23 subjects with poor metabolizer phenotype).
Study participants were administered 120 mg RUZURGI in 4 equal doses of 30 mg
at 4-hour intervals (Dose 1, 2, 3, and 4)[see Pharmacogenomics]. RUZURGI
did not prolong the QTc interval to any clinically relevant extent. In vitro,
RUZURGI did not inhibit the human ether-Ã -go-go-related gene ion channel.
Pharmacokinetics
The pharmacokinetics of amifampridine form RUZURGI is
approximately dose proportional. Steady state was generally reached within 1
day of dosing. Multiple dosing resulted in no accumulation of amifampridine and
only moderate accumulation of the 3-N-acetyl amifampridine metabolite [see Pharmacogenomics].
Absorption
The absolute bioavailability of RUZURGI has not been assessed.
RUZURGI is absorbed in an approximately dose-proportional manner with a median
time to maximum concentration (tmax) of 0.5 hours post administration.
Effect Of Food
Compared to administration of RUZURGI in the fasting
state, administration of the 20 and 30 mg dose levels of RUZURGI with a
standard high fat meal resulted in significant decreasein Cmax (41% and 52%,
respectively) and an increase in median tmax to 1.0 hour; AUC0-last was only
significantly reduced for the 30 mg dose (23%) [see DOSAGE AND
ADMINISTRATION].
Distribution
In healthy volunteers, the volume of distribution for
plasma amifampridine indicated that RUZURGI is a drug with a moderate to high
volume of distribution.
In vitro human plasma protein binding of amifampridine
and 3-N-acetyl amifampridine was 25.3% and 43.3%, respectively.
Elimination
Metabolism
In vitro studies with complimentary DNA expressed human
N-acetyltransferase (NAT) enzyme preparations indicate that amifampridine is
rapidly metabolized by the N-acetyltransferase 2 (NAT2) enzyme to the
3-N-acetyl amifampridine metabolite. Metabolism of amifampridine by
Nacetyltransferase 1 (NAT1) may also occur but at a much slower rate.
Amifampridine does not undergo glucuronidation or
sulfonation.
Excretion
Following oral administration of a single 20 or 30 mg
dose of RUZURGI to healthy volunteers, amifampridine apparent oral clearance
(CL/F) was 149 to 214 L/h, the average elimination half-life (t½) was 3.6 to
4.2 hours. The average t½ of the 3-N-acetyl amifampridine metabolite was 4.1
to 4.8 hours.
The majority (>65%) of RUZURGI administered to healthy
volunteers was recovered in urine as either the parent compound or the
3-N-acetyl amifampridine metabolite.
Specific Populations
Pediatric Patients (6 To Less Than 17 Years Of Age)
A population pharmacokinetic analysis showed that body
weight significantly correlates with the clearance of amifampridine; clearance
increased with an increase in body weight. A weight-based dosing regimen is
necessary to achieve amifampridine exposures in pediatric patients 6 to less
than 17 years of age similar to those observed in adults at effective doses of
RUZURGI [see INDICATIONS AND USAGE and Clinical Studies].
Drug Interaction Studies
In Vitro Studies
Amifampridine is not metabolized by cytochrome P450
(CYP)1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, or CYP3A4.
In vitro studies with human liver microsomes indicated
that amifampridine and 3-N-acetyl amifampridine were not direct or
time-dependent inhibitors of CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19,
CYP2D6, CYP2E1, or CYP3A4.
In vitro studies in cryopreserved human hepatocytes
indicated that amifampridine did not induce CYP isoforms CYP1A2, CYP2B6, or
CYP3A4.
Based on in vitro studies with Caco-2 cells amifampridine
is unlikely to act as a substrate or inhibitor of the P glycoprotein
transporter. Amifampridine is not an inhibitor of the BCRP transporter.
In vitro studies with Chinese hamster ovary cells
expressing human OATP1B1, OATP1B3, OAT1, and OCT2 and Madin-Darby canine kidney
cells expressing human OAT3 indicated that amifampridine is a weak inhibitor of
OCT2, but is not an inhibitor of OAT1, OAT3, OATP1B1, or OATP1B3. The studies
also indicated that amifampridine is not a substrate for OAT1, OAT3, or OCT2
transporters.
In Vivo Studies
Controlled clinical drug interaction studies have not
been performed with RUZURGI.
Co-administration of intravenous amifampridine and
intravenous pyridostigmine led to a 21% elevation in maximum pyridostigmine
serum concentrations, but did not significantly affect the pharmacokinetics of
amifampridine [see DRUG INTERACTIONS].
Pharmacogenomics
Genetic variants in the N-acetyltransferase gene 2 (NAT2)
affect the rate and extent of RUZURGI metabolism. In normal healthy volunteers,
poor metabolizers, also referred to as “slow acetylators” (i.e., carriers of
two reduced function alleles) had higher average plasma amifampridine
concentrations than intermediate metabolizers, also referred to as
“intermediate acetylators” (i.e., carriers of one reduced and one normal
function alleles), and normal metabolizers, also referred to as “fast/rapid
acetylators” (i.e., carriers of two normal function alleles).
In the TQT study [see Pharmacodynamics], poor
metabolizers (N=19) had 1.1 to 3.7 times higher AUC0-4h and 1.3 to 3.7 times
higher Cmax than intermediate metabolizers (N=21), following the first dose.
Poor metabolizers had 6.0 to 8.5 times higher AUC0-4h and 6.1 to 7.6 times
higher Cmax than normal metabolizers (N=3), following the first dose.
In the general population, the NAT2 poor metabolizer phenotype
prevalence is 40–60% in the White and African American populations, and in
10–30% in Asian ethnic populations (individuals of Japanese, Chinese, or Korean
descent).
Clinical Studies
The efficacy of RUZURGI for the treatment of LEMS was
established by Study 1, a randomized, double-blind, placebo-controlled,
withdrawal study (NCT: 01511978). Study 1 enrolled patients with an established
diagnosis of LEMS, confirmed by documentation and an independent neurologist
review. Patients were required to be on an adequate and stable dosage (30 mg to
100 mg daily for at least 3 months) of RUZURGI prior to entering the study.
The primary measure of efficacy was the categorization of
the degree of change (e.g., greater than 30% deterioration) in the Triple Timed
Up and Go test (3TUG) upon withdrawal of active medication, when compared with
the time-matched average of the 3TUG assessments at baseline.
The 3TUG is a measure of the time it takes a person to
rise from a chair, walk 3 meters, and return to the chair for 3 consecutive
laps without pause. Higher 3TUG scores represent greater impairment.
The secondary efficacy endpoint was the self-assessment
scale for LEMS-related weakness (WSAS), a scale from -3 to 3 assessing a
person's feeling of weakening or strengthening from baseline. A higher positive
W-SAS score indicates a perceived greater improvement of strength. A more
negative score indicates perceived greater weakening.
After an initial open-label run-in phase, 32 patients
were randomized in a double-blind fashion to either continue treatment with
RUZURGI (n = 14) or switch to placebo over a 3-day downward titration (n = 18)
period. Following the downward titration period, patients remained on blinded
RUZURGI or placebo for 16 more hours. Efficacy was assessed 2 hours after the
last dose of the downward titration period. Patients were allowed to use stable
dosages of peripherally-acting cholinesterase inhibitors or oral
immunosuppressants. Seventy-nine percent of patients randomized to RUZURGI were
receiving cholinesterase inhibitors, versus 83% in the placebo group, and 29%
of patients randomized to RUZURGI were receiving an immunosuppressant therapy,
versus 39% in the placebo group.
Randomized patients had a median age of 56 years (range:
23 to 83 years), 66% were female, and 91% were White. Ninety-seven percent of
patients had a diagnosis of autoimmune LEMS, and 3% of patients had a diagnosis
of paraneoplastic LEMS.
All 32 patients completed the study. None of the patients
randomized to continue RUZURGI experienced a greater than 30% deterioration in
the final post-dose 3TUG test. In contrast, 72% (13/18) of those randomized to
placebo experienced a greater than 30% deterioration in the final 3TUG test (p <
0.0001). Patients who were randomized to placebo returned to baseline after
restarting RUZURGI. Figure 1 shows the time course of the mean percent change
from baseline on the 3TUG during the double-blind phase and with reinitiation
of RUZURGI.
Figure 1: Mean Percent Change From Baseline in
Post-dose 3TUG Time During the Double-blind Phase of the Study and Return to Baseline
Upon Reinitiation of RUZURGI
The W-SAS score showed a significantly greater decrease
in patients randomized to placebo (2.4) than in those who continued treatment
with RUZURGI ( -0.2; p < 0.0001), indicating that patients who were
randomized to placebo perceived a worsening of weakness compared to those who
remained on RUZURGI.