CLINICAL PHARMACOLOGY
Mechanism Of Action
The mechanism by which amifampridine exerts its therapeutic effect in LEMS patients has not beenfully elucidated. Amifampridine is a broad spectrum potassium channel blocker.
Pharmacodynamics
The effect of FIRDAPSE on QTc interval prolongation was studied in a double blind, randomized, placebo and positive controlled study in 52 healthy individuals who are slow acetylators. At an exposure 2-fold the expected maximum therapeutic exposure of amifampridine, FIRDAPSE did not prolongQTc to any clinically relevant extent.
Pharmacokinetics
The pharmacokinetics of amifampridine are similar between healthy individuals and LEMS patients. Following single and multiple doses, AUC, Cmax and Cmin were highly variable between individuals. FIRDAPSE exposure increased proportionally with dose across the range of 20 mg to 80 mg single oral doses.
Absorption
Amifampridine peak plasma concentration is reached 20 minutes to 1 hour after administration. Fooddoes not have a clinically
significant effect onthe exposure of amifampridine.
Elimination
Amifampridine is eliminated primarily through metabolism to 3-N-acetyl-amifampridine and to a smaller extent through the kidneys.
The terminal half-life ranges from 1.8 to 2.5 hours in healthy subjects. Metabolism Amifampridine is extensively metabolized by N-acetyltransferase 2 (NAT2) to 3-N-acetyl-amifampridine, which is consideredan inactive metabolite.
Excretion
Following administration of FIRDAPSE to healthy subjects, 93% to 100% of the administered dose was eliminated in the urine as amifampridine or 3-N-acetyl amifampridine over 24 hours.
Specific Populations
Patients With Renal Impairment
Pharmacokinetic data are available from a study of 24 otherwise healthy subjects with impaired renal functionwho received a single 10-mg dose of FIRDAPSE. The exposure of amifampridine (measuredas AUC) was 2-to 3-foldhigher in subjects withmoderate (CLcr 30-59 mL/min) or severe (CLcr 15-29 mL/min) renal impairment than in subjects with normal renal function (CLcr greater than or equal to 90 mL/min). Compared with subjects with normal renal function, subjects withmild renal impairment (CLcr 60-89 mL/min) had a 36% increase inexposure. Therefore, FIRDAPSE should be initiatedat the lowest recommended starting dosage (15 mg/day) in patients with renal impairment, and such patients should be closely monitored for adverse reactions [see DOSAGE AND ADMINISTRATION and Use In Specific Populations]. Cmax was marginally affected by renal impairment.
Pharmacogenomics
Genetic variants in the N-acetyltransferase gene 2 (NAT2) affect the rate and extent of FIRDAPSE metabolism. Poor metabolizers, also referred to as “slow acetylators” (i.e., carriers of two reduced function alleles), have 3.5-to 4.5-fold higher Cmax, and 5.6-to 9fold higher AUC than normal metabolizers, also referred to as “fast/rapid acetylators” (i.e., carriers of two normal function alleles). Therefore, FIRDAPSE should be initiated at the lowest recommended starting dosage (15 mg/day) in known NAT2 poor metabolizers, and such patients should be closely monitored for adverse reactions [see DOSAGE AND ADMINISTRATION and Use In Specific Populations]. 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 FIRDAPSE for the treatment of LEMS was demonstrated in two randomized, double-blind, placebo-controlled discontinuation studies. A total of 64 adults (age 21 to 88 years) with LEMS were enrolled (Study 1 and Study 2). The studies enrolled patients with a confirmed diagnosis of LEMS based on either neuro physiology studies or a positive anti-P/Q type voltage-gated calcium channel anti body test. Patients were required to be on an adequate and stable dosage (30 to 80mg daily) of amifampridine phosphate prior to entering the randomized discontinuation phases of both studies.
The two co-primary measures of efficacy in both studies were the change from baseline to the end of the discontinuation period in the Quantitative Myasthenia Gravis (QMG) score and in the Subject Global Impression (SGI) score.
The QMG is a 13-item physician-rated categorical scale assessing muscle weakness. Each item is assessed on a 4-point scale, where a score of 0 represents no weakness, and ascore of 3 represents severe weakness (total score 0-39). Higher scores represent greater impairment.
The SGI is a 7-point scale on which patients rated their global impression of the effects of the study treatment on their physical well being. Lower scores on the SGI represent lower perceived benefit with the study treatment.
A key secondary efficacy endpoint was the clinical global impression improvement (CGI-I) score, a 7-point scale on which the treating physician rated the global impression of change in clinical symptoms. A higher CGI-I score indicates a perceived worsening of clinical symptoms.
Study 1 (NCT01377922)
After an initial open-label run-in phase, 38 patients were randomized in a double-blind fashion to either continue treatment with FIRDAPSE (n=16) or to a downward titration to placebo (n=22) over 7 days. Following the downward titration period, patients remainedonblindedFIRDAPSE orplacebofor7moredays.Efficacywas assessedatDay14ofthedouble-blindperiod.Patients were allowed to use stable dosages of peripherally acting cholinesterase inhibitors or oral immunosuppressants. Twenty-six percent of patients randomized to FIRDAPSE were receiving cholinesterase inhibitors, versus 36% in the placebo group, and 28% of patients randomized to FIRDAPSE were receiving oral immunosuppressant therapies, versus 34% in the placebo group.
Patients had a median age of 54 years (range: 21 to 88 years), 61% were female, and 90% were White. Eighty-four percent of patients had a diagnosis of autoimmune LEMS, and 16% of patients had a diagnosis of paraneoplastic LEMS.
During the double-blind period (from Baseline to Day 14), the QMG scores tended to worsen in both treatment groups, but there was significantly greater worsening in the placebo group than in the FIRDAPSE group (p=0.045). Similarly, the SGI score tended to worsen in both treatment groups during the double-blind period, but there was significantly greater worsening in the placebo group than in the FIRDAPSE group (p=0.003), as summarized in Table 2. These results indicate that in Study 1, patients randomized to placebo had a significantly greater worsening of muscle weakness and of global impression of the effects of the study treatment on their physical well-being,compared to patients who continued FIRDAPSE in the double-blind period.
Table 2.Change from Baseline to Day 14 in QMG Score and SGI Score in Study 1
Assessment |
FIRDAPSE (n=16) |
Placebo (n=21) |
PrimaryEndpoints |
QMG Scorea |
Baseline (mean) |
6.4 |
5.6 |
Change from Baseline (Least Square Mean) |
0.4 |
2.2 |
FIRDAPSE-placebo
Treatment Difference (Least Square Mean (95% CI)) |
–1.7 (–3.4, –0.0) |
p-valuec |
0.045 |
SGI Scoreb |
Baseline (mean) |
5.6 |
5.9 |
Change from Baseline (Least Square Mean ) |
-0.8 |
-2.6 |
FIRDAPSE-placebo
Treatment Difference, (Least Square Mean (95% CI)) |
1.8 (0.7, 3.0) |
p-valuec |
0.003 |
a.QMG Score range 0 (no impairment) to 39 (worst impairment)
b.SGI Score range 0 (least perceived benefit) to 7 (most perceived benefit)
c.Pairwise contrast at Day 14 from mixed-effects model with repeated measures. |
The CGI-I score was significantly greater for patients randomized to placebo than for patients who continued treatment with FIRDAPSE, with a mean difference between FIRDAPSE and placebo of -1.1 (p=0.02), indicating that clinicians perceived a greater worsening of clinical symptoms in patients who were randomized to placebo and discontinued from FIRDAPSE treatment, compared to patients who continued FIRDAPSE in the double-blind period.
Study 2 (NCT02970162)
Patients on stable treatment with FIRDAPSE were randomized 1:1 in a double-blind fashion to either continue treatment with FIRDAPSE (n=13) or change to placebo (n=13) for 4 days. Efficacy was assessed at the end of the 4-day double-blind discontinuation period. Patients were allowed to use stable doses of peripherally acting cholinesterase inhibitors or corticosteroids. Sixty-one percent ofpatientsrandomizedtoFIRDAPSE were receiving cholinesterase inhibitors,versus 54% of patients randomized to placebo. Corticosteroid use was similar between FIRDAPSE and placebo (8%). Patients with recent use of immunomodulatory therapies (e.g., azathioprine, mycophenolate, cyclosporine), rituximab, intravenous immunoglobulin G, and plasmapheresis were excluded from the study. Patients had a median age of 55.5 years (range: 31 to 75 years), 62% were female, and 88% were White.
From Baseline to Day 4, there was significantly greater worsening in the QMG score in the placebo group than in the FIRDAPSE group (p=0.0004), and also significantly greater worsening in the SGI score in the placebo group than in the FIRDAPSE group (p=0.0003), as summarized in Table 3. These results indicate that in Study 2, patients randomized to placebo had a significantly greater worsening of muscle weakness and of global impression of the effects of the study treatment on their physical well-being, compared to patients who continued FIRDAPSE in the double-blind period.
Table 3.Change from Baseline to Day 4 in QMG Scores and SGI Scores in Study 2
Assessment |
FIRDAPSE (n=13) |
Placebo (n=13) |
QMG Scoresa |
Baseline, Mean |
7.8 |
8.5 |
Change from Baseline, Least Square Meanc |
0.00 |
6.54 |
FIRDAPSE-placebo
Treatment Difference, Least Square Mean (95% CI) |
-6.54 (-9.78, -3.29) |
p-valued |
0.0004 |
SGI Scoresb |
Baseline, Mean |
6.1 |
5.8 |
Change from Baseline, Least Square Meanc |
-0.64 |
-3.59 |
FIRDAPSE-placebo
Treatment Difference, Least Square Mean (95% CI) |
2.95 (1.53, 4.38) |
p-valued |
0.0003 |
a.QMG Score range 0 (no impairment) to 39 (worst impairment)
b.SGI Score range 0 (least perceived benefit) to 7 (most perceived benefit)
cChange from baseline for QMG total score was modeled as the response, with fixed effects terms for treatment and QMG at Baseline.
d p-value based on the Wilcoxon Rank Sum Test for treatment differences. |
The clinical global impression improvement (CGI-I) score was significantly greater for patients randomized to placebo than for patients who continued treatment with FIRDAPSE, with a mean difference between FIRDAPSE and placebo of -2.7 (p=0.002), indicating that clinicians perceived a greater worsening of clinical symptoms in patients who were randomized to placebo and discontinued from FIRDAPSE treatment,compared to patients who continued FIRDAPSE in the double -blind period.