Amifampridine

Amifampridine is a drug, predominantly in the treatment of a number of rare muscle diseases.

Genetic variation in aryl N-acetyltransferase results in significant differences in the pharmacokinetic and safety profiles of amifampridine (3,4-diaminopyridine) phosphate.[Pubmed:25692017]

Pharmacol Res Perspect. 2015 Feb;3(1):e00099.

The clinical use of Amifampridine phosphate for neuromuscular junction disorders is increasing. The metabolism of Amifampridine occurs via polymorphic aryl N-acetyltransferase (NAT), yet its pharmacokinetic (PK) and safety profiles, as influenced by this enzyme system, have not been investigated. The objective of this study was to assess the effect of NAT phenotype and genotype on the PK and safety profiles of Amifampridine in healthy volunteers (N = 26). A caffeine challenge test and NAT2 genotyping were used to delineate subjects into slow and fast acetylators for PK and tolerability assessment of single, escalating doses of Amifampridine (up to 30 mg) and in multiple daily doses (20 mg QID) of Amifampridine. The results showed that fast acetylator phenotypes displayed significantly lower C max, AUC, and shorter t 1/2 for Amifampridine than slow acetylators. Plasma concentrations of the N-acetyl metabolite were approximately twofold higher in fast acetylators. Gender differences were not observed. Single doses of Amifampridine demonstrated dose linear PKs. Amifampridine achieved steady state plasma levels within 1 day of dosing four times daily. No accumulation or time-dependent changes in Amifampridine PK parameters occurred. Overall, slow acetylators reported 73 drug-related treatment-emergent adverse events versus 6 in fast acetylators. Variations in polymorphic NAT corresponding with fast and slow acetylator phenotypes significantly affects the PK and safety profiles of Amifampridine.

Effects of Food Intake on the Relative Bioavailability of Amifampridine Phosphate Salt in Healthy Adults.[Pubmed:26101174]

Clin Ther. 2015 Jul 1;37(7):1555-63.

PURPOSE: Amifampridine (3,4-diaminopyridine) has been approved in the European Union for the treatment of Lambert-Eaton myasthenic syndrome. Amifampridine has a narrow therapeutic index, and supratherapeutic exposure has been associated with dose-dependent adverse events, including an increased risk for seizure. This study assessed the effect of food on the relative bioavailability of Amifampridine in healthy subjects and informed on conditions that can alter exposure. METHODS: This randomized, open-labeled, 2-treatment, 2-period crossover study enrolled 47 healthy male and female subjects. Subjects were randomly assigned to receive 2 single oral doses of Amifampridine phosphate salt (20 mg base equivalents per dose) under fed or fasted conditions separated by a washout period. Blood and urine samples for pharmacokinetic analyses were taken before and after dosing. Plasma concentrations of Amifampridine and an inactive 3-N-acetyl metabolite were determined. The relative bioavailability values of Amifampridine and metabolite were assessed based on the plasma PK parameters AUC0-infinity, AUC0-t, and Cmax in the fed and fasted states using noncompartmental pharmacokinetic analysis. Parent drug and metabolite excretion were calculated from urinary concentrations. A food effect on bioavailability would be established if the 90% CI of the ratio of population geometric mean value of AUC0-infinity, AUC0-t, or Cmax between fed and fasted administration was not within the bioequivalence range of 80% to 125%. Tolerability was assessed based on adverse-event reporting, clinical laboratory assessments, physical examination including vital sign measurements, 12-lead ECG, and concurrent medication use. FINDINGS: Food slowed and somewhat decreased the absorption of Amifampridine. There was a decrease in exposure (Cmax, 44%; AUC, 20%) after oral administration of Amifampridine phosphate salt in the presence of food, and mean Tmax was 2-fold longer in the fed state. The extent of exposure and plasma elimination half-life of the major metabolite was greater than those of Amifampridine in the fed and fasted conditions. Mean AUCs in the fed and fasted states were slightly greater in women than men, with no difference in mean Cmax. Orally administered Amifampridine was renally eliminated (>93%) as the parent compound and metabolite within 24 hours. Single oral doses of 20 mg of Amifampridine phosphate salt were considered well tolerated in both the fed and fasted conditions. High intersubject variability (%CVs, >30%) in Amifampridine pharmacokinetic parameter values was observed. IMPLICATIONS: At the intended dose under fasting conditions, Amifampridine exposure may be increased. European Union Drug Regulating Authorities Clinical Trials identifier: 2011-000596-13.

Amifampridine phosphate (Firdapse((R))) is effective and safe in a phase 3 clinical trial in LEMS.[Pubmed:26852139]

Muscle Nerve. 2016 May;53(5):717-25.

OBJECTIVE: We evaluated the efficacy and safety of Amifampridine phosphate (Firdapse((R))) for symptomatic treatment in Lambert-Eaton myasthenic syndrome (LEMS). METHODS: Phase 3, randomized, double-blind, study. Patients were treated initially with Amifampridine phosphate for 7-91 days, followed by randomization to continue Amifampridine phosphate for 14 days or placebo (7-day taper, 7-day placebo). The primary efficacy endpoints were changes from baseline at day 14 in Quantitative Myasthenia Gravis and Subject Global Impression scores. RESULTS: The coprimary efficacy end points and 1 of the secondary efficacy end points were met, showing a significant benefit of aminfampridine phosphate over placebo at Day 14. All 5 primary, secondary, and tertiary endpoints achieved statistical significance at Day 8. Amifampridine phosphate was well tolerated; the most common adverse events were oral and digital paresthesias, nausea, and headache. CONCLUSIONS: This study provides Class I evidence of efficacy of Amifampridine phosphate as a symptomatic treatment for LEMS.