Amifampridine

Amifampridine blocks voltage-dependent potassium channels, thereby prolonging pre-synaptic cell membrane depolarisation. Prolonging the action potential enhances the transport of calcium into the nerve ending. The resulting increase in intra-cellular calcium concentrations facilitates exocytosis of acetylcholine-containing vesicles, which in turn enhances neuromuscular transmission.

It improves muscle strength and resting compound muscle action potential (CMAP) amplitudes with an overall weighted mean difference of 1.69 mV (95% CI 0.60 to 2.77).

Pharmacodynamic effects

The pharmacodynamic profile of amifampridine has been studied for a range of doses. A prospective, placebo-controlled, randomised study in 26 patients with Lambert-Eaton myasthenic syndrome (LEMS) reported clinical efficacy for amifampridine at the standard recommended maximum dose of 60 mg/day (Sanders et al 2000). Two further studies in a total of 57 patients with LEMS have reported data from higher doses of amifampridine. McEvoy et al 1989 reported data from a short-term study in 12 patients with LEMS, which demonstrated that administration of amifampridine at doses up to 100 mg/day for a period of 3 days was effective in treating the autonomic and motor symptoms of LEMS. Sanders et al 1998 presented data on efficacy and safety of amifampridine treatment at doses up to 100 mg/day in 45 patients with LEMS who were treated for an average of 31 months. Therefore, in exceptional circumstances higher doses up to a maximum of 80 mg/day may be of benefit when given with the appropriate safety monitoring. It is recommended that dose titration from 60 mg/day to 80 mg/day is performed in 5 mg increments every 7 days. Upward dose titration should be discontinued if any adverse reaction or ECG abnormality is observed.

The effect of a single dose of 30 mg or 60 mg of amifampridine phosphate was used to evaluate the pharmacokinetic-QTc relationship of amifampridine concentration on cardiac repolarization exposure in healthy volunteers. This evaluation was conducted in a Phase 1, double-blind, randomized, crossover study to define the ECG effects of amifampridine phosphate at these doses compared to placebo and moxifloxacin (a positive control) in healthy men and women who are slow acetylators (n=52). There was no effect of amifampridine phosphate on heart rate, atrioventricular conduction or cardiac depolarization as measured by the heart rate, PR and QRS interval durations. No subjects developed new clinically relevant ECG morphological changes following administration of amifampridine phosphate. No effect was observed of amifampridine phosphate on cardiac repolarization as assessed using the QTc interval.

Absorption

Orally administered amifampridine is rapidly absorbed in humans, reaching peak plasma concentrations by 0.6 to 1.3 hours (mean values).

In humans, the rate and extent of absorption of amifampridine is influenced by food (see Table 1). There was a decrease in C_max and AUC, and an increase in the time to reach maximum plasma concentrations when amifampridine phosphate was administered with food as compared to without food. A 2-fold increase in the time to reach C_max (T_max) was observed in the presence of food. Similarly C_max and AUC_0-∞ were greater in the fasted state than in the fed state. Overall, food slowed and decreased the absorption of amifampridine with a lowering of exposure by C_max on average by ~44% and lowered exposure by AUC ~20%. based on geometric mean ratios (fed-to-fasted).

Apparent plasma terminal elimination half-life differences were 3-4 fold between subjects in the food effect study. Bioavailability is approximately 93-100% based on recoveries of unmetabolised amifampridine and a major 3-N-acetylated amifampridine metabolite in urine.

Table 1. PK Parameters for Amifampridine in Fed and Fasted Subjects Following Administration of a Single Oral Dose of Amifampridine Phosphate:

^* Eating a standardised high-fat meal

In a study of healthy volunteers, systemic exposure of amifampridine was notably influenced by the overall metabolic acetylation activity of NAT enzymes and NAT2 genotype. The NAT genes are highly polymorphic and result in phenotypes with variable acetylation activity rates ranging from slow to fast. In the healthy volunteer study, fast acetylators were defined by having a caffeine metabolite ratio >0.3 and slow acetylators with a caffeine metabolite ratio <0.2. There was significantly higher exposure to amifampridine in slow acetylators compared to fast acetylators. Statistically significant differences in amifampridine PK parameters C_max, AUC_0-∞, t_1/2 and apparent clearance was observed between fast and slow acetylators at all dose levels. In this study, slow acetylators experienced more adverse reactions than the fast acetylators. The safety profile in this study is consistent with adverse reactions observed with patients on amifampridine.

Table 2. Mean PK Parameters of Amifampridine in Healthy Subjects after Single Oral Doses (5-30mg) in Slow and Fast Acetylator Phenotypes:

The mean caffeine acetylator ratio for these 12 subjects receiving four escalating doses were 0.408 and 0.172 for fast and slow acetylators types respectively.

Distribution

Distribution of amifampridine was studied in the rat. Following oral administration of radiolabelled [¹⁴C] amifampridine, radioactive material is rapidly absorbed from the gastrointestinal tract and widely distributed throughout the body. Concentrations in tissues are generally similar to or greater than concentrations in plasma, with the greatest concentration in organs of excretion (liver, kidney and the gastrointestinal tract) and some tissues of glandular function (lacrimal, salivary, mucous, pituitary and thyroid glands).

Biotransformation

In vitro and in vivo studies in humans indicate that amifampridine is metabolised to a single major 3-N-acetylated amifampridine metabolite.

Elimination

In humans, 93.2% to 100% of amifampridine is excreted into the urine within 24 hours after dosing as amifampridine (19%) and its 3-N-acetylated amifampridine metabolite (74.0% to 81.7%). The plasma elimination half-life is approximately 2.5 hours for the amifampridine and 4 hours for the 3-N-acetylated amifampridine metabolite.

The overall clearance of amifampridine is predominantly due to metabolism by N-acetylation and acetylator phenotype has a greater effect on an individual’s metabolism and elimination of amifampridine than does elimination by renal function (See Table 3).

Renal impairment

Exposure of amifampridine was generally higher in subjects with renal impairment than in subjects with normal renal function; however, NAT2 phenotype had a greater effect on an individual’s exposure to amifampridine than renal function status (See Table 3). Amifampridine exposure by AUC_0-∞ was up to 2-fold higher in slow acetylators and up to 3-fold higher in fast acetylators with severe renal impairment compared to subjects with normal renal function. Exposure by C_max was marginally affected by renal impairment regardless of acetylation status.

In contrast, the 3-N-acetyl metabolite exposure levels were affected to a greater extent by renal impairment than those for amifampridine. The 3-N-acetyl metabolite exposure by AUC_0-∞ was up to 6.8-fold higher in slow acetylators and up to 4-fold higher in fast acetylators with severe renal impairment compared to subjects with normal renal function. Exposure by C_max was only marginally affected by renal impairment regardless of acetylation status. Although the metabolite is inactive at potassium channels, potential off target effects due to accumulation are unknown.

Table 3. Mean PK Parameters of Amifampridine in Normal and Renal Impaired Subjects after Single Oral Dose Administration (10mg) in Slow and Fast Acetylator Phenotypes:

Hepatic impairment

There are no data on the pharmacokinetics of amifampridine in patients with hepatic impairment.

Paediatric population

There are no data on the pharmacokinetics of amifampridine in paediatric patients. The effect of age on the pharmacokinetics of amifampridine has not been studied.

In safety pharmacology studies in rats, no respiratory system related effects were seen up to 10 mg/kg or on the central nervous system up to 40 mg/kg.

In a repeat-dose toxicity studies in rats and dogs, effects on the central and autonomic nervous system, increased liver and kidney weights and cardiac effects (second degree atrioventricular block) were seen. No safety margins to human exposure were achieved in the animal studies due to the sensitivity of the animal models used.

In a 2-year rat dietary carcinogenicity study, amifampridine caused small but statistically significant dose-related increases in the incidence of Schwannomas in both genders and of endometrial carcinomas in females. The clinical relevance of these results is unknown.

Amifampridine was not genotoxic in a standard battery of in vitro and in vivo tests.

Animal studies evaluating the reproductive and developmental toxicity of amifampridine were conducted in rats and rabbits at doses up to 75 mg/kg/day. Amifampridine had no adverse reaction on male or female fertility in rats at doses up to 75 mg/kg/day, and no effect on post-natal development or fertility was observed in the offspring of the treated animals. In a perinatal/postnatal reproduction study in pregnant rats treated with amifampridine, a dose-related increase in the percentage of mothers with stillborn offspring (16.7%-20%) was observed at 22.5 mg/kg/day and 75 mg/kg/day (1.1 and 2.7 times the 80 mg per day dose in humans based on C_max). However, in a similar study in pregnant rabbits, there was no effect on embryo-foetal viability when evaluated just prior to birth at doses up to 57 mg/kg/day.