The use of antisense oligonucleotides in treating MG is largely based on pioneering research carried out by Soreq et al18
at the Hebrew University, Israel.
Antisense oligonucleotides are designed to selectively hybridize with AChE m-RNA: this activates RNAses with subsequent degradation of the mRNA-antisense complex and aborted synthesis of the encoded protein. shows the role of AChE-R, its mechanism of action and its interferences by antisense treatment. The antisense molecule then released binds to another strand of mRNA, thus creating a recycling mechanism.16
Figure 1 Schematic representation of AChE-R role and mechanism of action and its interferences by antisense treatment. (A) AChE inhibition leads to the overstimulation of AChRs, with the subsequent induction of AChE-R pre mRNA. The rise of soluble AChE-R enhances (more ...)
EN101, or Monarsen, is an antisense molecule 2′-oxymethylated at its 3′-terminal position, targeting at exon 2 of the AChE mRNA, a position common to all the AChE m-RNA splicing isoforms;18
yet AChE-R m-RNA appears to be more susceptible to destruction compared with AChE-S m-RNA, since the transcript is longer, less G-C rich,19
and devoid of polyribosomes. Monarsen, both by intravenous or oral administration, has been shown to reduce AChE-R in EAMG rats muscle and plasma, enhancing muscle strength as indicated by electromyography, in that it corrected the compound muscle action potentials (CMAPs) during repeated stimulation, in a dose-dependent manner, and induced improved task performance.12
Furthermore, in Phase Ib of an open-label study in patients with MG, orally administered Monarsen was shown to improve the Quantitative MG (QMG) score in 14 of the 16 patients examined. Inclusion criteria for the study were the following: MG with class II or higher-stage disease based on Myasthenia Gravis Foundation of America (MGFA) classification; and ACh-R-positive patients or patients with a clearly evident EMG neuromuscular function study. The patients had been on stable concomitant immunomodulatory treatment for at least 3 months, receiving at least 180 mg of pyridostigmine per day, in stable clinical conditions. Patients with major muscle impairment or with exclusively mild ocular disease were excluded from the study.
Monarsen was orally administered for 4 days, after a 12–18-hour washout from pyridostigmine. Throughout the entire period, patients underwent a full check up and complete physical examinations including QMG score. Steroids treatment was continued during the whole period, while pyridostigmine was suspended and reintroduced when it was required. A follow-up assessment was made after 7, 14, and 28 days. Evaluation consisted of comparing baseline QMG (pretreatment) with QMG obtained during the experimental period. The QMG score summarizes 10 measurements: fatigue in each limb (4 measurements) and neck (1 measurement), swallowing rate (1 measurement), speech/counting (1 measurement), power in hands (2 measurements), and vital capacity on spirometry (1 measurement); plus three clinical observations: double vision, ptosis, and facial weakness. The maximum total QMG score is 39.
The overall mean percent of improvement from baseline was 46.5% (QMG score). More than 90% of the patients felt an improvement in symptoms during the treatment period. One patient failed to respond to Monarsen, remaining responsive to pyridostigmine, and was not included in the efficacy analysis. Adverse events were generally considered to be mild and they did not require suspension of the therapy.16
This study was not placebo-controlled. Neither the initial dose of pyridostigmine nor steroids/immunosuppressants were given, and, in contrast to animal trials,20
no plasma levels of AChE-R and AChR-S were given. Compared with pyridostigmine, Monarsen appeared to be more effective in enhancing muscle strength and providing a sustained QMG improvement, mainly in vital capacity and swallowing rate, over the course of the trial period.16
The latest publication regarding this compound is an announcement from Amarin Corporation (2009) about the encouraging results of an exploratory phase IIa multicenter, dose-ranging, crossover clinical trial with 31 participants. 22
The study was performed in six centers in UK, Israel, and Serbia. The primary objective of the exploratory study was to assess the efficacy and safety of three doses of EN101 each given orally once daily for 1 week in patients with MG, after a 1-week wash-out on pyridostigmine.
The final results of the study indicate that 10 mg, 20 mg, and 40 mg of EN101 resulted in a statistically significant reduction in QMG score from a baseline of 11.8% (P
= 0.001), 16.8% (P
< 0.001), and 20.3% (P
< 0.001), respectively. Furthermore, EN101 was shown to be safe and well tolerated.22