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WASHINGTON -- An investigational drug improved muscle strength and function in patients with Lambert-Eaton myasthenic syndrome (LEMS), researchers reported here.

In a randomized, controlled trial, amifampridine phosphate (Firdapse) slowed disease worsening compared with placebo, according to , of the University of Alabama at Birmingham, and colleagues.

The drug "should be the standard symptomatic drug in LEMS on the basis of this study," Oh said during a presentation at the American Academy of Neurology annual meeting.

LEMS is a rare autoimmune disease characterized by proximal muscle weakness. Autoantibodies attack voltage-gated calcium channels, causing a reduction in the amount of acetylcholine released by nerves.

The condition can mimic myasthenia gravis, but it has a different etiology and course, Oh said. It is estimated to affect some one in 100,000 patients, and about half of cases also develop small-cell lung cancer.

Although Firdapse has been approved in Europe since December 2009, there is no FDA-approved drug for symptomatic treatment in the U.S., although some drugs are used to mitigate symptoms, including pyridostigmine (Mestinon), compounded 3.4-DAP, and guanidine.

However, Firdapse won both orphan drug and breakthrough therapy designation from the FDA in 2013.

To evaluate its efficacy in LEMS, Oh and colleagues conducted a phase III multicenter, double-blind, placebo-controlled trial of 38 patients at 14 sites.

The primary endpoints were a quantitative exam of muscle strength called QMG score and outcomes on the Subjective Global Impression (SGI) test. Secondary endpoints were Clinical Global Impression-Improvement (CGI-I) and 25-foot walk test.

Oh and colleagues found greater disease worsening as measured by QMG score in placebo patients compared with those on the drug over a 2-week period (2.2 versus 0.4, P=0.0452).

Placebo patients also had greater worsening as measured by SGI score (-2.6 versus -0.8, P=0.028).

For secondary outcomes, placebo patients also had greater worsening compared with drug patients in terms of CGI scores (4.7 versus 3.6, P=0.0267).

However, there was no significant change in 25-foot walking scores between the two groups.

Oh noted that drug-treated patients had a positive, rapid response that was seen within 8 days of starting on the drug, and an excellent overall response rate throughout the study.

Given the results, the researchers conducted an open-label extension phase in which 34 of 38 patients stayed at the same or lower dose for up to 2 years. That study is ongoing.

There were no serious adverse events attributable to the drug, which was generally safe and well tolerated. The side effects were benign, consisting of perioral and digital paresthesia, gastrointestinal disorders, and infections.

of NYU Langone Medical Center in New York City, who was not involved in the study, called the trial well-designed and said it demonstrates that the drug can "provide patients with [LEMS] a real treatment choice for improving the walking and breathing difficulties they typically have."

"These positive findings will hopefully lead to an FDA-approved therapy for this rare disease," Bhatt said.

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