A higher dose of Adamas Pharmaceuticals Inc.'s experimental multiple sclerosis drug improved patients' walking speed in a late-stage trial, but the company opted not to immediately commence a planned second late-stage study due to a smaller-than-expected clinical benefit.

Emeryville, Calif.-based Adamas' stock was down 42% to $4.23 as of 11:18 a.m. ET on Dec. 17.

The phase 3 trial called Inroads evaluated Adamas' drug candidate ADS-5102 against placebo in 594 multiple sclerosis patients with walking impairment, the company said in a Dec. 17 press release. Patients were randomized to receive 274 milligrams of ADS-5102, 137 milligrams of ADS-5102 or placebo for 12 weeks.

Among patients treated with the higher dose, 21.1% saw at least a 20% improvement in walking speed from baseline compared to 11.3% of placebo-treated patients, achieving statistical significance. However, only 17.6% of patients receiving the lower dose saw the same degree of improvement, missing the primary goal.

Neither dose met statistical significance on secondary goals, which included mean change in a timed walk, a mobility test and a two-minute walk.

During the trial, 20.5% of patients receiving the higher dose discontinued treatment due to adverse events, compared to 6.4% of patients treated with the low dose and 3.8% taking placebo. The most common adverse events were dry mouth, falling, constipation, urinary tract infection, insomnia and swelling.

Adamas will complete data analyses of the Inroads trial to better evaluate the efficacy and safety of ADS-5102, and hold off on initiating a second identical phase 3 study.

"As we did not see the scale of clinical benefit we had hoped for in this study we will fully assess the potential for ADS-5102 in MS patients before determining the extent of our continued investment in this program," Adamas CEO Neil McFarlane said.

The company previously won U.S. Food and Drug Administration approval for the therapy, marketed as Gocovri, for involuntary muscle movements known as dyskinesia in certain Parkinson's disease patients.