(Adds analyst comment, background; updates shares)

By Amrutha Penumudi

Sept 29 (Reuters) - Tonix Pharmaceuticals Holding Corp said its lead drug for pain management failed the main goal of a mid-stage study, sending its down by more than half in early trading.

The drug, TNX-102, is being tested to treat fibromyalgia, a chronic syndrome characterized by widespread pain and lack of sleep.

The treatment failed the main goal of reducing pain experienced by patients by the 12th week of the trial, Tonix said.

"I think it's a major setback. Don't forget, the drug is already available as a generic. To me, that means the data had to be outstanding. Very tough when you miss your primary endpoint," SeeThruEquity analyst, Ajay Tandon told Reuters.

However, TNX-102 was shown to improve sleep quality in patients, with some responders also experiencing a 30 percent reduction in pain, the company added.

The company said that despite the drug failing the main goal of the study, data on TNX-102 would support late-stage clinical trials and that it plans to discuss the design for a late-stage trial with the FDA soon.

Analysts believe the company may now redesign the late-stage study of TNX-102 to focus on sleep-quality improvement instead of pain management.

The company said mid-stage data also showed TNX-102 could be beneficial in treating post-traumatic stress disorder (PTSD), considering its positive effects on sleep quality. Tonix expects to begin a mid-stage trial on the drug for the PTSD indication in the fourth quarter.

"We expect the shares to trade down significantly at the open, but we do believe that the drug has a future," Roth Capital Partners analysts said in a note.

Fibromyalgia is said to be associated with an imbalance of certain chemicals in the brain.

According to the National Fibromyalgia Association, 3-6 percent of the world's population suffers from fibromyalgia syndrome.

Other drugs that have been approved for the condition include Savella, made by a subsidiary of Actavis Plc, and Cymbalta, which is manufactured by Eli Lilly and Co.

Tonix shares were trading down about 45.3 percent at $7.63 on the Nasdaq. The stock fell about 58 percent to a low of $5.85 earlier in the session.

(Editing by Simon Jennings)