By Natalie Grover

(Reuters) - Corbus Pharmaceuticals Holdings Inc said its only drug outperformed a placebo in a mid-stage study involving patients with a form of systemic sclerosis, an incurable autoimmune condition caused by abnormal growth of connective tissue.

Shares of the Norwood, Massachusetts-based company shot up nearly 125 percent to $13.15 in premarket trading on Monday.

The 42-patient trial tested the drug, resunab, against a placebo in patients with diffuse cutaneous systemic sclerosis, the less common form of the rare condition that has a higher risk of death.

Patients on resunab scored an average of 33 percent on an index that evaluates changes in skin hardening, lung capacity and assessments of benefit by patients and physicians, while those on the placebo scored zero after 16 weeks, Corbus said.

A score of 20 percent or more is considered clinically meaningful.

"No drug has ever shown has shown efficacy in scleroderma," Chief Executive Yuval Cohen told Reuters. "What we've seen here is unprecedented - to get these types of responses in a study that is so short and so small."

Systemic sclerosis, also called scleroderma, comes from the Greek words for "hard skin" and mostly affects women.

The chronic, rheumatic disease affects about 90,000 people in the United States and Europe but its cause is unclear.

The disease is characterized by vascular damage and scarring or fibrosis of the skin, the gastrointestinal tract, heart, lungs and other internal organs.

No existing drug can stop the progression of systemic sclerosis, but there are medicines that can relieve certain symptoms and reduce organ damage.

Typical anti-inflammatory drugs suppress the immune system, which makes users susceptible to infection.

Resunab, which is derived from cannabis but synthesized chemically to bind to the CB2 receptor found on immune cells, is designed to mimic a natural process to "turn off" chronic inflammation and fibrosis, without causing immuno-suppression.

Multiple attempts to successfully develop a drug that targets CB2 have failed in the past.

Resunab, which has already secured "fast-track" and "orphan drug" status from the U.S. Food and Drug Administration, is also being evaluated in three other inflammatory diseases, including cystic fibrosis, dermatomyositis and lupus.

Corbus is well positioned to secure an accelerated path towards approval, Cohen said, adding that the company now planned to discuss its next steps with regulators.

The company expects to submit a marketing application for scleroderma by 2021, pending further trials.

Up to Friday's close, Corbus's stock had more than tripled this year.

(Reporting by Natalie Grover in Bengaluru; Editing by Ted Kerr)