Updated from 11:28 am EDT with Celgene response:

SUMMIT, NJ (TheStreet) -- Mongersen is one of Celgene's (CELG - Get Report) most important pipeline drugs and a potential blockbuster treatment for Crohn's disease, an autoimmune disorder affecting the bowel. Investors, however, will wait longer than expected for data necessary to resolve a lingering concern about mongersen's potential.

On Sunday, Celgene said results from a recently started phase II study in which doctors are using an endoscope to examine the colons of Crohn's patients to confirm the benefit of mongersen would be available in the first half of 2017.

Investors were expecting to see the mongersen "endoscopy data" in 2016, and if positive, possibly allow Celgene to seek early, accelerated approval for the drug in the U.S.

The ability of mongersen to demonstrate high rates of clinical remission in Crohn's disease patients was called "unprecedented" by a physician in a recent editorial published in the New England Journal of Medicine. However, the same physician reviewing Celgene's previous phase II study also raised a concern that enrolled patients may have had more moderate Crohn's disease, making it easier for mongersen to show an outsized treatment effect.

In that study, Celgene defined remission as a Crohn's Disease Activity Index (CDAI) score of less than 150, maintained for at least two weeks. The study did not call for doctors to use an endoscope to measure patients' Crohn's disease at baseline or after treatment was concluded.

The ongoing endoscopy study -- which Celgene refers to as CD-001 -- is important because it will provide a more rigorous assessment of mongersen's ability to treat Crohn's patients.

On Sunday, Celgene said enrollment of 48 patients with Crohn's disease into the study should be completed by summer. All patients will have have their Crohn's disease measured with an endoscope at baseline. Then, the patients are split into three groups for treatment with mongersen for four, eight, and 12 weeks, respectively. After 12 weeks, a second endoscopic exam is performed to determine mongersen's effect on Crohn's disease.

Instead of closing the study after the 12 weeks and announcing results, Celgene is following all the patients for another year without treatment, then re-treating them with mongersen for another six months, the company said Sunday. The longer follow up and retreatment is why results are pushed into 2017.

While the smaller, phase II endoscopy study is underway, Celgene will also be enrolling 2,000 Crohn's patients into two, phase III studies of mongersen.

As a pill, mongersen has the potential to become a preferred Crohn's therapy over the currently approved injectable drugs like Abbvie's (ABBV - Get Report) Humira or Johnson & Johnson's (JNJ - Get Report) Remicade. Mongersen is an antisense drug which prevents a gene from producing a protein known as Smad7 believed to play a role in inflammation and the runaway autoimmune response seen in Crohn's patients.

Celgene purchased mongersen in April 2014 from a small Irish drug maker, paying $710 million upfront. Including future milestones and sales payouts, Celgene committed $2.6 billion to acquire rights to the drug.

Mongersen peak sales potential in Crohn's could top $3 billion per year if the positive results observed to date hold up and the drug is approved, analysts forecast. 

Unfortunately, the endoscopy data which could help answer lingering questions about mongersen's future will be delivered later than expected.

Update: Celgene responds:

The CD-001 study is made of five phases (screening, induction, observation, extension and follow-up) that will last up to 97 weeks. The main two objectives (endoscopic response and durability of response) of this study are measured as part of the first three phases which can last up to 69 weeks. These three phases will remain blinded until the last patient finishes the observation phase. Therefore, Celgene plans on releasing the data at a scientific meeting after the observation phase is complete. 

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.