There are few stocks that I am as bullish about as Celgene (NASDAQ:CELG). The biotech has enjoyed strong earnings growth thanks to dominance of its blood cancer treatment Revlimid and the tremendous success of autoimmune disease drug Otezla. However, like any other company, Celgene faces potential threats. Here are three of the biggest risks ahead for the company.

Risk Knob

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A challenger in multiple myeloma

When Johnson & Johnson (NYSE:JNJ) won regulatory approval for Darzalex in 2015 as treatment for patients with multiple myeloma who had received at least three prior therapies, the news didn't bother Celgene. Patients who were prescribed Darzalex typically would have already tried Revlimid, and probably its other treatment, Pomalyst, too.

Nor did it likely worry too much in October of this year when the Food and Drug Administration approved Darzalex as a second-line treatment for multiple myeloma. That approval required Darzalex to be administered in combination with Revlimid and dexamethasone, or with Velcade and dexamethasone.   

Now, though, there could be reason for Celgene to begin getting at least a little nervous. Results from a phase 1/2 study announced in October found that patients taking a combo regimen with Darzalex, Revlimid, and dexamethasone experienced significantly better results than those taking just Revlimid and dexamethasone.

This combo still includes Revlimid, so why should Celgene worry? Because Darzalex's performance was so positive that it could become a standard of care for multiple myeloma down the road. The drug's efficacy, safety, and convenience just might present a serious challenge to Revlimid's dominance. However, even if that shift materializes, it probably won't happen until at least 2020. 

Generic entrants

Celgene also faces the risk that generic rivals could enter some of its markets sooner than expected.

In March, Allergan (NYSE:AGN) submitted an Abbreviated New Drug Application (ANDA) to the FDA seeking approval for a generic version of Celgene's cancer drug Abraxane.

There are multiple patents for Abraxane, with the last U.S. patent expiring in 2026. Allergan is challenging the validity of some of Celgene's key patents for the drug. The two companies are fighting this battle in the courts, where it's likely that the litigation will stretch out beyond 2017, so this isn't an imminent threat.

Revlimid faces generic challenges as well. Celgene continues to defend its patents for the drug in Europe. The European Patent Office (EPO) ruled last year that one of the Revlimid patents was invalid. Celgene has appealed the ruling, but the company doesn't expect a decision by the EPO Board of Appeals for several years.

India-based pharmaceutical Dr. Reddy's Laboratories also filed an ANDA for approval in September of a generic version of Revlimid in the U.S. Celgene quickly responded by filing an infringement action, which would delay any FDA approval of the ANDA until 2019 unless the drug's patent protection is overturned. 

Pipeline failures

Celgene is counting on several of its pipeline candidates to drive growth in the years ahead. However, clinical studies often don't go as well as hoped. Celgene experienced this earlier in 2016 when Revlimid failed in a study evaluating the drug as a treatment for diffuse large B-cell lymphoma (DLBCL). 

Late-stage clinical trial failures hurt the worst. Celgene has 17 such studies in progress now. A clinical setback for ozanimod might be the most damaging of all. Celgene thinks the drug could reach peak annual sales between $4 billion and $6 billion if approved.

However, the experimental drug is in the midst of  two late-stage studies, one targeting treatment of multiple sclerosis and the other on treatment of ulcerative colities, and a mid-stage study focused on Crohn's disease. Ozanimod would have to succeed and win regulatory approval for all three indications to meet Celgene's lofty sales expectations.

Not every candidate in Celgene's pipeline will pan out. However, too many failures -- or a few flops for the biggest potential stars like ozanimod -- could make the biotech's growth goals difficult to achieve.

Riskiest risk?

Although J&J's Darzalex could cause problems for Celgene, that threat to the bottom line is still years away. I suspect the entrances of generic rivals to Abraxane or Revlimid are still well in the future also.

The riskiest risk for Celgene, in my view, is the possibility of one or more pipeline failures. There are countless abandoned drugs that  once seemed promising, but ultimately flamed out. Even drugs with very positive phase 2 results can often turn into disappointments in late-stage studies.

Despite these risks, I still like Celgene's chances for success. I think the biotech will continue growing earnings at impressive rates, with solid performances from Revlimid, Otezla, and new drugs on the way.

Keith Speights owns shares of Celgene. The Motley Fool owns shares of and recommends Celgene. The Motley Fool recommends Johnson and Johnson. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.