Regeneron Pharmaceuticals (Nasdaq: REGN) has gained Food and Drug Administration approval of its third drug, Zaltrap, which treats metastatic colorectal cancer. Three approvals in four years is no small feat.

And yet, it'll be its middle child that will get all the glory for now.

Arcalyst was approved to treat cryopyrin-associated periodic syndromes, but sales have never amounted to much -- $6 million in the second quarter -- because CAPS is an ultra-orphan disease that few people have. Splitting CAPS patients with Novartis' (NYSE: NVS) Ilaris, which was approved a year later, didn't help.

The drug could have gained some traction if it had been approved to prevent gout flares, but the FDA panned that idea last week. Regeneron was light on the details beyond that it was rejected, which we knew was coming. Investors should assume that the expansion plans for Arcalyst are dead until further notice.

Zaltrap will certainly sell better than Arcalyst. Even as a second-line indication there will be patients that fail Roche's Avastin or Eli Lilly (NYSE: LLY) and Bristol-Myers Squibb's (NYSE: BMY) Erbitux, which are both approved in the first-line setting.

But sales of Zaltrap won't contribute to Regeneron's profits initially because under its partnership agreement with Sanofi (NYSE: SNY), Regeneron is required to repay half of the cost of developing Zaltrap out if its profits from the joint venture. The repayment schedule allows Regeneron to keep any quarterly profits that exceed 5% of the outstanding obligation, but it'll be awhile before its share of the profits reaches that level. Zaltrap failed quite a few clinical trials in other types of tumors, which ratcheted up the expenses to $763 million as of the end of last year.

That leaves Regeneron's macular degeneration drug, Eylea, as the immediate future of the biotech. No pressure, kid.

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