Image source: Regeneron Pharmaceuticals.

What: Despite news that the FDA has accepted it and co-developer Sanofi's (SNY -1.42%) application for approval of a key drug last month, shares in Regeneron Pharmaceuticals (REGN -1.94%) fell 22.6% in January, according to S&P Capital IQ.

So what: On January 8, the FDA accepted the application for approval of sarilumab, a human monoclonal antibody therapy for use in patients with moderate to severe rheumatoid arthritis.

The rheumatoid arthritis market is valued at $18 billion annually (and growing), and if approved, sarilumab will compete against AbbVie Inc.'s (ABBV -2.36%) multibillion-dollar blockbuster Humira, which controls 22% market share in the indication.

Although sarilumab's opportunity is real, investors looked beyond that potential and sold their shares in Regeneron last month as part of a broader de-risking of portfolios that resulted in a 28% drop in the S&P Biotech ETF (XBI -1.62%).

Now what: A lot of investor attention this year will focus on how quickly demand and revenue grows for Sanofi and Regeneron's cholesterol-fighting drug Praluent.

Since winning FDA approval for Praluent last summer, the two companies have announced a slate of contract wins with pharmacy benefit managers that could help sales climb in 2016.

Because tens of millions of people suffer from high cholesterol, including millions who are categorized as tough-to-treat who could conceivably benefit from Praluent, industry watchers think Praluent could be a multibillion-dollar drug. If so, it would mean Regeneron could potentially have three billion-dollar blockbusters on the market in 2017.

That would be enviable product line, but it may be a while before investors send Regeneron's shares back to their prior highs. Despite January's retreat, Regeneron's shares still trade at about 30 times forward EPS. That's not that expensive for biotech, but it's not a bargain, either.

Regardless, Regeneron is building a first-class lineup of medicine that should provide revenue and profit tailwinds for years to come, and for that reason, I think Regeneron is one company growth investors should be socking away in portfolios for the long haul.