Amarin (NASDAQ:AMRN) has moved one step closer to substantially expanding the potential market for its fish oil Vascepa. On Tuesday, the biotech said it submitted its application to the Food and Drug Administration to market Vascepa in patients with high triglycerides levels, between 200 mg/dL and 500 mg/dL.

Vascepa is already approved to treat patients with extremely high triglyceride levels above 500 mg/dL. The data for patients with triglyceride levels from 200 mg/dL to 500 mg/dL has been out for nearly two years. Vascepa reduced triglycerides by a placebo-adjusted 21.5% at the higher dose. The drug also lowered bad cholesterol levels when used in combination with a statin such as Pfizer's (NYSE:PFE) Lipitor, AstraZeneca's (NYSE:AZN) Crestor, or Merck's (NYSE:MRK) Zocor. That's pretty impressive since statins do a pretty good job of lowering cholesterol on their own.

What took Amarin so long to file for approval? Blame it on the FDA.

The agency compromised with Amarin, agreeing to accept laboratory tests as surrogate endpoints as long as the company had substantially enrolled a clinical trial called Reduce-It that tests Vascepa's ability to lower heart problems -- strokes, heart attacks, and the like.

Clinical outcomes data will be nice to have when we get it, but just the ability to market Vascepa to patients with triglyceride levels as low as 200 mg/dL is a huge win for Amarin even if it can only claim changes in triglyceride and cholesterol levels. The number of patients with triglyceride levels between 200 mg/dL and 500 mg/dL is nearly ten times higher than the number of patients Vascepa is currently approved to treat.

The clinical laboratory data are solid, but investors should keep in mind there's a possibility, however slight, that the FDA reverses course, demanding outcomes data before making the expanded approval.

That would be a travesty; what's the point of coming to an agreement if the FDA is just going to change its mind? But the FDA always reserves the right to change its mind if new data become available, and there have been a couple of recent high-profile cardiovascular studies where drugs have shown to improve laboratory tests but not clinical outcomes, most recently with Merck's Cordaptive. Those studies will be in the back of the FDA's mind as it reviews the expanded approval.