Yesterday, the Food and Drug Administration approved Sanofi (NYSE: SNY) and Isis Pharmaceuticals' (NASDAQ: ISIS) Kynamro for treating homozygous familial hypercholesterolemia, or HoFH, a genetic disorder that results in dangerously high cholesterol levels.

In and of itself, the approval is a relatively minor event for both companies. At one point, it looked like Kynamro was going to be a wonder drug, lowering cholesterol for anyone that couldn't control their cholesterol with Pfizer's (NYSE: PFE) Lipitor or Merck's (NYSE: MRK) Vytorin. But side effects have limited its potential use. The drug will had a fairly extensive Risk Evaluation and Mitigation Strategy plan that will have to be followed by doctors to watch for liver toxicity, and the FDA is requiring the companies to run four post-marketing studies on the drug. Sanofi and Isis are testing the drug in less severe patients that only have one inherited mutation instead of two -- heterozygous vs. homozygous -- but even with positive efficacy data, the safety problems might hamper an expansion.

Even among HoFH patients Sanofi and Isis are going to have a hard time capturing market share. Aegerion Pharmaceuticals' (NASDAQ: AEGR) recently got Juxtapid approved for the same patient population. The FDA advisory committee was more impressed with Juxtapid than it was with Kynamro, suggesting that a vast majority of doctors will follow suit. Of course, Sanofi has a little more marketing muscle than Aegerion, so don't expect Kynamro to go down without a fight.

Regardless of how Kynamro does, the approval is big news, because it marks the first approval of a next-generation antisense drug. Isis got Vitravene  approved with partner Novartis many years ago, but new HIV medications quickly swamped out sales. Behind Kynamro, Isis has more than 20 antisense drugs in the clinic.

Will they all work? Probably not, but the approval of Kynamro should give investors confidence in the mechanism of action -- antisense drugs bind to specific mRNAs, promoting their degradation and preventing synthesis of the protein they code for -- and that Isis has the wherewithal to get a drug approved.