So what: This supply issue is only the latest hiccup in Keryx's quest to steal market share away from Sanofi 's ( SNY -0.32% ) rival drugs Renagel and Renvela. For instance, Auryxia generated a meager $8.3 million in net U.S. sales for the second quarter of 2016, compared to a far more impressive $232 million (at current exchange rates) for Sanofi's market-leading drugs during the same period -- despite Auryxia being on the market for over a year and a half at this point. Put simply, Keryx doesn't appear to be convincing doctors to switch patients to its drug at a significant rate, and a supply interruption could make things even worse moving forward. Now what: Per its press release, Keryx expects this supply issue for Auryxia to be resolved sometime during the fourth quarter of 2016. That's not exactly a reassuring assessment of the situation, and investors should probably expect short-sellers to pile into this stock as a result.