(Hopefully, with a new brand name in place.) But Keryx is moving into the "Zerenex" post-approval marketing period on its own, having failed to find an interested marketing partner. The "Zerenex" label is weak and offers little to differentiate it from entrenched phosphate binder competitors like Sanofi's ( SNY ) Renvela. With the expiration of key Renvela patents, low-priced generic phosphate binders are expected to enter the $1 billion-plus market, making it even more difficult for a premium-priced branded product to win market share. Keryx's stock price had a really nice run from $12 to 18 this year as the widely expected "Zerenex" approval decision neared. On Friday, Keryx shares closed down 5% to $17.01. Call it a "sell on the news" reaction to the "Zerenex" decision. If the bears are right -- and I think they will be -- Friday's weakness is just the start of a more significant retrenchment in the value of Keryx's stock.