Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of NPS Pharmaceuticals (UNKNOWN:NPSP.DL) sank 15% today after its sales guidance disappointed Wall Street.

So what: NPS' third-quarter results managed to top estimates -- a loss of $0.01 beat the consensus by a penny while revenue surged 45% -- but an in-line outlook for its bowel disease drug Gattex is forcing investors to lower their valuation estimates. To be sure, NPS actually raised its guidance from a prior view, but today's price action suggests the drug isn't growing fast enough to justify the seemingly lofty price-to-sales multiple.

Now what: Management now expects full-year Gattex sales of $28 million-$32 million, bracketing Wall Street's estimate of $32 million.

"We are excited by the continued success of the Gattex launch in the U.S.," said President and CEO Francois Nader in a press release. "We are seeing strong demand and sales have exceeded our expectations so far with 235 patients now on Gattex."

So while NPS remains just too speculative for average Fools, today's double-digit drop -- the stock is now off about 35% from its 52-week highs -- might be an attractive opportunity for biotech bargain hunters.