It could be a couple of things in my opinion.
Most likely - caught up in recent mkt correction, if you haven't noticed biotech has been sold off lately. However, you would still expect a higher pps. Let's face it, we need revenue.
Somewhat likely - delay in negotiations on reimbursement, worse year end numbers than anticipated.
Least likey- undisclosed production problems of Atryn
Who knows - the markets are fickle, if the story remains good and gets better so will the pps. If it goes south, then all the articles of how great antithrombin is won't mean squat.
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