What happened

Shares of the large-cap biotech Regeneron Pharmaceuticals (REGN -0.09%) were up by a healthy 19.2% on exceptionally heavy volume as of 1:46 p.m. ET Thursday afternoon. The drugmaker has thus gained over $12 billion in market capitalization through the first half of Thursday's trading session. 

What's sparking this double-digit rise? Earlier today, Regeneron and Bayer announced that their jointly developed anti-blindness medication Eylea (generic name: aflibercept) met the primary endpoint in two late-stage trials. The big deal is that these positive trial results support the use of Eylea at higher dosages (8 mg compared to 2 mg) at longer dosing intervals (16 weeks versus 8 weeks).

So what

Regeneron and Bayer's megablockbuster therapy could face biosimilar competition (generic versions of biologic drugs) as soon as next year. These dosing data might slam the door shut on the copycat threat for the time being, and make it more competitive against rivals such as Roche's new eye disease drug, Vabysmo.

A firmer competitive moat would be a huge win for Regeneron and Bayer. Eylea, after all, is forecast to generate well over $6 billion in U.S. sales for each of the next two years. 

Now what

Is Regeneron's stock a buy on this news? Under a best-case scenario, the biotech's stock is trading north of five times 2023 sales after this sizable uptick. While that's not an outlandish valuation for a top biotech company, it isn't exactly cheap, either. So, in short, this clinical trial news appears to be priced into Regeneron's shares now.