Shares of Regeneron (REGN) - Get Report are up a little more than 5%, but the gains are coming from another stock's decline -- shares of Ophthotech (OPHT) - Get Report have plunged more than 80% on Monday. 

Ophthotech tried to beat Regeneron's eylea's drug, but that clearly didn't work, TheStreet's Jim Cramer, co-manager of the Action Alerts PLUS portfolio, said on CNBC's "Mad Dash" segment. 

Ophthotech tried to combine its Fovista drug with Roche's Lucentis drug to create a superior drug to Regeneron's eylea drug, which is used for the treatment of wet age-related macular degeneration. Eylea is also used to treat swelling in a patient's retina, among other uses.

However, the Phase 3 results did not go in Ophthotech's favor, which explains the stock's huge drop and Regeneron's move higher, Cramer noted. Regeneron's eylea treatment is very tough to beat and it has a lot of market share, he added. 

While the news is good for Regeneron, don't get too optimistic, Cramer reminded viewers. President-elect Trump is still taking aim at drug companies and it is now a question of whether eylea may be too expensive. 

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At the time of publication, Cramer's Action Alerts PLUS had no position in companies mentioned.