Israel "Izzy" Englander has made plenty of smart stock picks during his career. The co-founder of the Millennium Management hedge fund is worth more than $5 billion as a result.

Englander just added to his fund's positions in three drug stocks -- Gilead Sciences (NASDAQ:GILD), Synergy Pharmaceuticals (NASDAQ:SGYP), and Valeant Pharmaceuticals (NYSE:VRX). Here's why the billionaire likely bought more shares of these three stocks that haven't performed very well so far in 2017. 

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Gilead Sciences

Millennium Management bought another 480,000 shares of Gilead Sciences stock during the first quarter of 2017. This puts the hedge fund's total stake in Gilead at nearly $129 million. In addition, Millennium bought 200,000 call options for the biotech. 

What makes Englander more bullish about Gilead Sciences? My guess is he likes three things about the biotech. One is its promising pipeline. Gilead recently submitted its bictegravir/emtricitabine/tenofovir alafenamide combo to the U.S. Food and Drug Administration for approval. This combo could be the biotech's next megablockbuster in treating HIV.

Second, I suspect Englander thinks that Gilead's valuation won't go much, if any, lower than it already is. The stock has fallen more than 20% in the last 12 months, with shares now trading at less than nine times expected earnings.

Third, like most investors, Englander probably expects Gilead to make one or more transformative acquisitions that could rejuvenate interest in the stock. Gilead COO Kevin Young recently stated that the big biotech is "looking far and wide" at potential assets in a number of therapeutic areas. 

Synergy Pharmaceuticals

At the end of 2016, Millennium Management held close to 1.9 million shares of Synergy Pharmaceuticals. You can add another 1.39 million shares to that count during the first quarter. 

Englander obviously thinks highly of Synergy. One reason for this positive opinion could be that the billionaire likes the prospects for Synergy's gastrointestinal-disease drug Trulance.

Weekly prescription volume for Trulance in treating chronic idiopathic constipation (CIC) is steadily growing after Synergy launched the drug in March 2017. The company recently stated that over 60% of patients with commercial insurance now have unrestricted access to Trulance.

Looking ahead, Synergy could see even more impressive results with Trulance in an additional indication. The FDA is scheduled to make a decision on approval of the drug in treating irritable bowel syndrome with constipation (IBS-C) by Jan. 24, 2018.  

Valeant Pharmaceuticals

While another billionaire, Pershing Square Capital's Bill Ackman, threw in the towel on Valeant Pharmaceuticals, Englander has apparently become even more positive about the stock. During the first quarter, Millennium Management bought more than 320,000 additional shares of Valeant. 

What does Englander see in Valeant that Ackman doesn't? The best answer might be that Englander believes that the drugmaker's CEO, Joe Papa, can deliver on his promises.

So far, Papa appears to be doing just that. He promised in August 2016 that he would reduce Valeant's debt by $5 billion within 18 months. Since then, the company has sold off multiple non-core assets, most recently announcing the sale of iNova Pharmaceuticals for $930 million in cash.

Surprisingly, Valeant stock has turned in a better performance than either Gilead Sciences or Synergy so far in 2017. That's not all that great, since Valeant's share price is still below where it started the year. However, the company's asset sales have helped prop up the stock to some extent. Whether or not Englander's bet pays off, though, will probably depend more on how Valeant's new products like psoriasis drug Siliq perform.

Keith Speights owns shares of Gilead Sciences. The Motley Fool owns shares of and recommends Gilead Sciences and Valeant Pharmaceuticals. The Motley Fool has a disclosure policy.