Acquisitions, anticipation, and analyst presentations. It's not that the reasons behind huge gains for biotech stocks are always alliterative. That's the case for three of the best-performing biotech stocks on the market this week, though.

Bioverativ (NASDAQ:BIVV), The Medicines Company (NASDAQ:MDCO), and Adverum Biotechnologies (NASDAQ:ADVM) soared over the last five days. Here's why these stocks took off, and whether or not they're smart picks now. 

Man looking through skyscraper window at three chart lines going up

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Biogen spun off its hemophilia business as a separate entity in early 2017 in an effort to unlock additional value for shareholders. The new company, Bioverativ, certainly accomplished that goal. Bioverativ stock skyrocketed 60% this week on news that it would be acquired by Sanofi.

Sanofi announced on Monday that it was buying Bioverativ for $11.6 billion. The French drugmaker hasn't made a major acquisition in several years, but it beat the pack in scooping up the Biogen spinoff, which was viewed by some as a prime takeover candidate even before Sanofi's announcement. 

Although both companies' boards of directors have approved the deal, Sanofi must first clear antitrust regulators before finalizing the acquisition of Bioverativ. That shouldn't be a problem, however. With the purchase, Sanofi will gain a couple of top hemophilia drugs -- Eloctate and Alprolix. The big pharma company will also get Bioverativ's promising pipeline, which includes a late-stage candidate targeting treatment of cold agglutinin disease.

Adverum Biotechnologies

Adverum Biotechnologies stock jumped more than 40% this week. So far in 2018, the biotech's valuation has nearly doubled. Why? There are a couple of factors.

The primary catalyst driving Adverum stock higher is investors' anticipation over the company's promising gene-therapy candidate, ADVM-043. Adverum initiated a phase 1/2 study for the therapy in December targeting treatment of alpha-1 antitrypsin (A1AT) deficiency, a genetic disorder that is associated with premature emphysema.

An extension of a research collaboration with hot gene-editing company Editas Medicine (NASDAQ:EDIT) announced on Thursday added to the anticipation. The two biotechs extended their research cooperation targeting retinal diseases through the third quarter of this year. Editas can also exercise options on Adverum's products from the collaboration through August 2020. 

The Medicines Company

Trailing Adverum was The Medicines Company, which saw its stock jump 35% this week. The biotech presented to analysts on Tuesday -- and investors liked what they saw and heard.

Much of the discussion centered on The Medicines' promising lead pipeline candidate, inclisiran. The company partners with Alnylam Pharmaceuticals (NASDAQ:ALNY) on the drug, which is an RNA interference therapy that inhibits PCSK9 protein in liver cells. The Medicines Company told investors that patient randomization in the first pivotal study of inclisiran would finish ahead of schedule this week -- and on Thursday announced that's exactly what had happened. 

If all goes well in this pivotal study, The Medicines Company and Alnylam hope to file for regulatory approval in the U.S. and in Europe by the end of 2019. Although already-approved PCSK9-inhibiting cholesterol drugs haven't been as successful as initially predicted, The Medicines Company thinks it has a different strategy that will pay off down the road. 

Are they buys?

After massive gains this week, are any of these stocks good candidates to buy? Let's start with the easiest one to tackle -- Bioverativ. The horse is already out of the barn on this biotech. It's a pass, since the Sanofi buyout seems likely to go through at the announced price tag.

What about Adverum? I really like the potential for gene therapies. However, I think it's still too early to jump on board with Adverum. This is a stock to keep your eyes on, though.

The Medicines Company is intriguing. Inclisiran looks to be a powerful drug for lowering cholesterol. I think it's smart for The Medicines and Alnylam to initially target homozygous familial hypercholesterolemia, a rare genetic disease resulting in high cholesterol levels. However, I'm not sold yet on how successful inclisiran will be in the broader cholesterol market when other PCSK9 inhibitors have disappointed so far. My view is that The Medicines' partner, Alnylam, is the better pick right now.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.