Some biotech stocks have enjoyed a tremendous year so far in 2018. But many haven't. More than 100 biotech stocks have dropped 30% or more.That total, however, includes many tiny biotech stocks. But there have also been quite a few relatively larger biotechs that have performed dismally in 2018 thus far.

The three worst biotech stocks of 2018 so far that claim market caps of at least $500 million are Prothena Corporation (NASDAQ:PRTA), Tesaro (NASDAQ:TSRO), and Acadia Pharmaceuticals (NASDAQ:ACAD). Here's why these stocks have taken a beating so far this year.

2018 in red with bar chart and line trending downward

Image source: Getty Images.

1. Prothena 

Prothena's share price has plunged 60% so far in 2018. Nearly all of Prothena's huge loss came on one day

On April 23, Prothena announced the worst news in the company's history. The biotech's lead candidate, NEOD001, failed miserably in a phase 2b clinical study targeting treatment of amyloid light-chain (AL) amyloidosis. Prothena's investigational antibody didn't meet the primary or secondary endpoints of the study. The company had no other option but to discontinue all clinical development for NEOD001, including a phase 3 study that had enrolled 260 patients.

In May, Prothena reorganized, slashing its workforce by 57%. The company refocused on its two remaining clinical programs. Experimental Parkinson's disease drug PRX002/RG7935 is being evaluated in a phase 2 clinical study. PRX004 is in a phase 1 study targeting transthyretin (ATTR) amyloidosis.

2. Tesaro

Tesaro stock has tanked 46% year to date. Unlike the situation for Prothena, though, it's been a relatively steady decline for the biotech throughout 2018 thus far.

The biotech started out the year on a down note, updating the label for chemotherapy-induced nausea and vomiting drug Varubi to warn about potential adverse reactions. In February, Tesaro announced disappointing Q4 results. Investors were especially concerned that AstraZeneca's Lymparza presented greater competition than expected for Tesaro's Zejula in treating ovarian cancer.

Tesaro has had some good news -- and good rumors -- that helped make the year be not as bad as it would have otherwise been. In March, the company reported positive results from a phase 2 study of Zejula in combination with Keytruda. More recently, there has been rampant speculation that Tesaro could be acquired, with the rumors driving the biotech stock higher.

3. Acadia Pharmaceuticals

Acadia Pharmaceuticals' share price has fallen more than 40% so far in 2018. Like a Shakespearean play, the biotech's decline came in three acts.

The first of these "acts" was Acadia's announcement of its Q4 results on Feb. 27, 2018. Investors were displeased that sales for the company's Parkinson's disease psychosis drug Nuplazid weren't growing as quickly as expected. A bad situation got much worse on April 9, though, when CNN reported patient deaths and adverse reactions potentially related to the use of Nuplazid. This CNN report led to the third act behind Acadia's miserable 2018 performance. On April 25, CNN reported that the Food and Drug Administration was reexamining the safety profile of Nuplazid.

It hasn't been a total tragedy for Acadia this year, however. The biotech's Q1 numbers announced in early May and reiteration of its full-year 2018 guidance helped reassure investors. Acadia also announced FDA approval for two new dosing formulations of Nuplazid on June 29, which provided a much-needed boost to the stock.

Most likely to bounce back

Which of these beaten-down biotech stocks is most likely to bounce back in a major way? Let's start first with the one that probably has the hardest path ahead -- Prothena. It's very difficult for a clinical-stage biotech to lose its lead candidate then come back. That's not saying it can't be done, but Prothena has the toughest climb of these three biotechs.

Tesaro's Zejula is a good ovarian cancer drug. I suspect that the rumors of an acquisition could become a reality. If so, Tesaro could again become a winner for investors.

However, my pick for most likely to bounce back in a big way is Acadia. Granted, a bad surprise from the FDA on Nuplazid could devastate the stock. I think, though, that it's more likely that the safety data on which Nuplazid won approval will hold up in the FDA's review. If I'm right and the FDA removes this dark cloud that's hanging over Acadia, the stock should skyrocket. 

Keith Speights has no position in any of the stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.