Biotech hasn't been a great place to invest your money in 2018. The iShares Nasdaq Biotechnology Index, after all, is down by over 4% at present, and summer has historically been a down time for this high-risk space. So, there's little reason to believe a widespread rally is coming anytime soon. 

Even against this dour backdrop, however, several former top biotech stocks have turned out to be particularly disappointing investing vehicles this year. Acadia Pharmaceuticals (NASDAQ:ACAD) and Celgene Corporation (NASDAQ:CELG), for instance, are both trading near their 52-week lows right now. That's a surprising turn of events given that Acadia and Celgene were two of the absolute best-performing growth stocks since about the start of 2010. 

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Should investors take advantage of this weakness in these two former highfliers? Let's dig deeper to find out. 

Is Acadia's risk overstated?

Acadia stock has suffered from two interrelated issues this year. First off, the company's valuation at the start of the year seemed to reflect an unwarranted optimism surrounding the commercial potential of Nuplazid, Acadia's FDA-approved drug for Parkinson's disease psychosis (PDP).

In short, investors were clearly expecting Nuplazid to achieve blockbuster status almost right out of the gate. As the drug has a questionable risk-to-reward ratio, however, it's not all that surprising that Nuplazid has so far failed to live up to these lofty expectations. 

And the drug's murky risk-to=reward ratio is at the heart of the company's second major issue. A recent report by CNN suggested that Nuplazid's substantial risks outweigh its modest rewards, which, in turn, caused the FDA to take a deeper look at the drug's emerging safety profile since coming on the market.

Although the FDA is unlikely to pull Nuplazid for a variety of reasons, investors are still concerned about this existential threat. Nuplazid, after all, is Acadia's only product, and it doesn't have much of a clinical pipeline to fall back on. 

On the bright side, Acadia's management has stuck by its annual 2018 sales guidance for Nuplazid of between $255 million and $270 million, and the company has been adamant about its position that this drug is an important new treatment option for PDP.

The core problem here, though, is that Acadia's valuation is still way out of line with its peers -- even after this year's downward move. At present, Acadia's shares are trading at a forward price-to-sales ratio of 7.9 at the high end of its estimated revenue guidance. The historical average for biotechs with traditional small-molecule drugs like Nuplazid, by contrast, is closer to 6. 

With a possible black swan event lurking in the shadows (Nuplazid getting pulled from the market), it's hard to justify buying this stock right now. At best, Acadia's shares are now only slightly overvalued, and a buyout scenario appears to be moot issue with Nuplazid's current regulatory risk.    

Can Celgene overcome these missteps? 

Celgene stock has plunged this year due to the unexpected regulatory setback for its multiple sclerosis drug candidate ozanimod, as well as the questionable acquisitions of Impact Biomedicines and Juno Therapeutics that cut deeply into the biotech's cash reserves. The long and short of it is that Celgene is under immense pressure to find an heir apparent to its flagship multiple myeloma drug Revlimid.

Now, ozanimod was supposed to be that drug, given its potential to generate upwards of $6 billion in peak sales. With the drug's commercial launch delayed by at least a year, however, ozanimod could end facing a far tougher competitive landscape by the time it finally hits the market.

And that's why Celgene's management was quick to note the sizable commercial potential of Juno's and Impact's lead product candidates -- JCAR017 and fedratinib respectively -- during its first-quarter conference call last week. Unfortunately, these two product candidates do have their own set of unique problems that could significantly dampen their commercial prospects, too.    

Overall, the picture that's emerging with Celgene is a company that's lost its way to some degree. The ozanimod fiasco triggered a change in key leadership positions, and the biotech's acquisitions of Juno and Impact appear, in hindsight, to be questionable moves at best. Valued at a mere 8.55 times forward earnings, though, Celgene stock is probably worth the risk at these levels.  

Are these two stocks worth owning right now?

Acadia could be in for some dark days in the near future. There's no telling what the ultimate fallout from that CNN report regarding Nuplazid's risk-to-reward ratio will be, and there's now the remote chance that the FDA could even pull the drug from the market. As such, this speculative biotech stock arguably isn't worth the potential headaches that could come with owning it. 

Celgene, on the other hand, probably deserves the benefit of the doubt. The company has made some regrettable errors this year, but management has been proactive at dealing with them head-on. The company also has a top-notch clinical pipeline that should be able to keep the growth going for at least another five to 10 years -- even if Juno and Impact both fail to live up to expectations. So with Celgene's shares trading at rock-bottom prices from a historical perspective, investors may want to consider taking advantage and scooping up some shares.

George Budwell has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Celgene. The Motley Fool has a disclosure policy.