Biotech stocks across the board have had a great year so far, but Cara Therapeutics Inc. (CARA 1.76%), Rigel Pharmaceuticals, Inc. (RIGL -7.05%), and Omeros Corporation (OMER -6.15%) have flown under the radar. Sure, these companies are developing therapies for conditions you've probably never heard of, but outlooks for their respective drug candidates are plenty exciting.

The fates of small-cap biotechs are always shrouded with enough uncertainty that these three stocks fall outside of my personal risk zone, but more adventurous investors might want to put them in their portfolios. To see why, let's look at potential catalysts that make these three stocks screaming buys right now.

Three technicians in a biotechnology lab.

Image source: Getty Images.

They don't like it, and that's a good thing

According to the American Society of Addiction Medicine, in 2015 about 2 million Americans had a substance abuse disorder involving prescription painkillers. Attempts to make opioids addiction-resistant have so far been laughable, but one biotech could have an interesting solution in its pipeline.

The lead candidate in Cara Therapeutics' pipeline, called CR845, is an opioid that doesn't pass the blood-brain barrier. This allows it to act on opioid receptors where pain signals originate instead of where they're interpreted, in the brain. Results from a safety study prove patients don't "like" the drug nearly as much as a popular opioid painkiller.

Luckily, patients with severe rashes and itching related to kidney disease don't need to "like" CR845 for it to help them stop feeling itchy. Kidney failure might be more prevalent than you realize. Cara estimates a potential patient population of around 4.2 million patients with various kidney disorders suffer from chronic itching and are underserved by existing treatments.

Later this year, Cara Therapeutics intends to begin a study that could support a New Drug Application for itching related to kidney disorders. If the results fall in line with previous observations, an approval seems likely. In the meantime, investors will want to keep their eyes open for results from a late-stage trial to see if CR845 has a shot in the postoperative setting.

With demand for non-habit-forming painkillers at an all-time high, annual CR845 sales could cross into $1 billion blockbuster territory. At recent prices, though, Cara's market cap is just $558 million. The average biotech stock trades at prices about 5.8 times its annual sales. That means even modest success in the commercial stage could make this a multibagger.

Laboratory researcher with a pipette.

Image source: Getty Images.

Another unmet need

You probably haven't heard of immune thrombocytopenic purpura (ITP), but between 60,000 and 125,000 people in the U.S. are all too familiar with the autoimmune disorder. False recognition of platelets as foreign invaders to be dealt with by the immune system causes a handful of debilitating symptoms.

At the moment there aren't any therapies specifically approved to treat ITP, but Rigel Pharmaceuticals has a candidate in late-stage development that could be the first. The company submitted a New Drug Application for Tavalisse, formerly fostamatinib, to the Food and Drug Administration last month.

The agency can take up to two months to formally accept the submission for review or reject it. A single patient response in a placebo group during a trial supporting the application is a noteworthy sticking point. I agree with my Foolish colleague David Liang that the overall application should satisfy regulators, but investors should remain braced for a longer timeline if the FDA asks for more data.

At recent prices, Rigel's enterprise value is a sprightly $221 million, which is far less than peak annual sales estimates of about $350 million for Tavalisse. Apply the average price-to-sales ratio and this biotech stock has long-term, multibagger potential written all over it.

Two roads to success

Rigel and Cara are burning through cash in hopes they'll have a product to sell, but Omeros Corporation is funneling sales of its lead product into the development of a candidate with blockbuster potential. The company markets Omidria to eye surgeons, who use it to keep pupils dilated while they remove cataracts or replace lenses. 

A 69.2% year-to-year increase in first-quarter Omidria revenue suggests the drug is quickly becoming popular among eye surgeons, who perform millions of these procedures each year. At a list price of $465 per vial, though, its annualized run rate of $49.2 million suggests its climb is just getting started.

At recent prices, Omeros Corporation's $652 million market cap is so low it's almost like getting its drugs in development for free. In the company's pipeline, you'll find a rare-disease candidate in late-stage development, called OMS721. During earlier studies, this candidate provided a significant benefit for patients with rare blood disorders, including atypical hemolytic uremic syndrome (aHUS).

Currently, Soliris is the only FDA-approved therapy for the treatment of aHUS, and the drug generated about $2.8 billion in sales for Alexion Pharmaceuticals last year. Omeros is running a late-stage aHUS trial right now with OMS721 and is poised to begin two more late-stage studies for separate indications later this year. If positive results seen earlier are repeated, Omeros could have a second drug with blockbuster potential generating sales.