Biotech stocks in general enjoyed a great first half of 2019. The biggest winners have been clinical-stage biotech stocks. Several of these smaller biotechs have quadrupled or more this year

Of course, small clinical-stage biotechs also tend to be very risky. But are there biotech stocks that should perform well going forward but don't have super-high risk levels? I think so. Here's why Alexion Pharmaceuticals (NASDAQ:ALXN), Cara Therapeutics (NASDAQ:CARA), and Vertex Pharmaceuticals (NASDAQ:VRTX) are top biotech stocks to buy for the second half of 2019.

Scientists picking up a test tube from a rack of test tubes

Image source: Getty Images.

1. Alexion Pharmaceuticals

Alexion's rare-disease drug Soliris continues to pick up steam more than 12 years after winning its first FDA approval. Sales for Soliris soared 20% year over year in Q1, helping pad Alexion's cash stockpile (including cash, cash equivalents, and marketable securities) to more than $1.6 billion.

But Alexion's growth should accelerate even more. Last month, the biotech won FDA approval for the fourth indication for Soliris, this time in treating neuromyelitis optica spectrum disorder (NMOSD), a rare central nervous system disorder.

Earlier this year, Alexion launched a worthy successor to Soliris. Ultomiris won FDA approval in December 2018 as a treatment for rare blood disorder paroxysmal nocturnal hemoglobinuria (PNH). Alexion also awaits FDA approval for the drug in treating atypical hemolytic uremic syndrome (aHUS). Market researcher EvaluatePharma ranks Ultomiris as the top new drug launch of 2019 and projects sales of close to $3.5 billion by 2024.

Wall Street likes Alexion, too. Analysts expect average annual earnings growth of more than 16% over the next five years. The average one-year price target for Alexion stock reflects a premium of more than 30% over the biotech's current share price.

2. Cara Therapeutics

Cara Therapeutics doesn't have any approved products yet. As a result, the biotech isn't in nearly as strong of a financial position as Alexion is. However, Cara's future looks very bright.

In May, Cara reported positive results from a phase 3 clinical study evaluating Korsuva injection in treating hemodialysis patients with moderate-to-severe chronic kidney disease-associated pruritus (CKD-aP), itching that's associated with kidney disease. These results bode well for another late-stage clinical study of Korsuva in the same indication that's expected to wrap up later this year.

Assuming the results from the second late-stage study are successful, Cara should have a really good shot at winning FDA approval. And it already has a major partner lined up to commercialize Korsuva for the CKD-aP indication in countries other than the U.S., Japan, and South Korea. Cara is also evaluating an oral version of the drug in treating CKD-aP patients who aren't on hemodialysis, which presents an even bigger opportunity.

Korsuva could potentially generate peak sales in the CKD-aP hemodialysis indication alone of around $500 million. Cara's market cap is less than twice that peak sales figure. It's not surprising that the average one-year price target set by Wall Street analysts is 34% higher than Cara's current share price.

3. Vertex Pharmaceuticals

Vertex Pharmaceuticals dominates the cystic fibrosis (CF) treatment market with its three approved drugs. Kalydeco and Orkambi are already blockbusters. Vertex's newest drug, Symdeko, is already on pace to top the $1 billion sales mark this year.

However, Vertex's most impressive CF drug could be on the market soon. The company plans to file for U.S. approval of a triple-drug combo of VX-445 (elexacaftor), tezacaftor, and ivacaftor in the third quarter with a European regulatory submission following in Q4. Vertex thinks that this triple-drug combo could enable it to treat 90% of all CF patients compared to only around 25% of CF patients that can be treated by its current drugs.

The biotech is also venturing outside of CF. Vertex teamed up with CRISPR Therapeutics to develop gene-editing therapies targeting rare blood diseases beta-thalassemia and sickle cell disease. In June, Vertex announced that it was acquiring Exonics Therapeutics and expanding its partnership with CRISPR Therapeutics to focus on developing gene-editing therapies for rare genetic diseases Duchenne Muscular Dystrophy (DMD) and Myotonic Dystrophy Type 1 (DM1).

Wall Street analysts think that Vertex stock could jump 20% in the next year. But with the realistic potential of generating average annual earnings growth of more than 40% for years to come and no serious competition yet, I think Vertex's longer-term prospects make it arguably the best biotech stock on the market.

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.