The SPDR S&P Biotech index, which tracks the world's largest biotech stocks, has lost over a quarter of its value since it peaked in February. Despite an overall stock market that soured on the largest biotech stocks, shares of two have soared.

Just because they're bucking the trend now doesn't necessarily mean they'll keep climbing once they find a way into your portfolio. Have a look at what makes this pair so special before trying to follow their ascent:

Company (Symbol) Gain (Loss) in 2021 Market Cap
BioCryst Pharmaceuticals (NASDAQ:BCRX) 72% $2.3 billion
Tonix Pharmaceuticals (NASDAQ:TNXP) 57% $346 million
SPDR S&P Biotech ETF (NYSEMKT:XBI) (-10%) N/A

Data source: Yahoo! Finance.

1. BioCryst Pharmaceuticals

This company focuses on developing new small-molecule drugs that treat rare diseases caused by faulty enzymes. BioCryst made the transition to a commercial-stage business last December when the Food and Drug Administration approved its first drug, Orladeyo, to prevent hereditary angioedema (HAE) attacks.

Bradykinin is an immune-system protein that performs many functions, including dilation of tiny blood vessels at the site of an injury. Production of bradykinin is largely unregulated in HAE patients, which leads to painful, uncontrolled swelling for no good reason. Orladeyo is a once-daily capsule that prevents another enzyme, kallikrein, from producing excess bradykinin.

Smiling scientist holding the eyepieces of a microscope

Image source: Getty Images.

Sales of Orladeyo got off to an encouraging start, producing $11 million in top-line revenue for BioCryst during the first three months of 2021; recent approvals in Europe should accelerate sales before the year's finished.

BioCryst's late-stage development pipeline also includes an orally available treatment for paroxysmal nocturnal hemoglobinuria (PNH) called BCX9930. In the second half of 2021, the company will begin pivotal studies that could make the drug the first needle-free treatment option for this life-threatening blood disorder.

2. Tonix Pharmaceuticals Holding

This clinical-stage drugmaker doesn't have any products it can sell yet, but progress with drugs to address the COVID-19 pandemic keeps driving its stock price to new peaks. Shares of Tonix Pharmaceuticals soared in February after the company reported progress with a potential new COVID-19 skin test. In March, the company also reported positive preclinical data from a study with TNX-1800, a potential coronavirus vaccine.

A new way to test for exposure to SARS-CoV-2 isn't the only new venture Tonix Pharmaceuticals has undertaken this year. In April, the company in-licensed sangivamycin, an experimental antiviral for the potential treatment of COVID-19.

Tonix Pharmaceuticals also has a program in late-stage clinical development for fibromyalgia called TNX-102. Last December, the company showed investors that TNX-102 significantly reduced pain for fibromyalgia patients.

Good stocks to buy now?

Using sales from one drug to try developing lots of new products is fine for commercial-stage businesses. As a clinical-stage company with very limited resources, though, Tonix Pharmaceuticals' unbridled ambition is downright disturbing.

Tonix Pharmaceuticals still hasn't started a clinical trial with its antiviral treatment for COVID-19, so it's going to be a long time before we know if it's worth more than an opportunity to issue a positive-looking press release. Despite plenty of time since its inception, we can say the same for the company's COVID-19 vaccine program.

BioCryst's first drug to earn approval is facing competition from an antibody treatment that's probably more effective. Further ahead, BCX9930 could run into competition from AstraZeneca (NASDAQ:AZN) and its Soliris/Ultomiris franchise, which the pharmaceutical giant picked up in its recent acquisition of Alexion Pharmaceuticals. But since BioCryst's drug comes in easy-to-swallow capsules, sales will probably be sufficient to fuel the company's effective new drug-discovery engine without tapping investors again.

In addition to having one potential blockbuster on the market and another ready for a pivotal trial, BioCryst appears capable of engineering new small-molecule drugs to fix faulty proteins at will. This is an extremely valuable skill rarely seen among drugmakers with market values as low as BioCryst's right now. That makes BioCryst look like a good stock to buy now and hold on to for the long run.

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.