It isn't easy to feel bullish when the overall stock market is falling, but zigging when others zag can lead to big gains over the long run. Shares of Vertex Pharmaceuticals (NASDAQ:VRTX) have been under pressure for reasons that have nothing to do with the financial catastrophe in China or the COVID-19 pandemic that's been dragging global markets lower.

Vertex Pharmaceuticals stock has fallen 8% so far this September. Biotech investing can be incredibly risky, but Vertex Pharmaceuticals stock at recent knockdown prices is about as close to a sure thing as it gets in this business. Here's why.

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Cystic fibrosis monopoly

Vertex Pharmaceuticals is the first and only company to develop drugs that address the root cause of a rare inherited lung disorder called cystic fibrosis. This is a progressive and ultimately fatal disease caused by a dysfunctional protein that transports sodium in and out of cells that line the lungs.

Hundreds of different mutations to the cystic fibrosis transmembrane conductance regulator gene ultimately lead to the formation of thick, sticky mucus that obstructs airways. Treatment with drugs from Vertex Pharmaceuticals can turn this fatal disease into a manageable condition.

Cystic fibrosis is a rare disorder that only affects around 30,000 Americans. Vertex's drug, Trikafta, has a list price of more than $300,000 annually, meaning there are enough patients in the U.S. and around the world to drive blockbuster franchise sales. Vertex Pharmaceuticals expects product sales to reach around $7.3 billion this year.

A good value stock

Shares of this biotech have fallen around 8% during the September sell-off. At recent prices, you can buy Vertex Pharmaceuticals for just 15 times forward earnings expectations. The average stock in the Nasdaq 100 index, which includes Vertex, trades at around 29.1 times forward earnings expectations.

A valuation like the one the market has placed on Vertex Pharmaceuticals is what you would expect for a business growing by a mid-single-digit percentage from year to year. Vertex, though, isn't plodding along like a giant pharmaceutical company that has to constantly fight off generic competition for aging brand name drugs. The company expects top-line sales to climb 18% this year over last and patents for Trikafta won't expire until 2037 at the earliest.

CRISPR Therapeutics collaboration

The company's own attempts to develop candidates beyond cystic fibrosis haven't worked out as well as investors hoped. Luckily, a collaboration program with gene editing pioneer CRISPR Therapeutics (NASDAQ:CRSP) appears ready to bear fruit. 

This June, the partners presented results from 22 patients with hemoglobin-related blood diseases who have been treated with CTX001. After a single administration of this experimental gene therapy, all patients with more than three months of follow-up observations are free from the problematic blood transfusions they used to rely on.

Reasons to tread lightly

Treatment with Trikafta is expensive enough to inspire a combination of competition and payer pushback. Last October, pharmaceutical giant Roche (OTC:RHHBY) acquired a portfolio of experimental cystic fibrosis candidates that aim to treat all CF patients regardless of which mutations are responsible for their disease.

Vertex Pharmaceuticals invests the huge cash flows its CF drugs generate into efforts to create new ones with less success than investors would like. Earlier this year the stock sold off after the company scrapped its second experimental treatment for a rare condition called alpha-1 antitrypsin deficiency. 

Vertex Pharmaceuticals might be one of the safest biotech stocks you can buy, but investors probably don't want to make Vertex a core holding in their portfolio. Remember, clinical trial failures are a constant risk for any biotech trying to grow over 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.