Source: Flickr user Elliott Brown.

Gilead Sciences (GILD 3.18%), Celgene Corp. (CELG), and Regeneron Pharmaceuticals (REGN 0.84%) aren't immune to market whims and whispers, but they do share one thing in common; a track record for developing multibillion dollar blockbuster drugs. Given that the NASDAQ Biotechnology ETF (IBB 1.21%) has plummeted 26.7% this year, is now a good time to buy shares in these industry titans? Read on to find out why I think adding them to your portfolio may be a profit-friendly move. 

No. 1: Gilead Sciences
After launching not one but two multibillion-dollar blockbuster therapies for hepatitis C, Gilead Sciences has seen its annual revenue and earnings surge to $32 billion and $18 billion from $9.4 billion and $2.6 billion three years ago, respectively. 

That's envy-inspiring growth, but Gilead Sciences shares have lost 15.7% of their value this year on fear that the company's hepatitis C dominance will be short-lived. As the argument goes, new therapies that work as well as Gilead Sciences' drugs but are cheaper will jeopardize the $19 billion in HCV revenue the company reported last year.

Although that's a risk, it may be foolish to bet against the company. In January 2015, AbbVie Inc(ABBV -1.01%) launched Viekira Pak, a cheaper HCV therapy, yet Gilead Sciences still held onto 90% market share in 2015. A similar scenario may play out this year following the recent approval of Merck & Co.'s (MRK 0.81%) HCV therapy, Zapatier.

Source: Gilead Sciences.

Zepatier offers solid efficacy and safety, but it's far from a slam dunk that doctors will shift patients to it from Gilead Sciences' Harvoni, a therapy that's arguably performed better in real-world use than it did in its already impressive clinical trials.

Zepatier requires testing to determine if polymorphisms are present in patients that could reduce its efficacy and its contraindicated in patients with moderate to severe liver damage and even if Zepatier does make headway, Gilead Sciences expects an FDA decision on its next generation HCV drug this summer. If that new therapy is approved, then it could allow Gilead Sciences to sidestep Zepatier's threat because Zepatier is only approved for use in genotype 1 and 4 patients and Gilead Sciences' new drug could be approved for use in all genotypes.

Given Gilead Sciences' track record and the fact that its shares trade at a paltry forward P/E ratio that's south of 7, long term investors might want to embrace its shares now that they're on sale.

No. 2: Celgene Corporation
Celgene Corporation has made a mint on the back of its top-selling multiple myeloma drug Revlimid and remarkably, Revlimid's sales, which totaled $5.8 billion in 2015, are still growing at a double digit pace.

Revlimid's approval last year for use as a first-line treatment means it's being prescribed to more patients than ever before, and given additional studies are evaluating Revlimid's use in other indications, sales could still be heading higher. Celgene's management forecasts that Revlimid sales will reach $6.6 billion this year and could eclipse $7 billion in 2017.

Celgene also markets the pancreatic cancer drug Abraxane and the third-line multiple myeloma drug Pomalyst -- two drugs that are on pace to achieve blockbuster status this year --  and Celgene thinks its proriasis drug Otezla could generate peak annual sales of $2 billion someday.

Celgene's ozanimod, a phase 3 oral therapy under development for multiple sclerosis, is also incredibly intriguing. Celgene got ozanimod when it bought Receptos last year and if phase 3 trials pan out, then ozanimod could hit the market with best-in-class safety that allows it to capture a big chunk of the $5 billion plus being spent annually on oral MS drugs.

Interestingly, Celgene's long-range guidance, which calls for a doubling of its sales to $21 billion and for EPS of at least $13 in 2020, only includes $1 billion in ozanimod sales. Therefore, if ozanimod hits the market early and does better than hoped, then Celgene's already heady forecast could end up being too low.

No. 3: Regeneron Pharmaceuticals
It's uncommon to see drugs already bringing in billions of dollars in sales grow by 40%, but that's exactly what happened to Regeneron's Eylea last year.

Greater use of Eylea in patients with vision loss tied to diabetes helped propel Eylea's global net sales to $4.1 billion in 2015. Sales of Eylea in the U.S. jumped 54% to $2.67 billion and sales outside the U.S. grew 36% to $1.41 billion.

Eylea's sales may keep heading higher because the drug is used to treat age related macular degeneration (AMD), a condition that's becoming more common as baby boomers become seniors. Eylea's sales could also increase if Regeneron wins market share away from Lucentis and Avastin, both of which are also multibillion-dollar drugs. 

Additionally, the company won approval for its cholesterol-busting drug Praluent last summer and while sales of Praluent were a trickle at $11 million last quarter, the addressable market of patients who could benefit from additional cholesterol lowering therapy is massive. As reimbursement headwinds that may be holding back sales ease, it wouldn't shock me if Praluent sales take a big leap forward this year.

The company may also end up with a third top seller on the market later this year. The FDA is set to make a decision on Regeneron's rheumatoid arthritis drug sarilumab on Oct. 30. If regulators give sarilumab a green light, then it will compete against AbbVie's Humira -- the planet's best selling drug. Given Humira racks up $14 billion a year in sales and results from a trial comparing sarilumab head to head against Humira are expected this year, sarilumab could soon be one more reason investors will want to take advantage of the recent sell-off to add Regeneron to their portfolios.