The 9 Best-Selling Prescription Drugs in 2016 (Each One Topped $6 Billion)

Prescription drug costs in the U.S. are rising. And costs are going up despite a record number of generic drugs that are on the market.

One primary reason behind the continued increase of prescription drug costs is the greater number of specialty drugs that have been introduced by biopharmaceutical companies. These specialty drugs are used to treat complex or rare chronic conditions. Many of them are biologics, which are drugs derived from living cells.

The flip side of the rising costs of prescription drugs is that some drugmakers are making a lot of money. Here are the nine best-selling prescription drugs of 2016, all of which generated sales of more than $6 billion.

No. 9: Lantus

Sanofi's (NYSE: SNY) Lantus made $6.05 billion last year. The insulin product is used to treat adults and children with type 1 diabetes and adults with type 2 diabetes. Lantus is the world's top-selling brand in the insulin market.

Sales for Lantus are declining, however, with a 10.6% drop in 2016 compared to the prior year. The drug faces stiff competition from a biosimilar version in the U.S. and in Europe. A major health insurer and a top pharmacy benefits manager have stopped providing coverage for Lantus in favor of the new biosimilar, which means that nearly 35 million Americans no longer have access to the insulin drug.

No. 8: Herceptin

Roche's (NASDAQOTH: RHHBY) Herceptin generated sales totaling $6.75 billion in 2016. The drug is approved for treating breast, stomach, and esophageal cancer that are Human Epidermal growth factor Receptor 2-positive (HER2+).

Herceptin's sales continue to grow -- up 3.7% last year -- despite losing patent exclusivity in Europe several years ago. The drug loses patent exclusivity in the U.S. in 2019. The key for sustained sales growth from what is now an "old-timer" among cancer treatments is that Roche is marketing Herceptin in combination with another one of its cancer drugs, Perjeta.

No. 7: Avastin

Another Roche drug takes the No. 7 spot on our list. Avastin generated sales of $6.75 billion in 2016. That reflected a slight increase of 1.5% compared to the previous year. The drug treats a laundry list of cancers, including colorectal, lung, brain, kidney, cervical, and ovarian cancer.

Although Roche won't lose patent exclusivity for Avastin until 2020, several new drugs have entered the market in the past few years that have taken market share in several indications targeted by Avastin. For now, Avastin continues to perform well internationally, particularly in China.

No. 6: Revlimid

Celgene's (NASDAQ: CELG) Revlimid made $6.97 billion last year. The drug is currently approved to treat multiple myeloma, myelodysplatic syndromes (MDS), and mantle cell lymphoma.

Sales for Revlimid continue to grow at a strong pace, jumping 20% in 2016. This growth isn't likely to slow down anytime soon. While there are some new blood cancer drugs on the market and in development, many of them are taken in combination with Revlimid. Celgene enjoys patent protection for its top-selling drug through 2024 in Europe and through 2027 in the U.S., but the biotech has struck a deal that allows a generic version to enter the U.S. market on a limited basis in 2022.

No. 5: Remicade

Remicade is co-marketed by Johnson & Johnson (NYSE: JNJ) and Merck. The drug generated total sales of nearly $7.83 billion last year, down 10.6% from 2015.

Pfizer's (NYSE: PFE) biosimilar to Remicade launched in the U.S. last year. J&J and Merck already faces biosimilar competition in Europe. This is especially problematic for Johnson & Johnson, as Remicade remains the company's top-selling drug. J&J is using price discounts and other promotional strategies in an attempt to preserve market share for Remicade.

No. 4: Rituxan

Roche again makes the list of the world's top-selling prescription drugs with Rituxan, with partner Biogen taking a share of the drug's revenue. The two companies reported combined sales in 2016 of Rituxan of $8.58 billion, up 2.7% from the previous year.

Rituxan first won FDA approval in 1997 for treating non-Hodgkin's lymphoma. In subsequent years, the drug gained approval for other indications, including chronic lymphocytic leukemia and rheumatoid arthritis. Several key U.S. patents for Rituxan begin to expire next year.

No. 3: Enbrel

Amgen (NASDAQ: AMGN) and Pfizer posted combined 2016 sales of Enbrel totaling $8.87 billion. Enbrel is approved for treating several autoimmune diseases, including rheumatoid arthritis, plaque psoriasis, and psoriatic arthritis.

Overall sales for Enbrel increased 2.7% last year. However, Pfizer, which markets the drug outside of the U.S. and Canada, saw revenue from Enbrel drop in 2016 due to competition from biosimilars. Amgen encountered the same challenge in the first quarter of 2017, with Enbrel revenue dropping from the entrance of a biosimilar in the U.S. market.

No. 2: Harvoni

Gilead Sciences (NASDAQ: GILD) reported sales of $9.08 billion for hepatitis C drug Harvoni last year. However, sales for Harvoni plunged 34.5% from the prior year.

There are two primary challenges for Harvoni. Most important is that new patient starts have fallen as a result of so many patients being cured by the drug and its predecessor, Sovaldi. Another big factor is competition, both from other companies' drugs as well as Gilead's newer hepatitis C treatment, Epclusa.

No. 1: Humira

AbbVie (NYSE: ABBV) claims the top-selling prescription drug in the world with Humira. Sales for the drug totaled $16.08 billion in 2016, up 14.7% from the prior year. Humira is approved for treating autoimmune diseases including rheumatoid arthritis, psoriatic arthritis, psoriasis, Crohn's disease, and ulcerative colitis.

Like some of the other companies on our list, AbbVie faces the prospects of biosimilar competition. Amgen won FDA approval last year for its biosimilar version to Humira. However, that biosimilar hasn't launched yet. AbbVie is fighting Amgen in court over potential patent infringement and thinks that it can keep biosimilar rivals from entering the U.S. market through 2022

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Keith Speights owns shares of AbbVie, Celgene, Gilead Sciences, and Pfizer. The Motley Fool owns shares of and recommends Biogen, Celgene, Gilead Sciences, and Johnson & Johnson. The Motley Fool has a disclosure policy.