Merck & Co. Inc.'s Keytruda surpassed Roche Holding AG's Herceptin as the top-selling cancer medicine in 2018 as oncology grabbed the majority of top drugmakers' sales for the year.

Keytruda, an immuno-oncology drug indicated for a variety of cancers, led oncology sales with $7.17 billion — almost doubling its 2017 revenue — while Roche's Herceptin had revenues of $7.14 billion, relatively flat compared to 2017. Merck's blockbuster cancer drug also overtook the sales of its chief immuno-oncology rival, Bristol-Myers Squibb Co.'s Opdivo, which made $6.74 billion during the year on 36% growth.

Keytruda and Opdivo — both tumor-targeting drugs known as checkpoint inhibitors — are locked in a race for the fast-growing immuno-oncology market. Lung cancer is the "main area of focus" between Keytruda and Opdivo — an area where the latter has lagged behind — but other indications have also driven the growth of both immuno-oncology therapies, according to Credit Suisse analyst Vamil Divan.

Merck's drug has been approved in the U.S. for 10 tumor types, including head and neck, kidney, bladder and skin cancers, which the company believes to be top growth drivers. On the other hand, Bristol-Myers sees melanoma and renal cell carcinoma as key growth opportunities for Opdivo.

Overall, the biggest cancer drugs made $36.37 billion in 2018, up from $31.35 billion in 2017.

Biosimilar competition plagues immunology drugs

Immunology still had strong growth in 2018 as sales of AbbVie Inc.'s top-selling drug Humira — indicated for autoimmune disorders from rheumatoid arthritis to plaque psoriasis — grew 8.2% from 2017 to $19.94 billion. The drug is facing cheaper competition in the EU from biosimilars made by companies Amgen Inc., Samsung Biologics Co. Ltd., Mylan NV, Novartis AG, Pfizer Inc. and Momenta Pharmaceuticals Inc., which are all looking to chip away at Humira's market share.

Biosimilars are lower-cost copies of biologic therapies.

AbbVie's executives struggled to articulate the future of their blockbuster drug during the company's fourth-quarter earnings call Jan. 25. CEO Richard Gonzalez said AbbVie plans to dampen the impact of biosimilar competition by building its pipeline, which also includes immunology drugs risankizumab and upadacitinib. Both are expected to launch in 2019.

Novartis' Cosentyx, another immunology therapy, saw its sales rise 37.2% from 2017 to $2.84 billion, in line with analysts' expectations. Other top-selling immunology therapies, however, did not see the same upward trend as Johnson & Johnson's Remicade and Amgen's Enbrel had sales of $5.33 billion and $5.01 billion, respectively — both down from their previous-year revenues as a result of pressure from biosimilars.

With their best-sellers facing declining sales, combined with having stores of cash on hand, top drugmakers may be poised to snap up smaller biotechnology companies in 2019, according to analysts. "The industry remains hungry for compelling new prospects and many pharma companies face looming patent expirations on key products," said S&P Global Ratings credit analyst Arthur Wong.

Combined, immunology generated sales of $33.26 billion for the largest pharmaceutical companies in 2018, up from $30.82 billion in 2017.

Vaccines followed as the third-highest sales driver for top drugmakers with $8.95 billion, a 13.1% growth year over year. Merck & Co.'s human papillomavirus vaccine Gardasil recorded a 36.4% growth year over year, while sales of Pfizer's pneumococcal conjugate vaccine Prevnar 13 — the company's largest revenue driver in 2018 — rose 3.6%.

During Pfizer's full-year 2018 earnings call, CEO Albert Bourla attributed Prevnar 13's sales growth to the vaccine's launch in China.

Diabetes medicines were the fourth-largest revenue driver, with sales falling year over year to $7.90 billion from $8.96 billion in 2017. Sanofi's Lantus, the year's top-selling diabetes drug, saw its sales drop by 19.5%, while Novo Nordisk A/S' Levemir saw a 17.3% decline.

Like Remicade and Enbrel, Lantus is facing cheaper competition from biosimilars. Sanofi, which was sued in 2018 and accused of deceptively raising insulin prices, hiked the U.S. list price of 35 drugs, including Lantus, during the year.

Rising insulin costs have been the subject of a U.S. probe where lawmakers pressed Eli Lilly and Co., Sanofi and Novo Nordisk — which account for over 90% of the global insulin market — to explain why the prices of their diabetes medicines keep increasing.

Meanwhile, hepatitis C vaccines recorded the largest sales jump for 2018, booking $3.44 billion compared to 2017's $490 million.