Medications that treat inflammatory conditions such as rheumatoid arthritis were the most expensive therapy class in 2016, when evaluated by annual spend per patient, according to Express Scripts’ annual drug trend report.

On average, employers paid $3,587.83 for each inflammatory drug prescription, and patients covered by those plans paid $118.21 in out-of-pocket costs per year, a 26.5% spending increase compared to 2015.

The pharmacy benefit manager found that AbbVie’s Humira and Amgen’s Enbrel accounted for about 70% of market share, even though there are more than 15 therapies in the class available on the market. Delays in the launch of biosimilar versions of those drugs contributed to a lack of competition as well, the report noted.

See also: The FDA approves Amgen’s biosimilar version of Humira

Overall spending on medications, including cost and use of drug, increased 3.8%, compared to a 5.2% increase in 2015. But the PBM still expects an increase in spending over the next few years, saying that drug spending, net of rebates, is likely to increase between 10% and 13% between 2017 and 2019.

The diabetes and oncology drug classes ranked second and third, respectively, as the most expensive therapeutic classes in 2016. Diabetes patients paid on average $108.80 in 2016, a 19.4% increase from 2015. Oncology patients paid 21.5% more in 2016, $60.70 compared to $49.50 in 2015.

Those three therapeutic classes accounted for a combined 26.7% of drug spending among Express Script members in 2016. The PBM said increases in both utilization and the cost of the therapies contributed to the cost spike of inflammatory and oncology medications.

See also: Prescription drug spending rises 9% in 2015, to $324 billion

“New expensive biologics will continue to come to market, many for diseases with very limited treatment options, but diabetes, inflammatory conditions, and oncology will continue to be the main drivers of spending over the next three years,” said Glen Stettin, SVP of clinical, research and new solutions at Express Scripts, in an email.

Hepatitis C drugs were the only specialty therapy class that reported a spending decline, with patients paying 34% less in 2016 than in 2015, due to lower utilization and lower prices. AbbVie’s Viekira Pak and Gilead Sciences’ Harvoni generated 43% of the market share, but the report noted that additional competition led to 6.7% decline in unit cost in the hepatitis C therapy class, which the PBM sees as an ongoing trend.