Analyst Umer Raffat cited patent concerns as the reason for the downgrade.
Amgen, which develops and manufactures human therapeutics, is scheduled report fourth-quarter and full-year results on Tuesday after the close of the U.S. financial markets. Analysts expect the company to report a fourth-quarter profit of $3.27 a share on sales of $5.84 billion.
Shares of Amgen have declined 1% so far in 2019.
The Thousand Oaks, California-based company's products include Evenity to treat osteoporosis in postmenopausal women; Prolia to treat postmenopausal women with osteoporosis; and Repatha to treat coronary diseases.
Last year, Xgeva gained approval in both United States and EU for the prevention of skeletal-related events in patients with multiple myeloma. In the third quarter, Blincyto was approved in Japan and for the pediatric patient population in the United States.
In October, Amgen said third quarter revenue rose 2% to $5.9 billion from the same period last year. Product sales grew 1% globally. Adjusted earnings per share increased 13% to $3.69, driven by higher total revenue, a lower tax rate and lower weighted-average shares outstanding.