The firm lowered Amgen to neutral from buy while also raising its price target to $212 from $208. The stock closed Friday's session at $204.02, with the new price target representing a potential 4% upside from the closing price.
Amgen shares were down 1.23% to $201.52 on Monday.
Analyst Salim Syed's downgrade is based on valuation as the company approaches where Mizuho believes it is appropriately priced.
Amgen won a patent dispute with rival Novartis (NVS) - Get Report over its rheumatoid arthritis drug, Enbrel, last week. A judge ruled that two patents that protect the drug from biosimilar competition were valid, leading a 10% run up in the stock.
Amgen is a holding in Jim Cramer's Action Alerts PLUS charitable trust.
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