Shares of Diplomat, Flint, Mich., on Monday slumped nearly a third from their closing price Friday at $5.81.
Diplomat Pharmacy's Securities and Exchange Commission Form 10-Q for the third quarter shows that as of Nov. 8, the company had nearly 76 million shares outstanding. A $4-a-share deal based on that number would be valued at about $304 million.
UnitedHealth will assume Diplomat Pharmacy's debt outstanding as well, a statement from the companies on Monday said.
UNH plans to combine Diplomat with its OptumRx unit.
Diplomat, which started up in 1975, focuses on providing patients with specialty medications for complex diseases including cancer and immunological disorders.
The deal expands the specialty-pharmacy and infusion solutions that OptumRx can offer to consumers, the company said.
For the nine months ended Sept. 30, Diplomat Pharmacy reported a loss of $4.69 a share compared with a loss of 6 cents a share in the year-earlier period. Revenue fell 6.9% to $3.85 billion.
Diplomat Pharmacy's board unanimously approved the deal with OptumRx, the statement said. The board had assessed a number of strategic options and considers this deal in the best interests of the company's stakeholders, Diplomat Chairman and CEO Brian Griffin said.
Diplomat's co-founder and chairman emeritus, Philip Hagerman, and certain people and entities affiliated with him own 23% of Diplomat's shares and have agreed to tender their stakes into the offer.
The deal is being done through OptumRx, an Optum unit that provides pharmacy-care services and manages prescription-drug benefits for Medicare and Medicaid as well as for health plans of employers and unions.
Parent Optum is a health-services subsidiary of UnitedHealth, the Minnetonka, Minn., managed-health giant.
The deal is subject to conditions including antitrust clearance, the companies said.
Diplomat Pharmacy shares traded as high as $15.69 one year ago. They were $2.43 in mid-November. They traded past $50 in July 2015.