FLINT—Diplomat Specialty Pharmacy, a provider of specialty pharmacy and infusion services, will be purchased by Eden Prairie, Minn.-based OptumRx, the pharmacy care business of Optum, a unit of the health insurance giant United Health Group, for $4 a share in cash and assumption of outstanding debt, valuing the company at about $300 million.

Diplomat stock plunged nearly 32 percent to $3.96 a share on the news. The stock closed at $5.81 a share Friday. The company’s 52-week stock price high was $16.35 a share, but the company posted a much larger than expected loss last month for the third straight quarter, prompting a “going concern” warning that it was running short on cash and was exploring a sale. News reports said Diplomat, once valued at $3 billion, was struggling to compete against other healthcare giants like CVS, Cigna, and United Health.

Diplomat provides management of specialty medications that treat patients with complex diseases, such as oncology and immunology, and provides specialized infusion therapies offered in all 50 states and Washington, D.C.

“With its focus in specialty and infusion services, Diplomat has a proven track record of solving the unique challenges facing patients with complex health care needs,” said John Prince, CEO of OptumRx. “This combination will expand the innovative specialty pharmacy and infusion solutions OptumRx can offer to the consumers and clients we serve, helping ensure people get the right medications and services at the right time, in the right setting.”

The transaction was unanimously approved by the board of directors of Diplomat.

“Our Board of Directors carefully considered a variety of strategic options and concluded that joining OptumRx is in the best interests of our shareholders, employees and the clients and patients we serve,” said Brian Griffin, chairman and CEO of Diplomat.

“Since co-founding the company in 1975 with my father, I have had the honor of helping to transform Diplomat into a specialty pharmacy services leader,” said Diplomat co-founder and chairman emeritus Philip R. Hagerman. “This combination will create significant value for Diplomat’s specialty pharmacy consumers and clients, and I look forward to the combined companies’ future success.”

Hagerman and people and entities affiliated with him own about 23 percent of Diplomat stock, and have agreed to tender their shares in connection with the offer.

There was no immediate word on the future of Diplomat’s Flint headquarters, a former General Motors office complex, or its employees.

More at diplomat.is or www.optum.com.