Shares of Amylin Pharmaceuticals Inc. jumped March 28 in the wake of a news report that the diabetes-focused pharmaceutical company had rejected a $3.5 billion unsolicited offer from Bristol-Myers Squibb Co. earlier this year.

The San Diego-based company’s stock closed March 28 at $23.77, up 54 percent from the previous day’s close.

The increase stemmed from a report by Bloomberg, citing “two people with knowledge of the matter,” that Bristol-Myers proposed an acquisition at $22 a share in a letter to Amylin.

The board turned down the offer last month, and Bristol-Myers hasn’t approached Amylin since, Bloomberg said in the report.

Amylin is the maker of the drug Bydureon, a once-weekly treatment for type 2 diabetes, and other drugs that help with blood sugar control.

“This is a major development for (Amylin), as an approved drug has always been the favorite target of Big Pharma,” the Berkeley-based Medical Technology Stock Letter said in a special update March 28. “That being said, it looks like Big Pharma is going to have to pay up for an approved type 2 diabetes drug that has significant potential.”

The Medical Technology Stock Letter said it expects Amylin to be bought this year, with Roche, Takeda, AstraZeneca, and GlaxoSmithKline, among others, lining up to buy the company.

The stock is listed as AMLN on Nasdaq.