AstraZeneca and Bristol-Myers Squibb are among several pharmaceutical giants that have submitted initial bids for Amylin Pharmaceuticals, the diabetes drugmaker that is exploring a sale, according to people familiar with the matter.

Other companies that also made initial bids for Amylin last week include Merck & Co and Sanofi, the sources said.

Amylin put itself up for sale after spurning a $3.5 billion takeover bid from Bristol earlier this year, under pressure from activist investor Carl Icahn. It is expected to seek second-round bids in the next several weeks, the sources said.

Two of the sources said all the parties made bids of at least $25 a share, valuing Amylin at more than $4 billion. It was not clear whether any other drugmakers in addition to the four companies also submitted bids.

Representatives for Amylin, Bristol-Myers, AstraZeneca, Merck and Sanofi all declined to comment.

Amylin, which makes diabetes drugs Byetta and Bydureon, started reaching out to potential buyers in April after hiring Credit Suisse and Goldman Sachs as its financial advisers and Skadden Arps as its legal adviser, sources told Reuters previously.

Big pharmaceutical companies, flush with cash and easy access to debt, have been making aggressive bids for promising biotech firms and other targets as they look to find new products and replenish their pipelines.

But often they find the targets, whose shares are sometimes depressed, holding out for a better price.

Other such bids in the healthcare sector in recent months include an aborted $6.8 billion hostile takeover attempt by Swiss drugmaker Roche for genetic specialist Illumina, and a bid by GlaxoSmithKline to buy long-time partner Human Genome Sciences for about $2.6 billion.

Amylin has been considered a possible takeover target for some time, with the diabetes market being one of the fastest-growing due to rising rates of obesity.

The disease affects more than 300 million people worldwide, including nearly 26 million Americans. Diabetics run a high risk of heart disease, stroke, kidney failure, blindness and limb loss.

Pressure on Amylin increased after news of the failed Bristol-Myers bid leaked and prompted Icahn to turn his sights on the company. Icahn, whose 8.94% stake has made him Amylin's third-largest shareholder, has called for a sale.

The opportunity for bidders to make their move on Amylin also got a boost when Amylin ended a nearly decade-long partnership with Eli Lilly in November last year.

Amylin and Lilly introduced Byetta in 2005. Bydureon, a longer-acting version, was approved recently. The dissolution of the partnership with Lilly made Amylin responsible for selling its products in the United States, while leaving the company to find a new partner outside the country.

Buying Amylin would complement the diabetes portfolios of many large drug companies, according to analysts, although acquirers will be wary of the increasingly competitive landscape for so-called GLP-1 drugs like Byetta and Bydureon, several more of which could hit the market in the next couple of years.

Merging the Amylin operation with a large business would create opportunities to rationalize marketing costs, by reducing the number of U.S. sales representatives, and pushing Amylin's products through a larger international sales force would increase their commercial reach.

Leerink Swann analyst Seamus Fernandez said AstraZeneca's European presence might give it an advantage relative to Bristol, but Sanofi and Merck could be better placed to fully capitalize on Amylin's diabetes business.