AstraZeneca has opted to pay a further $135 million to Bristol-Myers Squibb to get an equal say in decision-making regarding drugs developed by Amylin Pharmaceuticals, the diabetes specialist acquired jointly by the two partners.

The decision by AstraZeneca to exercise its option to governance rights had been widely expected, following the three-way takeover agreement clinched on June 29.

Under that deal, Bristol is the acquirer of Amylin, paying $5.3 billion in cash for the company, plus an additional $1.7 billion to cover Amylin's debt and its obligations to Eli Lilly from ending an earlier collaboration.

AstraZeneca will then pay Bristol $3.4 billion for a half share in Amylin's products and these medicines will be added to an existing diabetes alliance between the two major drugmakers. The $135 million option payment is in addition to this.

The companies said on Thursday that AstraZeneca had now made an initial payment of approximately $3.2 billion to its partner. A further payment will be made in the near future to take the amount up to $3.4 billion, an AstraZeneca spokeswoman said.

Amylin's products for the fast-growing type 2 diabetes market included Byetta, launched in 2005, and the longer-acting version Bydureon, which can be taken once a week and was approved by U.S. regulators earlier this year.