Biogen Inc. is planning what may be its biggest bond offering that will go to boost shareholders who have seen the value of the biotech company’s stock plummet by more than 8 percent this year.

The drug maker said Thursday in a regulatory filing that it plans to issue bonds in as many as four parts. The portion of the proposed deal with the longest maturity will be 30-year securities that may yield as much as 2.375 percentage points more than similar-maturity Treasuries, according to a person with knowledge of the sale, who asked not to be identified because the information isn’t public.

Proceeds will finance share repurchases under Biogen’s $5 billion buyback program and finance working capital.

Biogen reduced its forecast for 2015 profit and sales in July after revising its expectations for the growth of top-selling multiple sclerosis treatment Tecfidera.

Shares of the Cambridge, Massachusetts-based company have fallen by more than 8.3 percent this year compared to a 1.9 percent gain in the Standard and Poor’s 500 Health Care Index.

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