By Nick Kostov

PARIS--Sanofi SA on Thursday said it has made an all-cash offer to acquire Medivation Inc. in a deal valued at $9.3 billion, the French drugmaker's latest effort to expand its cancer-treatment business.

Sanofi decided to go public with its takeover approach after the U.S. company failed to respond to its earlier advances. Sanofi shares fell 1.1% in early trading Thursday.

Chief Executive Olivier Brandicourt said the $52.50 a share offer--a 50% premium to Medivation's average share price for the two months before takeover speculation emerged--was "very attractive."

In a letter to Medivation CEO David Hung dated April 28, Mr. Brandicourt writes that Medivation, which focuses on treatments for hard-to-cure cancers, would play a key part in the French drugmaker's efforts to expand its cancer-treatment business.

Competitors such as Roche Holding AG and AstraZeneca PLC have also been expanding their oncology portfolios and developing immuno-oncology treatments that harness patients own immune systems to fight cancer.

Medivation is a Nasdaq-listed company based in San Francisco with one marketed prostate cancer therapy, called Xtandi, and two additional oncology assets in clinical development.

"We do not understand the delay in responding to our letter," Mr. Brandicourt wrote. "In these circumstances we believe it is appropriate to make this letter public, which we are doing today."

In the letter, Mr. Brandicourt writes that he held a first call with Mr. Hung to discuss a takeover on March 25, when Mr. Hung said he was unwilling to meet. In a second call on April 3, Mr. Hung said that having discussed the possible combination with his board, Medivation concluded that it had no interest in discussing a transaction, according to Mr. Brandicourt's letter.

Sanofi subsequently sent a letter on April 15 setting out its proposal to buy the company for $52.50 a share. Medivation acknowledged its receipt but didn't follow up, Mr. Brandicourt wrote.

A spokeswoman for Medivation didn't immediately respond to a request for comment.

Write to Nick Kostov at Nick.Kostov@wsj.com

(END) Dow Jones Newswires

April 28, 2016 03:56 ET (07:56 GMT)

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