* Move follows U.S. firm's refusal to discuss takeover offer

* Cancer drug company says $52.50 a share inadequate (Adds details on Medivation bylaws)

By Ben Hirschler and Greg Roumeliotis

May 25 (Reuters) - French drugmaker Sanofi named eight candidates to replace the entire board of Medivation Inc on Wednesday, stepping up pressure on the U.S. cancer drug company which has rejected its $9.3 billion takeover approach.

Reuters reported earlier that Sanofi was about to propose a new board line-up, taking advantage of a so-called written consent rule that gives Medivation shareholders the ability to act at any time to replace directors.

Sanofi filed documents with the U.S. Securities and Exchange Commission (SEC) seeking the approval of Medivation shareholders for the board's overthrow.

"Despite multiple attempts, both prior to and following the public disclosure of Sanofi's proposal, Medivation has thus far refused to engage with us regarding the merits of a value creating transaction," said Sanofi CEO Olivier Brandicourt.

"Unfortunately, this has left us with no choice but to commence a process to elect directors who are more open to supporting the best interests of Medivation shareholders regarding a potential transaction."

Medivation urged its stockholders to reject Sanofi's election attempt and said it would promptly file materials with the SEC to try and block it.

Efforts to remove a board usually occur through dissident shareholders at a company's annual meeting.

But some companies also grant shareholders the right to act by written consent in lieu of an annual meeting, where the results of such a vote are either binding or non-binding, depending on what bylaws permit.

Medivation's bylaws allow written consent votes to be immediately binding, as long as at least a majority, or 50.1 percent, of shareholders vote in favor of the proposal, according to people familiar with the matter.

Written consents are rare and usually employed by activist investors for various reasons, which include pressing a campaign after a director nomination deadline has passed. Carl Icahn threatened consent last year at AIG to get a director on the insurer's board.

Since the start of 2010, there have been only 37 activism campaigns that pursued written consent, according to FactSet.

"HAND-PICKED NOMINEES"

Sanofi's nominees for Medivation's board are Michael Campbell, Barbara Deptula, Wendy Lane, Ronald Rolfe, Steven Shulman, Charles Slacik, James Tyree and David Wilson -- a roster of independent candidates with experience in the drug industry, finance and the law.

Campbell led Arch Chemicals, before it was sold to Lonza in 2011 for $1.2 billion, while Deptula headed corporate development at Shire. Lane chairs investment firm Lane Holdings and Rolfe retired from Cravath's litigation department in 2010.

Medivation, which rejected Sanofi's approach last month, said again that its $52.50 per share cash offer was "substantially inadequate".

"Its proposal to replace our existing directors with its own hand-picked nominees is simply a tactical manoeuvre to facilitate a transaction that will transfer value that rightly belongs to Medivation stockholders to Sanofi," said CEO David Hung.

In a new letter to Medivation's board, Brandicourt reiterated that Sanofi could raise its offer if the U.S. firm engaged in talks.

Medivation shares were down 0.7 percent at $61.46 in New York on Wednesday.

The U.S. company has signed non-disclosure agreements to share confidential information with other potential acquirers, including Pfizer and Amgen, sources told Reuters earlier this month.

Brandicourt expressed frustration at this. "There have been published reports that you have signed confidentiality agreements with other parties. If that is accurate, we cannot see how you have not done so with us," he wrote.

Sanofi wants Medivation - which sells a successful prostate cancer drug called Xtandi and has others in development - to expand in the lucrative oncology sector, as it seeks new businesses to compensate for flagging diabetes revenues.

Brandicourt said Sanofi was in a position to provide more value than any other party, given the strategic importance of the transaction to the Paris-based company.

(Additional reporting by Michael Flaherty in New York; Editing by Louise Heavens and Alan Crosby)