Paris : Sanofi and its takeover target Medivation dug in for trench warfare on Friday, with the French drugmaker confident of winning over investors and the U.S. cancer firm insisting it was better off staying independent.

Sanofi, which has a track record of winning hostile fights in the biotech sector, said it was ready to speak directly to Medivation shareholders about its spurned $9.3 billion offer.

"While to date Medivation has chosen not to enter into discussions regarding this value-creating transaction, Sanofi remains committed to the combination," it said.

In its first pronouncement on the subject since Sanofi confirmed its approach on Thursday, Medivation rejected the $52.50-a-share proposed offer outright, which it said undervalued the company "substantially".

"Our board strongly believes that Medivation's business plan will deliver value to our stockholders that is far superior to Sanofi's offer and unanimously rejects your proposal," it said.

Sanofi went public with its offer for the maker of the blockbuster prostate cancer drug Xtandi after failing to get Medivation management to engage.

"We are confident that Medivation shareholders will ultimately share our strong belief that our offer would provide significant and immediate cash value," Sanofi Chief Executive Olivier Brandicourt said on a conference call.

He was speaking as Sanofi reported higher quarterly profit on Friday, boosted by its Genzyme division, another U.S. biotech business acquired after a protracted fight in 2011.

Chief Financial Officer Jerome Contamine declined to say whether Sanofi was ready to engage in a bidding war for Medivation and raise its offer, as many investors expect it to do.

Still, shares in Sanofi fell 5 percent, with worries about it getting sucked into a costly and lengthy takeover battle fuelling the decline, traders said.

Industry analysts and healthcare bankers believe Sanofi will need to sweeten its opening offer if is to win its prey, since Medivation shares are already trading $4 above its $52.50 offer.

"I imagine you'll see a fair bit of movement," said one banker not directly involved in the situation. "It's fairly typical to see an average 10 percent premium to the original offer I think this is going to get much higher."

CVRs, or contingent value rights, might also be deployed, the banker said, as happened when Sanofi bought Genzyme for $20 billion after a similar unsolicited approach.

CVRs, which pay out in specific circumstances, are sometimes used in biotech to compensate shareholders in a target company if, for example, a drug sells particularly well.

There is also potential for other companies to get involved, including Japan's Astellas Pharma, Medivation's partner on Xtandi, or Britain's AstraZeneca, which has a big focus on cancer.

AstraZeneca Chief Executive Pascal Soriot declined to comment on Medivation in a post-results call with reporters on Friday but said he had a high bar for acquisitions, since his company's drug pipeline was now full. Officials at Astellas have also declined comment.

Sanofi said first-quarter business net profit grew 3.5 percent at constant exchange rates to 1.72 billion euros ($1.96 billion), equivalent to a 0.2 percent drop on a reported basis.

Sales rose 0.7 percent at constant exchange rates to 8.54 billion euros, down 1.9 percent on a reported basis, Sanofi said, adding that it was confirming its full-year forecasts.

Analysts polled by Reuters had on average expected business net profit of 1.7 billion euros and net sales of 8.73 billion.

Revenue at biotech arm Genzyme rose 20.5 percent, with a 134 percent rise in proceeds from multiple sclerosis treatment Lemtrada. Diabetes sales fell 4.5 percent, reflecting the trend of weaker revenue from blockbuster Lantus in the United States.

Sanofi's "main diabetes franchise remains in flux, with the company lowering estimates twice for this business over the last 18 months," Bernstein analyst Tim Anderson said in a note.

"(Sanofi's) research and development track record is not great, and it is guiding for no meaningful earnings per share growth in 2016/2017," said Anderson, who downgraded the stock to market-perform in November.

(Additional reporting by Freya Berry, Ben Hirschler and Andrew Callus; Editing by Elaine Hardcastle and David Evans)