More than a month after begging to keep his job, Sanofi CEO Christopher Viehbacher has been ousted by the company’s board following differences about the company’s performance and his management style.

The board’s official statement announcing the firing offered few clues to its underlying reasons for dismissing Viehbacher, saying little beyond the fact that it “decided unanimously to remove” him: “The group needs to pursue its development with a management aligning the teams, harnessing talents and focusing on execution with a close and confident cooperation with the board.”

Close and confident cooperation was hardly evident in Viehbacher’s dealings with the board—especially when it became apparent to him in September that chairman Serge Weinberg was siding against the CEO and with his opponents on the board. Weinberg has taken on the additional position of interim CEO pending the naming of a permanent chief executive. Ironically, Weinberg once served as a CEO, for Gucci parent company PPR, until he was ousted by that company’s board.

Viehbacher’s ouster came after shares in Sanofi suffered their biggest one-day decrease in 17 years on Tuesday, following the company’s warning that its core diabetes business would likely remain flat next year even though it grew 8.3% during Q3 over a year ago, to €1.799 billion (about $2.293 billion). Sanofi said it was compelled due to increased competition to cut U.S. prices for its anchor diabetes treatment, Lantus—whose U.S. patent on its active ingredient expires on February 12, 2015.

The diabetes caution came in a mostly-upbeat set of Q3 results that included word of a successful Phase III efficacy study in Latin America for a Dengue vaccine; a reassurance that 2014's full-year earnings-per-share growth target range of 6% to 8% would be met; and a 9.4% rise in net income over Q3’13.

In September, Viehbacher fought back against the board in a letter offering his arguments for staying on the job: “Changing the CEO would be against a backdrop of a company and a leader perceived to be succeeding strongly. Such a change would be difficult to understand,” Viehbacher wrote.

The consequences, he added, would include destabilizing Sanofi’s senior leadership; and its relationships with partners—citing by name Regeneron—as well as investors, regulatory agencies, and industry groups. Viehbacher is chairman of the European Federation of Pharmaceutical Industries and Associations (EFPIA), and also chairs the FDA and Biomedical Research Committee of the industry group Pharmaceutical Research and Manufacturers of America (PhRMA).

“My abrupt departure will, in my view, raise questions with key stakeholders,” Viehbacher added.

Viehbacher has served as Sanofi’s CEO since 2008, the first non-French CEO in a Paris-headquartered company whose predecessors stretch back to the founding of Laboratoires Midy by a family of pharmacists in 1718.

Viehbacher’s tenure coincided with the company’s stock price roughly doubling—the result of shifting the company away from chemical manufacturing to biotech, including vaccines and over-the-counter drugs. He steered Sanofi’s successful $20.1 billion acquisition of Genzyme, one of several moves to rebuild what was acknowledged to have been a depleted pipeline when he took over the company’s helm.

But Viehbacher generated friction with board members with some of his actions, and according to numerous news reports, his failure to inform directors of many of those actions. In June, he opted to move with his family from Paris to Boston, angering some French directors, as did Viehbacher’s pursuit of operations and job cuts in France, touching off conflicts with labor unions and the country’s labor-friendly government.

In 2012, for example, Sanofi announced plans to slice up to 2,500 jobs, before opposition from leaders of affected unions and France’s President François Hollande forced the company to retreat, saying only a net 207 jobs would be eliminated.

Earlier this year, Viehbacher was accused by some on the board of not cluing in directors on the company’s review of an $8 billion portfolio of off-patent drugs, mostly made in France, until labor leaders leaked the plans because it included the elimination of 2,600 jobs. The review was hurtful to France given the country’s high unemployment and frustration over erosion of its manufacturing base, an unnamed source complained to Reuters.

That friction apparently led Sanofi's board to look for another chief executive, according to the French newspaper Les Echos, which on Monday reported: “Contacts have been made with at least a top executive of a pharmaceutical competitor, but they were unsuccessful. This obviously creates a climate of uncertainty in the management team itself.”

Just yesterday, according to Reuters, Viehbacher confirmed that Weinberg declined to clarify his future during a meeting the previous day. When he asked the chairman about his position at the helm of France's second-biggest listed company, Weinberg brushed aside the question as being “not on the agenda.”

As with other biopharma CEOs, Viehbacher cut R&D expenses in favor of drug development through collaborations with smaller partners. Among the most successful has been Sanofi’s partnership with Regeneron Pharmaceuticals, launched with Sanofi predecessor Aventis in 2003 to develop Zaltrap® (ziv-aflibercept), indicated for metastatic colorectal cancer (mCRC) that is resistant to or has progressed following an oxaliplatin-containing regimen.

Since 2007, Regeneron has collaborated with Sanofi to discover, develop, manufacture, and commercialize fully human monoclonal antibodies utilizing Regeneron's VelociSuite of technologies. After five antibodies entered clinical development In the first three years of the collaboration, the companies expanded their partnership in 2009 with a goal of advancing 20–30 additional antibodies into clinical development by 2017. Sanofi has agreed to provide Regeneron with $160 million in annual research funding through 2017.

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