Sanofi ($SNY) CEO Chris Viehbacher is often pressed to answer why he doesn't just buy Regeneron ($REGN), a biotech outfit closely bonded with Big Pharma on several important development initiatives. So far, Viehbacher insists that the collaborative relationship is ideal, and he's underscored his appreciation for Regeneron by highlighting the big stake Sanofi has in Regeneron stock. Today, that stake got bigger. According to a filing with the SEC, Sanofi has snapped up $71 million worth of Regeneron stock, bumping its stake to 22.5%. Filing