>Tysabri enjoyed a 172 percent sales increase, hitting $171 million for the quarter.<
First of all, the $171M figure you cited is the amount of Tysabri sales booked by BIIB, specifically, rather than the actual in-market sales, which were $236M in 3Q08 (#msg-33002433). When discussing market share, the in-market sales figure is the one that matters.
Second, as masterlongevity just said, the year-over-year growth rate is not especially meaningful when the prior-year base is so small.
The salient point in all this is that Tysabri is clearly underperforming the prior sales projections of BIIB and ELN.
<font size=3><font color=red> “The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”