Dew,
<<However, two of those four (Rituxan and Tarceva) were developed by other companies.>>
Why does it matter whether dna developed or not? DNA shares a $2B drug with idec and has 50% of the US for tarceva. Seems like shrewd business development to me.
<<the survival benefit of Tarceva, modest as it is>>
Regarding tarceva, I wouldn't call a statistically significant >33% survival benefit "modest."
We'll see how it turns out in real life, but the inference on tarceva has been that it has greater potency than iressa which may confer greater efficacy. There's also other indications to come such as glioma where tarceva data has looked good.
Agree with you that tarceva won't automatically take 100% share.