AstraZeneca (NASDAQ:AZN) and Rigel Pharmaceuticals' (NASDAQ:RIGL) rheumatoid arthritis drug fostamatinib works.

It just might not work well enough to produce meaningful sales.

In a phase 2b trial, fostamatinib beat placebo using a couple of different dosing combinations. The drug produced a superior change in DAS28 score -- a measurement of signs and symptoms of rheumatoid arthritis -- from baseline to 6 weeks compared to placebo. We'll have to wait for the full results to be presented -- likely at a medical meeting -- to know how much better fostamatinib performed compared to placebo.

AstraZeneca also included Abbott Labs' (NYSE:ABT) Humira in the clinical trial. Unfortunately, patients taking the market-leading drug performed better than the patients taking fostamatinib. The companies only needed fostamatinib to match Humira's efficacy because fostamatinib is taken orally while Humira has to be injected. Alas, it couldn't even manage the non-inferiority claim.

If fostamatinib were the only new oral rheumatoid arthritis medication, perhaps working well-but-not-great would be enough. Fostamatinib might be used before Humira and the rest of the injected/infused drugs simply because patients don't like needles. There are plenty of oral generics, such as methotrexate, that get used as a first-line drug simply because they don't require needles.

But Pfizer (NYSE:PFE) recently gained Food and Drug Administration approval for its oral rheumatoid arthritis drug, Xeljanz, and Eli Lilly (NYSE:LLY) and Incyte (NASDAQ:INCY) are developing baricitinib, which is also taken orally.

Results for pivotal phase 3 trials testing fostamatinib in combination with other rheumatoid arthritis drugs are expected in the first half of next year. Fostamatinib only needs to beat placebo with a reasonable safety profile to gain FDA approval, but it's unlikely that would be enough for it to grab a substantial piece of the market.

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