The good news: Human Genome Sciences' (NASDAQ:HGSI) Benlysta still looks good in its phase 2 clinical trial.

The bad news: Benlysta treats systemic lupus erythematosus (SLE) -- an autoimmune disease that flares up and wanes. And lupus is where good drugs go to die. 

There hasn't been a drug approved to treat lupus in more than 30 years, despite many companies' best efforts. Genentech and Biogen Idec's (NASDAQ:BIIB) Rituxan failed a phase 3 trial earlier this year against lupus nephritis. La Jolla Pharmaceutical persuaded BioMarin Pharmaceutical (NASDAQ:BMRN) to take a flyer on its phase 3 drug, Riquent, for lupus nephritis and SLE, which failed just a month later. Teva Pharmaceutical's (NASDAQ:TEVA) edratide, for SLE, only made it to phase 2 before biting the dust. And those are just some recent ones I can recall off the top of my head.

The Benlysta results released yesterday were a continuation of a phase 2 trial. Patients out as far as four years seem to be having a positive response to the drug.

While the results are promising, they in no way guarantee success for the two phase 3 trials run by Human Genome Sciences and partner GlaxoSmithKline (NYSE:GSK), which are expected to finish up in July and November of this year. Plenty of drugs have done fine in phase 2, only to die when a larger phase 3 trial is performed. And the long-term effectiveness of the drug may not have a bearing on whether the drug works after one year, which is the primary endpoint for both of these phase 3 trials.

Weighing in at a market cap of $430 million after yesterday's 18% move, but now down to about $390 million after an analyst downgrade this morning, Human Genome Sciences has the potential to pop if Benlysta's phase 3 trials are a success. But please, be Foolish, and only keep HGS as a small fraction of your portfolio if you decide to invest. History is definitely not on your side.

Historical Foolishness:

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Biogen Idec is a Stock Advisor recommendation. The Fool has a disclosure policy.