UPDATE 3-Immunomedics soars as lupus drug shows promise

* Positive Phase IIb results for UCB, Immunomedics drug

* Treatment advantage of epratuzumab vs placebo 24.9 pct

* Analysts say result promising but drug still high risk

* Immunomedics shares up 57 pct, UCB adds 1 pct

(Recasts with Immunomedics shares)

By Ben Hirschler and Anne Jolis

LONDON/BRUSSELS, Aug 27 (Reuters) - Immunomedics (IMMU.O) became the second biotech company in just over a month to report success in the fight against lupus on Thursday, sending its shares more than 50 percent higher.

Immunomedics and Belgium's UCB (UCB.BR), which bought rights to the U.S. firm's lupus medicine epratuzumab three years ago, said the drug's treatment advantage over placebo reached 24.9 percent after 12 weeks of therapy in a Phase IIb clinical trial.

The study analysed the drug's impact on a number of measures of disease activity.

The result comes five weeks after Human Genome Sciences (HGSI.O) reported its lupus drug Benlysta, which it has partnered with GlaxoSmithKline (GSK.L), succeeded in a late-stage Phase III trial, sending Human Genome shares soaring.

Benlysta was the first drug ever to succeed in Phase III trials against lupus.

Analysts at Jefferies said the headline data for UCB and Immunomedics' rival product was encouraging but, given the high failure rate of lupus drugs, they remained cautious. The brokerage is retaining its conservative $150 million peak sales and 30 percent likelihood of success until full details of the Phase IIb results are released.

Still, investors in Immunomedics jumped on the news, boosting the stock 57 percent to the highest level in more than five years.

Epratuzumab is an antibody-based drug that was licensed to UCB by Immunomedics for all autoimmune disease indications in 2006.

It is the most advanced product in UCB's immunology disease pipeline and is significant for the Belgian group as it increases its focus on biotech medicines. Shares in UCB were up 1 percent at 1443 GMT.

UCB and Immunomedics said in-depth analysis of the data was now ongoing in preparation for a final-phase Phase III programme.