CAMBRIDGE, Mass. ( TheStreet) -- European regulators confirmed that 24 multiple sclerosis patients treated with Tysabri have contracted a serious brain infection. Four of those patients have died, according to the investment bank UBS.

The new disclosure concerning the link between Tysabri treatment and progressive multifocal leukoencephalopathy, or PML, pushed shares of Biogen Idec ( BIIB - Get Report) and Elan ( ELN), makers of Tysabri, lower in Thursday trading.

Last week, European regulators launched a new safety review of Tysabri after it was disclosed that 23 multiple sclerosis patients treated with Tysabri had contracted PML since the drug was relaunched in 2006. The number of PML cases tied to Tysabri was much higher than investors had previously known.

The 24 cases of Tysabri-linked PML and four patient deaths was disclosed Thursday in an email sent by European regulators to UBS drug analyst Martin Wales, who was seeking more information about the link between Tysabri and PML cases. The email has been circulated across Wall Street trading desks this morning. A copy of the email, obtained by TheStreet, states:

"We can confirm that to date, 24 confirmed cases of PML have been reported worldwide since Tysabri has been on the marked in the EU in 2006. Fourteen of these ocurred in the EU, 8 in the US and 2 in Switzerland. Four patients died. Most of the cases were reported in patients who have had 24 or more infusions of Tysabri."

Biogen Idec shares were down 2% to $42.71 while Elan shares were off 4% to $4.81.

Genta Melanoma Drug Study Disappoints

(At 7:57 AM EDT)

The resurrection of Genta ( GETA.OB) doesn't appear to have a happy ending.

The drug maker reported results Thursday from its latest phase III study of Genasense in melanoma and the data aren't encouraging.

Treatment with Genasense failed to demonstrate a statistically significant improvement in progression-free survival -- a co-primary endpoint of the study. Secondary endpoints of the study, including response rate and overall disease control rate, also failed to show a benefit favoring Genasense.

The only positive thing Genta had to say about the Genasense study was that the drug cleared a futility test for overall survival -- the other co-primary endpoint. But Genta said it was still too early to determine whether Genasense patients lived longer than those in the study's control arm.

Plans to seek Food and Drug Administration approval for Genasense in melanoma before the end of the year have now been put on hold given these results, said Genta CEO Ray Warrell.

Detailed data from the study will be presented on Nov. 15 at a meeting of cancer researchers in Boston.

In 2004, the FDA rejected Genta's application for Genasense in melanoma on the basis of a previous phase III study that also failed to demonstrate a survival benefit favoring the drug.

For this latest phase III study, Genta limited enrollment to a subset of melanoma patients with low levels of LDH, a blood enzyme, who appeared to live longer after treatment with Genasense during the first study. Conversely, patients with elevated levels of LDH were not helped at all by Genasense.

Safety Issues Delay Transcept Sleep Drug Approval

The FDA wants more safety data from Transcept Pharmaceuticals ( TSPT) before approving the company's insomnia drug, the company said Thursday.

Transcept was seeking FDA approval for Intermezzo as a sleep aid for people who wake up in the middle of the night but can't fall back to sleep. The drug's active ingredient is a low dose of the popular sleeping pill Ambien formulated into a tablet that dissolves quickly in the mouth.

But the FDA said it was not comfortable with Intermezzo's safety profile, particularly the risk that people might suffer lingering effects from the drug after they woke up. In particular, regulators want Transcept to show that Intermezzo has no negative effect on next-day driving ability, the company said.

Transcept said it may have to conduct one or more additional safety studies to provide FDA with the data it needs to approve Intermezzo.

Transcept shares closed Wednesday at $11.47 but appear to be trading sharply lower in pre-market trading Thursday.

-- Reported by Adam Feuerstein in Boston

Adam Feuerstein writes regularly for TheStreet.com. In keeping with TSC's editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet.com. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.