NEW YORK
(AP) — Shares of Idenix Pharmaceuticals Inc. plunged after the company said federal regulators have temporarily halted
studies of an HIV and AIDS drug that the company developed.

Late Wednesday, Idenix said it was informed by its partner
ViiV Healthcare that the Food and Drug Administration placed a clinical hold on
studies of the drug. It did not disclose reasons for the clinical hold, which
prevents further human testing of the drug until the unspecified problem is
resolved. ViiV Healthcare was running a mid-stage clinical trial of the drug
candidate.

Shares of Idenix dropped $1.01, or 25.2 percent, to $3 in
premarket trading. The stock closed at $4.01 Wednesday and has traded between
$2.57 and $6.11 over the last year.

The drug is designated GSK2248761. ViiV Healthcare licensed
the product from Idenix in early 2009, and it is responsible for all clinical
development. ViiV Healthcare is a partnership between GlaxoSmithKline PLC and
Pfizer Inc. that focuses on HIV and AIDS drugs.

Idenix also said it expects to resume clinical testing of a
potential hepatitis C drug called IDX184 in the second half of 2011. The FDA
stopped trials of IDX184 in September because some healthy patients who were
treated with the drug and another Idenix drug, IDX320, experienced elevated
levels of liver enzymes. The FDA now has a partial clinical hold on IDX184. The
upcoming study will be a mid-stage trial evaluating IDX184 and two standard
hepatitis C therapies.

The Cambridge,
Mass., company said it is
discontinuing development of IDX320 because it believes that drug was
responsible for the side effect.

Oppenheimer
& Co. analyst Bret Holley said the HIV drug is “critical” to the
value of Idenix’s stock. He downgraded the shares to “Perform” from
“Outperform,” and removed his price target of $5 per share. The
developments for the hepatitis C drugs are good news, Holley wrote, but since
additional news on IDX184 is not likely to come until the end of the year, it
is unlikely the stock will trade much higher.