Shares of Endo Pharmaceuticals Holdings Inc. jumped Wednesday after an analyst said Endo can keep generic versions of its painkiller Opana ER from being approved, which would sustain sales until Endo starts selling a new version.

Jefferies & Co. analyst Corey Davis said rival Purdue Pharma LP started selling a new version of its pain drug OxyContin last week. He said Purdue withdrew the regulatory application on the older version, which prevents generics of that version from being approved. Earlier this month, Endo said it filed for regulatory approval of a tamper-resistant version of Opana ER. Davis said Endo may copy Purdue, keeping competing versions of the old form of Opana ER from getting approved.

Once generic versions of a drug reach the market, sales of the older brand-name drugs drop off because of greater competition and lower prices.

Purdue, Endo, and other companies have been developing new versions of opiate-based pain drugs that are harder to abuse. Purdue's OxyContin is designed to gradually release its medication, but in the older version, the time-release element could be defeated if the drug was crushed or dissolved. That allowed users to get the full dose all at once.

Davis said the Food and Drug Administration wants to curtail abuse of opioid painkiller so it does not want to approve generic versions of the older, more abuse-prone drugs. He now believes Endo could keep generic version of Opana off the market for years, with annual sales of Opana rising to $800 million more than double his prior estimate of $300 million.

Davis kept a "Buy" rating on shares of the Chadds Ford, Pa., company, and raised his price target to $44 from $29.

In afternoon trading Endo shares rose $2.52, or 9.6 percent, to $28.66. Earlier the stock rose to $28.95, which was its highest price since November 2007.