NEW YORK (AP) — Shares of Pain Therapeutics Inc. and Durect Corp. tumbled Friday after their partner Pfizer Inc. said it may not pursue further development of Remoxy, an experimental pain drug that has been plagued with delays in regulatory approval.

Durect developed Remoxy and licensed it to Pain Therapeutics. Pfizer now holds a sublicense from Pain Therapeutics. Pain Therapeutics and Durect said Friday that Pfizer has told them it is evaluating its options on the drug, and if it decides to continue development, it will not file a new marketing application with the Food and Drug Administration until mid-2015 at the earliest.

Shares of Pain Therapeutics, based in Austin, Texas, sank $2.60, or 48.9 percent, to $2.72 in morning trading, while shares of Durect, Cupertino, Calif., fell 65 cents, or 40.9 percent, to 94 cents.

Pfizer told Pain Therapeutics it met with the FDA on March 28 and will be evaluating its plans for Remoxy. Pfizer asked the FDA to approve the drug in 2008 and again in late 2010, and its letter emphasized the problems in getting approval. The FDA's concerns mainly are over the manufacturing process for the drug.

Remoxy is an immediate-release painkiller. Its main ingredient is oxycodone, the primary ingredient in OxyContin. Remoxy tablets include the drug in a thick liquid form. The developers say that makes it harder to abuse. OxyContin was taken off the market in 2010 because of widespread concerns about people who were abusing the drug to get high. The FDA said it won't approve any generic versions of the drug that don't include some kind of tamper-resistant technology.

"Given the years of delay, additional cost incurred to bring the program to this point and development work left in the program, there will be much to consider when we seek management endorsement," Pfizer wrote.

In a form filed with the Securities and Exchange Commission, Pfizer said it will need to run additional clinical studies to get the drug approved.

During a conference call with investors and analysts on April 30, Pfizer said its meeting with the FDA was "productive" and that it believed it has a path forward for the drug. Prior to Friday, Pain Therapeutics shares were up 63 percent since then and Durect shares rose 27 percent.

Purdue Pharmaceuticals, which sells OxyContin, now sells a generic version that is designed to be harder to abuse. Pfizer sells an abuse-deterrent pain drug called Oxecta. That drug was developed using technology from Acura Pharmaceuticals Inc.

Shares of New York-based Pfizer rose 7 cents to $28.66.