Shares of Pain Therapeutics plummeted yesterday after the company reported that a Phase III trial of its painkiller Oxytrex (oxycodone plus naltrexone) failed due to high dropout rates among patients in the study.

This high dropout rate meant that the trial, conducted in patients with moderate-to-severe pain caused by osteoarthritis, failed to prove that Oxytrex was less likely than oxycodone alone to cause dependency. Pain Therapeutics said it will now have to carry out an additional study before it can file for approval of Oxytrex.

Oxycodone is the active ingredient in Purdue Pharma's OxyContin, a branded, controlled release narcotic painkiller with annual sales exceeding $1.9 billion that has been the subject of a number of cases of diversion and abuse, according to the US Food and Drug Administration (FDA). Oxytrex combines oxycodone with an ultra low-dose opioid antagonist, naltrexone, that is designed to inhibit the potential for dependency.

The news was not all bad for Pain Therapeutics, however. Oxytrex met its secondary objective, and was shown statistically to be non-inferior to oxycodone during the three-month treatment period. It also reduced physical dependency by 75% in patients older than 50. Pain Therapeutics said that the data still demonstrated that Oxytrex was 28% less addictive than oxycodone.

Researchers had expected a 40% dropout rate overall but instead saw an attrition rate of between 48% and 60% in the oxycodone groups, compared to 37% among those on placebo.

Remi Barbier, Pain Therapeutics president and chief executive, said: "There are ways to design clinical trials that overcome the statistical limitations imposed by very high dropout rates." He noted that the company plans to discuss with the US Food and Drug Administration early next year how to design clinical trials to correct for the dropout rates.

Shares in Pain Therapeutics closed down 19% yesterday on news of the trial results. The company insisted that the disappointing results will not affect its recently announced collaboration with King Pharmaceuticals, which earlier this month announced it would invest up to $400 million in Pain Therapeutics' Remoxy, a controlled-release formulation of oxycodone that is harder to abuse.

IMS Health data position oxycodone as the leading opioid used in the treatment of moderate-to-severe pain, with US sales of nearly $2 billion for the 12-months ending August 2005.