Pain Therapeutics watched its stock plunge more than 50% as investors reacted with horror to news that Pfizer is pulling out of an agreement to develop and commercialise an extended-release oral formulation of the analgesic oxycodone.

Shares were trading down 53% after the drug giant said it was handing back all rights, including responsibility for regulatory activities, for tamper-resistant Remoxy Capsules.

The decision, Pfizer said, came following an internal review of top-line data from five recently completed clinical studies required to address the Complete Response Letter received from the US Food and Drug Administration in June 2011. Consequently, current activities under the agreement will be continued but only for the next six months until the scheduled termination date. 

Despite the shock to its stock, Pain Therapeutics remained upbeat. “We are grateful to Pfizer for their substantial investment in REMOXY over the years and now for this opportunity to regain full control of REMOXY", said chief executive Remi Barbier. 

"It's a privilege to reacquire worldwide rights to an unencumbered Phase III asset that targets a $2 billion market,” he added, also noting that the firm is “enthusiastic about initiating discussions with potential pharmaceutical partners around this drug asset”.