NEW YORK (AP) — Nektar Therapeutics said Monday it licensed a pair of experimental pain treatments to AstraZeneca PLC in a deal valued at about $$1.12 billion. AstraZeneca will pay Nektar $125 million upfront for the rights to two drugs: NKTR-118, an oral product intended to treat constipation caused by opioid pain drugs, and NKTR-119, which is intended to combine NKTR-118 with opioid pain drugs to create a painkiller that doesn't cause constipation. AstraZeneca, which is based in London, will handle further development of the drugs, including the start of late-stage clinical trials for NKTR-118, as well as manufacturing and marketing. It expects to file for regulatory approval in 2013. NKTR-119 is in early-stage development. Nektar, of San Carlos, Calif., could get as much as $235 million in payments if the constipation drug advances through regulatory review, and another $375 million if it reaches sales goals. Nektar could also receive $75 million in development payments for NKTR-119, and $310 million in sales milestone payments, according to a filing with the Securities and Exchange Commission. If the drugs are approved, it will receive royalty payments of at least 10 percent on each. About 230 million prescriptions for opioid drugs were written in 2007 in the U.S., according to IMS Health. It is estimated that between 40 and 90 percent of the patients who take opioid drugs develop constipation.