NEW YORK (AP) — GlaxoSmithKline and XenoPort Inc. said Monday their developing pain drug failed in a midstage study, citing a high patient response to placebo. The companies are developing GSK1838262, or XP13512, as a treatment for diabetic peripheral neuropathy, or pain associated with diabetes. They compared the drug candidate with a placebo, and cited a high placebo response rate as a potential reason for the failure. The 14-week study involved 421 patients with either Type 1 or Type 2 diabetes. Santa Clara, Calif.-based Xenoport and U.K.-based Glaxo said they will evaluate the results to determine the next step for the drug candidate.