PF-04455242

PF-04455242 is an experimental small molecule drug developed by Pfizer as a selective κ-opioid receptor (KOR) antagonist. It was investigated for the treatment of bipolar depression but never reached the market due to discontinuation in early clinical trials following unfavorable toxicological findings in animal studies. The compound acts as a moderately efficacious partial antagonist at the KOR, with approximately 10–20 times higher affinity for KOR than for μ-opioid receptors and negligible affinity for δ-opioid receptors. In preclinical models, it reversed effects of KOR agonists and showed potential antidepressant activity. The drug is orally administered[1][2][6].