ml138ML138 is a probe for MLPCN, an agonist of the κ opioid receptor. Popular Searches. Cell Counting Kit-8 (CCK-8) Approved Drug Library MG-132 Ferrostatin-1 Z-VAD-FMK. mole. 1ML138 (CID 44601470) is a hhly selective agonist at kappa opioid receptors (KOR) that potently activate G protein coupling but weakly interact with β arrestin2.