ChEBI: Quinine is a cinchona alkaloid that is cinchonidine in which the hydrogen at the 6-position of the quinoline ring is substituted by methoxy. It has a role as an antimalarial, a muscle relaxant and a non-narcotic analgesic. It is a conjugate base of a quinine(1+). It derives from a hydride of an (8S)-cinchonan.NCI Thesaurus (NCIt): Quinine is a quinidine alkaloid isolated from the bark of the cinchona tree. Quinine has many mechanisms of action, including reduction of oxygen intake and carbohydrate metabolism; disruption of DNA replication and transcription via DNA intercalation; and reduction of the excitability of muscle fibers via alteration of calcium distribution. This agent also inhibits the drug efflux pump P-glycoprotein which is overexpressed in multi-drug resistant tumors and may improve the efficacy of some antineoplastic agents. (NCI04)