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. Check for active clinical trialsusing this agent.