Quinidine

Quinidine is a pharmaceutical agent that acts as a class I antiarrhythmic agent in the heart. Quinidine is a stereoisomer of quinine, originally derived from the bark of the cinchona tree. Quinidine is used to treat abnormal heart rhythms. Like all other class I antiarrhythmic agents, quinidine primarily works by blocking the fast inward sodium current. At micromolar concentrations, quinidine inhibits Na+/K+-ATPase by binding to the same receptor sites as the digitalis glycosides such as ouabain. Quinidine is also used to treat malaria. The half life of oral quinidine is 6 to 8 hours, and it is eliminated by the cytochrome P450 system in the liver. About 20% is excreted unchanged via the kidneys. Quinidine is also an inhibitor of the cytochrome P450 enzyme 2D6, and can lead to increased blood levels of lidocaine, Beta blockers, opioids, and some anti-depressants. Quinidine also inhibits the transport protein P-glycoprotein and so can cause some peripherally acting drugs such as loperamide to have CNS side effects such as respiratory depression if the two drugs are co-administered.