VERAPAMIL COMPETES WITH DOXORUBICIN FOR BINDING TO ANIONIC PHOSPHOLIPIDS RESULTING IN INCREASED INTERNAL CONCENTRATIONS AND RATES OF PASSIVE TRANSPORT OF DOXORUBICIN

It is well documented that the Ca2+ channel antagonist verapamil can r everse multidrug resistance in cancer cells by decreasing P-glycoprote in mediated drug efflux. However, less information is available about effects of verapamil on drug-phospholipid interactions and on passive diffusion of drugs across the membrane, which both may play an importa nt role in resensitizing cells to anti-cancer drugs. Therefore we stud ied the binding of verapamil to model membranes (large unilamellar ves icles) composed of various phospholipids and biological membranes. An increase of the amount of anionic phospholipids resulted in an enhance d binding of verapamil. Competition between verapamil and the anti-can cer drug and P-glycoprotein substrate doxorubicin for binding to anion ic phospholipids was observed in model membranes composed of synthetic lipids, or composed of native Escherichia coli phospholipid mixtures, and in cytoplasmic membrane vesicles of this organism. Furthermore, v erapamil specifically increased the rate of passive diffusion of doxor ubicin across model membranes containing anionic phospholipids. It can be concluded that besides the decrease of P-glycoprotein mediated eff lux at least two other effects may account for an increase of the inte rnal (free and DNA-bound) doxorubicin concentration in the presence of verapamil; (i) a decrease of binding to anionic phospholipids in plas ma-and intracellular membranes and (ii) an increase of the rate of pas sive import of doxorubicin across the plasma membrane.