Interactions between verapamil and neutral and acidic liposomes: effects of the ionic strength.
Patients with cancer often develop major electrolyte disorders, which are aggravated by radiation therapy and chemotherapy and by the concomitant impairment of the renal function and the development of drug resistance. In addition, tumour cells have membranes with more negative charges than normal eukaryotic cells. This study was designed to test the hypothesis that the ability of the Ca(2+) blocker verapamil to mediate the reversal of multidrug resistance (MDR) by interacting with the membrane phospholipids may be correlated with the ionic strength and membrane surface potential in resistant tumours. The permeation properties of verapamil, which is the best-known MDR-modulator, were therefore studied by quantifying its ability to induce the leakage of carboxyfluorescein through unilamellar liposomes containing various mole fractions of phosphatidic acid (x(EPA)=0, 0.1 and 0.3), at four different ionic strengths (I=0.052, 0.124, 0.204 and 0.318 M). The dye leakage induced by verapamil varied greatly with I, depending on x(EPA). The permeation process was a co-operative one (1.3<Hill coefficient<3.5) and the permeation doses inducing 50% dye leakage (PD(50)) ranged between 0.2 and 1.8 mM. A highly significant multiple correlation was found to exist between the variations of log(1/PD(50)) with those of 1/ radical I and x(EPA) (dlog(1/PD(50))/d(1/ radical I)=0.15+/-0.01, dlog(1/PD(50))/dx(EPA)=2.07+/-0.08, y-intercept=2.46+/-0.03, P<0.000001). Kinetic studies on the permeation process showed that it involved two steps. The apparent rate constants of the slow and fast kinetic steps, which were driven by electrostatic and hydrophobic interactions, respectively, increased with the verapamil concentrations, depending on x(EPA). The results provide evidence that in resistant tumours (high negative membrane surface potential), the MDR reversal by verapamil might be enhanced by favourable drug-membrane interactions in patients with severe hypo-electrolytic (Na(+) and K(+)) disorders, whereas the MDR reversal might be reduced by unfavourable drug-membrane interactions in patients with severe hyper-electrolytic (Ca(2+), Na(+) and K(+)) disorders.
Castaing
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GERCTOP-UMR6009, Faculté de Pharmacie, 27 Boulevard Jean Moulin, 13385 Marseille Cedex 05, France. gerctop@pharmacie.univ-mrs.fr
Loiseau
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Mulliert
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D002121:Calcium Channel Blockers / Q000737:chemistry*; D018432:Drug Resistance, Multiple; D005456:Fluorescent Dyes; D006023:Glycoproteins; D006863:Hydrogen-Ion Concentration; D008081:Liposomes / Q000737:chemistry*; D008564:Membrane Potentials; D009994:Osmolar Concentration; D010539:Permeability; D011228:Prealbumin; D012044:Regression Analysis; D014700:Verapamil / Q000737:chemistry*
D002121:Calcium Channel Blockers; D005456:Fluorescent Dyes; D006023:Glycoproteins; D008081:Liposomes; D011228:Prealbumin; C031458:embryonic prealbumin; D014700:Verapamil