INTERACTION OF THE MACROLIDE AZITHROMYCIN WITH PHOSPHOLIPIDS .2. BIOPHYSICAL AND COMPUTER-AIDED CONFORMATIONAL STUDIES

In a companion paper, we show that azithromycin causes a lysosomal pho spholipidosis in cultured cells, binds in vitro to negatively charged bilayers without causing aggregation or fusion, and inhibits lysosomal phospholipase A,. In this paper, we show that azithromycin decreases the mobility of the phospholipids in negatively charged liposomes (usi ng P-31 nuclear magnetic resonance) and that it increases the fluidity of the acyl chains close to the hydrophilic/hydrophobic interface, bu t not deeper into the hydrophobic domain (assessed by measuring the fl uorescence polarization of trimethylammonium-diphenylhexatriene and di phenyhexatriene, respectively). Computer-aided conformational analysis of mixed monolayers of azithromycin and phosphatidylinositol shows th at the drug can be positioned largely in the hydrophobic domain, but c lose to the interface, with the macrocycle facing the C-1 of the fatty acids (allowing the N-9a endocyclic tertiary amine to interact with t he phospho-groups), the cladinose located on the hydrophobic side of t he lipid/water interface and the desosamine projected into the hydroph obic domain. This position is consistent with the experimental data. A nalysis of virtual molecules shows that this unanticipated behavior is due to the shielding of the ionizable N-3' amino-group in the desosam ine by methyl-groups, and to the wide dispersion of hydrophobic domain s all over the molecule. The interaction of azithromycin with phosphol ipids may account for some of its unusual pharmacokinetic properties a nd for its potential to cause lysosomal phospholipidosis.