Interaction Of The Macrolide Azithromycin With Phospholipids .2. Biophysical And Computer-Aided Conformational Studies

In a comparison paper, we show the azithromycin causes a lysosomal
phospholipidosis in cultured cells, binds in vitro to negatively charged bilayers
without causing aggregation or fusion, and inhibits lysosomal phospholipase A1.
In this paper, we show that azithromycin decreases the mobility of the
phospholipids in negatively charged liposomes (using 31P nuclear magnetic
resonance) and that it increases the fluidity of the acyl chains close to the
hydrophilic/hydrophobic interface, but not deeper into the hydrophobic domain
(assessed by measuring the fluorescence polarization of
trimethylammonium-diphenylhexatriene and diphenyhexatriene, respectively).
Computer-aided conformational analysis of mixed monolayers of azithromycin and
phosphatidylinositol shows that the drug can be positioned largely in the
hydrophobic domain, but close to the interface, with the macrocycle facing the C1
of the fatty acids (allowing the N9a endocyclic tertiary amine to interact with
the phospho-groups), the cladinose located on the hydrophobic side of the
lipid/water interface and the desosamine projected into the hydrophobic domain.
This position is consistent with the experimental data. Analysis of virtual
molecules shows that this unanticipated behavior to the shielding of the
ionizable N3' amino-group in the desosamine by methyl-groups, and to the wide
dispersion of hydrophobic domains all over the molecule. The interaction of
azithromycin with phospholipids may account for some of its unusual
pharmacokinetic properties and for its potential to cause lysosomal
phospholipidosis.