Jp. Montenez et al., INTERACTION OF THE MACROLIDE AZITHROMYCIN WITH PHOSPHOLIPIDS .2. BIOPHYSICAL AND COMPUTER-AIDED CONFORMATIONAL STUDIES, European journal of pharmacology, 314(1-2), 1996, pp. 215-227
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.