Rp. Austin et al., THE EFFECT OF IONIC-STRENGTH ON LIPOSOME-BUFFER AND 1-OCTANOL-BUFFER DISTRIBUTION COEFFICIENTS, Journal of pharmaceutical sciences, 87(5), 1998, pp. 599-607
The distribution of salmeterol and proxicromil between unilamellar ves
icles of dioleoylphosphatidylcholine (DOPC) and aqueous buffer at pH 7
.4 has been studied, using an ultrafiltration method, as a function of
compound concentration, DOPC concentration, and buffer ionic strength
. The binding of these ionized lipophilic compounds to neutral DOPC ve
sicles induces a surface charge, which causes the observed membrane di
stribution coefficient (DOmemobs) to vary significantly with bound com
pound to DOPC ratio and with ionic strength. This variability is shown
to be well-described with use of the Gouy-Chapman theory of the ionic
double layer and is contrasted with the ideal behavior shown by the n
eutral compound clofibrate. Increasing ionic strength is also shown to
increase the observed 1-octanol-buffer distribution coefficients (D-o
/w(obs)) of proxicromil but through a very different mechanism involvi
ng the extraction of ion pairs. This study highlights the experimental
difficulty in determining concentration-independent liposome distribu
tion coefficients of ionized lipophilic compounds and describes when d
eviations will be significant and how observed values may be corrected
for such effects. The general effect of ionic strength on membrane-bu
ffer distribution and 1-octanol-buffer distribution is discussed with
particular reference to the very different propensity for ion pair for
mation shown by the two systems, and the most suitable experimental co
nditions that should be used with each system.