Doxil — The First FDA-Approved Nano-Drug: From an Idea to a Product - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 372

so that it will control the state of aggregation and precipitation/
crystallization of the drug-counterion salt in the intraliposome
aqueous phase, thereby contributing to control the efficiency and
stability of remote loading, as well as drug release rate at various
temperatures (Clerc and Barenholz, 1998; Barenholz et al., 2001;
Wasserman et al., 2007).
It is important to note that the successful application of this
nano-chemical engine benefits from the very small trapped aqueous
volume of nanoliposomes (i.e., 2.21 × 10 5 nm 3 for a 37.5 nm radius
liposome), which supports faster and higher accumulation, as well
as intraliposome precipitation of drug-counterion salt in crystalline
or noncrystalline forms.
12.4.6.4 Ammonium sulfate transmembrane gradient driven
doxorubicin loading into nSSL
For the remote loading of doxorubicin into nSSL we applied the
transmembrane ammonium sulfate gradient under conditions that
[(NH 4 ) 2 SO 4 ] lip >> [(NH 4 ) 2 SO 4 )] med (lip is the nSSL and med is the
extraliposome medium). Figure 12.6 describes the overall mechanism
of this loading process. The drug loading is actually a base exchange
of the amphipathic weak base drug with the ammonium ions. For
doxorubicin >90% drug encapsulation was obtained. Doxorubicin is
accumulated in the intraliposome aqueous phase, where it reaches a
concentration > 100-fold the drug level in the loading medium (this
explains why we refer to it as active loading, as it goes against the
drug concentration gradient). Based on various spectral analyses
including X-ray diff raction ( Barenholz and Haran, 1993; Haran et al.,
1993; Lasic et al., 1992, 1995) almost all the encapsulated doxorubicin
is in the intra-liposome aqueous phase and most of it is in the form
of aggregated (crystalline) [doxorubicin] 2 SO 4 salt. The loading
is dependent on the ammonium ion gradient, while the loading
stability is dependent on the liposome lipid composition and on the
level of [doxorubicin] 2 SO 4 precipitation, as well as temperature. The
transmembrane ammonium-sulfate-gradient-driven drug loading
diff ers from most other remote loading approaches since it neither
requires fabrication of liposomes in acidic pH, nor alkalinization of
the extra-liposome aqueous phase.
Doxil is a good example of remote loading by an ammonium sulfate
gradient under conditions that [(NH 4 ) 2 SO 4 ] lip >> [(NH 4 ) 2 SO 4 )] med.
Figure 12.6 describes the overall mechanism of this loading process.