The release of conjugated drugs from a dendrimer system has
been investigated under various external stimuli such as
temperature and especially pH18−23 on the basis of the fact
that the microenvironments around a tumor and inside cells are
typically acidic.24 In general, the release efficiency of covalently
conjugated drugs is lower than that of electrostatically bound
drugs. For example, when DOX molecules were covalently
bound to PEG-modified PAMAM by thiolated transferrin
molecules via Michael addition, the release of DOX was only
30% at pH 4.5.25 Recent work on the electrostatic conjugation
of DOX with various generations (e.g., G3, G5, and G6) of
dendrimers demonstrated that the release efficiency of DOX
was 40% in PBS buffer at pH 5.226,27 but was increased to 80%
when the pH was reduced to 4.2,27 although the latter pH is
unlikely to be in an extracellular environment in vivo. Note that
acidic pH around 5 is found neither extracellularly (normally)
nor within most of the cell cytoplasm, but this is the pH of the
lysosome, which is where many internalized molecules end up
and where they are broken down by acid hydrolases and other
acid-active enzymes. The primary objective of this work was to
optimize the efficiency of pH-responsive PAMAM-Fe3O4-DOX
triads. To this end, different generations (G3, G5, and G6) of
PAMAM modified with PEG were used to encapsulate Fe3O4,
and DOX was electrostatically conjugated on the surface of the
PAMAM-Fe3O4 nanocarriers.