Biomaterial-Based Particulate Drug Carriers - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 6

provide drug targeting. This is usually sufficient also for small drugs
that operate inside the cell.
The third step may be necessary for macromolecules that operate
inside the cell. In this respect, it is imperative to diff erentiate — at
the cellular level — between drug targeting and drug internalization.
Frequently and especially for carrier-mediated drug targeting,
eff orts are made to achieve internalization of the complete system.
For drugs that operate on the cell membrane, internalization is
detrimental and should be avoided, as any drug molecule that has
entered the cell is probably lost to its extracellular sites of action.
When it comes to drug targeting, the use of the term “homing”
should be avoided as it is erroneous and misleading. Unlike a weapon
that can be equipped with homing devices that recognize their target
from afar and lock onto it, self-targeting drugs or targeted drug-
carrier systems cannot home onto the target immediately upon
administration (local or systemic). The forces that drive recognition
and binding between the targeting moiety and its counterpart at the
target operate at short range, when the administered system is at
very close proximity to the target-provided binding counterpart.
Focusing, from here on, on carriers, the question of whether the
steps in drug targeting illustrated in Figure 1.4 (obviously mediated
by a carrier) have been, and can be, achieved is discussed in
Section 1.3.
Figure 1.4 Steps in drug targeting.
Finally, we would like to stress that a major share of the
experimental work done in the area of drug targeting has been
performed in vitro , in cell cultures. The extensively investigated case
of immunoliposomes — liposomes that are surface modified to carry