Biomedical Engineering Reference
In-Depth Information
delineations of what targeted drug delivery is and is not would help, there are
challenges with the current state of targeted drug delivery that extend beyond
defining the term. These challenges are present in both the carrier and the
target. Carrier technology has improved greatly, yet still has trouble delivering
drug to the target. The target, cancer in the clinical setting, is still resisting
treatment. The challenges associated with this problem need to be addressed in
order to move forward. The systems may be targeted by design, but they are
not hitting the target fully and exclusively.
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1.1.1 Origins of Targeted Drug Delivery
The current paradigm of targeted drug delivery is linked to its origins, which
are tied to chemotherapy and immunology. Paul Ehrlich was a pioneer in
chemotherapy and is known for the metaphor of a magic bullet. The vision of
the magic bullet was inspired from his ability to selectively stain bacteria
cultures.
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He reasoned a toxic molecule could be tied to the stains to selectively
kill only that target. Targeted drug delivery has been guided by Ehrlich's
vision, particularly in the field of cancer therapy.
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In the context of cancer and
chemotherapy, a magic bullet carrying an anticancer drug is administered to
the patient to provide exclusive delivery to the cancer. In contemplating the
metaphor of a magic bullet, perhaps the body's immune system fits best. It has
both mobility and specificity. Immune cells can follow chemical gradients and
have specificity with antibodies and cell receptors. Ehrlich himself stated that
cancer would be more prevalent if not for the immune system.
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1.1.2 Progress in Targeted Drug Delivery
Advances in immunology and the advent of monoclonal antibodies have
become an important part of pursuing the vision of a magic bullet. With
immunostaining, the targeting application seems flawless and provides
motivation for application in cancer therapy. In targeted drug delivery there
is a targeting aspect and a therapeutic aspect. Sometimes these monoclonal
antibodies can provide a therapeutic effect on their own. However,
monoclonal antibodies and other targeting designs are usually incorporated
in a variety of therapeutic carriers, such as microemulsions, inorganic
nanoparticles, viruses, and polymers. Liposomes are composed of lipids which
assemble into vesicles with a bilayer capable of carrying drug molecules. Gold
and iron oxide nanoparticles are among the more popular inorganic molecules
used. Viral carriers, made by modifying existing viruses or by using certain
aspects from them, have also been used in targeted drug delivery of
chemotherapeutics.
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All of these have dimensions on the scale of nanometers
and can be described as nanoparticles. While colloidal chemistry and even
targeted drug delivery have a long history, nanotechnology and its application
in nanomedicine are quickly becoming popular topics. Polymer therapeutics is
another major trend in research that seeks after the properties of a magic
