Conclusions - Nanomaterials for Medical Applications - page 415

One of the most promising applications of biocompatible NMs is drug delivery.
Today, a number of nano-enabled drugs are commercially available for various
applications. 312 Aside from the use of NMs for MRI contrast agents and drug
encapsulation, some are also being studied for vaccine delivery. 64,91,197,229,313 Pusic
and her group focused on the use of the semiconductor nanoparticles, in particular
QDs, as an alternative vaccine delivery platform. 154 For the delivery of peptide vac-
cines, they used <15 nm QDs with a crystal shell of alternating cationic and anionic
layers, being the most widely used CdSe/ZnS. Their results indicated that the QDs
they used were nonimmunogenic, stable, and when coated with an organic layer
allowed for an array of proteins, DNA, and other biomolecules to be conjugated
to their 60 surfaces. Leveraging on the small size and surface modification, the
QDs were conjugated with vaccine candidates against malaria. 154 The QDs served
as a delivery platform for protein antigens and also activated key immune cells to
further increase the immunogenicity of the vaccine. The in vivo tests demonstrated
no immediate toxic effects but the system uses QDs that contain Cd which are not
compatible for human use. The toxicity of the CdSe/ZnS QDs was contained by
coating the core with a shell layer of ZnS that is further coated with an amphiphilic
polymer. 12,18,144,202,406,447 Currently, because of the possibility of eventual polymer
and nanoparticle degradation, thereby releasing the toxic Cd, attempts is currently
being made to develop cadmium free QD nanoparticles as well as the use of other
nanoparticles that do not contain Cd such as IOMNPs and AuNPs.
The delivery of drugs to manage incurable diseases such as cancer, tumor,
other diseases of similar nature, and those which currently have no cure such
AIDS and HIV are important to optimize the effect of drugs and to reduce toxic
side effects. 75,117,314 To date, several nanotechnologies, mostly based on NMs
can facilitate drug delivery to these diseases. These NMs have to be carefully and
meticulously engineered to provide a design that allows efficient performance
of the intended functions. Excellent candidates as drug nanocarriers must be
small (less than 100 nm), nontoxic, biodegradable, biocompatible, do not aggre-
gate, avoids the RES and escapes opsonization, noninflammatory, supports pro-
longed circulation time, and cost effective. 73,77,88,110,119,124,125,138,176,240,271,294,
315-321 Many drugs in the market that are now available for human use are already
nano-enabled. The ability of these drugs to minimize the side effects that are
normally observed in conventional drugs open doors for applications of NMs as
safer alternatives to drug delivery.
Aside from the physical and chemical properties including drug-loading to
effectively deliver drugs using the NM carrier systems other challenges need fur-
ther attention. More studies need to focus on the interaction of NMs with their
hosts in terms of biodistribution, organ accumulation, degradation while in cir-
culation, damage of cellular structures or inflammatory foreign body effects, and
genetic damage. Aggregation or precipitation upon contact with biological fluids
in animals or human hosts must be carefully evaluated and its prevention be estab-
lished to prevent adverse effects. Preliminary studies on possible aggregation in
fresh human whole blood and in animals have indicated that some NMs loaded