What are Drug Delivery Systems?
The term "drug delivery systems" refers to both the
construct of the drug – its delivery vehicle – and its method of administration.
New Drug Delivery Vehicles
Some of the major vehicles being researched for
delivering drugs are:
- Organic and synthetic
polymers, and other chemical constructs that can release drugs at a
sustained rate, or release them only in certain environments; and,
- Liposomes.
| New Methods of
Administration |
- Medicated skin patches,
- Implanted devices that can release drugs with an
external remote control; and,
- Powder forms of traditional drugs which can be
inhaled and absorbed through the lungs.
The main focus of this document will be on the new
drug delivery vehicles.
The Science – How do Drug Delivery Systems
Work?
Liposomes
Liposomes encapsulate active drugs to improve their
delivery. With liposomes, the active drug is carried within its layers or in the
hallow space in the middle of the liposome. The immune system, which seeks out
foreign material for destruction, can be a major obstacle to liposomes. In 1992,
researchers discovered that coating liposomes with inactive polymers, such as
polyethylene glycol (PEG), drastically increased the liposome's ability to evade
recognition by the immune system.
The main mechanism of a liposome is simply fusing to
the cell membrane or through
endocytosis.
Polymers are made in different shapes and sizes. They
can come in a rod-like shape, with the drug attached to it, or be circular with
the drug inside the polymer construct. They can also be biodegradable or
non-biodegradable. Biodegradable polymers like PLA and PLGA are commonly used
since they do not have to be surgically removed from the body after the drug is
released.
Polymers can deliver drugs through dissolution,
diffusion, or osmosis.
In dissolution, the drug is released
over time as the polymer dissolves in the gastrointestinal tract. Mixing and
layering polymers with varying dissolutions rates controls the rate of release.
In diffusion, the release of the drug is controlled by its rate
of diffusion out of the polymer.
In osmosis, the drug is contained in a polymer consisting of
two compartments: one compartment contains the drug and the other contains a
biologically inactive agent that can push out the drug under certain conditions
– a push layer. When an individual takes the drug in pill form, water penetrates
the pill through the membrane of the polymer. This step activates the push
layer, which then drives the active ingredient into the gastrointestinal tract
through one or more tiny holes on the other side of the pill.
Upon swallowing, the biologically inactive parts of
the pill remain intact during its voyage through the gastrointestinal tract and
are eliminated in the feces as an insoluble shell.
Biotechnology Drug Delivery Systems
Biotechnology is improving the mechanism of two drug
delivery vehicles called liposomes and polymers.
Liposomes are fatty droplets made artificially in the
lab by the addition of a water solution to a
phospholipid gel. Liposomes encapsulate active drugs to improve their
delivery. Depending on the construction of the liposome, the active drug can be
carried within its layers or in the hallow space created by the encapsulation.
Liposomes mimic the natural phospholipid cell membranes in the human body.
Liposomes are currently being used for intravenous
delivery of small molecules and are being investigated for oral, transdermal
(through skin), and sustained-release delivery of drugs. Liposomes are often
attached to other molecules, like polyethylene glycol (PEG), that will prevent
or lessen detection by the immune system.
Polymers
Polymers are formed by the linkage of a large number
of smaller molecules, called monomers. Natural polymers include proteins, DNA,
and latexes, such as rubber. Synthetic polymers include glass, concrete and
plastics. The polymers used to deliver drugs are usually lactic acid based, such
as polylactic acid (PLA) and polylactic-co-glycolic acid (PLGA).
Lactic acid is a compound that is naturally produced
when glucose is metabolized in the body. PLA and PLGA have been used as
bioabsorbable sutures for more than 20 years, so their safety is well
established.
Polymers can be designed to either release a drug at a
steady rate for a long time, or it can release the drug after a prolonged
interval. Polymers can also be attached to antibodies to deliver drugs to a
specific target.
| The Role of Biotechnology |
Biotechnology is contributing to advances in drug
delivery through gene and protein discovery, and the resulting knowledge of
human biological systems. This allows researchers to create synthetic systems
that mimic the already existing biological processes in the body. Also, as
researchers learn more about certain diseases, they can discover drug delivery
targets that are more specific to the particular disease.
The current development of delivery systems as well as
methods of administration are the result of chemical, technical, and biological
advances and the subsequent understanding of the body.
Drug Delivery Systems in Development
The next step in drug delivery will be the creation of
vehicles or constructs that can target the disease tissue more accurately. There
are several developments currently underway:
Smart Drugs – Also known as "prodrugs,"
these compounds are designed to work only when activated by certain components
in the body. For example, a smart drug designed to be activated by a certain
enzyme will be activated only in tissues that produces that specific enzyme.
Monoclonal Antibodies – These are
antibodies made in the lab that can target antigens with extreme specificity.
They are attached to a drug in order to guide it to a specific cell. For
example, cancer drugs can be attached to monoclonal antibodies made against
tumour cells, which helps the drug target only tumour cells. This reduces the
toxic effects of cancer drugs.
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