Lipids are a major
class of biomolecules that includes fatty acids, waxes, glycerol
and triacylglycerols, phospholipids and cholesterols. Unlike other
classes of biomolecules such as protein and DNA, the structure
of lipids varies tremendously and lipids are involved in a wide
array of processes from compartmentalization of the cell with
membranes to energy storage and cell signaling3.
The membranes that
define the exterior edges of animal cells and divide the cells
into compartments are composed of amphipathic phospholipids. This
means a portion of the molecule is charged, while the rest is
uncharged or hydrophobic3.
The lipids align to
form a bilayer, with the charged portions facing the aqueous environment
inside the cell and in the exterior, and the uncharged portions
interacting with each other within the bilayer2,3.
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Figure
2. A lipid bilayer. |
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In a sense, bilayer
formation is analogous to mixing oil and water. Adding drops of
oil to a container of water causes oil droplets to aggregate spontaneously
with each other to minimize the surface area in contact with water.
The same hydrophobic (literally "water-hating") forces
are involved in membrane assembly. In the case of cells, most
of the interior is composed of water with a few structures enclosed
by lipids that tend to aggregate together to form membranes3.
In an active, biological membrane, there are many different types
of lipids present and many other components besides lipids: these
include proteins for transporting substances across the membrane
or linking the membrane to the cell's cytoskeleton, signaling
molecules and a variety of other structures3.
The membrane is flexible and can self-seal because of the strong
interactions between the hydrophobic portions of the lipids3.
It is this ability to self-seal to form relatively stable structures
that is the basis for Inex's technology.
Lipids isolated from
cellular membranes or synthesized artificially will spontaneously
form a variety of structures when place in an aqueous environment2.
Some examples are shown below:
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Figure
3. A variety of lipid structures.
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Inex investigates lipids
capable of forming liposomes, self-closing spherical particles
where one or several lipid membranes encapsulate part of the solvent
in which they freely float in their interior4.
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Figure
4. A schematic of a liposome. |
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Inex and other companies
use these liposomes for the targeting and intracellular delivery
of drugs, oligonucleotides (short stretches of DNA) and plasmids
encoding therapeutic genes2,4.
The lipid-encapsulated agent is transported through the bloodstream
to specific sites in the body: for example, areas of inflammation
or tumour growth. Drug delivery by liposomes has many advantages
over simply injecting the drug alone into the bloodstream. First
of all, the drug remains concentrated in the liposomes instead
of diffusing throughout the body. This means lower doses can be
administered. Since many drugs used in treatments for diseases
such as cancer have toxic side effects to healthy cells as well
as the diseased ones, this lower dose means fewer side effects
for the patient and lower cost for treatment4.
Secondly, the liposomes can sometimes be targeted to a tumour
or site of disease. Just as biological membranes in a normal cell
contain proteins and a variety of other materials, synthetic liposomes
can also be manufacturing to contain small proteins or carbohydrates
that will "lock on" to a specific target molecule when
they come in contact with it. Sites of inflammation and some tumours
often have specific molecules on their surfaces that are potential
targets. Including molecules in the liposome that bind these targets
increases the likelihood of the drug reaching the proper area
of the body and minimizes uptake by non-target cells4.
Lastly, the release of the drug at the site is prolonged as the
drug slowly releases from the liposome. Since the liposome is
composed of lipids like the membrane itself, the liposome can
fuse to the membrane and deliver the drug directly inside the
cell. The direct delivery and prolonged exposure increases the
effectiveness of the drug2,4.
INEX HAS A REALLY COOL
FLASH ANIMATION OF THIS PROCESS.
Targeted
Chemotherapy
-- NB: Requires Macromedia Flash
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Figure
6. An overview of Oligovax.
(Image Source: Reproduced from the Inex web
site) |
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The short piece of
DNA also included in the liposome is called an oligonucleotide
or "oligo". The oligo can cause immune stimulation in
a variety of ways. One method commonly used is to include so-called
Immune Stimulatory Sequences (ISS) or CpG sequences. These are
DNA sequences with a high frequency of repeated CG bases. In vertebrates
such as humans, these sequences are rare and furthermore, are
modified by the addition of a methyl residue1,4.
However, in bacteria,
these sequences are more common and are less likely to be methylated.
In nature, when a bacterium invades a vertebrate and tries to
infect it, one of the ways in which the vertebrate's immune system
can recognize the bacteria as an invader is because its DNA has
these unmethylated CpG motifs1.
The bacterial DNA itself causes an immune response leading to
the generation of killer T cells that can recognize and kill the
bacteria. By including such sequences from bacteria in the liposomes,
Inex hopes to boost the immune response and direct it against
the antigen that was also included in the liposome. Having these
two immune stimulators delivered together by liposome may result
in better tumour recognition and killing4.
The OligoVax targeted
immunotherapy method is being evaluated with an antigen from a
protein called tyrosine-related protein-2 (TRP-2) - found in melanoma
cells. Using an antigen from cancerous melanoma cells in liposomes
with immunostimulatory DNA may generate enough killer T cells
to eliminate the melanoma from the body. The company also plans
to develop the immunotherapy program for the treatment of other
cancers and infectious diseases4.
