Liposomes as drug delivery systems
Drugs have been made to enter the human body in order to bring about its effect. The traditional methods of drug delivery
have been topical, inhalational, enteral and parenteral. Most of these methods leads in the entry of the drugs into the body
but results in a uniform concentration and all the tissues of the body are exposed to the drug irrespective of whether the
disease is present there or not.
There have been research into developing drug delivery systems that deliver drugs to “where the disease is”. One
of these newer modalities is the Liposomal encapsulated drug delivery.
Liposomes
are subcellular structures which is made up of one or many concentrically arranged lipid bilayers. They are 10nm to 20 nm
in size. This lipid layer is amphipathic in nature consisting of a hydrophobic component which is directed inwards and the
hydrophilic component which is directed outwards.
Liposomes
consist mostly of phospholipids and has been divided into three basic types:
- Small unilamellar liposomes
- Large unilamellar liposomes
- Large multilamellar liposomes
There
have been various modifications and the newer liposomes contain synthetic substances as components of the bilayer.
Pharmacologic
utility of liposomes
The utility
of Liposomes is due to the amphipathic nature of the bilayer. This interacts with the nature of the drugs in the following
way.
- Hydrophilic drugs are encapsulated in the internal aqueous phase of the liposome due
to its hydrophobic nature
- Hydrophobic substances are incorporated into the lipid bilayer due to its hydrophilic
nature
- Amphipathic substances are adsorbed on to the lipid bilayer
The extent of this interaction can be modified by modifying the size as well as the composition of the lipid bilayer. The
time for which the liposomes are retained in the blood circulation can also be modified (eg., protective effect of polyethylene
glycol, stealth liposomes). The action of the liposomes can also be made specifically directed by attachment of antibodies
which are organ or tumour specific and also by the use of liposomes which are thermospecific. Because liposomal encapsulation
radically alters a drug’s absorption, distribution, metabolism and excretion, it can potentially address many of the
current problems with controlled drug delivery.
The pharmacologic
advantages of this mode of drug delivery are:
- Gradual release of the drug
- Dosage of some of the drugs can be reduced. Organs such as the heart, kidneys and
the brain which have a poor uptake of liposomes are spared of the toxic effects of some drugs
- As described above, attaching immunoglobulins can make the liposomes organ or tumour
specific and produce a very directed effect
Preparation
of liposomes
There are two main ways by which liposomes have been prepared:
- Hydration of dry lipid films to produce large multilamellar
liposomes.
- These large multilamellar liposomes are reduced in size by pushing them through polycarbonate
microfilters
The drug
is incorporated into these liposomes and are commercially available in two forms:
- As an aqueous suspension
- Lysophilised preparation
Most of the research concerning liposomal drug delivery have been focused on anti-cancer chemotherapy. This modality circumvents
the drawbacks of the conventional anti-cancer therapies like low efficacy, rapid elimination of the drugs before the maximum
effect is obtained, high toxicity
Some of the drugs which utilise liposomes for their delivery
are doxorubicin and amphotericin B. Other drugs for which liposomal delivery has been tried include antibiotics and anti-inflammatory
agents.
