Terry M. Allen & Pieter
Cullis talk with ScienceWatch.com and answer a few
questions about this month's Emerging Research Front Paper
in the field of Pharmacology & Toxicology. The authors
have also sent along images of their work.
Article: Drug delivery systems: Entering the
Journal: SCIENCE, 303 (5665): 1818-1822, MAR 19 2004
Addresses: Univ Alberta, Dept Pharmacol, Edmonton, AB T6G
Univ Alberta, Dept Pharmacol, Edmonton, AB T6G 2H7,
Univ British Columbia, Dept Biochem & Mol Biol,
Vancouver, BC V6T 1Z3, Canada.
Inex Pharmaceut Corp, Burnaby, BC V5J 5J8, Canada.
Why do you think your paper is highly
The paper summarizes the state of the art, in 2004, for drug delivery
systems (DDS) and provides examples of several DDS that have received
clinical approval. At that time, there was increasing publicity about, and
interest, in nanomedicines, and the field was in a stage of exponential
growth. It appears to have appealed to a captive audience, or maybe we
could say that we were fortunate that it was "the right paper at the right
Within a few pages, it summarized the major background information on the
mechanism of action of DDS, and attempted to provide the know-how on how to
select appropriate drugs, to design DDS with appropriate characteristics
such as size, stable drug retention, and appropriate release rates, and to
select applications where the DDS have a higher chance of success.
Does it describe a new discovery, methodology, or synthesis of
The paper is basically a synthesis of the knowledge that was contributed by
our laboratories and co-workers, along with our wonderful series of
colleagues, too many to mention, that have been working on DDS going back
several decades. Indeed, "we stood on the shoulders of giants."
Would you summarize the significance of your paper in layman's
These days, we are beginning to see a number of negative comments in the
press concerning the potential hazards of nanotechnology. In some cases,
these concerns may be justified. However, all applications of
nanotechnologies are not equal, and the paper clearly shows how the use of
nanomedicines—a subclass of nanotechnology—has been of enormous
benefit to, in particular, cancer patients, by improving their therapeutic
outcome and/or improving their quality of life by reducing the side effects
of the cancer drugs.
At the time of the writing of the paper, DDS had already experienced
thousands of patient-years of use, and over a decade of clinical experience
with few or no serious side effects associated with their use. This
remarkable record continues to this day, and shows that DDS will have an
important role in medicine as we go forward, if applications are chosen
judiciously and go through the proper regulatory controls.
How did you become involved in this research and were any
particular problems encountered along the way?
Theresa M. Allen:
As a young post-doctoral fellow, I worked in a laboratory that was among
the first to purify the acetylcholine receptor. I became interested in
studying the properties of the purified receptor by reconstituting it in a
lipid membrane. This led to a general interest in membrane structure and
function, and eventually to a continuing interest in the use of lipid
nanoparticles as DDS.
One of the big problems that plagued the field of DDS in the early days was
a destruction of the lipidic DDS, because they were seen as "foreign"
entities by the body and prematurely taken up into macrophages, which are
the body's main defense system.
By studying the surface structure of red blood cells, which the body allows
to circulate for many days, we devised a method to "coat" the liposomes
with hydrophilic molecules that increased their invisibility to the
macrophages. Once the DDS could avoid the macrophages, they circulated long
enough to concentrate in diseased tissues such as tumors—leading to
their clinical approval.
I became involved in drug delivery research as a result of my early studies
on the roles of lipids in membranes. In that work I employed simple "model
membrane" systems consisting of dispersions of lipid in water (also known
as liposomes) to gain insight into the potential roles of individual lipid
species in biological membranes.
In the 1980s I used the model membrane approach to investigate the
influence of trans-membrane ion gradients on the ability of lipids to move
across membranes, demonstrating that transbilayer pH gradients can lead to
asymmetric trans-membrane distributions of lipids such as are observed in
biological membranes. This work required the development of new procedures
to generate small liposomal systems in the 100 nanometer size range that
exhibited trans-membrane pH gradients.
In order to make these "liposomal nanoparticle" (LN) systems we developed a
rapid extrusion procedure whereby aqueous dispersions of lipid are extruded
under high pressure through filters with 100 nm pore-size filters to
rapidly (10 minutes or less) produce LN systems of 100 nm diameter. This
technique is now the preferred method for making LN worldwide.
We then showed that when these liposomal systems were subjected to a
trans-membrane pH gradient (e.g., pH 4 inside, pH 7 outside), weak base
drugs, which includes many anticancer drugs, could be loaded into the
interior of the liposome, achieving encapsulation efficiencies approaching
100% and the stable encapsulation of tens of thousands of drug molecules
per LN. This discovery had a major impact in the emerging field of drug
delivery using LN as it provided methods, which were previously not
available, for rapidly making liposomes and then efficiently loading them
with the desired drug.
In the 1990s, we investigated whether nucleic acid-based macromolecules
such as antisense oligonucleotides (OGN) and plasmids coding for
therapeutic genes could be encapsulated in LN, developing novel and
efficient ways of encapsulating such macromolecules into small (~100 nm),
well-defined LN systems that exhibited little surface charge.
The development of small uncharged LN systems is vital for in vivo
applications to reduce toxic side effects and to avoid rapid clearance by
the reticuloendothelial system (RES) following i.v administration. LN that
avoid rapid clearance are necessary to access non-hepatic tissues in the
body. Three kinds of therapeutic are resulting from this work. The first
concerns well-defined LN systems containing one plasmid per LN that exhibit
long circulation lifetimes following i.v. injection and preferentially
transfects tumor tissue with the plasmid gene.
Second, these techniques allow well-defined LN containing thousands of OGN
to be made. Subsequent work showed that LN containing OGN that are
immunostimulatory (such as those containing CpG sequences) dramatically
enhance the innate immune response to the encapsulated oligonucleotides. We
are currently investigating the utility of these systems as stand-alone
agents to fight disease by stimulating the innate immune system, as agents
to enhance the potency of MAb as well their properties as potent adjuvants
for vaccine applications.
A third and very exciting area concerns the extension of this work to
delivery of siRNA in vivo, which is the major focus in my lab
Where do you see your research leading in the future?
Nowadays, promising new therapeutics are coming out of genomics and
proteomics. But these new therapeutics, which are mostly macromolecules,
e.g., siRNA, can be fiendishly difficult to deliver in vivo for a
variety of reasons related to their premature destruction and/or
elimination from the body, and their difficulties in crossing cell
In recent years we and others have been researching ligand-targeted DDS,
where surface ligands such as antibodies can trigger the internalization of
the DDS into the target cells. It is now widely believed that these
targeted DDS may represent our best hope for delivering new therapeutics
such as siRNA to diseased cells.
Do you foresee any social or political implications for your
We have alluded above to the social and political implications of the
unfettered application of nanotechnology, and the fear of the unknown in
the form of unidentified toxicities associated with the unapproved,
unregulated use of products containing nanoparticles.
However, we want to state clearly and unambiguously that nanomedicines,
such as the DDS systems currently approved for clinical applications, have
been widely examined over many years in both humans and animals, and have
been subjected to the stringent guidelines and scrutiny of regulatory
authorities such as the FDA. We can be confident that these products are
safe and effective.
Dr. Theresa M. Allen, Ph.D., F.R.S.C.
Professor of Pharmacology & Oncology
University of Alberta
Edmonton, Alberta, Canada
Division Head, Drug Delivery
Centre for Drug Research & Development
University of British Columbia
Vancouver, BC, Canada
Pieter Cullis, Ph.D., F.R.S.C.
Centre for Drug Research and Development
Department of Biochemistry and Molecular Biology
Life Sciences Centre
University of British Columbia
Vancouver, BC, Canada
KEYWORDS: PEGYLATED LIPOSOMAL DOXORUBICIN; AMPHOTERICIN-B