Clinical Pharmacology and Therapeutics

HUMAN STEM CELL THERAPY 95
duration and benefit. Adenoviral vectors are more efficient
than liposomes but themselves cause serious inflammatory
reactions.
A dramatic example of the potential benefit and danger of
gene therapy has been seen in the treatment of severe combined
immunodeficiency (SCID) secondary to adenosine
deaminase deficiency by reinfusing genetically corrected
autologous T cells into affected children. Whilst the gene therapy
was effective in the immunological reconstitution of the
patients, allowing a normal life including socializing with
other children rather than living in an isolation ‘bubble’, T-cell
leukaemia has developed in some patients. This probably
reflects problems with the retrovirus vector.
A success in gene therapy has occurred with recipients of
allogenic bone marrow transplants with recurrent malignancies.
T cells from the original bone marrow donor can mediate
regression of the malignancy, but can then potentially damage
normal host tissues. A suicide gene was introduced into the
donor T cells, rendering them susceptible to ganciclovir
before they were infused into the patients, so that they could
be eliminated after the tumours had regressed and so avoid
future damage to normal tissues.
From the above, it will be appreciated that a major problem
in gene therapy is introducing the gene into human cells. In
some applications, ‘gene-gun’ injection of ‘naked’ (i.e. not
incorporated in a vector) plasmid DNA may be sufficient.
Minute metal (e.g. gold) particles coated with DNA are ‘shot’
into tissues using gas pressure (Figure 16.2). Some DNA is recognized
as foreign by a minority of cells, and this may be sufficient
to induce an immune response. This method underpins
DNA vaccines. The other major problem is that for most diseases
it is not enough simply to replace a defective protein, it
is also necessary to control the expression of the inserted gene.
It is for reasons such as these that gene therapy has been
slower in finding clinical applications than had been hoped,
but the long-term prospects remain bright.
Despite the inherent problems of gene therapy and societal
concerns as to how information from the genotyping of individuals
will be used, the development of gene therapy has
dramatic potential – not only for the replacement of defective
genes in disabling diseases such as cystic fibrosis, Duchenne
muscular dystrophy and Friedreich’s ataxia, but also for the
treatment of malignant disease, and for prevention of cardiovascular
disease and other diseases for which there is a genetic
predisposition or critical protein target.
Another gene-modulating therapy that is currently
being evaluated is the role of anti-sense oligonucleotides.
These are nucleotides (approximately 20mers in length)
whose sequence is complementary to part of the mRNA of the
gene of interest. When the anti-sense enters cells it binds to the
complementary sequence, forming a short piece of doublestranded
DNA that is then degraded by RNase enzymes,
thus inhibiting gene expression. Examples of such agents in
development or near approval include fomiversen, which
binds to cytomegalovirus (CMV) RNA (used intraocularly for
CMV infection) and anti-Bcl-2, used to enhance apoptosis in
lymphoma cells.
Gold particle
coated with
DNA
DNA
Nucleus
Antigen
presenting
cell
Transcribed
to mRNA
MHC
HUMAN STEM CELL THERAPY
Translated
to protein
Processed into
antigen peptides
MHC: antigen
presentation
Antigens
presented to
the immune
system invoke
humoral and
cellular
response
Figure 16.2: Particle-mediated epidermal delivery (PMED) of DNA
into an antigen presenting cell (APC). The DNA elutes from the
gold particle and enters the nucleus where it is transcribed into
mRNA. The mRNA is then translated using the cellular synthetic
pathways to produce the encoded protein of interest. This
intracellular foreign protein is then processed by proteasomes
into small antigenic peptides that are presented on the cell
surface by the major histocompatibility complex (MHC).
The discovery of stem cells’ ability to replace damaged cells
has led to much interest in cell-based therapies. Stem cells
retain the potential to differentiate, for example into cardiac
muscle cells or pancreatic insulin-producing cells, under
particular physiological conditions.
In the UK, stem cell therapy is already established in the
treatment of certain leukaemias and has also been used successfully
in skin grafting, certain immune system and corneal
disorders. Autologous and allogenic haemopoietic stem cells
collected from bone marrow or via leukophoresis from
peripheral blood following granulocyte colony-stimulating
factor (G-CSF) stimulation (see Chapter 49) have been used
for some years in the management of certain leukaemias.
Allogenic stem cell transplantation is associated with graftversus-host
disease, hence concomitant immunosuppressant
treatment with prophylactic anti-infective treatment including
anti-T-cell antibodies is required. Graft-versus-host
disease and opportunistic infections remain the principal
complications.
Non-myeloblastic allogenic stem cell transplantation is
being increasingly used, particularly in the elderly. This has
an additional benefit from a graft-versus-tumour effect as
immunosuppression is less severe.
Although there has been much publicity over the potential
of stem cell regenerative and reparative effects in chronic
central nervous system disorders, such as Parkinson’s disease,
Alzheimer’s disease, motor neurone disease and multiple
sclerosis, to date there is no convincing evidence of benefit
for these conditions. There is ongoing ethical debate over
the use of embryonic stem cells, which have more therapeutic