pressure powered displacement from a plastic
matrix occurs.
Nonviral vectors are mostly liposomes of one
type or another. Liposomal envelopes can transport
substances across cell membranes which would
otherwise be repelled by the hydrophilicity of the
gene construct. Liposomes may be constructed that
are either anionic or cationic. Complex liposomes,
coated with antibodies that will target specific
antigen presenting cells, can also be designed.
Human gene transfer experiments in lympho-
cyte marking studies began in 1989. These early
studies showed that gene transfer was feasible and
could be well tolerated although there was no
demonstrable therapeutic benefit. The first human
gene therapy clinical trial was in 1990, in a patient
with adenosine deaminase (ADA) deficiency;
initial responsiveness proved not to be uniform
when the series of cases was extended, possibly
due to the fact that the disease phenotype could be
elicited by a variety of genotypes.
Two-stage delivery systems for gene therapies
are also under development. This usually requires
manipulating somatic tissueex vivo. A good exam-
ple would be following the transformation of bone
marrow biopsies. The gene therapy can be intro-
duced into the biopsy materialex vivousing either a
viral or a nonviral vector. Successful expression
can then be definitely demonstrated in vitro,
following which the transformed marrow biopsy
can be infused as an autologous transplant, with the
intention of its proliferation and generation of the
desired protein productin vivo.
The pharmacokinetics of gene therapy, and its
relationship to dynamic effects, are very different
form the orthodox pharmaceutical situation
(Table 22.1). Ledley and Ledley (1994) have pro-
posed a corollary of traditional PK–PD modeling,
predicated upon the specific events in the cellular
response to gene uptake and activation. These
authors have developed a six-compartment model,
which appears to have general applicability, to eval-
uate the apparent kinetic properties of a therapeutic
geneproduct. Thisleadstothepossibilityofdesign-
ing dosing regimens and relating them to measure-
ments of expression and efficacy responses.
Acquired disorders may also be amenable to
gene therapy. Stimulating the production of some
cytokine that is a normal response to a tumor might
be one strategy, using an appropriate gene and
vector. Another example might be the differential
sensitization of cells in a tumor to a particular
cytotoxic drug, thus obtaining enhanced therapeu-
tic response, permitting the use of lower doses of
cytotoxic, and minimizing dose-limiting systemic
adverse effects.
There are two areas of specific tolerability con-
cerns associated with gene therapies, related to the
expressed gene product and the vector. Both are
immunological in nature, and may lead to thera-
peutic ineffectiveness.
If the gene therapy causes the production of a
protein that was previously absent in the body, then
an immune response to the novel protein is likely.
Resistance to gene therapy can result from immu-
nization against either the construct or the vector.
The former is analogous to the patients who used to
become resistant to xenobiotic insulins (see
above), and is also seen in the case of human factor
VIII in some patients with hemophilia. Escalating
doses may be needed to maintain efficacy, or effi-
cacy may be eventually lost. On the other hand,
viral vectors are liable to replicate and also to elicit
immune responses, just as for any vaccination,
creating many of the same problems.
One approach has been to develop strains of
many of the viruses listed above as ‘replication
defective’ or ‘replication incompetent’. These
viruses are mutations that are cultured initially in
conditions that provide some crucial nutrient or
element of the replicating machinery that neither
thevirusnor,importantly,thepatientcansynthesize.
These strains of virus are therefore replication-
incompetent after human administration. There is
nonetheless always the concern that after injection
the virus will find some way to overcome its incom-
petency, for example by recruitment of the host cell
machinery for this purpose.Safety issues in gene product
developmentAlthough issues surrounding sterility, mutageni-
city, stability and carcinogenicity, and the atten-
dant GLP and GMP issues are much the same for22.8 GENE THERAPY 287