the appropriate intracellular and intranuclear sites is
more difficult. The first antisense drug to be approved
is for the treatment of cytomegalovirus retinitis in
patients with AIDS (fomivirsen; Vitravene, ISIS
Pharmaceuticals, Inc.). The route of administration
of this drug is by direct intraocular injection, illus-
trating well the ADME complexities associated with
some biotechnology products.
To date, most regulatory authorities have trea-
ted antisense drugs in much the same way as
any other biological product, and without the
additional constraints that apply to gene thera-
pies. As these oligonucleotides have specific
binding activities, safety considerations are
usually dependent on the potential for nonspeci-
fic effects of protein synthesis inhibition. At
present, with the current limited experience,
there would appear to be soundin vitromethods
for the testing of the specificity of antisense drugs
to be predictive for their tolerability in man.
Furthermore, when the properties of the protein
which is inhibited are discrete and consistent
across individuals, then it is likely that the poten-
tial adverse effects will be predictable.
22.8 Gene therapy
Gene therapy may be defined as the administration of
exogenous DNA, in the form of intact gene(s) for
therapeutic purposes. There are someaprioricharac-
teristics for diseases that are likely to be attractive
targets for gene therapy, and a fundamental contrast
between the gene therapy of protein replacement, in
comparison with protein synthesis regulation.
The absoluteorrelativedeficiencyof aparticular
protein needed for health may be correctable, for
example the enzyme needed to reverse Gaucher
disease. If the gene product can be manufactured
and administered effectively and tolerably, then the
need for a gene therapy is reduced.
However, there are also congenital disorders
involving relative deficiencies of a particular pro-
tein, and, where, importantly, therapeutic-induced
overexpression can be as harmful as underexpres-
sion. Attempting to regulate gene expression then
becomes more difficult than merely inducing it.
The thallassemias area prioria good example of
this problem.Overproductionof themissing hemo-
globin chain is unlikely to be helpful to the patient.
Similarly, when the principal desired target for
gene therapy is a specific target organ, then over-
expression of genes in other tissues may create
tolerability problems.
Gene therapies usually have two major compo-
nents, the DNA molecule itself (the ‘construct’),
and an administration adjuvant (the ‘vector’). In
some cases constructs are injected directly, without
a vector (termed ‘naked DNA’), but vectors are
usually necessary because genes are large, hydro-
philic molecules that do not readily cross lipid
membranes. Vectors may be viral or nonviral.
Viral vectors include:Potentially pathogenic DNA viruses. These
include adenoviruses and pox or vaccinia
viruses. Both virus types can replicate in mam-
malian cytoplasm, whether or not the host cell is
in mitosis or quiescent, and usually elicit a host
immune response.Herpes simplex virus I (HSV1) also contains
double-stranded DNA, but it replicates in the
nucleus of cells that are successfully infected,
again without need for mitosis.Nonpathogenic adeno-associated viruses. These
parvoviruses carry single-stranded DNA and are
able to integrate into a broad range of nondivid-
ing cells.Retroviruses. These RNA-containing viruses
exist in an envelope derived from host cell mem-
brane, and thus do not usually elicit vigorous
immune responses. Retroviruses also tend to
replicate only in dividing cells.It is perhaps surprising that naked DNA can cause
gene expression at all. Current examples where this
concept has been proven includegenes injected into
skeletal and smooth muscle. DNA–protein conju-
gates can also be administered without a vector, and
seek to be internalized into the cell by specific
receptors during the ordinary processes of endocy-
tosis. Gold-coated DNA may also be inserted into
cells by a ‘gene gun’, where electrostatic or gas286 CH22 BIOTECHNOLOGY PRODUCTS AND DEVELOPMENT