Nucleic-acid- and cell-based therapeutics - Pharmaceutical Biotechnology: Concepts and Aapplications - page 425

The in vitro approach entails initial removal of the target cells from the body. These are then
cultured in vitro and incubated with vector containing the nucleic acid to be delivered. The geneti-
cally altered cells are then reintroduced into the patient's body. This approach represents the most
commonly adopted protocol to date. In order to be successful, however, the target cells must be
relatively easy to remove from the body, and reintroduce into the body. Such in vitro approaches
have successfully been undertaken utilizing various body cell types, including blood cells, stem
cells, epithelial cells, muscle cells and hepatocytes.
A second approach involves direct injection/administration of the nucleic-acid-containing vec-
tor to the target cell, in situ in the body. Examples of this approach have included the direct injec-
tion of vectors into a tumour mass, as well as aerosol administration of vectors (e.g. containing the
cystic fi brosis gene) to respiratory tract epithelial cells.
Although less complicated than the in vitro approach, direct in situ injection of vector into the imme-
diate vicinity of target cells is not always feasible. This would be true, for example, if the target cells are
not localized to one specifi c area of the body (e.g. blood cells). An alternative ( in vivo ) approach entails
the development of vectors capable of recognizing and binding only to specifi c, predefi ned cell types.
Such vectors could then be administered easily by, for example, i.v. injection. Through appropriate
biospecifi c interactions, they would only deliver their nucleic acid payload to the specifi ed target cells.
The simplicity and specifi city of this approach renders it the method of choice. However, thus far, no
such vector systems have been developed for routine therapeutic use. Intensive efforts to develop these
are underway, and a number of different strategies are being pursued. For example, the inclusion of an
antibody on the vector surface, which specifi cally binds a surface-antigen uniquely associated with the
target cell, would allow selective delivery. Another approach entails engineering the vector to display a
specifi c hormone that would bind only to cells displaying the hormone receptor. The feasibility of this
approach has been demonstrated using retroviral vectors engineered to display EPO on their surface.
14.2.2 Someadditionalquestions
The choice of vector, target cell and protocol used will depend upon a number of considerations.
The major consideration is obviously what the ultimate goal of the gene therapy treatment is in
any given case. For example, in some instances it may be to correct an inherited genetic defect,
whereas in other instances it may be to confer a novel function upon the recipient cell. An ex-
ample of the former would be the introduction of the cystic fi brosis transmembrane conductance
regulator (CFTR) gene (the cystic fi brosis gene) into the airway epithelial cells of cystic fi brosis
sufferers. An example of the latter would be the introduction of a novel gene into white blood cells
whose protein product is capable of in some way interfering with HIV replication. Such an ap-
proach might prove an effective therapeutic strategy for the treatment of AIDS.
An additional consideration that may infl uence the protocol used is the desired duration of sub-
sequent expression of the gene product. In most cases of genetic disease, long-term expression of
the inserted gene would be required. In other instances (e.g. some forms of cancer therapy or the
use of gene therapy to deliver a DNA-based vaccine), short-term expression of the gene introduced
would be suffi cient/desirable.
For most applications of gene therapy, straightforward expression of the gene product itself will suf-
fi ce. However, in some instances, regulation of expression of the transferred gene would be required (e.g.
if gene therapy combating insulin-dependent diabetes mellitus was to be considered). Achieving such
expressional control over transferred genes is a pursuit that is only in the early stages of development.