Page 273 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

252 PART III Therapeutic Modalities for the Cancer Patient

TABLE 15.1 Gene Therapy Vector Systems
Viral Vectors
VetBooks.ir Retroviruses Originally the gold-standard vector. Gene is packaged into replication-defective viral particles using a packaging cell line. The
(oncoviruses)
therapeutic gene is integrated into the host cell genome when the virus is delivered to the target cell. Limited by their inability to
infect postmitotic cells.
Retroviruses Many are based on HIV-1. These vectors have become safer in recent years and have many of the benefits of the oncoviruses
(lentivirus) and also will infect postmitotic cells. Construction of the vector takes place in a packaging cell line and the therapeutic gene is
integrated into the host cell genome.
Adenoviruses Have become the most popular viral delivery mechanism. Gene is packaged into a replication incompetent adenovirus (usually
E1 deleted). Gene expression remains episomal when delivered to the host cell. Concerns have been raised about the safety
of adenoviral vectors, in particular potential toxic side effects at high doses. Adenoviruses can infect a wide range of pre- and
postmitotic cells. Conditionally replicating adenoviruses are being explored as oncolytic vectors.
Adeno-associated Gaining popularity as a vector as they are potentially safer than adenoviruses. They infect a wide variety of mammalian cell types
viruses (AAV) but are limited by the amount of DNA they can deliver. Gene expression in the host cell is episomal. However, in the natural host,
integration is possible.
Nonviral Vectors
Naked DNA This is a simple form of gene delivery in which “naked” plasmid DNA is directly injected into the tumor. Vectors are derived from
bacterial plasmids and are engineered to express the therapeutic gene under the control of a strong promoter. Naked DNA can be
taken up by many tissues, but typically the efficiency for delivery is lower than that for viral gene delivery.
Particle bombardment This is a more sophisticated approach to the delivery of naked DNA. In this system plasmid DNA is typically adsorbed onto gold
(gene gun) particles. Helium is then used as a motive force to fire the gold particles into cells or tissues via a hand-held “gene gun.”
Liposome/DNA In this system naked DNA is coated with liposomes to improve uptake by endocytosis. This enhances the efficiency of gene delivery.
conjugates
Ligand/DNA Ligands are used to specifically target DNA to tumor tissue.
conjugates
HIV-1, Human immunodeficiency virus-1.

Retrovirus
Adeno-Associated Virus
Adenovirus
Promoter Transcription initiation
Gene
Viral Delivery
Nonviral Delivery

DNA
adsorbed on Target Cell
to gold
particles
Plasmid DNA Biolistic Gene Gun
• Fig. 15.1 Viral and nonviral gene delivery. Adenoviral vectors are produced in “producer cell lines.” They
enter the cell by transduction and their genetic material is transported to the nucleus. In contrast to retro-
viruses, the DNA is not integrated into the host genome, but gene expression is achieved episomally. Ret-
roviral vectors are also produced is specialized producer cell lines. They enter the cell by transduction but
their RNA genome is reverse transcribed into proviral DNA. This integrates into the host genome, where
expression of the transgene takes place. DNA plasmid vectors contain a gene cassette that incorporates
the therapeutic transgene under the control of a promoter. The plasmid can be delivered by direct injection
as naked or liposome encapsulated DNA, by direct injection or systemically wrapped in nanomedicine
particles, or by direct injection utilizing a helium-driven “biolistic” gene gun.