Page 274 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 15 Molecular/Targeted Therapy of Cancer 253

tumor cells through direct injection and systemic delivery. 15,16 the Food and Drug Administration (FDA) and the European
They have a selectivity for endothelial cells, are considered effi- Medicines Agency because of the risk of transfer and replica-
tion of resistance genes to bacteria in the human microbiome
cient gene delivery systems, and are relatively safe. Naked DNA
VetBooks.ir delivery (the delivery of plasmid DNA alone containing the gene • Residual antibiotics that remain from vector production may
and possibly into the environment.
of interest) results in uptake by tumor cells and antigen-presenting
17
cells after simple direct injection. A modification of this is par- trigger an immune reaction in patients.
ticle-mediated gene delivery using a “gene gun.” In this approach, In human gene therapy, these issues have led to the devel-
DNA is adsorbed onto gold particles and fired into tissues under opment of minimal plasmid vectors, with extensive deletions
high pressure (using helium as the motive force). 17,18 However, (minivectors). 17
the majority of these naked DNA approaches are still inefficient
and are not able to be given systemically (see Table 15.1). 19–23 Targeting Cancer Gene Therapy
Plasmid vectors for gene therapy have a number of
limitations 19–23 : In terms of delivery, one of the major barriers is the ability to give
• Plasmid DNA preparations contain several topological variants vectors systemically, to target them to cancer tissues, and to ensure
of the plasmid, including the unwanted open circular and lin- that therapeutic transgenes are not expressed in normal cells.
ear forms of the molecule. Targeting also ensures that enough vector can reach the target of
• Plasmids are inefficient at delivering genes compared with interest, without being dispersed to irrelevant sites. Numerous
viruses, thus requiring vehicles, physical forces, or specialized strategies have been attempted to provide levels of targeting and
modifications for uptake and nuclear localization. Some deliv- to spare normal tissue, and many have been explored in isolation
ery methods lead to breakage of the plasmid DNA backbone, or as combination strategies (e.g., dual and triple targeting).
which increases the likelihood of genome integration and/or 1. Transductional targeting. This involves surface modification
less efficient expression. (usually of virus), to allow delivery and/or entry to cells via spe-
• Plasmids are nonreplicating episomes, so transgene expression cific surface receptor. This approach has largely been applied to
is transient and is diluted by cell division. adenoviral vectors and has included modifications to the cox-
• Bacterial sequences in plasmids, such as unmethylated cyto- sackie and adenovirus receptor (CAR) (which gives adenovirus
sine-phosphate-guanine (CpG) dinucleotides, have the poten- its normal cellular tropism), and/or the use of “adaptors.” An
tial to be recognized by the mammalian immune system via adaptor molecule can ablate native CAR-based tropism and
toll-like receptor (TLR)-9, potentially precipitating not only target the virus to an alternate cellular receptor molecule. For
transgene silencing but also immune response. example, bispecific fusion proteins (e.g., diabodies, composed
• Plasmids typically encode antibiotic resistance-encoding genes of two single-chain antibodies [scFv], with one scFv recogniz-
for selection of plasmid-harboring bacteria. The use of antibi- ing the fiber knob and the other a tumor-associated antigen
otics and their resistance genes in the preparation of plasmid [TAA]) can be used as an adaptor to target adenovirus to a
vectors, however, is discouraged by regulatory bodies such as specific tumor type 24,25 (Fig. 15.2).

Cell or tissue- Transgene
Viral fiber specific promoter expression

Adapter

Receptor

Target cell Target cell Target cell
A B C
• Fig. 15.2 Vector targeting. The specificity of viral vectors can be improved utilizing either transductional
targeting (A), where the viral surface proteins are modified so they will only enter the cell of interest; trans-
ductional targeting, where the vector is modified using an “adapter” (B); or transcriptional targeting (C),
where the expression of the therapeutic transgene is under the control of a tissue or cell-specific promoter.
Transcriptional targeting can also be employed in nonviral vectors and directly delivered to the patient.