Antisense Oligonucleotides and RNA Interference - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 351

7.8 Formulation Considerations for Antisense
Drug Delivery
Naked AS ODNs and siRNA, being large and ionic, cannot diffuse freely across the
cell membrane, and hence, to facilitate their entry to intracellular targets, a suitable
delivery system is required. The degree of biological effect of an AS ODN is highly
dependent on the delivery vector used; these are generally categorized into viral and
nonviral vectors [12] . Viral vectors usually have better transfection efficiency; however,
nonviral vectors have been found to be superior to viral vectors in terms of toxicity,
immunogenicity, and insertional mutagenesis [153] . Here we discuss these vectors in
detail. Generally, a delivery vector includes a cationic group for efficient loading of oli-
gonucleotide, a nonionic group for steric hindrance, an endosomolytic group for endo-
somal escape, and a targeting ligand for site-specific delivery [20] . The delivery system
should be sufficiently large, in other words, greater than 5 nm, to avoid clearance by
glomerular filtration. Simultaneously, it should be greater than 100 nm, to avoid leak-
age to interstitial spaces of hepatic sinusoid and entrapment by hepatic Kuffer cells
[20] ; but smaller than 200 nm, to avoid uptake by organs of the RES, such as the liver
and spleen. Thus, the size requirement for systemic delivery of these delivery systems
is about 100 nm.
7.8.1 Viral Vectors
Intracellular delivery of AS ODNs using a viral vector is called transduction. A viral
vector usually consists of a viral genome with deletions in some or all essential
genes, into which a transgene is inserted. Viral vectors provide high tissue specificity
and result in highly efficient oligonucleotide expression. However, they pose severe
safety risks owing to their oncogenic potential, and immunogenic effects, and are
still being used widely for AS ODN delivery. In this section, we discuss some of the
most commonly employed viral vectors, such as retrovirus, lentivirus, herpesvirus,
adenovirus, and adeno-associated virus. However, some viral vectors like herpesvirus
and poxvirus have also been used to carry AS ODNs in a few applications.
7.8.1.1 Retrovirus
Retroviruses are the most widely used RNA viruses for delivering AS ODNs and
were the first vectors to be developed for intracellular gene delivery. These infect
the host cells via the help of the enzyme transcriptase and require dividing cells
to achieve high transduction. Hence, replication defective vectors are used for
transducing the host cells. These vectors require integration into the host genome,
resulting in a sustained expression of vector. However, this integration is highly
nonspecific, and by integrating into the host genome at random positions these vec-
tors possess high potential for mutagenic consequences. They possess high trans-
duction efficiency and can carry oligonucleotides up to 8 kB without expression of
viral proteins [154,155] .