The effectiveness of chemotherapeutic agents is often limited by various mechanisms that exist in cancer cells, often through overexpression of certain genes, in particular those coding for membrane-spanning ATP-binding cassette (ABC) transporters; examples include MDR
, ABCG2 
, and MRP1 
. The MDR
1 gene is frequently overexpressed in several drug-resistant cancers, such as acute myeloid leukemia 
, colon cancer 
, adrenal cancer 
, and kidney cancer 
. An over-abundance of P-glycoprotein (P-gp), the protein product of the MDR
1 gene 
, leads to multidrug resistance (MDR), because cancer cells become able to efflux a number of structurally diverse compounds including many chemotherapy agents, such as paclitaxel 
, doxorubicin 
, and vinblastine 
. Silencing the expression of genes such as MDR
1 is one potential way to address the problem of MDR in cancer.
Peptide nucleic acid (PNA) is a synthetic DNA analog in which the sugar-phosphate backbone is replaced with a polyamide backbone 
. The nucleobases covalently attached to the PNA backbone enable the PNA oligomers to bind to complementary DNA or RNA sequences via Watson-Crick base-pairing. Certain key properties of PNA make it ideal to target DNA or RNA sequences in vivo 
. First, because PNA is purely synthetic, it is resistant to degradation by nucleases and proteases 
, and thus these molecules may remain in cells for extended time periods 
. Second, the thermal stability of PNA
oligonucleotide complexes is significantly higher than corresponding oligonucleotide duplexes. This high stability when bound to its target oligonucleotide should enhance the ability of PNA to suppress protein or gene expression. Numerous studies have sought to develop PNA as an antisense agent to suppress protein expression by targeting an mRNA sequence 
. In this way, PNA inhibits translation by sterically blocking translation start sites along mRNA 
. More recently, PNA has been used as an antigene agent that suppresses gene expression by targeting a DNA sequence 
. The work of the Corey group 
has elegantly demonstrated that PNA designed to target the transcription initiation sites of genes may effectively suppress overall expression; by targeting the transcription start site of the gene, the whole gene is inhibited, including all splicing forms of the protein, making antigene PNA a powerful inhibitor.
To date, only a few genes have been targeted using this approach. A key hurdle in the development of PNA as an antisense or antigene agent is the effective delivery of PNA to cells 
. While some cellular systems are permeable to PNA, many cells lines are not 
, so various systems have been developed to deliver PNA to cells. Most of these systems involve covalent conjugation of PNA to another molecule that facilitates delivery into a cell, such as a lipid or cell-penetrating peptide 
. Others chemically modify the PNA backbone with multiple arginine side chains to gain entry 
. Unfortunately, there is no delivery system currently available that works with all cells. Therefore, a universally applicable system could facilitate the therapeutic use of PNA.
To that end, we investigated the simian virus 40 (SV40) in vitro
-packaging (IVP) system, in which pseudovirions are formed when the major capsid protein of SV40 (VP1) self-assembles around nucleic acids 
. Unlike other in-vitro
packaging systems, no viral genetic material or packaging signal sequence is required to form these pseudovirions. To date, SV40 pseudovirions have been shown to deliver reporter genes such as GFP, suicide genes such as Pseudomonas exotoxin
, plasmids encoding shRNA, and siRNA oligomers 
. These pseudovirions can transduce both dividing and quiescent cells in vitro
and tissues in vivo
. Therefore, we chose the SV40 system to deliver PNA molecules to cancer cells.
Here we demonstrate the effectiveness of the SV40 IVP system to deliver antigene PNA molecules against the MDR1 gene through the reduction of the expression of MDR1 at the transcriptional level leading to reduced total levels of P-gp within a cell. These changes result in a decreased ability of the cell to efflux xenobiotic compounds (efflux capacity). Additionally, drug-resistant cells transduced with PNA targeted against the MDR1 gene and treated with the chemotherapeutic agent paclitaxel were significantly less viable compared to those transduced with a scrambled sequence of PNA. Our in vitro results suggest that delivery of PNA via the SV40 delivery system would be a promising technique to treat certain cancers that exhibit MDR1-mediated MDR.