CA STEIN
Department of Medical Oncology and Experimental Therapeutics, City of Hope Medical Center, 1500 E. Duarte Rd., Duarte, CA, USA 91010

Abstract

The field of therapeutic oligonucleotides (oligos) has existed for more than 25 years, but few clinically therapeutic oligonucleotides have been FDA approved. None, so far, have been approved for a cancer indication. Using the example of antisense phosphorothioate oligos, this review will discuss delivery of unencapsulated, unconjugated oligos to cells, focusing on basic properties such as cell surface adsorption, adsorptive endocytosis and fluid-phase endocytosis. Examples of how newer chemistry, such as the LNA modification, aids the in vitro internalization process by enabling the process of gymnosis, will also be summarized. The barriers to in vivo therapeutic oligo internalization in cancer cells, which may originate in tumor vascular heterogeneity and intratumoral hypoxia, will also be discussed.

Phosphorothioate (PS) antisense oligonucleotides (oligos), short sequences of DNA in which a non-bridging oxygen atom is replaced by a sulfur atom at each phosphorus atom in the chain, have been generally available for about twenty-five years. However, at the present time, there are a mere handful of PS antisense oligos that are FDA-approved for therapeutic purposes; sadly, none of these are for a cancer indication. Why is it that the antisense and other gene-silencing strategies, which initially appeared so promising, have to date produced benefits only for niche indications and in small markets? 

I propose that the fundamental problem is the in vitro and in vivo delivery of these highly negatively charged, and hence hydrophilic, molecules into cells, and the requirement that they must cross hydrophobic lipid bilayers. For many years, the dogma in the field was ineluctable:for gene silencing to occur in vitro, the PS oligo must be delivered with a carrier (Note---as will be discussed below, this dogma, as many others in this field, has been shown to be false). This carrier was often a cationic lipid along with other lipids that were incorporated into the nanop ...