CATIONIC LIPIDS, PHOSPHATIDYLETHANOLAMINE AND THE INTRACELLULAR DELIVERY OF POLYMERIC, NUCLEIC ACID-BASED DRUGS (REVIEW)

Polymeric, nucleic acid drugs must be protected from endogenous nuclea ses and delivered to target cell nuclei in order to maximize their act ivity. Constructs expressing therapeutic genes, antisense oligonucleot ides and ribozymes can be delivered into cells by viral vectors, but c oncerns over safety and clinical utility have led to research into the development of alternative, non-viral delivery systems. Antisense and ribozyme drug development has focused upon modifications to the natur al oligonucleotide chemistry which make the molecules resistant to nuc lease degradation. These novel oligonucleotides cannot be generated by transgenes and must be administered in similar fashion to conventiona l drugs. However, oligonucleotides cannot cross membranes by passive d iffusion and intracellular delivery for these drugs is very inefficien t. Here we review the recent advances in forming lipid-DNA particles d esigned to mimic viral delivery of DNA. Most evidence now supports the hypothesis that lipid-DNA drugs enter target cells by endocytosis and disrupt the endosomal membrane, releasing nucleic acid into the cytop lasm. The mechanisms of particle formation and endosome disruption are not well understood. Cationic lipids are employed to provide an elect rostatic interaction between the lipid carrier and polyanionic nucleic acids, and they are critical for efficient packaging of the drugs int o a form suitable for systemic administration. However, their role in endosome disruption and other aspects of successful delivery leading t o gene expression or inhibition of mRNA translation are less clear. We discuss the propensity of lipid-nucleic acid particles to undergo lip id mixing and fusion with adjacent membranes, and how phosphatidyletha nolamine and other lipids may act as factors capable of disrupting bil ayer structure and the endosomal pathway. Finally, we consider the cha llenges that remain in bringing nucleic acid based drugs into the real m of clinical reality.