Polyamines and novel polyamine conjugates interact with DNA in ways that can be exploited in non-viral gene therapy

Abstract

As a part of our continuing studies on ‘Polyamines and their role in human disease’ we are investigating how polyamines, and especially how novel polyamine conjugates, interact with DNA. We are studying how these conjugates interact with circular plasmids in order to produce nanometre-sized particles suitable for transfecting cells. Our considerations of structure--activity relationships (SAR) within naturally occurring and synthetic polyamines have shown the significance of the inter-atomic distances between the basic nitrogen atoms. As these atoms are typically fully protonated under physiological conditions, they exist in equilibrium as polyammonium ions. The covalent addition of a lipid moiety, typically one or two alkyl or alkenyl chains, or a steroid, allows much greater efficiency in DNA condensation and in the cellular transfection achieved. Thus efficient DNA condensation and subsequently drug delivery (i.e. with DNA as the drug) can be brought about using novel polyamine conjugates. Taking further advantage of the functionalization of specific steroids (e.g. cholesterol and certain bile acids), we have designed and prepared novel fluorescent molecular probes as tools to throw light on the problematic steps in non-viral gene delivery which still impede efficient gene therapy. Thus, the current aims of our research are to understand, design and prepare small-molecule lipopolyamines for non-viral gene therapy (NVGT). The rational design and practical preparation of non-symmetrical polyamine carbamates and amides, based on steroid templates of cholesterol and the bile acid lithocholic acid as the lipid moiety, provides fluorescent molecular probes that condense DNA. These novel lipopolyamine conjugates mimic the positive charge distribution found in the triamine spermidine and the tetra-amine spermine alkaloids. After optimizing their SAR, these fluorescent probes will be useful in monitoring gene delivery in NVGT.