Cationic lipids for gene therapy part 4 - Physico-chemical analysis of cationic liposome-DNA complexes (lipoplexes) with respect to in vitro and in vivo gene delivery efficiency

Cationic liposomes are potential vectors for gene therapy applications. In previous work, our first generation cationic liposome system, formulated fr om cytofectin 3 beta-[N-(N',N'-dimethylaminoethyl)carbamoyl]cholesterol (DC -Chol) and the neutral phospholipid dioleoyl-L-alpha -phosphatidylethanolam ine (DOPE), was shown to transfect the lungs of mice in vivo. More recently , we described second generation cationic liposome systems including one fo rmulated from DOPE and the novel pentaamine cytofectin N-15-cholesteryloxyc arbonyl-3,7,12-triazapentadecane-1,15-diamine (CTAP). As a result of formul ation changes, CTAP-DOPE cationic liposomes are shown here to be approximat ely 400-fold more efficient at mediating gene delivery to mouse lung in viv o than DC-Chol-DOPE liposomes (2000-fold more effective than plasmid DNA al one). Physico-chemical analyses were performed on CTAP-DOPE, other second g eneration cationic liposome systems and DC-Chol-DOPE to determine how diffe rences in the structural and physical properties of cytofectins, cationic l iposomes and lipoplex mixtures might affect the efficiency of transfection both in vitro and in vivo. The data suggest that CTAP-DOPE cationic liposom es are effective in vivo for two reasons. (1)They are able to efficiently n eutralise, condense and encapsulate nucleic acids into lipoplex particles; (2) they present unprotonated amine functional groups (pK(a) < 8) at neutra l pH that could have the capacity for endosome buffering, thereby facilitat ing nucleic acid escape from endosome compartments into the cytosol followi ng cell entry, like polyethylenimine. Weak, inefficient neutralisation, con densation and encapsulation of nucleic acids and the presence of unprotonat ed amine functional groups appear to be desirable liposome characteristics for in vitro transfection. The inclusion of "natural" propylene and butylen e spacings between the amine functional groups of cytofectin head groups ap pears to promote efficient neutralisation, condensation and encapsulation. The inclusion of some "unnatural" ethylene spacings appears to be a useful way of lowering amine pK(a) values.