Intravenous (i.v.) administration of cationic lipid N-[(1-(2-3-dioleyloxy)p
ropyl)]-N-N-N-trimethylammonium chloride (DOTMA)-based transfection complex
es in mice with subcutaneous squamous cell tumors yielded plasmid delivery
and expression in tumor lesions. The efficiency of gene transfer in tumors
was significantly lower than in the lung. This was consistent with low plas
mid levels associated with the tumor, suggesting that plasmid delivery to t
he tumor site was a limiting factor. Lowering the lipid/DNA charge ratio fr
om 5:1 to 0.8:1 (+/-) did not change DNA levels in tumor but significantly
reduced DNA levels in lung. However, expression levels were significantly r
educed in both tissues at lower lipid/DNA charge ratios. Complexes prepared
from small unilamellar liposomes gave significantly lower expression level
s in the lungs but similar expression levels in tumors when compared to com
plexes prepared from larger unilamellar liposomes, The small liposome compl
exes were better tolerated than large liposome complexes. Varying the catio
nic lipid to colipid (cholesterol or DOPE) molar ratio from 4:1 to 1:1 sign
ificantly reduced expression levels in both tumor and lung. Cationic lipid
substitution, using a cholesterol cationic lipid, diethyldiamino-carbamyl-c
holesterol instead of DOTMA, produced reduced expression in all other tissu
es except tumor. Incorporation of PEG into preformed transfection complexes
reduced DNA delivery to lung, increased circulation half-life. and enhance
d DNA delivery to tumor. In a lung metastatic mouse tumor model, where the
accessibility of the i.v. administered transfection complexes to tumor lesi
ons should be less challenging, DOTMA:CHOL complexes (4:1 lipid to colipid
molar ratio, 3:1 +/- lipid to plasmid charge ratio) were preferentially loc
alized in tumor lesions. These data demonstrate that systemic gene transfer
to distal tumor sites by lipid DNA complexes may be limited by low plasmid
delivery. Modifying the chemical surface properties of transfection comple
xes enhanced both DNA delivery and expression in tumor and is one approach
that may overcome limitations.