Nucleic acid drugs for gene silencing remain under active development. An essential requirement for their successful employment in the clinic will be the ability to deliver these molecules, intact, to tissues, and the cells that comprise them, in sufficient concentrations for them to work. Many delivery systems have been investigated for this purpose but none has proved completely satisfactory. To this end, we are evaluating a novel polyethyleneimine (PEI)-polyethyleneglycol (PEG) conjugate with a biocleavable linker between the PEI and PEG. This material, called “Polybus (R),” was designed to incorporate into one polymer the highly desirable properties of PEI and PEG, i.e. the ability to condense DNA for delivery and to prolong the half-life of the incorporated material in circulation of the recipient. To test the performance of “Polybus (R)” in a living animal, we injected mice intraperitonealy with 200 μg of a phosphorothioate oligodeoxynucleotide (PS-ODN) that was first dissolved in 100 μl of PBS, and then mixed with 100 μl of “PolyBus (R)” compound. Final injection volume was therefore 200 μl/mice. At varying time points after injection, mice were sacrificed and their plasma assayed for the PS-ODN by slot blotting with a radiolabeled probe complementary to the ODN. The amount of ODN in the plasma was semi-quantitated by reference to known amounts of material also blotted to the membrane. On days 1, 3, and 7 post injection we were able to detect microgram amounts of DNA in the serum of the test animals (∼1-2 μg/250μl plasma). Of considerable interest, we were also able to detect material out to 21 days (∼500ng/250 μl plasma) in some animals. These results suggest that “Polybus (R)” prolongs the circulating half-life, and protects from degradation, nucleic acid compounds administered to live animals. Concentrations of material in various organs are presently being measured, as are the effectiveness of antisense PS-ODN delivered to tumor bearing animals.

[Proc Amer Assoc Cancer Res, Volume 45, 2004]