Oligonucleotide-entrapped immunoliposome delivered by mini-osmotic pump improves the survival of scid mice bearing human leukemia

A study is made of the efficacy of CD45-targeted immunoliposomes entrapping c-myb antisense phosphorothioate oligonucleotides to increase survival in a scid mouse model of human leukemia. The encapsulation efficiency of oligo nucleotides in backbone liposomes was optimized using a dehydration-rehydra tion procedure. Pharmacokinetic parameters indicate that the t(1/2) of free oligonucleotide (0.14 +/- 0.02 h) was increased 63-fold when the oligonucl eotide was encapsulated in small liposomes, whereas clearance decreased 50- fold accordingly. Multivalent liposomes for targeting were prepared by cova lently coupled streptavidin on the liposome surface. The ability of strepta vidin-liposomes to bind biotinylated antibodies was confirmed using biotin- peroxidase as tracer and size exclusion chromatography as it allows differe ntiation of the proportion of enzymatic activity in the liposome fraction v ersus the free enzyme. The in vivo efficacy of CD45-targeted liposomes was evaluated on scid mice transplanted with K562 human leukemia cells, Three w eeks after transplant, mice were treated with HEPES (N-[2-Hydroxyethyl] pip erazine-N'-[2-ethanesulfonic acid]), free antisense oligonucleotide or lipo somes encapsulating sense or antisense oligonucleotides, In order to improv e the pharmacokinetic properties of free and encapsulated oligonucleotides, administration was performed by continuous infusion employing mini-osmotic pumps, We observed that CD45-targeted liposomes entrapping antisense oligo nucleotides greatly increased survival, which was > 60% six months after tu mor transplant, However, free antisense or encapsulated sense oligonucleoti de in CD45-targeted liposome did little to increase survival with respect t o the animals treated with HEPES, These results suggest that targeted immun oliposomes can contribute to the success of antisense therapeutic strategie s in leukemia.