BIODISTRIBUTION OF LIPOSOMES AFTER EXTRADURAL ADMINISTRATION IN RODENTS

We have mapped over 24 h the biodistribution of Tc-99m-labelled multil amellar and small unilamellar liposomes in rabbits and rats by scintig raphic imaging after extradural injection. Multilamellar vesicles form ed a depot at the site of injection; small unilamellar vesicles spread immediately along the extradural space and entered the systemic compa rtment 30 min after injection. Well-delineated liver and kidney labell ings were seen after 24 h. The use of H-3-cholesterol-labelled small u nilamellar vesicles suggested hepatic capture of intact liposomes with sizes averaging 0.05 mu m drained unmodified into the systemic circul ation through the extradural lymphatics. These results have led to the selection of multilamellar vesicles (0.1-15 mu m size range) for clin ical trials using liposome-associated local anaesthetics.