The use of a new radioactive-iodine labeled lipid marker to follow in vivo disposition of liposomes: comparison with an encapsulated aqueous space marker.

The in vivo disposition of multilamellar liposomes extruded at 0.6 micrometers (PC/DPPA/CH/ alpha-T = 4:1:5:0.1 molar ratio) when injected i.v. into mice has been examined utilizing a novel iodinatable phospholipid derivative as a lipid phase marker (p-hydroxybenzamidine phosphatidylethanolamine: 125I-BPE) and compared to that using 14C-inulin as an aqueous phase marker. At times up to 5 h post-dose the disposition of both markers was essentially identical with the exception of blood and intestine, where 125I-BPE levels were consistently higher than 14C-inulin levels. At time intervals from 5-72 h post-dose 125I-BPE levels in all the organs examined were lower than those of 14C-inulin. These differences in the behaviour of the two labels may be explained in terms of exchange of the iodinated lipids, excretion of released inulin and long term metabolism of the lipid marker. We conclude tha 125I-BPE is a useful marker for following liposome disposition in short-term studies particularly in view of the easily quantifiable nature of gamma-radioactivity which obviates the need for sample preparation.