Kinetics of distribution and retention of 3H-oestradiol-17 beta in rat tissues: a comparative study with free oestradiol and after its incorporation into liposomes.

With a view to impart selective uptake of estrogen by the target tissues of rat, liposomes in which (6,7-3H) oestradiol-17 beta constituted a part of lipid bilayer were used as carriers of the hormone. The distribution and retention of the radioactivity was determined in blood plasma, uterus, liver, kidney, spleen and leg muscle of ovariectomized rat at different time intervals up to 72 hr following a single intravenous injection of free oestradiol (1.61 muCi) or an equivalent amount of liposomal-oestradiol. When free oestradiol was administered, uterus showed peak amount of radioactivity between 30 min to 2 hr and remained high up to 6 hr. In the other tissues examined, maximum amount of radioactivity was seen at 15 min followed by a marked fall at 30 min and also at other subsequent intervals. The pattern of uptake and retention of radioactivity after administration of liposomal-oestradiol was not much different from that of free oestradiol between 1 and 6 hr. A moderate increase in the amount of radioactivity in the nongenital tissues at 24 hr was the only difference noticed with liposomal-oestradiol. It is concluded that targeting of oestradiol preferentially to the uterus could not be achieved through liposomal delivery system.