Dipalmitoylphosphatidylcholine Liposomes with Soybean-Derived Sterols and Cholesterol as a Carrier for the Oral Administration of Insulin in Rats

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Dipalmitoylphosphatidylcholine (DPPC) liposomes with a soybean-derived sterol mixture (SS) or cholesterol (Ch) were examined as a carrier for the oral administration of insulin in rats. Four kinds of liposomes were prepared : liposomes containing SS or Ch (molar ratio of DPPC/X=7 : 2 or 7 : 4, X=SS or Ch), respectively. The pharmacological availability was greater and the lag time for the glucose reduction was later in the order of DPPC/SS (7 : 4)>DPPC/SS (7 : 2)>DPPC/Ch (7 : 4)»DPPC/Ch (7 : 2)-liposomes. This order appears to correspond well with that of the rigidity of the liposomal membrane. In particular, DPPC/SS (7 : 4)-liposomes reduced blood glucose levels for up to 21 h in rats after oral administration. The highest absolute pharmacological availability was 31.6% at a dose of 20.0 IU/kg of DPPC/SS (7 : 4)-liposomes among the liposomes in comparison to intravenous administration.