The present invention provides a preparation containing a compound of the formula (I) ##STR00001## wherein R.sup.1 is an alkyl group having 1 to 4 carbon atoms or an optionally substituted aryl group having 6 to 10 carbon atoms; R.sup.2 and R.sup.3 are the same or different and each is hydrogen or an alkyl group having 1 to 4 carbon atoms, or R.sup.2 and R.sup.3 in combination form a ring having 3 to 7 carbon atoms; and R.sup.4 is a lower alkyl group optionally substituted by aryl, cycloalkyl or aromatic heterocyclic residue, or a pharmaceutically acceptable salt thereof together with a lipid. The preparation shows improved stability and improved absorption and penetration into tissue.