The lipophilic co-enhancer may also be a fatty alcohol, in which case X is ???CH2O??? and R is H, a fatty ether, in which case X is ???CH2O??? and R is other than H, or an ester of a lower carboxylic acid with a fatty ether, in which case X is ???CH2O???(CO)??? and R is other than H. Fatty alcohols include, for example, stearyl alcohol and oleyl alcohol, while fatty ethers include compounds wherei