A new class of lipids, containing the closo-dodecaborate cluster, has been synthesized. Two lipids, S-(N, N-(2-dimyristoyloxyethyl)acetamido)thioundecahydro-closo-dodecaborate (2-) (B-6-14) and S-(N, N-(2-dipalmitoyloxyethyl)acetamido)thioundecahydro-closo-dodecaborate (2-) (B-6-16) are described. Both of them have a double-tailed lipophilic part and a headgroup carrying two negative charges. Differential scanning calorimetry shows that B-6-14 and B-6-16 bilayers have main phase transition temperatures of 18.8 and 37.9 degrees C, respectively. Above the transition temperature of 18.8 degrees C, B-6-14 can form liposomal vesicles, representing the first boron-containing lipid with this capability. Upon cooling below the transition temperature, stiff bilayers are formed. When incorporated into liposomal formulations with equimolar amounts of distearoyl phosphatidylcholine (DSPC) and cholesterol, stable liposomes are obtained. The zeta-potential measurements indicate that both B-6-14- and B-6-16-containing vesicles are negatively charged, with the most negative potential described of any liposome so far. The liposomes are of high potential value as transporters of boron to tumor cells in treatments based on boron neutron capture therapy (BNCT). Liposomes prepared from B-6-14 were slightly less toxic in V79 Chinese hamster cells (IC50 5.6 mM) than unformulated Na2B12H11SH (IC50 3.9 mM), while liposomes prepared from B-6-16 were not toxic even at 30 mM.

Representatives of N,N,N-trialkylammoniododecaborates, which are anions in a new class of ionic liquids, were tested for their hazard potential. As biological test systems, toxicity against V79 mammalian cells, reproduction inhibition of Scenedesmus vacuolatus algae, and inhibition of acetylcholinesterase were studied. EC 50 values for the toxicity against V79 cells range between 9.1 mM for the trimethylammonio derivative and 0.19 mM for the trihexyl derivative. Reproduction inhibition of S. vacuolatus range between over 3 mM for the trimethylammonio derivative down to 0.016 mM for the trihexyl derivative. Fifty percent inhibition of acetylcholinesterase was caused by 21.9 mM of the trimethylammonio derivative, as compared to 0.03 mM for the hexyl derivative The data demonstrate that increasing hydrophobicity leads to higher toxicity and inhibition for straight alkyl chains. All tested ABs are able to induce leakage in liposomes and the capability for triggering it strongly increases with the length of the alkyl chain and consequently with lipophilicity. The leakage experiments could be an indicator for toxic tendencies in vitro but they allow no quantitative prediction of EC 50 values. For branched chains and for derivatives with mixed substitution, a prediction of the toxic potential is not simple. The new class of ionic liquids is in general no more toxic than the ionic liquids presently in industrial applications.

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