Prolonged release from lipid nanoemulsions by modification of drug lipophilicity

In addition to the solubilization of poorly water-soluble, highly lipophilic drugs, lipid nanoemulsions bear potential for drug targeting approaches. This requires that the drug remains within the emulsion droplets until they reach the site of action. Since drug release is rather controlled by the lipophilicity of the drug than by the formulation, this study systematically investigated the influence of drug lipophilicity on the course of drug transfer in (physiological) acceptor media. An increase in drug lipophilicity, according to ClogD/P values, was achieved by the formation of lipophilic prodrugs of 5-phenylanthranilic acid – a potential pathoblocker. The range of substances was supplemented by orlistat, lumefantrine and cholesteryl acetate as model drugs. Drug transfer from supercooled trimyristin nanodroplets was determined via differential scanning calorimetry by monitoring their onset crystallization temperature, which decreases linearly with increasing drug content. Release of the model (pro)drugs ranged from burst to hardly any release in the order of the ClogD/P values. Except for cholesteryl acetate, the results were in line with the lipophilicity of the model (pro)drugs estimated by their retention times on a reversed-phase HPLC column under isocratic conditions. An approximate prediction of drug release kinetics was, thus, possible by logP calculations and, to a limited extent, also by reversed-phase HPLC. A further finding was the increased drug loading capacity of the lipid nanoemulsion for lipophilic prodrugs, if the structural changes of the parent compound were accompanied by a lower melting point.