Improving solubility and chemical stability of natural compounds for medicinal use by incorporation into liposomes

Natural bioactive compounds have been studied for a long time for their chemopreventive and therapeutic potential in several chronic inflammatory diseases, including cancer. However, their physicochemical properties generally result in poor chemical stability and lack of in vivo bioavailability. Very few human clinical trials have addressed absorption, distribution, metabolism, and excretion of these compounds in relation to efficacy. This limits the use of these valuable natural compounds in the clinic. In this study, we examined caffeic acid (derivatives), carvacrol (derivatives), thymol, pterostilbene (derivatives), and N-(3-oxo-dodecanoyl)-l-homoserine lactone. These are natural compounds with strong anti-inflammatory properties derived from plants and bacteria. However, these compounds have poor water solubility or are chemically unstable. To overcome these limitations we have prepared liposomal formulations. Our results show that lipophilic 3-oxo-C ₁₂-homoserine lactone and stilbene derivatives can be loaded into liposomal lipid bilayer with efficiencies of 50-70%. Thereby, the liposomes solubilize these compounds, allowing intravenous administration without use of solvents. When compounds could not be loaded into the lipid bilayer (carvacrol and thymol) or are rapidly extracted from the liposomes in the presence of serum albumin (3-oxo-C ₁₂-homoserine lactone and pterostilbene derivatives), derivatization of the compound into a water-soluble prodrug was shown to improve loading efficiency and encapsulation stability. The phosphate forms of carvacrol and pterostilbene were loaded into the aqueous interior of the liposomes and encapsulation was unaffected by the presence of serum albumin. Chemical instability of resveratrol was improved by liposome-encapsulation, preventing inactivating cis-trans isomerization. For caffeic acid, liposomal encapsulation did not prevent oxidation into a variety of products. Still, by derivatization into a phenyl ester, the compound could be stably encapsulated without chemical degradation. Despite the instability of liposome-association of 3-oxo-C ₁₂-homoserine lactone and resveratrol, intravenous administration of these compounds inhibited tumor growth for approximately 70% in a murine tumor model, showing that simple solubilization can have important therapeutic benefits.