The Polar Lipid Fraction E from Sulfolobus acidocaldarius Can Be Used as Liposomal Drug Stabilizing Agents to Reduce the Leakage of the Antivascular Drug Combretastatin A4 Disodium Phosphate from Tetraether/Diester Hybrid Archaeosomes

Liposomes have many advantages as therapeutic capsules over free drugs such as small molecule drugs and nucleic acids. Cholesterol is commonly used as a membrane stabilizing agent in liposomal drugs (e.g., mRNA-lipid nanoparticle COVID-19 vaccines). However, due to the vulnerability of cholesterol to oxidation and the etiological role of cholesterol in many disorders, it is desirable to find an alternative means to stabilize liposomal membranes for drug delivery. In this study, we demonstrated that the polar lipid fraction E (PLFE), which contains exclusively bipolar tetraether macrocyclic lipids, isolated from the thermoacidophilic archaeon S. acidocaldarius can greatly stabilize the liposomal formulation of the anti-vascular drug, combretastatin A4 disodium phosphate (CA4P). Stability was assessed by determining the leakage rate constant k of entrapped CA4P fluorometrically. We found that, at 37 °C, PLFE decreases the k value monotonically from 1.54 × 10−2 s−1 for 100% 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) liposomes to 3.4 × 10−5 s−1 for 100% PLFE archaeosomes, a change of k by two orders of magnitude. The changes in k of CA4P leakage are correlated well with the changes in liposomal CA4P’s cytotoxicity against MCF-7 breast cancer cells. We further showed that the reduction in spontaneous leakage of entrapped CA4P by PLFE can be attributed to the increased membrane surface charge and the increased membrane order and packing tightness in liposomes, as reflected by the zeta potential (−6.83 to −41.1 mV from 0 to 100 mol% PLFE) and diphenylhexatriene (DPH) fluorescence polarization (0.13 to 0.4 from 0 to 100 mol% PLFE) measurements. Moreover, we showed that PLFE slows down CA4P leakage more than cholesterol in POPC liposomes. These results together suggest that PLFE lipids can serve as an effective stabilizing agent for liposomal drugs and could potentially be useful for the optimization of liposomal CA4P for cancer treatment.