Molecular hydrophilicity dictates the membrane-disruptive efficacy of bile acid-modified lipids in secretory phospholipase A ₂ -responsive liposomes

The enzymatic degradation of bile acid-modified phospholipids by secretory phospholipase A₂ (sPLA₂) can induce membrane disruption in liposomes. However, the influence of bile acid hydrophilicity on this disruptive effect remains unclear. In this study, we synthesized four lipids by conjugating lithocholic acid (LCA: 3α-OH), chenodeoxycholic acid (CDCA: 3α-OH, 7α-OH) or its derivatives (7k-CDCA: 3α-OH, 7 = O; bk-CDCA: 3 = O, 7 = O) to a phosphocholine backbone. We found that sPLA₂-mediated release of 6-carboxyfluorescein (6-CF) from liposomes was significantly faster with LCA-PC (100% release) than with the more hydrophilic CDCA-, 7k-CDCA-, or bk-CDCA-PC (14-30% release) in 2 hours. Assays of enzymatic degradation rates indicated that the low efficacy of CDCA-PC and 7k-CDCA-PC correlated with their slow hydrolysis by sPLA₂. Although bk-CDCA-PC was degraded at a rate (13.01%) similar to LCA-PC (15.07%) in 2 hours, dialysis experiments revealed that its metabolite (bk-CDCA) readily diffused into the aqueous phase, unlike LCA, which remained anchored to the membrane. This demonstrates that the firm interfacial settlement of the bile acid metabolite is crucial for the membrane-disruptive effect. Our findings establish that increased hydrophilicity of the bile acid moiety attenuates the sPLA₂-induced pore-forming effect, providing a critical guideline for the future design of sPLA₂-responsive liposomal systems.