RESEARCH ARTICLE Molecular Structure-Affinity Relationship of Bufadienolides and Human Serum Albumin In Vitro and Molecular Docking Analysis

The development of bufadienolides as anti-tumor agents is limited due to poor pharmacoki-netic properties regarding drug half-lives and toxicity in vivo. These serious factors might be improved by increasing the drug/albumin-binding ratio. This study therefore investigated the relationship between the structural properties of nine bufadienolides and their affinities for human serum albumin (HSA) by a fluorescence spectroscopy-based analysis and molecu-lar docking. Fluorescence quenching data showed that the interaction of each bufadienolide with HSA formed a non-fluorescent complex, while thermodynamic parameters revealed negative ΔS and ΔH values, corresponding to changes in enthalpy and entropy, respective-ly. The structural differences between the various bufadienolides markedly influenced their binding affinity for HSA. With the exception of a C = O bond at the C12 position that de-creased the binding affinity for HSA, other polar groups tended to increase the affinity, espe-cially a hydroxyl (OH) group at assorted bufadienolide sites. The rank order of binding affinities for drugs with tri-hydroxyl groups was as follows: 11-OH> 5-OH> 16-OH; in addi-tion, 16-acetoxy (OAc), 10-aldehyde and 14-epoxy constituents notably enhanced the bind