Protein Binding of Macromolecular Anticancer Agent SMANCS: Characterization of Poly(styrene-co-maleic acid) Derivatives as an Albumin Binding Ligand

Abstract

We have studied the interaction of macromolecular anticancer agent SMANCS, a conjugate of partially half-butyl-esterified styrene-co-maleic acid polymer[butyl-SMA]- and neocarzinostatin (NCS), with various serum proteins by the fluorescence polarization method. Comparatively strong binding of FITC-labeled SMANCS (F-SMANCS) to human serum albumin (HSA) and weak binding to fibrinogen were observed, while other serum proteins did not exhibit any appreciable binding profile. From Scatchard polt analysis, the asso ciation constant of binding for F-SMANCS to HSA at 37 °C, pH 7.4 was calculated to be 2.19 × 10⁶ M^-1 and the number of moles of F-SMANCS bound to 1 mol of HSA was 3.2 Binding of F-NCS to HSA was not observed. The F-SMANCS bound to HSA was effectively displaced by butyl-SMA, but not by NCS. This evidence supports that SMANCS binds to HSA through butyl-SMA, not through NCS portion. A role of the alkyl ester groups of SMA derivatives to HSA binding was investigated by competitive inhibition using butyl-SMA, ethyl-SMA, H-SMA and short chain butyl-SMA. The degree of competitive in hibition was stronger in the following order: butyl-(n = 6) > ethyl (n = 6) = short butyl-(n = 4) > H-SMA. The number of carbons introduced by esterification and the hydrophobicity of SMA derivatives both positively in fluence the binding affinity to HSA. The binding site for SMANCS on HSA was investigated. From the competitive inhibition of known standard drugs (war farin, diazepam and digitoxin) and endogenous substance (bilirubin) in the albumin binding. The binding site on HSA appears to be in the close vicinity to the warfarin or diazepam binding site, and might partially overlap with the bilirubin binding site. These data support that the prolonged plasma half-life of SMANCS in vivo reported previously can be attributed to this albumin binding.