Probing Disaccharide Binding to Triplatin as Models for Tumor Cell Heparan Sulfate (GAG) Interactions.

In this study, we have used [ ¹ H, ¹⁵ N] NMR spectroscopy to investigate the interactions of the trinuclear platinum anticancer drug triplatin ( 1 ) (1,0,1 /t,t,t or BBR3464) with site-specific sulfated and carboxylated disaccharides. Specifically, the disaccharides GlcNS(6 S )-GlcA ( I ) and GlcNS(6 S )-IdoA(2S) ( II ) are useful models of longer-chain glycosaminoglycans (GAGs) such as heparan sulfate (HS). For both the reactions of ¹⁵ N -1 with I and II , equilibrium conditions were achieved more slowly (65 h) compared to the reaction with the monosaccharide GlcNS(6S) (9 h). The data suggest both carboxylate and sulfate binding of disaccharide I to the Pt with the sulfato species accounting for <1% of the total species at equilibrium. The rate constant for sulfate displacement of the aqua ligand ( k _L2 ) is 4 times higher than the analogous rate constant for carboxylate displacement ( k _L1 ). There are marked differences in the equilibrium concentrations of the chlorido, aqua, and carboxy-bound species for reactions with the two disaccharides, notably a significantly higher concentration of carboxylate-bound species for II , where sulfate-bound species were barely detectable. The trend mirrors that reported for the corresponding dinuclear platinum complex 1,1/ t,t , where the rate constant for sulfate displacement of the aqua ligand was 3 times higher than that for acetate. Also similar to what we observed for the reactions of 1,1/ t,t with the simple anions, aquation of the sulfato group is rapid, and the rate constant k _-L2 is 3 orders of magnitude higher than that for displacement of the carboxylate ( k _-L1 ). Molecular dynamics calculations suggest that extra hydrogen-bonding interactions with the more sulfated disaccharide II may prevent or diminish sulfate binding of the triplatin moiety. The overall results suggest that Pt- O donor interactions should be considered in any full description of platinum complex cellular chemistry.