Glycans as Ligands in Bioinorganic Chemistry. Probing the Interaction of a Trinuclear Platinum Anticancer Complex with Defined Monosaccharide Fragments of Heparan Sulfate

Abstract

We report herein a detailed NMR study of the aquation and subsequent covalent binding of the trinuclear clinical agent [{trans-PtCl( 1 5 NH 3 ) 2 } 2 {μ-trans-Pt-( 15 NH 3 ) 2 ( 15 NH 2 (CH 2 ) 6 15 NH 2 ) 2 }] 4+ (1, 1,0,1/t,t,t or Triplatin) with three D-glucosamine residues containing varied O-sulfate and N-sulfate or N-acetyl substitutions, which represent monosaccharide fragments present within the repeating disaccharide sequences of cell surface heparan sulfate (HS). The monosaccharides GlcNS(6S), GlcNS, and GlcNAc(6S) were synthesized in good yield from a common 4,6-diol αmethyl glucopyranoside intermediate. The reactions of 15 N-1 with sodium sulfate, GlcNS(6S), GlcNS, and GlcNAc(6S) were followed by 2D [ 1 H, 15 N] heteronuclear single quantum coherence (HSQC) NMR spectroscopy using conditions (298 K, pH ≈5.4) similar to those previously used for other anionic systems, allowing for a direct comparison. The equilibrium constants (pK 1 ) for the aquation of 1 in the presence of GlcNS(6S) and GlcNS were slightly higher compared to that of the aquation in a sulfate solution, while a comparable pK 1 value was observed in the presence of GlcNAc(6S). A comparison of the rate constants for sulfate displacement of the aqua ligand showed preferential binding to 2-N-sulfate compared to 6-O-sulfate but a more rapid liberation. For disulfated GlcNS(6S), equilibrium conditions were achieved rapidly (9 h) and strongly favored the dichloro form, with <2% sulfato species observed. The value of k L1 was up to 15-fold lower than that for binding to sulfate, whereas the rate constant for the reverse ligation (k −L1 ) was comparable. Equilibrium conditions were achieved much more slowly (∼ 100 h) for the reactions of 1 with GlcNS and GlcNAc(6S), attributed to covalent binding also to the N-donor of the sulfamate (GlcNS) group and the O-donor of the Nacetyl [GlcNAc(6S)] group. The rate constants (k L2 ) were 20−40-fold lower than that for binding to the 2-N-or 6-O-sulfate, but the binding was less reversible, so that their equilibrium concentrations (5−8%) were comparable to the 2-N-or 6-Osulfate-bound species. The results emphasize the relevance of glycans in bioinorganic chemistry and underpin a fundamental molecular description of the HS−Pt interactions that alter the profile of platinum agents from cytotoxic to metastatic in a systematic manner.

Conformational Modulation of Iduronic Acid‐Containing Sulfated Glycosaminoglycans by a Polynuclear Platinum Compound and Implications for Development of Antimetastatic Platinum Drugs

“…[44] Ap olyamine-linked Pt IV compound displays interesting anti-metastatic properties upon proposed reduction to Pt II and polyamine release. [45] Indeed, in the absence of aPt-Cl bond for covalent binding to target biomolecules, [46] the substitution-inert PPCs do have some similarities with polyamines.S permine itself binds to and modifies the structure of HS with effects on polyamine pathways. [47] In both solution and gas-phase DNAb inding studies,c omplexes such as Tr iPtNC have significantly higher affinity than the simple polyamines and are significantly more effective DNA-condensing agents.…”

“…A polyamine‐linked Pt IV compound displays interesting anti‐metastatic properties upon proposed reduction to Pt II and polyamine release [45] . Indeed, in the absence of a Pt‐Cl bond for covalent binding to target biomolecules, [46] the substitution‐inert PPCs do have some similarities with polyamines. Spermine itself binds to and modifies the structure of HS with effects on polyamine pathways [47] .…”

Biological relevance of interaction of platinum drugs with O-donor ligands

“…Secondly, the comparison of phosphate and sulfate [35] shows that the rate constant for sulfate displacement of the aqua ligand (kL) is approximately 3-fold higher than that of phosphate, whilst aquation of the bound sulfato ligand (k-L) is more than an order of magnitude faster in comparison to displacement of phosphate. The reaction of the trinuclear 1,0.1/t,t,t with sulfate similarly reaches equilibrium after 12 h and has a similar distribution at equilibrium of chloro, aqua and sulfato species [36]. However, inspection of the rate constants (Table 2) shows interesting differences attributed to a balance of ion-pairing (Cl -) and/or formation of "sulfate clamps" with the charged central {PtN4} 2+ unit.…”

“…To understand the kinetics of covalent binding with the monosaccharide fragments that represent those present within the repeating disaccharide units of HS, we have used 2D [ 1 H, 15 N] HSQC NMR spectroscopy to follow the interaction of 15 N-labelled 1,0,1/t,t,t with three D-glucosamine residues (GlcNS(6S), GlcNAc(6S) and GlcNS; see Fig. 7c) containing varied O-sulfate and N-sulfate or N-acetate substitutions at 6-O and/or 2-N positions [36]. The conditions were similar to those previously used for the reactions with anionic systems (see section 4).…”

Influence of geometric isomerism on the binding of platinum anticancer agents with phospholipids

“…For example, we have investigated the interaction of the trinuclear (Triplatin or 1,0,1/t,t,t) and dinuclear (1,1/t,t) PPCs with three D-glucosamine residues containing varied O-sulfate and N-sulfate or N-acetyl substitutions, which represent monosaccharide fragments present within the repeating disaccharide sequences of cell surface heparan sulfate. 9 We have studied also the interaction of these PPCs with 1,2-dihexanoyl-sn-glycero-3-phosphate (DHPA), a shorter, water-soluble model for the phospholipid DPPA (Chart 1). 10 These latter studies showed that the monofunctional DHPA adduct of 1,1/t,t coordinated to a second DHPA molecule in two different orientations, leading to formation of two conformers of the bifunctional adduct.…”