Glycosaminoglycans (GAGs) are negatively charged linear polysaccharides located on the cell surface and play an important role in cell signaling. Heparan sulfate (HS), a highly sulfated GAG, is essential for adhesion and migration processes in the extracellular environment [4,5]. Enzymatic cleavage of HS by mammalian heparanase (HPSE) is often considered the first step in the angiogenic cascade and is correlated with metastatic potential [9]. HS is overexpressed on the surface of many cancer cells and has also been implicated as a site for viral attachment and entry, including that of SARS-Cov-2 [[6], [7], [8]]. For these reasons, HS has emerged as a target for drug development.

Charged coordination complexes (CCCs) offer a distinct opportunity for HS targeting through metalloshielding, the result of covalent or noncovalent interactions of positively charged metal complexes with the negatively charged HS, with analogies to DNA binding [3,10]. Polynuclear platinum complexes (PPCs) bind DNA via a phosphate clamp and bind HS in an analogous sulfate cluster motif; their dual nature enables PPCs to inhibit both DNA and HS functions [3,10,11,12]. The biological consequences of metalloshielding are varied and have significant therapeutic potential. The metalloshielding of HS by PPCs prevents its cleavage by HPSE and the inhibition correlates with the affinity of the charged platinum complex for the anionic oligosaccharide substrate [11]. This approach – of template modification or blocking - differs from HS mimetics, which act broadly as competitive inhibitors toward the enzyme [9,13]. This approach is also valid in considerations of development of antiviral compounds and PPCs have demonstrated broad-spectrum antiviral activity by inhibiting HS-dependent viral attachment and entry [14,15].

The concept of metalloshielding by substitution-inert cationic complexes with HS should extend to examples besides those of platinum. In respect of this we have shown that the 6+ Co-containing Werner's Complex (WC) (I) affects HS-related biological function including wound healing and viral attachment [1]. The affinity of WC to the structurally homogeneous pentasaccharide Fondaparinux (FPX) is similar to that of the PPCs, with consequent inhibition of FPX cleavage by both mammalian heparanase and bacterial heparinase [1,11]. The GAG-related biological activities of I further include inhibition of cellular invasion and migration of TNBC MDA-MB-231 cells through a Matrigel membrane as measured by the “wound healing” assay. WC binds DNA and tRNA and condenses DNA as effectively as TriplatinNC [[16], [17], [18]]. Cellular accumulation of WC is also mediated through HSPGs and antiproliferative effects of WC correlated with cellular accumulation [1]. These results suggest a more general biological role for HSPGs in internalization of charged coordination compounds and metal cations [1,20]. In another study, ruthenium and cobalt complexes with the non‑hydrogen donating 2,2′-bipyridine (bpy) ligand found [Co(bpy)3]3+ binds to FPX to a greater extent than [Ru(bpy)3]2+, despite the similarity in complex structures [19]. However [Ru(bpy)3]2+ inhibited enzymatic cleavage of HS to a larger extent than [Co(bpy)3]3+, likely though direct interaction with the enzyme rather than metalloshielding.

To understand the molecular details of this biological action of Co complexes we have begun a structure-activity study of small molecule Co-ammine complexes and their interactions with HS or HS mimetics. Evidence of cobalt cation binding with sulfated GAGs appear in the literature as early as the 1950s [21]. Cobalt-ammine complexes have been shown to interact with and precipitate chondroitin sulfate and heparin out of aqueous solutions [22,23]. Enantiomeric resolution of the cobalt-based Werner's Complex (WC) is facilitated in the presence of amino sugars [24]. Co coordination complexes have been described for use in imaging, diagnostics and therapy [25,26]. However, the biological implications of Co-GAG interactions have remained largely unstudied. This work focuses on a selection of common Co-ammine complexes found in many inorganic laboratories, in order to emphasize the previously underappreciated ability of familiar Co CCCs to target HS.