Jon K. Fairweather

The Development of Inhibitors of Heparanase, a Key Enzyme Involved in Tumour Metastasis, Angiogenesis and Inflammation

Heparanase is an endo-beta-glucuronidase that degrades the glycosaminoglycan heparan sulfate, a major component of the extracellular matrix and basement membranes, and has been implicated in such processes as inflammation, angiogenesis and metastasis. The identification of inhibitors of heparanase is an attractive approach towards developing new therapeutics for metastatic tumours and chronic inflammatory diseases. This review focuses on heparanase inhibitors that have been isolated or synthesised to date. More recent developments in the understanding of heparanase structure and function that may ultimately aid in the future design of inhibitors with improved potency and specificity, are also discussed.

Some Approaches to Glycosylated Versions of Methyl β-Acarviosin

In a first approach to a glycosylated version of ‘methyl β-acarviosin’, a putative inhibitor of cellulases, cellobiose was converted into a carbocyclic enone that could not be transformed into the required amine for a subsequent alkylation. Alternatively, methyl β-acarviosin itself was glycosylated at C4′, using a ‘glycosynthase’, to provide the ‘trisaccharide’ (and some ‘tetrasaccharide’). Both of these molecules were effective inhibitors of various cellulases. In a related approach to a regioisomer of the above ‘trisaccharide’, a selectively protected derivative of 1-epivalienamine was alkylated with a carbohydrate triflate to give a ‘disaccharide’ that could not be glycosylated to give the desired ‘trisaccharide’. Another unsuccessful approach to this molecule is also reported.

Synthesis of Disaccharides Containing 6-Deoxy-a-L-talose as Potential Heparan Sulfate Mimetics

A 6-deoxy-α-L-talopyranoside acceptor was readily prepared from methyl α-L-rhamnopyranoside and glycosylated with thiogalactoside donors using NIS/TfOH as the promoter to give good yields of the desired α-linked disaccharide (69–90%). Glycosylation with a 2-azido-2-deoxy-D-glucosyl trichloroacetimidate donor was not completely stereoselective (α:β = 6:1), but the desired α-linked disaccharide could be isolated in good overall yield (60%) following conversion into its corresponding tribenzoate derivative. The disaccharides were designed to mimic the heparan sulfate (HS) disaccharide GlcN(2S,6S)-IdoA(2S). However, the intermediates readily derived from these disaccharides were not stable to the sulfonation/deacylation conditions required for their conversion into the target HS mimetics.

Synthesis of a heparan sulfate mimetic disaccharide with a conformationally locked residue from a common intermediate.

A simple mimetic of a heparan sulfate disaccharide sequence that binds to the growth factors FGF-1 and FGF-2 was synthesized by coupling a 2-azido-2-deoxy-D-glucopyranosyl trichloroacetimidate donor with a 1,6-anhydro-2-azido-2-deoxy-beta-D-glucopyranose acceptor. Both the donor and acceptor were obtained from a common intermediate readily obtained from D-glucal. Molecular docking calculations showed that the predicted locations of the disaccharide sulfo groups in the binding site of FGF-1 and FGF-2 are similar to the positions observed for co-crystallized heparin-derived oligosaccharides obtained from published crystal structures.

An improved synthetic route to the potent angiogenesis inhibitor benzyl manα(1→3)-Manα(1→3)-Manα(1→3) -Manα(1→2)-Man Hexadecasulfate

An improved synthetic route to α(1→3)/α(1→2)-linked mannooligosaccharides has been developed and applied to a more efficient preparation of the potent anti-angiogenic sulfated pentasaccharide, benzyl Manα(1→3)-Manα(1→3)-Manα(1→3)-Manα(1→2)-Man hexadecasulfate, using only two monosaccharide building blocks. Of particular note are improvements in the preparation of both building blocks and a simpler, final deprotection strategy. The route also provides common intermediates for the introduction of aglycones other than benzyl, either at the building block stage or after oligosaccharide assembly. The anti-angiogenic activity of the synthesized target compound was confirmed via the rat aortic assay.