Upregulation of sialyltransferases—the enzymes responsible for the addition of sialic acid to growing glycoconjugate chains—and the resultant hypersialylation of up to 40–60% of tumour cell surfaces are established hallmarks of several cancers, including lung, breast, ovarian, pancreatic and prostate cancer. Hypersialylation promotes tumour metastasis by several routes, including enhancing immune evasion and tumour cell survival, and stimulating tumour invasion and migration. The critical role of enzymes that regulate sialic acid in tumour cell growth and metastasis points towards targeting sialylation as a potential new anti-metastatic cancer treatment strategy. Herein, we explore insights into the mechanisms by which hypersialylation plays a role in promoting metastasis, and explore the current state of sialyltransferase inhibitor development.

No abstract

Sialyltransferase (ST) upregulation and the resultant hypersialylation of tumour cell surfaces is an established hallmark of many cancers including lung, breast, ovarian, pancreatic and prostate cancer. The role of ST enzymes in tumour cell growth and metastasis, as well as links to multi-drug resistance, has seen ST inhibition emerge as a target for potential antimetastatic cancer treatments. The most potent of these reported inhibitors are transition-state analogues. Although there are several examples of these in the literature, many have suspected poor pharmacokinetic properties and are not readily synthetically accessible. A proposed solution to these problems is the use of a neutral carbamate or 1,2,3-triazole linker instead of the more commonly used phosphodiester linker, and replacing the traditionally utilised cytidine nucleotide with uridine. Another issue in this area is the paucity of structural information of human ST enzymes. However, in late 2015 the structure of human ST8Sia III was reported (only the second human ST described so far), creating the opportunity for structure-based design of selective ST8 inhibitors for the first time. Herein, molecular docking and molecular dynamics simulations with the newly published crystal structure of hST8Sia III were performed for the first time with selected ST transition state analogues. Simulations showed that these compounds could participate in many of the key interactions common with the natural donor and acceptor substrates, and reveals some key insights into the synthesis of potentially selective ST inhibitors.

Key modifications of previous sialyltransferase inhibitors increased their activity against hST6Gal I and has further implications for synthetically accessible ST inhibitor design.

Sialic acid occupies a privileged position at the terminus of the glycan chain of many cell-surface glycoconjugates. Owing to both their structure and location, charged sialic acid residues mediate numerous critical interactions in cell–cell communication including cell recognition, invasion, migration, receptor binding, and immunological responses. Sialyltransferases (STs) are the enzymes involved in the biosynthesis of sialylated glycans and are highly upregulated, up to 40–60 %, in a range of cancers, with tumour hypersialylation strongly correlated with both tumour progression and treatment resistance. Accordingly, inhibiting sialylation is currently being explored by several research groups worldwide as a potential new cancer treatment strategy. However, to progress small molecule ST inhibitors into the clinic, issues around selectivity, synthetic accessibility, and cell permeability need to be addressed. Using computationally guided design principles, we produced a leading series of ST inhibitors by replacing the cytidine nucleoside with uridine and substituting the charged phosphodiester linker with a carbamate or triazole moiety. Biological evaluation of the newly developed inhibitors was performed using commercially available human ST enzymes, with the Ki inhibition values of the lead compounds ranging from 1 to 20 µM. Compared with earlier generations of sialylation inhibitors, our inhibitors are non-toxic in a range of cell studies, with improved synthetic accessibility.