Synthesis of α-Hydroxy-1,2,3-Triazole-linked Sialyltransferase Inhibitors and Evaluation of Selectivity Towards ST3GAL1, ST6GAL1 and ST8SIA2.

Tumour-derived sialoglycans, bearing the charged nonulosonic sugar sialic acid at their termini, play a critical role in tumour cell adhesion and invasion, as well as evading cell death and immune surveillance. Sialyltransferases (ST), the enzymes responsible for the biosynthesis of sialylated glycans, are highly upregulated in cancer, with tumour hypersialylation strongly correlated with tumour growth, metastasis and drug resistance. As a result, desialylation of the tumour cell surface using either targeted delivery of a pan-ST inhibitor (or sialidase) or systemic delivery of a non-toxic selective ST inhibitors are being pursued as potential new anti-metastatic strategies against multiple cancers including pancreatic, ovarian, breast, melanoma and lung cancer. Herein, we have employed molecular modelling to give insights into the selectivity observed in a series of selective ST inhibitors that incorporate a uridyl ring in place of the cytidine of the natural donor (CMP-Neu5Ac) and replace the charged phosphodiester linker of classical ST inhibitors with a neutral α-hydroxy-1,2,3-triazole linker. The inhibitory activities of the nascent compounds were determined against recombinant human ST enzymes (ST3GAL1, ST6GAL1, ST8SIA2) showing promising activity and selectivity towards specific ST sub-types. Our ST inhibitors are non-toxic and show improved synthetic accessibility and drug-likeness compared to earlier nucleoside-based ST inhibitors.

Synthesis and biological evaluation of selective phosphonate-bearing 1,2,3-triazole-linked sialyltransferase inhibitors.

The critical role of sialyltransferase (ST) enzymes in tumour cell growth and metastasis, as well as links to multi-drug and radiation resistance, has seen STs emerge as a target for potential antimetastatic cancer treatments. One promising class of ST inhibitors that improve upon the pharmacokinetic issues of previous inhibitors is the 1,2,3-triazole-linked transition-state analogues. Herein, we present the design and synthesis of a new generation of 1,2,3-triazole-linked sialyltransferase inhibitors, along with their biological evaluation demonstrating increased potency for phosphonate bearing compounds. The six most promising inhibitors presented in this work exhibited a greater number of binding modes for hST6Gal I over hST3Gal I, with K i ranging from 3-55 µM. This work highlights phosphonate bearing triazole-linked compounds as a promising class of synthetically accessible ST inhibitors that warrant further investigation.

Design, synthesis and evaluation of carbamate-linked uridyl-based inhibitors of human ST6Gal I.

Sialic acid at the terminus of cell surface glycoconjugates is a critical element in cell-cell recognition, receptor binding and immune responses. Sialyltransferases (ST), the enzymes responsible for the biosynthesis of sialylated glycans are highly upregulated in cancer and the resulting hypersialylation of the tumour cell surface correlates strongly with tumour growth, metastasis and drug resistance. Inhibitors of human STs, in particular human ST6Gal I, are thus expected to be valuable chemical tools for the discovery of novel anticancer drugs. Herein, we report on the computationally-guided design and development of uridine-based inhibitors that replace the charged phosphodiester linker of known ST inhibitors with a neutral carbamate to improve pharmacokinetic properties and synthetic accessibility. A series of 24 carbamate-linked uridyl-based compounds were synthesised by coupling aryl and hetaryl α-hydroxyphosphonates with a 5'-amino-5'-deoxyuridine fragment. The inhibitory activities of the newly synthesised compounds against recombinant human ST6Gal I were determined using a luminescent microplate assay, and five promising inhibitors with Ki's ranging from 1 to 20 µM were identified. These results show that carbamate-linked uridyl-based compounds are a potential new class of readily accessible, non-cytotoxic ST inhibitors to be further explored.

Computer-aided design of human sialyltransferase inhibitors of hST8Sia III.

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.

Advancement of Sialyltransferase Inhibitors: Therapeutic Challenges and Opportunities.

Hypersialylation of tumor cell surface proteins along with a marked upregulation of sialyltransferase (ST) activity is a well-established hallmark of cancer. Due to the critical role of STs in tumor growth and progression, ST inhibition has emerged as a potential new antimetastatic strategy for a range of cancers including pancreatic and ovarian. Human STs are divided into subtypes based on their linkage and acceptor molecule, with each subtype controlling the synthesis of specific sialylated structures with unique biological roles. This has important implications for inhibitor development, as STs also play significant roles in immune responses, inflammation, viral infection, and neurological disorders. Thus, the current goal in order to advance to the clinic is the development of subtype selective, cell-permeable and synthetically accessible, small-molecule ST inhibitors. Herein is a comprehensive review of the latest developments in ST inhibitors from design, Nature, and high-throughput screening, addressing both the challenges and opportunities in targeting cell surface sialylation. The review features an overview of the biological evaluation methods, computational and imaging tools, inhibitor molecular diversity, and selectivity toward ST subtypes, along with the emerging role of ST inhibitors as diagnostic tools for disease imaging.

Computational characterisation of the interactions between human ST6Gal I and transition-state analogue inhibitors: insights for inhibitor design.

Human ß-galactoside α-2,6-sialyltransferase I (hST6Gal I) catalyses the synthesis of sialylated glycoconjugates involved in cell-cell interactions. Overexpression of hST6Gal I is observed in many different types of cancers, where it promotes metastasis through altered cell surface sialylation. A wide range of sialyltransferase (ST) inhibitors have been developed based on the natural donor, cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-Neu5Ac). Of these, analogues that are structurally similar to the transition state exhibit the highest inhibitory activity. In order to design inhibitors that are readily accessible synthetically and with favourable pharmacokinetic properties, an investigation of the replacement of the charged phosphodiester-linker, present in many ST inhibitors, with a potential neutral isostere such as a carbamate or a 1,2,3-triazole has been undertaken. To investigate this, molecular docking and molecular dynamics simulations were performed. These simulations provided an insight into the binding mode of previously reported phosphodiester-linked ST inhibitors and demonstrated that targeting the proposed sialyl acceptor site is a viable option for producing selective inhibitors. The potential for a carbamate- or triazole-linker as an isosteric replacement for the phosphodiester in transition-state analogue ST inhibitors was established using molecular docking. Molecular dynamics simulations of carbamate- and phosphodiester-linked compounds revealed that both classes exhibit consistent interactions with hST6Gal I. Overall, the results obtained from this study provide a rationale for synthetic and biological evaluation of triazole- and carbamate-linked transition-state analogue ST inhibitors as potential new antimetastatic agents.