Identifying the Mechanism of Action of Bioactive 1,2-Cyclopropyl Carbohydrates
Cyclopropanes and carbohydrates have long been used in the field of drug development. Previous work has shown that 1,2-cyclopropyl carbohydrates display bioactivity in both HeLa cancer cell lines¹ and in yeast² with a tentatively proposed mechanism of inhibition occurring through an enzymatic cyclopropane ring opening reaction and subsequent formation of a covalent bond with a target enzyme.² A small library of 1,2-cyclopropyl carbohydrate derivatives were synthesised based on known pharmacophores to examine further the potential mechanism of inhibition of such compounds and confirm the occurrence of enzyme-catalysed cyclopropane ring-opening reactions. Initial synthetic efforts were focused on the synthesis of the 1,2-dichlorocyclopropyl carbohydrate 23, which, through the optimisation of an essential C-6 detritylation reaction, was achieved in moderate yields of 32% over 7 steps. Following this, the ethoxycarbonyl substituted 1,2-cyclopropyl carbohydrate 54 was synthesised over 7 steps in a 22% yield through a rhodium acetate-catalysed addition of ethyl diazoacetate (49) to the glucal substrate 40. It was envisioned that if enzymatic cyclopropane ring-opening was occurring to form a C-7 carbanion, this would in turn be stabilised through the potential enolate formation of 54. Use of N,N-ditosylhydrazine in the synthesis of propargyl diazoacetate (58) followed by a rhodium acetate-catalysed cyclopropanation of 58 with substrate 40 resulted in the successful synthesis of 61 over 7 steps in a total yield of 9%. The incorporation of the propargyl substituent in 61 was introduced as a molecular probe in an attempt to isolate the target protein through an affinity purification procedure. The bioactivity of the propargyl derivative 61 was consistent with the synthesised compounds 23 and 54. It was proposed that these compounds undergo an enzymatic cyclopropane ring opening reaction accompanied with a clear diastereoselective preference for the α-stereoisomer of the cyclopropane ring, consistent with a target-based activation of the compounds. Chemical genetic analysis of the resulting bioactive compounds was undertaken using a deletion mutant array of Saccharomyces cerevisiae to elucidate a potential mechanism of action. Analysis of the results showed that, of the 4800 homozygous deletion strains tested in the high-throughput screens, a total of 122 strains were found following validation to sensitise and 68 to give resistance against 23 and 54. These sensitive and resistant mutants were subjected to a validation assay. Following validation, genes whose deletion led to sensitivity or resistance were then subjected to gene ontology term enrichment analysis which showed enrichment in the cytosolic ribosome, SNARE complex and SNAP receptor activity for resistant strains and enrichment in endoplasmic reticulum and endomembrane systems was found for the sensitive strain. Genes whose deletion sensitised to both compounds showed strong enrichment in cellular protein localisation, intra-golgi vesicale-mediated transport and the endomembrane system. Target identification and isolation were attempted through an affinity purification procedure using compound 61 and an azide-modified agarose resin. However, this was without success, either through inaccessibility of the alkyne of the target probe or because the target resides in the membrane-associated fraction which was discarded prior to treatment with the probe. This study suggests that the 1,2-cyclopropyl carbohydrates synthesised function through a cyclopropane ring-opening reaction, assisted by an enzymatic nucleophile. Chemical genetic analysis showed that the target of these compounds is involved in protein transport and protein localisation most likely relating to the vesicle tethering. Although many aspects of this work still need further investigation, either through the synthesis of new 1,2-cyclopropyl carbohydrates to increase bioactivity and better understand the enzymatic target, or through further biological procedures to better understand the mechanism of action, the use of 1,2-cyclopropyl carbohydrates as a potential pharmaceuticals or probes of protein trafficking shows much promise.