‣ laboratory at Harvard University . This chemistry uses two appropriately positioned alkenyl side chain amino acids , which , upon treatment with Grubb ’ s catalyst ( a ruthenium-based reagent ), results in an intra-side chain cyclisation containing an alkene staple ( Figure 1 ). These staples can be in both cis and trans configurations and can also use stereoisomer amino acid derivatives , resulting in R and S configurations at this staple position . ⁹ ^, ¹⁰ The placement of the staple has been shown to stabilise α-helical configurations and they can be placed in the same i and i + 4 and i + 7 positions by varying the length of the alkenyl side chain ( Figure 1 ). Staples also can allow for intracellular transport through the cell membrane . ¹¹ In some laboratories , this alkene staple has been successfully reduced to the alkene by using a palladium catalyst . Reduction eliminates the cis and trans configurations which occur during the Grubbs catalyst formation

12 , 13 of the staple . ALRN-6924 is a first-in-class , hydrocarbon stapled , cell-permeating α-helical peptide being developed by Aileron Therapeutics . It mimics the p53 tumour-suppressor protein to disrupt p53 ’ s interactions with its endogenous inhibitors , MDMX and MDM2 . The stapling also improves stability against proteolytic degradation . This peptide is currently in Phase Ib-IIa trials for myelopreservation and in Phase II as a combination therapy with Pfiser ’ s palbociclib ( Ibrance ) for the treatment of patients with MDM2- amplified advanced solid tumours . ¹⁴

Click chemistry cyclisation

Cu + I-catalysed azide – alkyne cycloaddition ( CuAAC ), also called the click reaction or biocompatible ligation technique , is another means of introducing a constraint into a peptide . It was first described by the Sharpless group at Scripp ’ s and the Meldal group at the University of Copenhagen . ¹⁵ ^, ¹⁶ ^, ¹⁷ Introducing constraints into a peptide structure

is just one application of this versatile chemistry . The first research groups which applied CuAAC to generate α-helical structures between i , i + 4 spacing within peptides were in the labs of Chorev and D ’ Ursi , based on parathyroid hormone-related peptide . ¹⁸ For this chemistry , selective positioning of an alkyne containing component ( e . g . Propargyl-Gly ) and an azide component ( e . g . azido-Nle ) can be cyclised in the presence of Cu + 1 , resulting in a 1,2,3 substituted triazole ring ( Figure 1 ). More recently , strained cyclic alkynes , such as azodibenzocyclooctynes ( DBCO ), allow for click chemistry with azides without Cu + 1 . A click strategy has been exploited to replace disulfide bonds in peptides such as the antimicrobial peptide tachyplesin and to form bivalent cRGDfK compounds . 19 , 20

Figure 2 – Zilucoplan ( RA-101495 )

Thioether constraints

Brunel and Dawson have designed the reaction between Cys thiol and α-bromo methylene groups as a protocol for peptide stapling . ²¹ This linkage was designed to mimic the ring size of previously reported lactam staples , but a thioether link was hosted into gp41-peptide epitopes as an approach to establishing an HIV vaccine . Successful staples were created in both i , i + 3 and i , i + 4 linkages and a peptide with i , i + 3 stapling possessing a higher degree of helicity over unstapled and lactamstapled peptides i , i + 4 . Following this optimisation , the stapled peptide bound to a gp41-specific antibody more effectively than the linear peptide . ²² These findings illustrate the efficiency of thioether stapling with shorter distance i . e . i , i + 3 , while suggesting that lactam staples are more suitable for i , i + 4 stapling . Pharmaceutical peptide development with thioether uses 1,3,5 tris ( bromomethyl ) benzene as the scaffold for reaction with a peptide containing three Cys residues which spontaneously react and form bicyclic peptide thioethers ( Figure 4 ). Bicycle

46 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981

Speciality Chemicals Magazine JAN / FEB 2021 | Page 46