Development of macrocyclic β-strand calpain cysteine protease inhibitors
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The work in this thesis reports studies directed to developing a calpain cysteine protease inhibitor that could be of value in slowing cataract development in humans. The work focuses on the development of macrocyclic compounds which can have advantages over acyclic compounds due to their resistance to proteolytic hydrolysis, improved selectivity, bioavailability and membrane permeability. A review of X-ray crystal structures of natural and synthetic calpain inhibitors complexed with the cysteine protease calpain show the inhibitors generally bind in the enzyme active site in an extended β-strand conformation. The calpain inhibitor SJA-6017 has been identified as a suitable lead compound. The importance of the para-fluoro group in SJA-6017 has been investigated. Modifications have been made to constrain this basic structure within a macrocycle and restrict the peptide chain as a β-strand conformation. Macrocycle CAT811 is a potent calpain 1 and 2 inhibitor and shows promise in slowing the progression of cortical cataract in trials with sheep having a hereditary propensity towards the development of cataract. In this thesis I report studies directed to improve the yield of the key RCM macrocyclisation step in the synthesis of aldehyde CAT811 and of three ester analogues (2.1, 2.3 and 2.4). I also report the development of a more commercial route to CAT811 not involving RCM but using intramolecular nucleophilic cyclisation. This intramolecular nucleophilic cyclisation strategy was attempted for the preparation of a histidine containing macrocyclic ester (4.1a) but was unsuccessful. An alternate strategy involving intramolecular lactamization proved successful for the synthesis of histidine-based macrocyclic esters (4.1a-4.3a). Reduction to the corresponding alcohols (4.1b-4.3b) was successful and oxidation of (4.1b and 4.3b) afforded the corresponding aldehydes (4.1c and 4.3c) for biological assay against ovine calpain 2. Aldehyde 4.3c has an IC50 of 1 μM and the corresponding alcohol 4.3b shows no activity (IC50 > 50 μM) consistent with the modelling which indicated that these two compounds did not adopt a β-strand conformation in the docking studies. Aldehyde 4.1c, on the other hand, shows significant inhibitory activity with an IC50 of 238 nM but as expected the corresponding alcohol 4.1b shows little activity (IC50 = 29 μM). Modelling studies showed that both the aldehyde 4.1c and the alcohol 4.1b on docking can form a β-strand with appropriate H-bonding interactions. The aldehyde is more active than the alcohol due to the reactivity of the aldehyde warhead allowing for the reversible formation of a hemiacetal. A similar difference in reactivity is observed for CAT811 (30 nM) and its alcohol analogue (700 nM). These results demonstrate the value of molecular modelling as a screening mechanism before unproductive synthetic work is considered.