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.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/5582 |
| Date | January 2011 |
| Creators | Chen, Hongyuan |
| Publisher | University of Canterbury. Chemistry |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Tyler Chen, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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