In the Quest for Alzheimer’s Therapeutics – Glutaminyl Cyclase Inhibitors


I would like to begin this post on a personal note if I may. The truth is that as scientists involved in research it is sometimes easy to get fixated on the daily challenges of compound synthesis, assays and biological targets, inhibition results, efficacy and physiochemical properties without remembering why we and other research groups undertake the research that is so important. It’s not intentional but easy to sometimes forget the patients and families who need the progression of therapeutics to help them battle disease and improve quality of life. I find it’s nice to take a step back from the bench once in a while and look at the bigger picture.

As with many people, I have been witness to the devastating effects of neurodegenerative disease. The implications not only directly to the patients but on those around them who day to day have to see a loved ones’ health degrade in the knowledge that there is very little they can do.

In the last few years a close friend’s parent was diagnosed with early onset Alzheimer’s disease (AD) and after knowing them for many years it was sad to find that last time we met they didn’t know who I was. Even more heart breaking is that sooner or later the close family will also have the same experience. It is unfortunately inevitable.

Obviously this is not isolated, just one of millions of cases around the world. The charity Alzheimer’s society quote more than 520,000 people in the UK suffer with AD and more than 40,000 are under the age of 65, accounting for approximately 7% of AD suffers [1]. A report from the Alzheimer’s Association in 2014 cite a decrease in registered US deaths of heart disease, stroke and prostate cancer, 16%, 23% and 8% respectively between 2000 and 2010 however an increase of AD associated deaths of 68% in the same period. [2]

Currently treatment focuses on temporary symptomatic relief with Acetylcolinesterase inhibitors and methyl-D-Aspartate receptor agonists rather than targeting the amyloid plaque formation believed root cause of AD.

Recently failures of both Janssen and Pfizers Bapineuzumab and Eli Lilly’s Solanezumab in phase III clinical trials has given a further knockback to AD therapeutics. Both of these drugs were developed to target β-amyloid (Aβ) of which high levels in the brain can promote the formation of amyloid plaques leading to neuronal death. The moderation of Aβ levels should reduce the formation of amyloid plaques and hence the neuronal death in neurodegenerative disease.[3] Unfortunately, these drugs did not prove significantly efficacious in the clinical trials.

In response to the Aβ targeting therapeutics a recent publication from Van-Hai Hoang et al.[4] describes their research targeting Aβ and the employment of rational design to develop potential anti-Alzheimer’s treatments. In the paper they explain the possible ineffective outcomes of Bapineuzumab and Solanezumab were perhaps related to the multi functionality of Aβ peptides and their diverse structural features. They suggest targeting specific Aβ peptides that are neurotoxic and prone to aggregation may prove therapeutically more effective.

The paper goes on to explain that certain N-terminal Aβ peptides are of significant levels in AD patients and these are prone to cyclisation by glutaminyl cyclase (QC) to form Pyroglutamate. These cyclic Aβ peptides are more neurotoxic, rapidly aggregate and are seed for both amyloid and tau plaques. It has been reported that QC is overexpressed in the brains of AD patients and inhibition of QC in animal models reduces the amount of cyclic Aβ peptides and Aβ plaques.

Van-Hai Hoang et al. use a pharmacophoric model based upon the N-terminal of Aβ3E-42 to develop a known QC inhibitor (compound 1) (Fig 1.).

alex 1

Figure 1 – 4 pharmacophoric regions [4]

The model contains 4 key pharmacophore regions:

  • A-region consists of a zinc binding group, in this case a 5-methyl imidazole, which binds to an active zinc ion.
  • B-region, H-bond donor site which consists of a thiourea motif.
  • C-region mimics the phenylalanine of the N-terminus requiring an aromatic ring
  • D-region was postulated to require a mimic to the arginine motif.

The group had previously reported the SAR based upon Compound 1 (fig 1) giving an IC50 of 58 nM. [5] Keeping the A, B and C regions consistent with compound 1 the D-region was investigated via a number of synthesised analogues containing linked nitrogen containing groups. The length of linker and the nitrogen containing arginine mimic group were both varied to provide a large SAR landscape.

The group found that most analogues showed improved in vitro inhibitory activity against human QC compared to that of compound 1. For all compounds synthesised with the added D-region they saw  5-40 fold increases in potency and noted that nitrogen arrangement in this portion was key to the binding.

alex 2

Figure 2 – Compound 212 and docking ton QC active site [4]

Compound, 212 (fig 2) a 3-carbon linked amino pyridine gave vast improvement in activity (4.5 nM) and when screened against an isozyme of QC also showed selectivity yielding a 100 fold decrease in IC50. In an acute model mice compound 212 confirmed penetration and efficacy reducing both Aβ and cyclic Aβ concentrations.

To test therapeutic effects long term in vivo efficacy studies of 212 were established in two transgenic AD model mice where not only concentrations of Aβ were reduced but cognitive functions of the mice were also restored.

Molecular modelling studies on the x-ray crystal structure of QC’s active site were also undertaken to try and rationalise the improvement in potency with the addition of the D-region. By docking a selection of their most potent compounds into the active site it was found that all had favourable binding to the first three regions and compound 212 formed strong interactions with its D-region amino pyridine to a key glutamic acid residue.

The publication from Van-Hai Hoang et al. is a great example of utilising rational thought processes and logical steps to re-asses the idea of targeting β-amyloid via inhibition of glutaminyl cyclase. The paper backs up its findings with both successful in vivo and in silico modelling.

Another piece in the puzzle for β-amyloid inhibitors perhaps, but most importantly a step in the right direction for the patients and their families who rely on this research.

Finally, I would like to thank the family of whom I spoke at the beginning of this blog who gave me their complete support to publish this post and increase awareness around Alzheimer’s disease. As researchers in drug discovery, it is this family and those like them who in the end we should remember.

Blog  by Alexander Ashall-Kelly

References:

[1] http://www.alzheimers.org.uk

[2] Alzheimer’s Association / Alzheimer’s & Dementia 10 (2014) e47-e92

[3] Hardy, J.; Selkoe, D. J. The amyloid hypothesis of Alzheimer’s disease: progress and problems on the road to therapeutics. Science 2002, 297, 353−356.

[4] Van-Hai Hoang et al. Discovery of Potent human Glutaminyl Cyclase Inhibitors as Anti-Alzheimer’s Agents Based on Rational Design, J. Med. Chem. 2017, 60, 2573−2590

[5] Tran, P. T.; Hoang, V. H. et al, J.Structure-activity relationship of human glutaminyl cyclase inhibitorshaving an N-(5-methyl-1H-imidazol-1-yl)propyl thiourea template.Bioorg. Med. Chem. 2013, 21, 3821−3830.

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