Glutaminyl cyclase activity is a characteristic feature of human cerebrospinal fluid

Development and evolution of human glutaminyl cyclase inhibitors (QCIs): an alternative promising approach for disease-modifying treatment of Alzheimer's disease

Human glutaminyl cyclase (hQC) is drawing considerable attention and emerging as a potential druggable target for Alzheimer's disease (AD) due to its close involvement in the pathology of AD via the post-translational pyroglutamate modification of amyloid-β. A recent phase 2a study has shown promising early evidence of efficacy for AD with a competitive benzimidazole-based QC inhibitor, PQ912, which also demonstrated favorable safety profiles. This finding has sparked new hope for the treatment of AD. In this review, we briefly summarize the discovery and evolution of hQC inhibitors, with a particular interest in classic Zinc binding group (ZBG)-containing chemicals reported in recent years. Additionally, we highlight several high-potency inhibitors and discuss new trends and challenges in the development of QC inhibitors as an alternative and promising disease-modifying therapy for AD.

Glutaminyl Cyclase, Diseases, and Development of Glutaminyl Cyclase Inhibitors

Pyroglutamate (pE) modification, catalyzed mainly by glutaminyl cyclase (QC), is prevalent throughout nature and is particularly important in mammals including humans for the maturation of hormones, peptides, and proteins. In humans, the upregulation of QC is involved in multiple diseases and conditions including Alzheimer's disease, Huntington's disease, melanomas, thyroid carcinomas, accelerated atherosclerosis, septic arthritics, etc. This upregulation catalyzes the generation of modified mediators such as pE-amyloid beta (Aß) and pE-chemokine ligand 2 (CCL2) peptides. Not surprisingly, QC has emerged as a reasonable target for the development of therapeutics to combat these diseases and conditions. In this manuscript the deleterious effects of upregulated QC resulting in disease manifestation are reviewed, along with progress on the development of QC inhibitors.

A phase 1 study to evaluate the safety and pharmacokinetics of PQ912, a glutaminyl cyclase inhibitor, in healthy subjects

Introduction

Pyroglutamate-amyloid-β (pE-Aβ) peptides are major components of Aβ-oligomers and Aβ-plaques, which are regarded as key culprits of Alzheimer's disease (AD) pathology. PQ912 is a competitive inhibitor of the enzyme glutaminyl cyclase (QC), essential for the formation of pE-Aβ peptides.

Methods

A randomized, double-blind, placebo-controlled, single- and multiple-ascending oral dose study investigated the safety, pharmacokinetics, and pharmacodynamics of PQ912 in healthy nonelderly and elderly subjects.

Results

PQ912 was considered safe and well tolerated with dose-proportional pharmacokinetics up to doses of 200 mg. At higher doses up to 1800 mg, exposure was supraproportional and exposure in elderly subjects was approximately 1.5- to 2.1-fold higher. Exposure in cerebrospinal fluid (CSF) was approximately 20% of the unbound drug in plasma, and both serum and CSF QC activity was inhibited in a dose-related manner.

Discussion

This first-in-man study of a compound-targeting QC inhibition justifies further development of PQ912 for the treatment of AD.

Pyroglutamate-Aβ: role in the natural history of Alzheimer's disease

The accumulation of amyloid-beta (Aβ) peptides is believed to be a central contributor to the neurodegeneration typically seen in Alzheimer's disease (AD) brain. Aβ extracted from AD brains invariably possesses extensive truncations, yielding peptides of differing N- and C-terminal composition. Whilst Aβ is often abundant in the brains of cognitively normal elderly people, the brains of AD patients are highly enriched for N-terminally truncated Aβ bearing the pyroglutamate modification. Pyroglutamate-Aβ (pE-Aβ) has a higher propensity for oligomerisation and aggregation than full-length Aβ, potentially seeding the accumulation of neurotoxic Aβ oligomers and amyloid deposits. In addition, pE-Aβ has increased resistance to clearance by peptidases, causing these peptides to persist in biological fluids and tissues. The extensive deposition of pE-Aβ in human AD brain is under-represented in many transgenic mouse models of AD, reflecting major differences in the production and processing of Aβ peptides in these models compared to the human disease state.