Ahp-Cyclodepsipeptide Inhibitors of Elastase: Lyngbyastatin 7 Stability, Scalable Synthesis, and Focused Library Analysis

Peptide Human Neutrophil Elastase Inhibitors from Natural Sources: An Overview

Elastases are a broad group of enzymes involved in the lysis of elastin, the main component of elastic fibres. They are produced and released in the human body, mainly by neutrophils and the pancreas. The imbalance between elastase activity and its endogenous inhibitors can cause different illnesses due to their excessive activity. The main aim of this review is to provide an overview of the latest advancements on the identification, structures and mechanisms of action of peptide human neutrophil elastase inhibitors isolated from natural sources, such as plants, animals, fungi, bacteria and sponges. The discovery of new elastase inhibitors could have a great impact on the pharmaceutical development of novel drugs through the optimization of the natural lead compounds. Bacteria produce mainly cyclic peptides, while animals provide for long and linear amino acid sequences. Despite their diverse natural sources, these elastase inhibitors show remarkable IC₅₀ values in a range from nM to μM values, thus representing an interesting starting point for the further development of potent bioactive compounds on human elastase enzymes.

Ahp-Cyclodepsipeptides as tunable inhibitors of human neutrophil elastase and kallikrein 7: Total synthesis of tutuilamide A, serine protease selectivity profile and comparison with lyngbyastatin 7

We describe the total synthesis of tutuilamide A, a potent porcine pancreatic elastase (PPE) inhibitor and a representative member of the 3-amino-6-hydroxy-2-piperidone (Ahp) cyclodepsipeptide family, isolated from marine cyanobacteria. The Ahp unit serves as a pharmacophore and the adjacent 2-amino-2-butenoic acid (Abu) is a main driver of the selectivity among serine proteases. We adapted our previous convergent strategy to generate the macrocycle, common with lyngbyastatin 7 and related elastase inhibitors, and then appended the tutuilamide A-specific side chain bearing a vinyl chloride. Tutuilamide A and lyngbyastatin 7 were evaluated side by side for the inhibition of the disease-relevant human neutrophil elastase (HNE). Tutuilamide A and lyngbyastatin 7 were approximately equipotent against HNE, while tutuilamide A was previously shown to be more active against PPE compared with lyngbyastatin 7, further demonstrating that the side chain provides opportunities to not only modulate potency but also selectivity among proteases of the same function from different organisms. Profiling of tutuilamide A against mainly human serine proteases revealed high selectivity for HNE (IC₅₀ 0.73 nM) and pleiotropic activity against kallikrein 7 (KLK7, IC₅₀ 5.0 nM), without affecting other kallikreins, similarly to lyngbyastatin 7 (IC₅₀ 0.85 nM for HNE and 3.1 nM for KLK7). A comprehensive molecular docking study for elastases and KLK7 afforded deeper insight into the intricate differences between inhibitor interactions with HNE and PPE, accounting for the differential activities for both compounds. The synthesis and molecular studies serve as a proof-of-concept that the macrocyclic scaffold can be diversified to fine-tune the activity of serine protease inhibitors.

Targeting eukaryotic proteases for natural products-based drug development

Covering: up to April 2020 Proteases are involved in the regulation of many physiological processes. Their overexpression and dysregulated activity are linked to diseases such as hypertension, diabetes, viral infections, blood clotting disorders, respiratory diseases, and cancer. Therefore, they represent an important class of therapeutic targets. Several protease inhibitors have reached the market and >60% of them are directly related to natural products, even when excluding synthetic natural product mimics. Historically, natural products have been a valuable and validated source of therapeutic agents, as over half of the marketed drugs across targets and diseases are inspired by natural product structures. In the past two decades the number of new protease inhibitors discovered from nature has sharply increased. Additionally, the availability of 3D structural information for proteases has permitted structure-based design and accelerated the synthesis of optimized lead structures with improved potency and selectivity profiles, resulting in some of the most-potent-in-class inhibitors. These discoveries were oftentimes maximized by in-depth biological assessments of lead inhibitors, linking them to a relevant disease state. This review will discuss some of the current and emerging drug targets and their involvement in various disease processes, highlighting selected success stories behind several FDA-approved protease inhibitors that have natural products scaffolds as well as recent selected pharmacologically well-characterized inhibitors derived from marine or terrestrial sources.

The Natural Polypeptides as Significant Elastase Inhibitors

Human neutrophil elastase (HNE) is a major cause of the destruction of tissues in cases of several different chronic andinflammatory diseases. Overexpression of the elastase enzyme plays a significant role in the pathogenesis of various diseases including chronic obstructive pulmonary disease (COPD), acute respiratory distress syndrome, rheumatoid arthritis, the rare disease cyclic hematopoiesis (or cyclic neutropenia), infections, sepsis, cystic fibrosis, myocardial ischemia/reperfusion injury and asthma, inflammation, and atherosclerosis. Human neutrophil elastase is secreted by human neutrophils due to different stimuli. Medicine-based inhibition of the over-activation of neutrophils or production and activity of elastase have been suggested to mend inflammatory diseases. Although the development of new elastase inhibitors is an essential strategy for treating the different inflammatory diseases, it has been a challenge to specifically target the activity of elastase because of its overlapping functions with those of other serine proteases. This review article highlights the reported natural polypeptides as potential inhibitors of elastase enzyme. The mechanism of action, structural features, and activity of the polypeptides have also been correlated wherever they were available.

Marine Cyanobacteria: A Source of Lead Compounds and their Clinically-Relevant Molecular Targets

The prokaryotic filamentous marine cyanobacteria are photosynthetic microbes that are found in diverse marine habitats, ranging from epiphytic to endolithic communities. Their successful colonization in nature is largely attributed to genetic diversity as well as the production of ecologically important natural products. These cyanobacterial natural products are also a source of potential drug leads for the development of therapeutic agents used in the treatment of diseases, such as cancer, parasitic infections and inflammation. Major sources of these biomedically important natural compounds are found predominately from marine cyanobacterial orders Oscillatoriales, Nostocales, Chroococcales and Synechococcales. Moreover, technological advances in genomic and metabolomics approaches, such as mass spectrometry and NMR spectroscopy, revealed that marine cyanobacteria are a treasure trove of structurally unique natural products. The high potency of a number of natural products are due to their specific interference with validated drug targets, such as proteasomes, proteases, histone deacetylases, microtubules, actin filaments and membrane receptors/channels. In this review, the chemistry and biology of selected potent cyanobacterial compounds as well as their synthetic analogues are presented based on their molecular targets. These molecules are discussed to reflect current research trends in drug discovery from marine cyanobacterial natural products.