Synthesis and Evaluation of Macrocyclic Peptide Aldehydes as Potent and Selective Inhibitors of the 20S Proteasome

Macrocyclic Oxindole Peptide Epoxyketones-A Comparative Study of Macrocyclic Inhibitors of the 20S Proteasome

Peptide macrocycles have recently gained attention as protease inhibitors due to their metabolic stability and specificity. However, the development of peptide macrocycles with improved binding potency has so far been challenging. Here we present macrocyclic peptides derived from the clinically applied proteasome inhibitor carfilzomib with an oxindole group that mimics the natural product TMC-95A. Fluorescence kinetic activity assays reveal a high potency of the oxindole group (IC50 = 0.19 μM) compared with agents lacking this motif. X-ray structures of the ligands with the β5-subunit of the yeast 20S proteasome illustrate that the installed macrocycle forces strong hydrogen bonding of the oxindole group with β5-Gly23NH. Thus, the binding of our designed oxindole epoxyketones is entropically and enthalpically favored in contrast to more flexible proteasome inhibitors such as carfilzomib.

Recent Advances in Macrocyclic Drugs and Microwave-Assisted and/or Solid-Supported Synthesis of Macrocycles

Macrocycles represent attractive candidates in organic synthesis and drug discovery. Since 2014, nineteen macrocyclic drugs, including three radiopharmaceuticals, have been approved by FDA for the treatment of bacterial and viral infections, cancer, obesity, immunosuppression, etc. As such, new synthetic methodologies and high throughput chemistry (e.g., microwave-assisted and/or solid-phase synthesis) to access various macrocycle entities have attracted great interest in this chemical space. This article serves as an update on our previous review related to macrocyclic drugs and new synthetic strategies toward macrocycles (Molecules, 2013, 18, 6230). In this work, I first reviewed recent FDA-approved macrocyclic drugs since 2014, followed by new advances in macrocycle synthesis using high throughput chemistry, including microwave-assisted and/or solid-supported macrocyclization strategies. Examples and highlights of macrocyclization include macrolactonization and macrolactamization, transition-metal catalyzed olefin ring-closure metathesis, intramolecular C–C and C–heteroatom cross-coupling, copper- or ruthenium-catalyzed azide–alkyne cycloaddition, intramolecular S_NAr or S_N2 nucleophilic substitution, condensation reaction, and multi-component reaction-mediated macrocyclization, and covering the literature since 2010.

Natural product scaffolds as inspiration for the design and synthesis of 20S human proteasome inhibitors

The 20S proteasome is a valuable target for the treatment of a number of diseases including cancer, neurodegenerative disease, and parasitic infection. In an effort to discover novel inhibitors of the 20S proteasome, many reseaarchers have looked to natural products as potential leads for drug discovery. The following review discusses the efforts made in the field to isolate and identify natural products as inhibitors of the proteasome. In addition, we describe some of the modifications made to natural products in order to discover more potent and selective inhibitors for potential disease treatment.

Diaryl Ether: A Privileged Scaffold for Drug and Agrochemical Discovery

Diaryl ether (DE) is a functional scaffold existing widely both in natural products (NPs) and synthetic organic compounds. Statistically, DE is the second most popular and enduring scaffold within the numerous medicinal chemistry and agrochemical reports. Given its unique physicochemical properties and potential biological activities, DE nucleus is recognized as a fundamental element of medicinal and agrochemical agents aimed at different biological targets. Its drug-like derivatives have been extensively synthesized with interesting biological features including anticancer, anti-inflammatory, antiviral, antibacterial, antimalarial, herbicidal, fungicidal, insecticidal, and so on. In this review, we highlight the medicinal and agrochemical versatility of the DE motif according to the published information in the past decade and comprehensively give a summary of the target recognition, structure-activity relationship (SAR), and mechanism of action of its analogues. It is expected that this profile may provide valuable guidance for the discovery of new active ingredients both in drug and pesticide research.

Development of Macrocyclic Peptides Containing Epoxyketone with Oral Availability as Proteasome Inhibitors

Macrocyclization has been frequently utilized for optimizing peptide or peptidomimetic-based compounds. In an attempt to obtain potent, metabolically stable, and orally available proteasome inhibitors, 30 oprozomib-derived macrocyclic peptides with structural diversity in their N-terminus and linker were successively designed and synthesized for structure-activity relationship (SAR) studies. As a consequence, the macrocyclic peptides with N-methyl-pyrazole (24p, 24x), imidazole (24t), and pyrazole (24v) as their respective N-termini exhibited favorable in vitro activity and metabolic stability, which translated into their potent in vivo proteasome inhibitory activity after oral administration. In particular, compound 24v, as the most distinguished one among this series, displayed excellent chymotrypsin-like (ChT-L, β5) inhibitory potency (IC₅₀ = 16 nM), low nanomolar antiproliferative activity against all three of the tested cell lines, and superior metabolic stability in mouse liver microsome (MLM), as well as favorable inhibition against ChT-L compared to that of oprozomib in BABL/c mice following po administration at a comparatively low dose, thereby representing a promising candidate for further development.

The proteasome as a target to combat malaria: hits and misses

The proteasome plays a vital role throughout the life cycle as Plasmodium parasites quickly adapt to a new host and undergo a series of morphologic changes during asexual replication and sexual differentiation. Plasmodium carries 3 different types of protease complexes: typical eukaryotic proteasome (26S) that resides in the cytoplasm and the nucleus, a prokaryotic proteasome homolog ClpQ that resides in the mitochondria, and a caseinolytic protease complex ClpP that resides in the apicoplast. In silico prediction in conjunction with immunoprecipitation analysis of ubiquitin conjugates have suggested that over half of the Plasmodium falciparum proteome during asexual reproduction are potential targets for ubiquitination. The marked potency of multiple classes of proteasome inhibitors against all stages of the life cycle, synergy with the current frontline antimalarial, artemisinin, and recent advances identifying differences between Plasmodium and human proteasomes strongly support further drug development efforts.

A mechanistic study on the inhibition of α-chymotrypsin by a macrocyclic peptidomimetic aldehyde

NMR and X-ray crystallography reveals covalent attachment of the macrocyclic aldehyde to serine195 of α-chymotrypsin and that its backbone binds as a β-strand.