Tripeptide analogues of MG132 as protease inhibitors

Calpains, the proteases of two faces controlling the epithelial homeostasis in mammary gland

Calpain-1 and calpain-2 are calcium-dependent Cys-proteases ubiquitously expressed in mammalian tissues with a processive, rather than degradative activity. They are crucial for physiological mammary gland homeostasis as well as for breast cancer progression. A growing number of evidences indicate that their pleiotropic functions depend on the cell type, tissue and biological context where they are expressed or dysregulated. This review considers these standpoints to cover the paradoxical role of calpain-1 and -2 in the mammary tissue either, under the physiological conditions of the postlactational mammary gland regression or the pathological context of breast cancer. The role of both calpains will be examined and discussed in both conditions, followed by a brief snapshot on the present and future challenges for calpains, the two-gateway proteases towards tissue homeostasis or tumor development.

Self-Masked Aldehyde Inhibitors of Human Cathepsin L Are Potent Anti-CoV-2 Agents

Cysteine proteases comprise an important class of drug targets, especially for infectious diseases such as Chagas disease (cruzain) and COVID-19 (3CL protease, cathepsin L). Peptide aldehydes have proven to be potent inhibitors for all of these proteases. However, the intrinsic, high electrophilicity of the aldehyde group is associated with safety concerns and metabolic instability, limiting the use of aldehyde inhibitors as drugs. We have developed a novel class of compounds, self-masked aldehyde inhibitors (SMAIs) which are based on the dipeptide aldehyde inhibitor (Cbz-Phe-Phe-CHO, 1), for which the P₁ Phe group contains a 1′-hydroxy group, effectively, an o-tyrosinyl aldehyde (Cbz-Phe-o-Tyr-CHO, 2; (Li et al. (2021) J. Med. Chem. 64, 11,267–11,287)). Compound 2 and other SMAIs exist in aqueous mixtures as stable δ-lactols, and apparent catalysis by the cysteine protease cruzain, the major cysteine protease of Trypanosoma cruzi, results in the opening of the lactol ring to afford the aldehydes which then form reversible thiohemiacetals with the enzyme. These SMAIs are also potent, time-dependent inhibitors of human cathepsin L (K_i = 11–60 nM), an enzyme which shares 36% amino acid identity with cruzain. As inactivators of cathepsin L have recently been shown to be potent anti-SARS-CoV-2 agents in infected mammalian cells (Mellott et al. (2021) ACS Chem. Biol. 16, 642–650), we evaluated SMAIs in VeroE6 and A549/ACE2 cells infected with SARS-CoV-2. These SMAIs demonstrated potent anti-SARS-CoV-2 activity with values of EC₅₀ = 2–8 μM. We also synthesized pro-drug forms of the SMAIs in which the hydroxyl groups of the lactols were O-acylated. Such pro-drug SMAIs resulted in significantly enhanced anti-SARS-CoV-2 activity (EC₅₀ = 0.3–0.6 μM), demonstrating that the O-acylated-SMAIs afforded a level of stability within infected cells, and are likely converted to SMAIs by the action of cellular esterases. Lastly, we prepared and characterized an SMAI in which the sidechain adjacent to the terminal aldehyde is a 2-pyridonyl-alanine group, a mimic of both phenylalanine and glutamine. This compound (9) inhibited both cathepsin L and 3CL protease at low nanomolar concentrations, and also exerted anti-CoV-2 activity in an infected human cell line.

The C2 domain of calpain 5 contributes to enzyme activation and membrane localization

The enzymatic characteristics of the ubiquitous calpain 5 (CAPN5) remain undescribed despite its high expression in the central nervous system and links to eye development and disease. CAPN5 contains the typical protease core domains but lacks the C terminal penta-EF hand domain of classical calpains, and instead contains a putative C2 domain. This study used the SH-SY5Y neuroblastoma cell line stably transfected with CAPN5-3xFLAG variants to assess the potential roles of the CAPN5 C2 domain in Ca²⁺ regulated enzyme activity and intracellular localization. Calcium dependent autoproteolysis of CAPN5 was documented and characterized. Mutation of the catalytic Cys81 to Ala or addition of EGTA prevented autolysis. Eighty μM Ca²⁺ was sufficient to stimulate half-maximal CAPN5 autolysis in cellular lysates. CAPN5 autolysis was inhibited by tri-leucine peptidyl aldehydes, but less effectively by di-Leu aldehydes, consistent with a more open conformation of the protease core relative to classical calpains. In silico modeling revealed a type II topology C2 domain including loops with the potential to bind calcium. Mutation of the acidic amino acid residues predicted to participate in Ca²⁺ binding, particularly Asp531 and Asp589, resulted in a decrease of CAPN5 membrane association. These residues were also found to be invariant in several genomes. The autolytic fragment of CAPN5 was prevalent in membrane-enriched fractions, but not in cytosolic fractions, suggesting that membrane association facilitates the autoproteolytic activity of CAPN5. Together, these results demonstrate that CAPN5 undergoes Ca²⁺-activated autoproteolytic processing and suggest that CAPN5 association with membranes enhances CAPN5 autolysis.

An update on the therapeutic potential of calpain inhibitors: a patent review

INTRODUCTION

Calpain is a cytosolic proteinase that regulates of a wide range of physiological functions. The enzyme has been implicated in various pathological conditions including neurodegenerative disorders, cardiovascular disorders, cancer, and several other diseases. Therefore, calpain inhibitors are of interest as therapeutic agents and have been studied in preclinical models of several diseases in which the enzyme has been implicated.

AREAS COVERED

Calpain inhibitors that were disclosed over the last 5 years (2015-2019) include calpastatin-based peptidomimetics; thalassospiramide lipopeptides; disulfide analogs of alpha-mercaptoacrylic acids; allosteric modulators; azoloimidazolidenones; and macrocyclic/non-macrocyclic carboxamides. The effectiveness of some of the inhibitors in preclinical animal models is discussed.

EXPERT OPINION

Significant milestones that were made over this time frame include: a) disclosure of novel blood-brain barrier (BBB) permeable calpastatin analogs as calpain inhibitors; b) disclosure that potent calpain inhibitors can be obtained by targeting the hydrophobic pockets on chain A of PEF(S) of the small subunit of calpain; c) use of PEF(S) (PDB ID: 4WQ2) in virtual screening to identify novel structurally diverse calpain inhibitors; and d) mitigation of the metabolic instability of the alpha-ketoamide warhead of calpain inhibitors.