Mechanistic insights into the inhibition of prostate specific antigen by β-lactam class compounds

Synthesis and antimicrobial activities of some novel diastereoselective monocyclic cis-β-lactams using 2-ethoxy carbonyl DCPN as a carboxylic acid activator

A series of novel monocyclic cis-β-lactams were prepared from phenoxyacetic acid as ketene source and imines derived from 1-chloro-3,4-dihydronaphthalene-2-carbaldehyde and respective amine using ethyl 4,5-dichloro-6-oxopyridazine-1(6H)-carboxylate (or 2-ethoxy carbonyl DCPN) as a carboxylic acid activator. This is the first time 2-ethoxy carbonyl DCPN has been used as an acid activator in synthesis of β-lactams. The reaction was entirely diastereoselective leading to the formation cis-β-lactam derivatives. These newly synthesized cis-β-lactam were fully characterized by FT-IR, 1H NMR, 13C NMR, HRMS, CHNS and X-ray crystallography study. All this novel compound was also evaluated for their antibacterial and antifungal activities against certain strains of Gram-positive bacteria, Gram-negative bacteria and fungi. These compounds displayed moderate activity against using bacterial and fungal strains.

Mechanism of semen liquefaction and its potential for a novel non-hormonal contraception†

Semen liquefaction is a proteolytic process where a gel-like ejaculated semen becomes watery due to the enzymatic activity of prostate-derived serine proteases in the female reproductive tract. The liquefaction process is crucial for the sperm to gain their motility and successful transport to the fertilization site in Fallopian tubes (or oviducts in animals). Hyperviscous semen or failure in liquefaction is one of the causes of male infertility. Therefore, the biochemical inhibition of serine proteases in the female reproductive tract after ejaculation is a prime target for novel contraceptive development. Herein, we will discuss protein components in the ejaculates responsible for semen liquefaction and any developments of contraceptive methods in the past that involve the liquefaction process.

β-Lactams as promising anticancer agents: Molecular hybrids, structure activity relationships and potential targets

β-Lactam, commonly referred as azetidin-2-one, is a multifunctional building block for synthesizing β-amino ketones, γ-amino alcohols, and other compounds. Besides its well known antibiotic activity, this ring system exhibits a wide range of activities, attracting the attention of researchers. However, the structurally diverse β-lactam analogues as anticancer agents and their different molecular targets are poorly discussed. The purpose of this review is 3-fold: (1) to explore the molecular hybridization approach to design β-lactams hybrids as anticancer agents; (2) the structure activity relationship of the most active anticancer β-lactams and (3) to summarize their antitumor mechanisms.

Targeting the kallikrein-related peptidases for drug development

Kallikrein-related peptidases (KLKs) constitute a family of 15 serine proteases. Recent studies have shed light on key physiological functions of KLK enzymes and implicate their deregulation in major human pathologies such as neurodegenerative and inflammatory diseases, skin conditions, asthma, and cancer. Consequently, KLKs have emerged as novel targets for pharmacological intervention. Given the pleiotropic roles of KLKs, both activators and inhibitors of KLK activities are of therapeutic interest. For example, inhibitors of hyperactive KLKs in the epidermis would be effective against excess skin desquamation and inflammation, whereas KLK activators could benefit hyperkeratosis caused by diminished KLK proteolysis. Expression of active KLKs by cancer cells and tissues can be exploited to target prodrugs that are proteolytically cleaved to release a cytotoxic compound or a cytolytic toxin at the site of KLK protease activity. Here, we review current approaches for the design and testing of KLK-based therapeutics.

In vitro evaluation of the antielastase activity of polycyclic β-lactams

A series of bi- and tricyclic β-lactam compounds was synthesized and evaluated as inhibitors of cleavage of synthetic substrates in vitro by the serine proteases Human Leukocyte Elastase (HLE), Human Leukocyte Proteinase 3 (HLPR3) and Porcine Pancreatic Elastase (PPE). The obtained results have permitted us to describe a homobenzocarbacephem compound as HLE and HLPR3 inhibitor, to observe the positive effect that the styryl group exerts on the HLE inhibitory activity in polycyclic β-lactam compounds and to conclude that the hydroxyl function decreases the HLE inhibitory activity or rules it out completely.

Prostate-specific antigen: an overlooked candidate for the targeted treatment and selective imaging of prostate cancer

The role of prostate-specific antigen (PSA) or kallikrein-related peptidase 3 (KLK3) as a biomarker for prostate cancer is well known; however, the precise physiological role of it's serine protease activity in prostate cancer remains a mystery. PSA is produced at high levels by both androgen-dependent and -independent prostate cancers. Studies have documented high levels of active PSA in the milieu surrounding osseous and soft tissue metastases. This evidence, coupled with growing experimental evidence, suggests that PSA plays an important role in the pathobiology of prostate cancer. These observations support the development of PSA-selective inhibitors as useful tools for the targeted treatment and imaging of prostate cancer. Here, we review the research that has been conducted to date on developing selective inhibitors for PSA. The different approaches used to determine PSA substrate specificity and for creating inhibitors are discussed. In addition, the unique active site characteristics of PSA and how these motifs aided our research in developing PSA targeted agents are highlighted.

Molecular insights into substrate specificity of prostate specific antigen through structural modeling

Prostate Specific Antigen's (PSA) role as a biomarker for prostate cancer is well established but the physiological role of its serine protease activity in the pathobiology of normal prostate and prostate carcinogenesis remains largely unknown. In light of recent studies that implicate PSA's enzymatic activity in the initiation and/or progression of prostate cancer, we performed a molecular modeling study of substrate binding at the catalytic site of PSA wherein a PSA-selective substrate (HSSKLQ) was docked in an acyl-enzyme conformation to a three-dimensional homology model of PSA. Additionally, virtual positional scanning studies were conducted to gain mechanistic insights into substrate recognition of PSA. Subsequently, 13 novel peptide substrates of 6-aa length and four peptide substrates with varying length were synthesized and assayed for PSA hydrolysis to evaluate the experimental validity of docking insights. Additionally, six novel aldehyde-containing transition state analog inhibitors were synthesized and tested for their inhibitory potencies. The experimental data on the hydrolysis rates of the newly synthesized substrates and inhibitory potencies of the aldehyde peptides agreed with the docking predictions, providing validation of the docking methodology and demonstrating its utility towards the design of substrate-mimetic inhibitors that can be used to explore PSA's role in the pathobiology of prostate cancer.

Optimization of peptide-based inhibitors of prostate-specific antigen (PSA) as targeted imaging agents for prostate cancer

Prostate-specific antigen (PSA) is a serine protease biomarker that may play a role in prostate cancer development and progression. The inhibition of PSA's enzymatic activity with small molecule inhibitors is an attractive and, as of yet, unexploited target. Previously, we reported a series of peptidyl aldehyde and boronic acid based inhibitors of PSA. In this study, the structural requirements in the P2 and P3 positions of peptide-based PSA inhibitors are explored through the substitution of a series of natural and unnatural amino acids in these positions. This analysis demonstrated a preference for hydrophobic residues in the P2 position and amino acids with the potential to hydrogen bond in the P3 position. Using this information, a peptide boronic acid inhibitor with the sequence Cbz-Ser-Ser-Gln-Nle-(boro)-Leu was identified with a K(i) for PSA of 25nM. The attachment of a bulky metal chelating group to the amino terminal of this peptide did not adversely affect PSA inhibition. This result suggests that a platform of PSA inhibitor chelates could be developed as SPECT or PET-based imaging agents for prostate cancer.

Prostate-specific antigen is a "chymotrypsin-like" serine protease with unique P1 substrate specificity

Prostate-specific antigen (PSA), a serine protease belonging to the human kallikrein family, is best known as a prostate cancer biomarker. Emerging evidence suggests that PSA may also play a salient role in prostate cancer development and progression. With large amounts of enzymatically active PSA continuously and selectively produced by all stages of prostate cancer, PSA is an attractive target. PSA inhibitors, therefore, may represent a promising class of therapeutics and/or imaging agents. PSA displays chymotrypsin-like specificity, cleaving after hydrophobic residues, in addition to possessing a unique ability to cleave after glutamine in the P1 position. In this study, we investigated the structural motifs of the PSA S1 pocket that give it a distinct architecture and specificity when compared to the S1 pocket of chymotrypsin. Using the previously described PSA substrate Ser-Ser-Lys-Leu-Gln (SSKLQ) as a template, peptide aldehyde based inhibitors containing novel P1 aldehydes were made and tested against both proteases. Glutamine derivative aldehydes were highly specific for PSA while inhibitors with hydrophobic P1 aldehydes were potent inhibitors of both proteases with K(i) values <500 nM. The crystal structure of PSA was used to generate a model that allowed GOLD docking studies to be performed to further understand the critical interactions required for inhibitor binding to the S1 pockets of PSA and chymotrypsin. In conclusion, these results provide experimental and structural evidence that the S1 specificity pocket of PSA is distinctly different from that of chymotrypsin and that the development of highly specific PSA inhibitors is feasible.

Potent and selective peptidyl boronic acid inhibitors of the serine protease prostate-specific antigen

Prostate cancer cells produce high (microgram to milligram/milliliter) levels of the serine protease Prostate-Specific Antigen (PSA). PSA is enzymatically active in the extracellular fluid surrounding prostate cancers but is found at 1,000- to 10,000-fold lower concentrations in the circulation, where it is inactivated due to binding to abundant serum protease inhibitors. The exclusive presence of high levels of active PSA within prostate cancer sites makes PSA an attractive candidate for targeted imaging and therapeutics. A synthetic approach based on a peptide substrate identified first peptide aldehyde and then boronic acid inhibitors of PSA. The best of these had the sequence Cbz-Ser-Ser-Lys-Leu-(boro)Leu, with a K(i) for PSA of 65 nM. The inhibitor had a 60-fold higher K(i) for chymotrypsin. A validated model of PSA's catalytic site confirmed the critical interactions between the inhibitor and residues within the PSA enzyme.

Novel small molecule inhibitors for prostate-specific antigen

BACKGROUND

Prostate-specific antigen (PSA or KLK3) has been shown to inhibit angiogenesis, but it might also have tumor promoting activities. Thus, it may be possible to modulate prostate cancer growth by stimulating or inhibiting the activity of PSA. To this end we have previously identified peptides that stimulate the activity of PSA. As peptides have several limitations as drug molecules, we screened a chemical library to find drug-like compounds that could be used to modulate the function(s) of PSA.

METHODS

Almost 50,000 compounds were analyzed for their ability to modulate PSA activity towards a fluorescent PSA-substrate. The ability of the most active compounds to affect the anti-angiogenic activity of PSA was analyzed by human umbilical vein endothelial cell (HUVEC) tube formation assay.

RESULTS

In the initial screening we identified two compounds that inhibited PSA activity. Based on these, similar compounds were selected and tested for activity to define structure-activity relationships. Several compounds with micromolar IC50-values were found, but they were not entirely specific towards PSA, e.g., they inhibited chymotrypsin, which has similar substrate specificity as PSA. However, it was possibly to improve the selectivity of the compounds towards PSA by small structural changes. These compounds inhibited the anti-angiogenic activity of PSA in the HUVEC model, proving that the proteolytic activity of PSA is essential for inhibition of angiogenesis.

CONCLUSIONS

We found several PSA inhibitors that could be useful tools for studying the role of PSA in cancer models and in normal physiology as showed in angiogenesis model.