Semisynthetic Antimycobacterial C-3 Silicate and C-3/C-21 Ester Derivatives of Fusidic Acid: Pharmacological Evaluation and Stability Studies in Liver Microsomes, Rat Plasma, and Mycobacterium tuberculosis culture.

Fusidic acid (FA), a natural product fusidane triterpene-based antibiotic with unique structural features, is active in vitro against Mycobacterium tuberculosis, the causative agent of tuberculosis (TB). While possessing good pharmacokinetics in man, FA is rapidly metabolized in rodents, thus complicating proof-of-concept studies in this model. Toward the repositioning of FA as an anti-TB agent, we herein describe the synthesis, activity, and metabolism of FA and semisynthesized ester derivatives in rat liver microsomes, rat plasma, and mycobacterial cell culture. FA and derivative molecules with a free C-3 OH underwent species-specific metabolism to the corresponding 3-OH epimer, 3-epifusidic acid (3-epiFA). FA was also metabolized in rat plasma to form FA lactone. These additional routes of metabolism may contribute to the more rapid clearance of FA observed in rodents. C-3 alkyl and aryl esters functioned as classic prodrugs of FA, being hydrolyzed to FA in microsomes, plasma, and Mycobacterium tuberculosis culture. In contrast, C-3 silicate esters and C-21 esters were inert to hydrolysis and so did not act as prodrugs. The antimycobacterial activity of the C-3 silicate esters was comparable to that of FA, and these compounds were stable in microsomes and plasma, identifying them as potential candidates for evaluation in a rodent model of tuberculosis.

Molecular Typing of Staphylococcus aureus Isolated from Patients with Autosomal Dominant Hyper IgE Syndrome.

Autosomal dominant hyper IgE syndrome (AD-HIES) is a primary immunodeficiency caused by a loss-of-function mutation in the Signal Transducer and Activator of Transcription 3 (STAT3). This immune disorder is clinically characterized by increased susceptibility to cutaneous and sinopulmonary infections, in particular with Candida and Staphylococcus aureus. It has recently been recognized that the skin microbiome of patients with AD-HIES is altered with an overrepresentation of certain Gram-negative bacteria and Gram-positive staphylococci. However, these alterations have not been characterized at the species- and strain-level. Since S. aureus infections are influenced by strain-specific expression of virulence factors, information on colonizing strain characteristics may provide insights into host-pathogen interactions and help guide management strategies for treatment and prophylaxis. The aim of this study was to determine whether the immunodeficiency of AD-HIES selects for unique strains of colonizing S. aureus. Using multi-locus sequence typing (MLST), protein A (spa) typing, and PCR-based detection of toxin genes, we performed a detailed analysis of the S. aureus isolates (n = 13) found on the skin of twenty-one patients with AD-HIES. We found a low diversity of sequence types, and an abundance of strains that expressed methicillin resistance, Panton-Valentine leukocidin (PVL), and staphylococcal enterotoxins K and Q (SEK, SEQ). Our results indicate that patients with AD-HIES may often carry antibiotic-resistant strains that harbor key virulence factors.

Synthesis and biological characterisation of ester and amide derivatives of fusidic acid as antiplasmodial agents.

A series of novel fusidic acid (FA) derivatives was synthesized by replacing the carboxylic acid group with various ester and amide groups and evaluated in vitro for their antiplasmodial activity against the chloroquine-sensitive NF54 and multidrug-resistant K1 strains of the malarial parasite Plasmodium falciparum. Most of these derivatives showed a 4-49 and 5-17-fold increase in activity against NF54 and KI strains, respectively, as compared to FA and had a good selectivity index. These derivatives are stable over the incubation period and do not appear to be prodrugs of fusidic acid.

The Dynamic Nonprime Binding of Sampatrilat to the C-Domain of Angiotensin-Converting Enzyme.

Sampatrilat is a vasopeptidase inhibitor that inhibits both angiotensin I-converting enzyme (ACE) and neutral endopeptidase. ACE is a zinc dipeptidyl carboxypeptidase that contains two extracellular domains (nACE and cACE). In this study the molecular basis for the selectivity of sampatrilat for nACE and cACE was investigated. Enzyme inhibition assays were performed to evaluate the in vitro ACE domain selectivity of sampatrilat. The inhibition of the C-domain (Ki = 13.8 nM) by sampatrilat was 12.4-fold more potent than that for the N-domain (171.9 nM), indicating differences in affinities for the respective ACE domain binding sites. Interestingly, replacement of the P2 group of sampatrilat with an aspartate abrogated its C-selectivity and lowered the potency of the inhibitor to activities in the micromolar range. The molecular basis for this selective profile was evaluated using molecular modeling methods. We found that the C-domain selectivity of sampatrilat is due to occupation of the lysine side chain in the S1 and S2 subsites and interactions with Glu748 and Glu1008, respectively. This study provides new insights into ligand interactions with the nonprime binding site that can be exploited for the design of domain-selective ACE inhibitors.

In silico comparison of antimycobacterial natural products with known antituberculosis drugs.

The chemical space based on physicochemical properties and structural features of a diverse group of natural products with reported in vitro activity against different Mycobacterium tuberculosis strains is investigated using in silico tools. This is compared to the chemical space occupied by drugs currently recommended for the treatment of various forms of tuberculosis as well as compounds in preclinical and clinical development. Docking studies exploring possible binding affinities and modes of two main clusters of natural products on two different mycobacterial targets are also reported. Our docking results suggest that scytoscalarol, an antibacterial and antifungal guanidine-bearing sesterterpene, can inhibit arabinosyltransferase Mtb EmbC, and the ß-carboline alkaloids 8-hydroxymanzamine A and manzamine A can bind to the oxidoreductase of Mtb INHA. On this basis, these products showing high binding affinities to the two targets in silico could be rationally selected for in vitro testing against these targets and/or semisynthetic modification.

1-Benzyl-1,2,3,4-tetrahydro-ß-carboline as channel blocker of N-methyl-D-aspartate receptors.

N-methyl-d-aspartate (NMDA) receptors belong to the family of ligand-gated ion channels and are important for synaptic plasticity and memory function. The NMDA receptor consists of a voltage-dependent channel permeable to Ca(2+) and Na(+) . In Alzheimer's disease, neuronal degeneration is thought to cause an excessive release of glutamate to the extracellular space, which may in turn mediate prolonged stimulation of the NMDA receptor complex and, as a consequence, excessive calcium influx into neuronal cells, leading to subsequent cell death. This process is called glutamate-induced excitotoxicity, and its inhibition may present an effective antidementive therapy. We found that 1-benzyl-1,2,3,4-tetrahydro-ß-carboline (1a) blocked NMDA receptor-mediated, glutamate-induced excitotoxicity with an IC(50) value of 27.4 µm, whereas the closely related 1-phenyl-1,2,3,4-tetrahydro-ß-carboline (1b) had no effect. The binding modes of the reported compounds were studied by in silico docking simulations. We demonstrate that compounds (S)-1a and (R)-1a, but not (S)-1b and (R)-1b, have the same characteristics of potent NMDA receptor blockers, because they establish the main interactions inside the vestibule region of the receptor described previously for the high-affinity NMDA receptor blocker, MK-801.