Consequences of Postnatally Elevated Insulin-Like Growth Factor-II in Transgenic Mice: Endocrine Changes and Effects on Body and Organ Growth.

Insulin-like growth factor-II (IGF-II) is an important regulator of embryonic growth and differentiation, but its function in postnatal life is unclear. To address this point, we generated transgenic mice harboring fusion genes in which a human IGF-II complementary DNA is placed under the transcriptional control of the rat phosphoenolpyruvate carboxykinase promoter. Transgene-specific messenger RNA was detected in liver, kidney, and several parts of the gut. Serum IGF-II levels in transgenic mice were 2-3 times higher than those in controls and increased after starvation. Circulating IGF-I correlated negatively and IGF-binding protein-2 (IGFBP-2) positively with IGF-II levels, suggesting that IGF-I is displaced from IGFBPs by IGF-II and that IGFII is a major regulator of IGFBP-2. Serum levels of IGFBP-3 and IGFBP-4 tended to be higher in phosphoenolpyruvate carboxykinase- IGF-II transgenic mice than in controls, as evaluated by ligand blot analysis. Starvation reduced serum IGF-I, but increased IGFBP-2 in transgenic mice more markedly than in controls. Fasting insulin levels were significantly reduced in transgenic mice, whereas glucose levels were not influenced by elevated IGF-II. The body growth of 4- and 12- week-old mice was not significantly influenced by elevated IGF-II, but transgenic mice displayed increased kidney and testis weight at the age of 4 weeks, and increased adrenal weight at the age of 12 weeks. Our results demonstrate that elevated IGF-II in postnatal life has multiple endocrine consequences and subtle time-specific effects on organ growth

Cyclic boronates as versatile scaffolds for KPC-2 β-lactamase inhibition

Klebsiella pneumoniae carbapenemase-2 (KPC-2) is a serine-β-lactamase (SBL) capable of hydrolysing almost all β-lactam antibiotics. We compare KPC-2 inhibition by vaborbactam, a clinically-approved monocyclic boronate, and VNRX-5133 (taniborbactam), a bicyclic boronate in late-stage clinical development. Vaborbactam inhibition is slowly reversible, whereas taniborbactam has an off-rate indicating essentially irreversible complex formation and a 15-fold higher on-rate, although both potentiate β-lactam activity against KPC-2-expressing K. pneumoniae. High resolution X-ray crystal structures reveal closely related binding modes for both inhibitors to KPC-2, with differences apparent only in positioning of the endocyclic boronate ester oxygen. The results indicate the bicyclic boronate scaffold as both an efficient, long-lasting, KPC-2 inhibitor and capable of supporting further iterations that may improve potency against specific enzyme targets and pre empt the emergence of inhibitor resistant KPC-2 variants

Sideromimic Modification of Lactivicin Dramatically Increases Potency against Extensively Drug-Resistant <i>Stenotrophomonas maltophilia </i>Clinical Isolates

Acetamido derivatives of the naturally antibacterial non-β-lactam lactivicin (LTV) have improved activity against their penicillin binding protein targets and reduced hydrolysis by β-lactamases, but penetration into Gram-negative bacteria is still relatively poor. Here we report that modification of the LTV lactone with a catechol-type siderophore increases potency 1,000-fold against Stenotrophomonas maltophilia, a species renowned for its insusceptibility to antimicrobials. The MIC90 of modified lactone compound 17 (LTV17) against a global collection of extensively drug-resistant clinical S. maltophilia isolates was 0.063 μg · ml(-1) Sideromimic modification does not reduce the ability of LTVs to induce production of the L1 and L2 β-lactamases in S. maltophilia and does not reduce the rate at which LTVs are hydrolyzed by L1 or L2. We conclude, therefore, that lactivicin modification with a siderophore known to be preferentially used by S. maltophilia substantially increases penetration via siderophore uptake. LTV17 has the potential to be developed as a novel antimicrobial for treatment of infections by S. maltophilia More generally, our work shows that sideromimic modification in a species-targeted manner might prove useful for the development of narrow-spectrum antimicrobials that have reduced collateral effects

High-throughput screen with the l,d-transpeptidase LdtMt2 of Mycobacterium tuberculosis reveals novel classes of covalently reacting inhibitors

Disruption of bacterial cell wall biosynthesis in Mycobacterium tuberculosis is a promising target for treating tuberculosis. The L,D-transpeptidase LdtMt2, which is responsible for the formation of 3 → 3 cross-links in the cell wall peptidoglycan, has been identified as essential for M. tuberculosis virulence. We optimised a high-throughput assay for LdtMt2, and screened a targeted library of ∼10 000 electrophilic compounds. Potent inhibitor classes were identified, including established (e.g., β-lactams) and unexplored covalently reacting electrophilic groups (e.g., cyanamides). Protein-observed mass spectrometric studies reveal most classes to react covalently and irreversibly with the LdtMt2 catalytic cysteine (Cys354). Crystallographic analyses of seven representative inhibitors reveal induced fit involving a loop enclosing the LdtMt2 active site. Several of the identified compounds have a bactericidal effect on M. tuberculosis within macrophages, one with an MIC50 value of ∼1 μM. The results provide leads for the development of new covalently reaction inhibitors of LdtMt2 and other nucleophilic cysteine enzymes

Molecular Basis of Class A β-lactamase Inhibition by Relebactam

β-Lactamase production is the major β-lactam resistance mechanism in Gram-negative bacteria. β-Lactamase inhibitors (BLIs) efficacious against serine β-lactamase (SBL) producers, especially strains carrying the widely disseminated class A enzymes, are required. Relebactam, a diazabicyclooctane (DBO) BLI is in phase-3 clinical trials in combination with imipenem, for treatment of infections by multi-drug resistant Enterobacteriaceae. We show that relebactam inhibits five clinically-important class A SBLs (despite their differing spectra of activity), representing both chromosomal and plasmid-borne enzymes, i.e. the extended spectrum β-lactamases L2 (inhibition constant 3 μM) and CTX-M-15 (21 μM); and the carbapenemases, KPC-2, -3 and -4 (1 - 5 μM). Against purified class A SBLs, relebactam is an inferior inhibitor compared to the clinically approved DBO avibactam, (9 to 120-fold differences in IC50). Minimum inhibitory concentration assays indicate relebactam potentiates β-lactam (imipenem) activity against KPC-producing Klebsiella pneumoniae with similar potency to avibactam (with ceftazidime). Relebactam is less effective than avibactam in combination with aztreonam against Stenotrophomonas maltophilia K279a. X-ray crystal structures of relebactam bound to CTX-M-15, L2, KPC-2, KPC-3 and KPC-4 reveal its C2 linked piperidine ring can sterically clash with Asn104 (CTX-M-15) or His/Trp105 (L2 and KPCs), rationalizing its poorer inhibition activity compared to avibactam, which has a smaller C2 carboxyamide group. Mass spectrometry and crystallographic data show slow, pH-dependent relebactam desulfation by KPC-2, -3 and -4. This comprehensive comparison of relebactam binding across five clinically-important class A SBLs will inform the design of future DBOs with the aim of improving clinical efficacy of BLI:β-lactam combinations

Mechanistic Insights into β-Lactamase-Catalysed Carbapenem Degradation Through Product Characterisation

β-Lactamases are a major threat to the clinical use of carbapenems, which are often antibiotics of last resort. Despite this, the reaction outcomes and mechanisms by which β-lactamases degrade carbapenems are still not fully understood. The carbapenem bicyclic core consists of a β-lactam ring fused to a pyrroline ring. Following β-lactamase-mediated opening of the β-lactam, the pyrroline may interconvert between an enamine (2-pyrroline) form and two epimeric imine (1-pyrroline) forms; previous crystallographic and spectroscopic studies have reported all three of these forms in the contexts of hydrolysis by different β-lactamases. As we show by NMR spectroscopy, the serine β-lactamases (KPC-2, SFC-1, CMY-10, OXA-23, and OXA-48) and metallo-β-lactamases (NDM-1, VIM-1, BcII, CphA, and L1) tested all degrade carbapenems to preferentially give the Δ² (enamine) and/or (R)-Δ¹ (imine) products. Rapid non-enzymatic tautomerisation of the Δ² product to the (R)-Δ¹ product prevents assignment of the nascent enzymatic product by NMR. The observed stereoselectivity implies that carbapenemases control the form of their pyrroline ring intermediate(s)/product(s), thereby preventing pyrroline tautomerisation from inhibiting catalysis

Profiling interactions of vaborbactam with metallo-β-lactamases

β-Lactams are the most successful antibacterials, yet their use is threatened by resistance, importantly as caused by β-lactamases. β-Lactamases fall into two mechanistic groups: the serine β-lactamases that utilise a covalent acyl-enzyme mechanism and the metallo β-lactamases that utilise a zinc-bound water nucleophile. Achieving simultaneous inhibition of both β-lactamase classes remains a challenge in the field. Vaborbactam is a boronate-based inhibitor that reacts with serine-β-lactamases to form covalent complexes that mimic tetrahedral intermediates in catalysis. Vaborbactam has recently been approved for clinical use in combination with the carbapenem meropenem. Here we show that vaborbactam moderately inhibits metallo-β-lactamases from all 3 subclasses (B1, B2 and B3), with a potency of around 20–100 fold below that by which it inhibits its current clinical targets, the Class A serine β-lactamases. This result contrasts with recent investigations of bicyclic boronate inhibitors, which potently inhibit subclass B1 MBLs but which presently lack activity against B2 and B3 enzymes. These findings indicate that cyclic boronate scaffolds have the potential to inhibit the full range of β-lactamases and justify further work on the development of boronates as broad-spectrum β-lactamase inhibitors

Crystal structures of VIM-1 complexes explain active site heterogeneity in VIM-class metallo-β-lactamases

Metallo‐β‐Lactamases (MBLs) protect bacteria from almost all β‐lactam antibiotics. Verona integron‐encoded MBL (VIM) enzymes are among the most clinically important MBLs, with VIM‐1 increasing in carbapenem‐resistant Enterobacteriaceae (Escherichia coli, Klebsiella pneumoniae) that are among the hardest bacterial pathogens to treat. VIM enzymes display sequence variation at residues (224 and 228) that in related MBLs are conserved and participate in substrate binding. How they accommodate this variability, while retaining catalytic efficiency against a broad substrate range, has remained unclear. Here, we present crystal structures of VIM‐1 and its complexes with a substrate‐mimicking thioenolate inhibitor, ML302F, that restores meropenem activity against a range of VIM‐1 producing clinical strains, and the hydrolysed product of the carbapenem meropenem. Comparison of these two structures identifies a water‐mediated hydrogen bond, between the carboxylate group of substrate/inhibitor and the backbone carbonyl of the active site zinc ligand Cys221, that is common to both complexes. Structural comparisons show that the responsible Cys221‐bound water is observed in all known VIM structures, participates in carboxylate binding with other inhibitor classes, and thus effectively replicates the role of the conserved Lys224 in analogous complexes with other MBLs. These results provide a mechanism for substrate binding that permits the variation at positions 224 and 228 that is a hallmark of VIM MBLs

Bicyclic Boronate VNRX-5133 Inhibits Metallo- and Serine-β-Lactamases

The bicyclic boronate VNRX-5133 (taniborbactam) is a new type of β-lactamase inhibitor in clinical development. We report that VNRX-5133 inhibits serine-β-lactamases (SBLs) and some clinically important metallo-β-lactamases (MBLs), including NDM-1 and VIM-1/2. VNRX-5133 activity against IMP-1 and tested B2/B3 MBLs was lower/not observed. Crystallography reveals how VNRX-5133 binds to the class D SBL OXA-10 and MBL NDM-1. The crystallographic results highlight the ability of bicyclic boronates to inhibit SBLs and MBLs via binding of a tetrahedral (sp3) boron species. The structures imply conserved binding of the bicyclic core with SBLs/MBLs. With NDM-1, by crystallography, we observed an unanticipated VNRX-5133 binding mode involving cyclization of its acylamino oxygen onto the boron of the bicyclic core. Different side-chain binding modes for bicyclic boronates for SBLs and MBLs imply scope for side-chain optimization. The results further support the "high-energy-intermediate" analogue approach for broad-spectrum β-lactamase inhibitor development and highlight the ability of boron inhibitors to interchange between different hybridization states/binding modes