Pamela Hill

Pyrrolamide DNA gyrase inhibitors: optimization of antibacterial activity and efficacy.

The pyrrolamides are a new class of antibacterial agents targeting DNA gyrase, an essential enzyme across bacterial species and inhibition results in the disruption of DNA synthesis and subsequently, cell death. The optimization of biochemical activity and other drug-like properties through substitutions to the pyrrole, piperidine, and heterocycle portions of the molecule resulted in pyrrolamides with improved cellular activity and in vivo efficacy.

Potent and selective inhibitors of Helicobacter pylori glutamate racemase (MurI): pyridodiazepine amines.

An SAR study of an HTS screening hit generated a series of pyridodiazepine amines as potent inhibitors of Helicobacter pylori glutamate racemase (MurI) showing highly selective anti-H. pylori activity, marked improved solubility, and reduced plasma protein binding. X-ray co-crystal E-I structures were obtained. These uncompetitive inhibitors bind at the MurI dimer interface.

Optimization of pyrrolamide topoisomerase II inhibitors toward identification of an antibacterial clinical candidate (AZD5099).

AZD5099 (compound 63) is an antibacterial agent that entered phase 1 clinical trials targeting infections caused by Gram-positive and fastidious Gram-negative bacteria. It was derived from previously reported pyrrolamide antibacterials and a fragment-based approach targeting the ATP binding site of bacterial type II topoisomerases. The program described herein varied a 3-piperidine substituent and incorporated 4-thiazole substituents that form a seven-membered ring intramolecular hydrogen bond with a 5-position carboxylic acid. Improved antibacterial activity and lower in vivo clearances were achieved. The lower clearances were attributed, in part, to reduced recognition by the multidrug resistant transporter Mrp2. Compound 63 showed notable efficacy in a mouse neutropenic Staphylococcus aureus infection model. Resistance frequency versus the drug was low, and reports of clinical resistance due to alteration of the target are few. Hence, 63 could offer a novel treatment for serious issues of resistance to currently used antibacterials.

Design of Helicobacter pylori glutamate racemase inhibitors as selective antibacterial agents: a novel pro-drug approach to increase exposure.

High-throughput screening uncovered a pyrazolopyrimidinedione hit as a selective, low micromolar inhibitor of Helicobacter pylori glutamate racemase (MurI). Variation of the substituents around the scaffold led to low nanomolar inhibitors and improved antibacterial activity. The challenge in this program was to translate excellent enzyme inhibition into potent antibacterial activity and pharmacokinetics suitable for oral therapy. Compounds were profiled for MurI inhibition, activity against H. pylori, microsomal stability, and pharmacokinetics in mice. Iterative cycles of analog synthesis and biological testing led to compounds with substituents optimized for both low MICs (2 microg/ml) and good microsomal stability. In order to achieve high bioavailability, a novel pro-drug approach was implemented wherein a solubilizing sulfoxide moiety is oxidized in vivo to a sulfone.

Exploring the UDP pocket of LpxC through amino acid analogs.

Lipopolysaccharide (LPS) biosynthesis is an attractive antibacterial target as it is both conserved and essential for the survival of key pathogenic bacteria. Lipid A is the hydrophobic anchor for LPS and a key structural component of the outer membrane of Gram-negative bacteria. Lipid A biosynthesis is performed in part by a unique zinc dependent metalloamidase, LpxC (UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase), which catalyzes the first non-reversible step in lipid A biosynthesis. The UDP portion of the LpxC substrate-binding pocket has been relatively unexplored. We have designed and evaluated a series of hydroxamate based inhibitors which explore the SAR of substitutions directed into the UDP pocket with a range of substituted α-amino acid based linkers. We also provide the first wild type structure of Pseudomonas aeruginosa LpxC which was utilized in the design of many of these analogs.

Selective Inhibitors of Bacterial t-RNA-(N(1)G37) Methyltransferase (TrmD) That Demonstrate Novel Ordering of the Lid Domain.

The tRNA-(N(1)G37) methyltransferase (TrmD) is essential for growth and highly conserved in both Gram-positive and Gram-negative bacterial pathogens. Additionally, TrmD is very distinct from its human orthologue TRM5 and thus is a suitable target for the design of novel antibacterials. Screening of a collection of compound fragments using Haemophilus influenzae TrmD identified inhibitory, fused thieno-pyrimidones that were competitive with S-adenosylmethionine (SAM), the physiological methyl donor substrate. Guided by X-ray cocrystal structures, fragment 1 was elaborated into a nanomolar inhibitor of a broad range of Gram-negative TrmD isozymes. These compounds demonstrated no activity against representative human SAM utilizing enzymes, PRMT1 and SET7/9. This is the first report of selective, nanomolar inhibitors of TrmD with demonstrated ability to order the TrmD lid in the absence of tRNA.

Characterization of novel respiratory syncytial virus inhibitors identified by high throughput screen

Respiratory Syncytial Virus (RSV) is a major cause of lower respiratory tract infections with no effective treatment available. Finding novel inhibitors of RSV is an important first step towards developing an efficacious RSV therapy. Here we report the characterization of three novel classes of RSV replication inhibitors identified through a high throughput RSV replicon screen of ∼1million compounds in the AstraZeneca compound collection. These inhibitors, cpd 1, 2, and 3, specifically targeted RSV and were not active against other viruses tested. Resistance selection in RSV A2 with cpd 1 identified escape viruses with mutations mapped to the RSV L protein, an RNA-dependent RNA polymerase (Y1631C and I1413T). Recombinant RSV containing the L Y1631C substitution conferred resistance towards cpd 1, suggesting that the RSV polymerase is the target of this inhibitor. Interestingly, cpd 3, a nucleoside analog, induced a single resistant mutation in the P protein (D231V), indicating a novel mode of action not previously reported. cpd 2 affected host cell cycle and no frequent mutation was isolated following resistance selection, suggesting its possible involvement of a host-targeted mechanism. Taken together, we have identified three novel RSV inhibitors with different modes of action, providing new chemistry starting points for the discovery and development of future RSV therapeutic treatment.

Design of inhibitors of Helicobacter pylori glutamate racemase as selective antibacterial agents: Incorporation of imidazoles onto a core pyrazolopyrimidinedione scaffold to improve bioavailabilty

Structure-activity relationships are presented around a series of pyrazolopyrimidinediones that inhibit the growth of Helicobacter pylori by targeting glutamate racemase, an enzyme that provides d-glutamate for the construction of N-acetylglucosamine-N-acetylmuramic acid peptidoglycan subunits assimilated into the bacterial cell wall. Substituents on the inhibitor scaffold were varied to optimize target potency, antibacterial activity and in vivo pharmacokinetic stability. By incorporating an imidazole ring at the 7-position of scaffold, high target potency was achieved due to a hydrogen bonding network that occurs between the 3-position nitrogen atom, a bridging water molecule and the side chains Ser152 and Trp244 of the enzyme. The lipophilicity of the scaffold series proved important for expression of antibacterial activity. Clearances in vitro and in vivo were monitored to identify compounds with improved plasma stability. The basicity of the imidazole may contribute to increased aqueous solubility at lower pH allowing for improved oral bioavailability.

Overexpression of Pseudomonas aeruginosa LpxC with its inhibitors in an acrB-deficient Escherichia coli strain

In Gram-negative bacteria, the cell wall is surrounded by an outer membrane, the outer leaflet of which is comprised of charged lipopolysaccharide (LPS) molecules. Lipid A, a component of LPS, anchors this molecule to the outer membrane. UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase (LpxC) is a zinc-dependent metalloamidase that catalyzes the first committed step of biosynthesis of Lipid A, making it a promising target for antibiotic therapy. Formation of soluble aggregates of Pseudomonas aeruginosa LpxC protein when overexpressed in Escherichia coli has limited the availability of high quality protein for X-ray crystallography. Expression of LpxC in the presence of an inhibitor dramatically increased protein solubility, shortened crystallization time and led to a high-resolution crystal structure of LpxC bound to the inhibitor. However, this approach required large amounts of compound, restricting its use. To reduce the amount of compound needed, an overexpression strain of E. coli was created lacking acrB, a critical component of the major efflux pump. By overexpressing LpxC in the efflux deficient strain in the presence of LpxC inhibitors, several structures of P. aeruginosa LpxC in complex with different compounds were solved to accelerate structure-based drug design.