Heinz E. Moser

Physicochemical Properties of Antibacterial Compounds: Implications for Drug Discovery

Antibacterial drug discovery peaked shortly after the middle of the past century with the discovery of most compound classes that are still in clinical use. After the introduction of streptogramins and quinolones in 1962, no novel class of antibiotics was identified and approved for clinical use until linezolid was launched in 2000. This fact is rather surprising because not only was research heavily supported during this time but also novel technologies such as genomics and high-throughput screening were introduced and applied to improve productivity. With antibacterial resistance on the rise, we will have to replenish the arsenal of antibacterial drugs to provide physicians with the tools to successfully treat infections in the future. Part of the difficulty associated with the discovery of novel antibacterial compound classes has been defined by stringent requirements for a safe, broad spectrum antibiotic: The target must be essential, highly conserved among various bacterial species, and absent, different, or nonessential in humans. The inhibitor must be potent and should ideally display target-related whole cell activity with a low propensity for the emergence of resistance. Furthermore, the initial “hit” scaffold should be amenable to structural changes to allow for optimization of the potency, efficacy, and safety of later-stage “lead” compounds. A number of authors have discussed not only the necessity of novel antibacterial drugs to ensure future treatment options but also difficulties previously encountered during the hit identification and lead optimization steps. While the declining pipeline of antibacterial compounds arguably reflects the technical complexities of these requirements, relatively little has been published providing detailed information about the difficulties encountered at various companies over the past years. A recent review from the group at GlaxoSmithKline added valuable information on multiple issues related to this topic and provided insight into the successes and failures of a target-based, genomic approach. Relatively little though has been published on the nature of the compounds themselves as a possible source for the paucity of new agents. The analysis of Payne and co-workers illustrates the challenge of antibacterial drug discovery and suggests that multiple parameters contribute to the high attrition rate. With the rise of multi-drug-resistant pathogens and the need for novel antibiotics, it is critical to understand as much as possible from prior efforts and to apply learned lessons to the discovery of future antibiotics. One important parameter in particular has previously been mentioned but, in our view, not sufficiently analyzed: the physicochemical property space of antibacterial drugs. Lipinski’s landmark study represented the first systematic attempt to correlate the physicochemical properties of drugs with the predicted successful matriculation of initial hits and subsequent late-stage leads. It was this work that connected for the first time physicochemical properties of drugs with both their oral bioavailability and their subsequent difficulties and attrition rates during preclinical and clinical development. Major findings from this analysis were the recognition of an ideal property space for orally available drug candidates (MW, lipophilicity, hydrogen bond donors and acceptors), as well as the fact that corporate compound archives had been slowly moving away from an optimal area of this physicochemical space, most likely driven by synthetic convenience rather than by design. This awareness had a major impact on drug discovery, and today it is common to analyze these properties (the “rule of five” or “Lipinski’s rules”) prior to synthesizing novel candidates.

Synthesis and spectrum of the neoglycoside ACHN-490.

ABSTRACT ACHN-490 is a neoglycoside, or “next-generation” aminoglycoside (AG), that has been identified as a potentially useful agent to combat drug-resistant bacteria emerging in hospitals and health care facilities around the world. A focused medicinal chemistry campaign produced a collection of over 400 sisomicin analogs from which ACHN-490 was selected. We tested ACHN-490 against two panels of Gram-negative and Gram-positive pathogens, many of which harbored AG resistance mechanisms. Unlike legacy AGs, ACHN-490 was active against strains expressing known AG-modifying enzymes, including the three most common such enzymes found in Enterobacteriaceae. ACHN-490 inhibited the growth of AG-resistant Enterobacteriaceae (MIC90, ≤4 μg/ml), with the exception of Proteus mirabilis and indole-positive Proteae (MIC90, 8 μg/ml and 16 μg/ml, respectively). ACHN-490 was more active alone in vitro against Pseudomonas aeruginosa and Acinetobacter baumannii isolates with AG-modifying enzymes than against those with altered permeability/efflux. The MIC90 of ACHN-490 against AG-resistant staphylococci was 2 μg/ml. Due to its promising in vitro and in vivo profiles, ACHN-490 has been advanced into clinical development as a new antibacterial agent.

DNA binding ligands targeting drug-resistant Gram-positive bacteria. Part 2: C-terminal benzimidazoles and derivatives.

The synthesis and in vitro potency of DNA minor-groove binding antibacterials lacking the C-terminal amide bond are described. The crescent shaped molecules bear the positively charged amino group at an internal pyrrole unit instead of the C-terminus. Three structural parameters were investigated: the N-terminal unit, the internal amino group, and the C-terminal ring system. Several compounds demonstrated good in vitro potency against various Gram-positive bacteria and some molecules were moderately active against Escherichia coli, a representative Gram-negative strain.

Efficient sequence-specific cleavage of RNA using novel europium complexes conjugated to oligonucleotides

BACKGROUND A general method allowing the selective destruction of targeted mRNA molecules in vivo would have broad application in biology and medicine. Metal complexes are among the best synthetic catalysts for the cleavage of RNA, and covalent attachment of suitable metal complexes to oligonucleotides allows the cleavage of complementary single-stranded RNAs in a sequence-specific manner. RESULTS Using novel europium complexes covalently linked to an oligodeoxyribonucleotide, we have achieved the sequence-specific cleavage of a complementary synthetic RNA. The complexes are completely resistant to chemical degradation under the experimental conditions. The cleavage efficiency of the conjugate strongly depends on the nature of the linker between the oligonucleotide and the complex. Almost complete cleavage of the RNA target has been achieved within 16 h at 37 degrees C. CONCLUSIONS The results will be important for improving the efficacy of antisense oligonucleotides and will provide a basis for the design of synthetic RNA restriction enzymes. Conjugates of the kind described here may also find application as chemical probes for structural and functional studies of RNA.

The Identification of Indacaterol as an Ultralong-Acting Inhaled β2-Adrenoceptor Agonist

Following a lipophilicity-based hypothesis, an 8-hydroxyquinolinone 2-aminoindan derived series of beta(2)-adrenoceptor agonists have been prepared and evaluated for their potential as inhaled ultralong-acting bronchodilators. Determination of their activities at the human beta(2)-adrenoceptor receptor showed symmetrical substitution of the 2-aminoindan moiety at the 5- and 6-positions delivered the targeted intermediate potency and intrinsic-efficacy profiles relative to a series of clinical reference beta(2)-adrenoceptor agonists. Further assessment with an in vitro superfused electrically stimulated guinea-pig tracheal-strip assay established the onset and duration of action time courses, which could be rationalized by considering the lipophilicity, potency, and intrinsic efficacy of the compounds. From these studies the 5,6-diethylindan analogue indacaterol 1c was shown to possess a unique profile of combining a rapid onset of action with a long duration of action. Further in vivo profiling of 1c supported the long duration of action and a wide therapeutic index following administration to the lung, which led to the compound being selected as a development candidate.

Pharmacokinetics in Animals and Humans of a First-in-Class Peptide Deformylase Inhibitor

ABSTRACT BB-83698, a potent and selective inhibitor of peptide deformylase, was the first compound of this novel antibacterial class to progress to clinical trials. Single- and/or multiple-dose studies with doses ranging from 10 to 50 mg of BB-83698/kg of body weight were done with mice, rats, and dogs. Intravenous pharmacokinetics were characterized by low to moderate clearances and moderate volumes of distribution for all species. In dogs, but not in rodents, central nervous system (CNS) effects were dose limiting for intravenously administered BB-83698 and were suspected to be related to a high maximum concentration of the agent in plasma (Cmax) rather than to total systemic exposure. Controlled infusion studies with dogs demonstrated that CNS effects could be avoided without compromising systemic exposure by reducing the Cmax. A randomized, double-blind, placebo-controlled, five-way-crossover, single-dose-escalation, phase I study to explore the safety, tolerability, and pharmacokinetics of intravenous BB-83698 at doses ranging from 10 to 475 mg was performed with healthy male volunteers. Systemic exposures were generally in linear relationships with administered doses in animals and humans. Pharmacokinetics were consistent, predictable, and exhibited good allometric scaling among all species (r2 >0.98). Moreover, BB-83698 dosing in humans proceeded to a predicted efficacious exposure (the area under the concentration-time curve/MIC ratio, up to 184) without any clinically significant adverse effects.

DNA Binding Ligands with Excellent Antibiotic Potency Against Drug-Resistant Gram-Positive Bacteria

An efficient synthesis of DNA binding molecules consisting of four heterocyclic carboxamide units and various substituents at both termini is described. The minor-groove binding ligands showed excellent activity against a broad range of Gram-positive bacteria; no cross-resistance to known antibacterial drugs was observed.

The evaluation of 2′- and 6′-substituted carbocyclic nucleosides as building blocks for antisense oligonucleotides

Abstract Oligodeoxyribonucleotides incorporating 2′- or 6′-alkoxy substituted carbocyclic nucleotide units are shown to bind to complementary RNA with lower affinity than their unmodified parent compounds, while the presence of stretches of contiguous 6′-hydroxy substituted building blocks enhances RNA-binding affinity. 6′-substituted carbocyclic nucleotides are generally found to increase oligonucleotide resistance to enzymatic degradation, the effect being more pronounced for larger substituents.

Pharmacology of Novel Heteroaromatic Polycycle Antibacterials

ABSTRACT Heteroaromatic polycycle (HARP) compounds are a novel class of small (Mw, 600 to 650) DNA-binding antibacterials. HARP compounds exhibit a novel mechanism of action by preferentially binding to AT-rich sites commonly found in bacterial promoters and replication origins. Noncovalent binding in the minor groove of DNA results in inhibition of DNA replication and DNA-dependent RNA transcription and subsequent bacterial growth. HARP compounds have previously been shown to have potent in vitro activities against a broad spectrum of gram-positive organisms. The present report describes the extensive profiling of the in vitro and in vivo pharmacology of HARP antibacterials. The efficacies of representative compounds (GSQ-2287, GSQ-10547, and GSQ-11203), which exhibited good MIC activity, were tested in murine lethal peritonitis and neutropenic thigh infection models following intravenous (i.v.) administration. All compounds were efficacious in vivo, with potencies generally correlating with MICs. GSQ-10547 was the most potent compound in vitro and in vivo, with a 50% effective dose in the murine lethal peritonitis model of 7 mg/kg of body weight against methicillin-sensitive Staphylococcus aureus (MSSA) and 13 mg/kg against methicillin-resistant S. aureus (MRSA). In the neutropenic mouse thigh infection model, GSQ-11203 reduced the bacterial load (MRSA and MSSA) 2 log units following administration of a 25-mg/kg i.v. dose. In a murine lung infection model, treatment with GSQ-10547 at a dose of 50 mg/kg resulted in 100% survival. In addition to determination of efficacy in animals, the pharmacokinetic and tissue disposition profiles in animals following administration of an i.v. dose were determined. The compounds were advanced into broad safety screening studies, including screening for safety pharmacology, genotoxicity, and rodent toxicity. The results support further development of this novel class of antibiotics.

Solid‐Phase Synthesis of 2,4,6‐Triaminopyrimidines

A library of various 2,4,6-triaminopyrimidines was prepared by a solid-phase approach with a ring-activation/amination process (see scheme). Regio- and chemoselectivity could be controlled by a careful choice of leaving groups. The use of high temperature (140 °C) and neat amines show the advantages of solid-phase synthesis, since high purities and acceptable yields of substituted pyrimidines were obtained.