Martin S. Linsell

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.

2-(2-Aminophenyl)-acetaldehyde Dimethyl Acetal: A Novel Reagent for the Protection of Carboxylic Acids.

Abstract The synthesis and use of 2-(2-aminophenyl)-acetaldehyde dimethyl acetal 1 are described. The amides 2, derived from this amine and carboxylic acids, are stable under basic conditions and thus can be regarded as the protected carboxylic acids. The corresponding carboxylic acids are regenerated by conversion of 2 into indolylamides 3 by treatment with CSA and subsequent hydrolysis with LiOOH or NaOH. In addition, 3 can be easily converted to esters, amides, and aldehydes.

Exploring the Positional Attachment of Glycopeptide/β-lactam Heterodimers

Further investigations towards novel glycopeptide/β-lactam heterodimers are reported. Employing a multivalent approach to drug discovery, vancomycin and cephalosporin synthons, 4, 2, 5 and 10, 18, 25 respectively, were chemically linked to yield heterodimer antibiotics. These novel compounds were designed to inhibit Grampositive bacterial cell wall biosynthesis by simultaneously targeting the principal cellular targets of both glycopeptides and β-lactams. The positional attachment of both the vancomycin and the cephalosporin central cores has been explored and the SAR is reported. This novel class of bifunctional antibiotics 28∼36 all displayed remarkable potency against a wide range of Gram-positive organisms, including methicillin-resistant Staphylococcus aureus (MRSA). A subset of compounds, 29, 31 and 35 demonstrated excellent bactericidal activity against MRSA (ATCC 33591) and 31 and 35 also exhibited superb in vivo efficacy in a mouse model of MRSA infection. As a result of this work compound 35 was selected as a clinical candidate, TD-1792.

A Multivalent Approach to Drug Discovery for Novel Antibiotics

The design, synthesis and antibacterial activity of novel glycopeptide/β-lactam heterodimers is reported. Employing a multivalent approach to drug discovery, vancomycin and cephalosporin synthons, A and B respectively, were chemically linked to yield heterodimer antibiotics. These novel compounds were designed to inhibit Gram-positive bacterial cell wall biosynthesis by simultaneously targeting the principal cellular targets of both glycopeptides and β-lactams. The antibiotics 8a˜f displayed remarkable potency against a wide range of Gram-positive organisms including methicillin-resistant Staphylococcus aureus (MRSA). Compound 8e demonstrated excellent bactericidal activity against MRSA (ATCC 33591) and initial evidence supports a multivalent mechanism of action for this important new class of antibiotic.

Toxicity modulation, resistance enzyme evasion, and A-site X-ray structure of broad-spectrum antibacterial neomycin analogs.

Aminoglycoside antibiotics are pseudosaccharides decorated with ammonium groups that are critical for their potent broad-spectrum antibacterial activity. Despite over three decades of speculation whether or not modulation of pKa is a viable strategy to curtail aminoglycoside kidney toxicity, there is a lack of methods to systematically probe amine-RNA interactions and resultant cytotoxicity trends. This study reports the first series of potent aminoglycoside antibiotics harboring fluorinated N1-hydroxyaminobutyryl acyl (HABA) appendages for which fluorine-RNA contacts are revealed through an X-ray cocrystal structure within the RNA A-site. Cytotoxicity in kidney-derived cells was significantly reduced for the derivative featuring our novel β,β-difluoro-HABA group, which masks one net charge by lowering the pKa without compromising antibacterial potency. This novel side-chain assists in evasion of aminoglycoside-modifying enzymes, and it can be easily transferred to impart these properties onto any number of novel analogs.

Toward Overcoming Staphylococcus aureus Aminoglycoside Resistance Mechanisms with a Functionally Designed Neomycin Analogue.

Deoxygenation of the diol groups in rings A and D of neomycin in combination with the introduction of an N1-(l)-HABA group in the 2-deoxystreptamine subunit (ring B) leads to a novel and potent antibiotic (1) with activity against strains of S. aureus carrying known aminoglycoside resistance determinants, as well as against an extended panel of Methicillin-resistant S. aureus isolates (n = 50). Antibiotic 1 displayed >64 fold improvement in MIC50 and MIC90 against this MRSA collection when compared to the clinically relevant aminoglycosides amikacin and gentamicin. The synthesis was achieved in six steps and 15% overall yield.

Discovery of TD-4306, a long-acting β2-agonist for the treatment of asthma and COPD.

A multivalent approach focused on amine-based secondary binding groups was applied to the discovery of long-acting inhaled β2-agonists. Addition of amine moieties to the neutral secondary binding group of an existing β2-agonist series was found to provide improved in vivo efficacy, but also led to the formation of biologically active aldehyde metabolites which were viewed as a risk for the development of these compounds. Structural simplification of the scaffold and blocking the site of metabolism to prevent aldehyde formation afforded a potent series of dibasic β2-agonists with improved duration of action relative to their monobasic analogs. Additional optimization led to the discovery of 29 (TD-4306), a potent and selective β2-agonist with potential for once-daily dosing.