Manuka Ghosh

Iron transport-mediated drug delivery using mixed-ligand siderophore-β-lactam conjugates

BACKGROUND Assimilation of iron is essential for microbial growth. Most microbes synthesize and excrete low molecular weight iron chelators called siderophores to sequester and deliver iron by active transport processes. Specific outer membrane proteins recognize, bind and initiate transport of species-selective ferric siderophore complexes. Organisms most often have specific receptors for multiple types of siderophores, presumably to ensure adequate acquisition of the iron that is essential for their growth. Conjugation of drugs to synthetic hydroxamate or catechol siderophore components can facilitate active iron-transport-mediated drug delivery. While resistance to the siderophore-drug conjugates frequently occurs by selection of mutants deficient in the corresponding siderophore-selective outer membrane receptor, the mutants are less able to survive under iron-deficient conditions and in vivo. We anticipated that synthesis of mixed ligand siderophore-drug conjugates would allow active drug delivery by multiple iron receptor recognition and transport processes, further reducing the likelihood that resistant mutants would be viable. RESULTS Mixed ligand siderophore-drug conjugates were synthesized by combining hydroxamate and catechol components in a single compound that could chelate iron, and that also contained a covalent linkage to carbacephalosporins, as representative drugs. The new conjugates appear to be assimilated by multiple active iron-transport processes both in wild type microbes and in selected mutants that are deficient in some outer membrane iron-transport receptors. CONCLUSIONS The concept of active iron-transport-mediated drug delivery can now be extended to drug conjugates that can enter the cell through multiple outer membrane receptors. Mutants that are resistant to such conjugates should be severely impaired in iron uptake, and therefore particularly prone to iron starvation.

Synthesis and in vitro antibacterial activity of spermidine-based mixed catechol- and hydroxamate-containing siderophore—Vancomycin conjugates

The first antibiotic conjugates of vancomycin (1) and siderophore analogues containing spermidine-based catechol ligands (conjugate 11) as well as mixed catechol and hydroxamate ligands (conjugate 13) are described. The design of the conjugates was based on the earlier observation that conjugation of siderophore components to beta-lactam antibiotics induced active iron transport-mediated drug delivery. The novel conjugates (11 and 13) were synthesized by selective acylation of the primary amino group of 1. Preliminary biological studies indicated that siderophore modified vancomycins lost some activity (4- to 16-fold) against Gram-positive bacteria relative to vancomycin itself, and were generally similar to vancomycin in activity against Gram-negative bacteria under iron-sufficient conditions. However, under iron-depleted conditions which mimic human serum, conjugate 11 displayed enhanced antibacterial activity against an antibiotic hypersensitive strain of Pseudomonas aeruginosa.

Design, synthesis, and biological evaluation of isocyanurate-based antifungal and macrolide antibiotic conjugates: Iron transport-mediated drug delivery

The syntheses and preliminary biological evaluation of conjugates of a synthetic isocyanurate-based trihydroxamate siderophore with two antifungal agents, 5-FU (conjugate 9) and norneoenactin (conjugate 12), and a macrolide antibiotic, erythromycylamine (conjugate 18), are described. A 19F NMR study was used to determine the hydrolytic stability of conjugate 9 under assay conditions. Preliminary biological studies with ferric complexes of conjugates 9 and 12 indicated that these antifungal agents are recognized by Candida and perhaps are actively transported into the cell by the siderophore-transport mechanisms. While conjugate 18 did not show any significant antibacterial activity, presumably due to size restriction, the 5-FU conjugate 9 appeared to be moderately active against a variety of Gram-positive strains, and was more active than the 5-FC control against some strains of Staphylococcus.

SELECTIVE GROWTH PROMOTION AND GROWTH INHIBITION OF GRAM-NEGATIVE AND GRAM-POSITIVE BACTERIA BY SYNTHETIC SIDEROPHORE-BETA -LACTAM CONJUGATES

Conjugates of a carbacephalosporin with hydroxamate, spermexatol, N,N-bis(2,3-dihydroxybenzoyl)-L-lysine, mixed catecholate/hydroxamate and cyanuric acid-based siderophores were investigated for their potential to promote growth of siderophore indicator strains of Gram-negative and Gram-positive bacteria under iron depleted conditions, for their antibacterial activity and for their ability to use iron transport path-ways to penetrate the Gram-negative bacterial outer membrane. The selective growth promotion of enter-obacterial and pseudomonas strains by hydroxamate, spermexatol and mixed catecholate-hydroxamate siderophore-based conjugates bearing a L- or D-amino acid spacer was correlated with TonB dependent uptake routes. The preferred outer membrane siderophore receptor used in Escherichia coli was found to be Fiu, followed by Cir. Antagonistic effects of siderophores administered with the conjugates to determine antibacterial activity confirmed the active transport of conjugates via siderophore receptors. All of the conjugates were still able to diffuse through the porin proteins OmpC and OmpF. Nevertheless, strong inhibition of E. coli and Pseudomones aeruginosa outer membrane mutants DC2 and K799/61 compared to the parent strains indicated inefficient penetrability of all types of conjugates tested. Mycobacterium smegmatis SG 987 was able to use all of the siderophore-cephalosporin conjugates as growth promotors. Consequently there was no growth inhibition of this strain.

Synthesis, bioactivity, and DNA-cleaving ability of desferrioxamine B-nalidixic acid and anthraquinone carboxylic acid conjugates

Abstract Synthesis and in vitro activity of two novel conjugates ( 4 and 5 ) of desferrioxamine B, with nalidixic acid ( 2 ) and anthraquinone carboxylic acid ( 3 ), are described.

Synthesis of a bicyclic oxamazin. A novel heteroatom activated β-lactam

Abstract The synthesis of the bicyclic oxamazin 23 was accomplished by an intramolecular cyclization process. Allyl protected N-hydroxy β-lactam 22 served as the key intermediate during a simultaneous deprotection and cyclization in the presence of Pd(0).

Siderophore Conjugates of Daptomycin are Potent Inhibitors of Carbapenem Resistant Strains of Acinetobacter baumannii

Development of resistance to antibiotics is a major medical problem. One approach to extending the utility of our limited antibiotic arsenal is to repurpose antibiotics by altering their bacterial selectivity. Many antibiotics that are used to treat infections caused by Gram-positive bacteria might be made effective against Gram-negative bacterial infections, if they could circumvent permeability barriers and antibiotic deactivation processes associated with Gram-negative bacteria. Herein, we report that covalent attachment of the normally Gram-positive-only antibiotic, daptomycin, with iron sequestering siderophore mimetics that are recognized by Gram-negative bacteria, provides conjugates that are active against virulent strains of Acinetobacter baumannii, including carbapenemase and cephalosporinase producers. The result is the generation of a new set of antibiotics designed to target bacterial infections that have been designated as being of dire concern.