Design, synthesis, crystal structures, and antimicrobial activity of sulfonamide boronic acids as β-lactamase inhibitors

Abstract

We investigated a series of sulfonamide boronic acids that resulted from the merging of two unrelated AmpC β-lactamase inhibitor series. The new boronic acids differed in the replacement of the canonical carboxamide, found in all penicillin and cephalosporin antibiotics, with a sulfonamide. Surprisingly, these sulfonamides had a highly distinct structure-activity relationship from the previously explored carboxamides, high ligand efficiencies (up to 0.91), and K(i) values down to 25 nM and up to 23 times better for smaller analogues. Conversely, K(i) values were 10-20 times worse for larger molecules than in the carboxamide congener series. X-ray crystal structures (1.6-1.8 Å) of AmpC with three of the new sulfonamides suggest that this altered structure-activity relationship results from the different geometry and polarity of the sulfonamide versus the carboxamide. The most potent inhibitor reversed β-lactamase-mediated resistance to third generation cephalosporins, lowering their minimum inhibitory concentrations up to 32-fold in cell culture.

Figure 1

Penicillin and cephalosporin antibiotics, and two clinically used β-lactamase inhibitors.

Figure 2

Boronic acids as transition-state analogs. A Hydrolytic attack on the β-lactam ring of a cephalosporin and formation of the high-energy intermediate. B Binding of β-lactamase to a boronic acid and formation of a transition-state analog.

Figure 3

Inspiration for sulfonamide boronic acids. A The carboxamide of boronic acid 18c hydrogen bonds with Asn152 and Ala318 of AmpC β-lactamase (PDB code 1MXO). Carbon atoms of inhibitor colored green, oxygens red, nitrogens blue, sulfur yellow, boron atom amaranth pink. B The sulfonamide of a non-covalent β-lactamase inhibitor hydrogen bonds with Asn152, Ala318, Ser64 and Lys67 (PDB code 1L2S). Carbon atoms of inhibitor colored cyan, oxygens red, nitrogens blue, sulfur yellow, chloride green.

Figure 4

Stereo views of initial F_o-F_c electron densities (green, 2.5 σ contour level) of sulfonamide boronic acids and final 2F_o-F_c densities (blue, 1 σ contour level) of AmpC and conserved water molecules. A Compound 4 in chain A. B Compound 9. C Compound 17. Inhibitor carbons in green, enzyme carbons in gold, oxygens red, nitrogens blue, sulfurs yellow, borons amaranth pink.

Figure 5

Polar interactions of sulfonamide boronic acids with AmpC and comparison of conformations. Enzyme residues depicted in atom-colors (grey carbons, red oxygens, blue nitrogens), hydrogen bonds shown as red dashes, water molecules as red spheres. A Compound 4 (green carbon atoms) in chain A. B Compound 9 (cyan carbon atoms). C Compound 17 (magenta carbon atoms). D Superposition of compound 9 on compound 4. E Superposition of compound 17 on compound 4. F Superposition of a model of 17 (cyan carbon atoms) on its actual crystal structure (magenta carbon atoms). See discussion.

Figure 6

Inhibition of bacterial growth and potentiation effect. A Activity of compounds 4 (CP4) and 9 (CP9) in combination with ceftazidime (CAZ) and alone against a Klebsiella pneumoniae strain producing AmpC β-lactamase. Solvent (SOL): DMSO/Water 1:1. B Potentiation of β-lactamase inhibition in an E. cloacae strain in which ampC gene expression is inducible by β-lactam antibiotics, such as cefoxitin (FOX). C Potentiation of β-lactamase inhibition in a Klebsiella pneumoniae isolate producing the inducible class C enzyme DHA-1.

Scheme 1

The conversion of carboxamide boronic acids into sulfonamide boronic acids.

Scheme 2

General scheme of the synthesis of sulfonamidomethaneboronic acids.

Scheme 3

Asymmetric synthesis of 1-sulfonamido-2-phenylethaneboronic acids 16 and 17.