Structure-guided functional studies of plasmid-encoded dihydrofolate reductases reveal a common mechanism of trimethoprim resistance in Gram-negative pathogens.

Two plasmid-encoded dihydrofolate reductase (DHFR) isoforms, DfrA1 and DfrA5, that give rise to high levels of resistance in Gram-negative bacteria were structurally and biochemically characterized to reveal the mechanism of TMP resistance and to support phylogenic groupings for drug development against antibiotic resistant pathogens. Preliminary screening of novel antifolates revealed related chemotypes that showed high levels of inhibitory potency against Escherichia coli chromosomal DHFR (EcDHFR), DfrA1, and DfrA5. Kinetics and biophysical analysis, coupled with crystal structures of trimethoprim bound to EcDHFR, DfrA1 and DfrA5, and two propargyl-linked antifolates (PLA) complexed with EcDHFR, DfrA1 and DfrA5, were determined to define structural features of the substrate binding pocket and guide synthesis of pan-DHFR inhibitors.

Toward Broad Spectrum Dihydrofolate Reductase Inhibitors Targeting Trimethoprim Resistant Enzymes Identified in Clinical Isolates of Methicillin Resistant Staphylococcus aureus.

The spread of plasmid borne resistance enzymes in clinical Staphylococcus aureus isolates is rendering trimethoprim and iclaprim, both inhibitors of dihydrofolate reductase (DHFR), ineffective. Continued exploitation of these targets will require compounds that can broadly inhibit these resistance-conferring isoforms. Using a structure-based approach, we have developed a novel class of ionized nonclassical antifolates (INCAs) that capture the molecular interactions that have been exclusive to classical antifolates. These modifications allow for a greatly expanded spectrum of activity across these pathogenic DHFR isoforms, while maintaining the ability to penetrate the bacterial cell wall. Using biochemical, structural, and computational methods, we are able to optimize these inhibitors to the conserved active sites of the endogenous and trimethoprim resistant DHFR enzymes. Here, we report a series of INCA compounds that exhibit low nanomolar enzymatic activity and potent cellular activity with human selectivity against a panel of clinically relevant TMP resistant (TMPR) and methicillin resistant Staphylococcus aureus (MRSA) isolates.

Synthesis and structure of a carbohydrate-fused [15]-macrodilactone.

The design, synthesis and structural characterization of a new α-d-glucose fused [15]-macrodilactone is reported. The macrolide was synthesized by a route involving sequential acylations of glucose at the C4' and C6' hydroxyl groups followed by an intramolecular Stille reaction previously established for other [15]-macrodilactones. Analysis of the X-ray crystallographic structure of the macrolide revealed a unique conformation of this macrocycle that differs from earlier models for [13]- and [15]-macrodilactones. Organizing the three planar units and the pyranose moiety into a macrocyclic ring resulted in a cup-shaped structure with planar chirality. Further, the gt conformation of the exocyclic hydroxymethyl group in the glucose unit was found to be crucial for controlling the planar chirality and, hence, governing the molecular shape and overall topology of the compound.

Synthesis, structure and reactivity of [15]-macrodilactones.

A strategy for utilizing parameters such as the ring size, planar units and the connections between them, and the location of asymmetric centers has been applied to the design and synthesis of a new class of 15-membered macrocycles. The interplay between three planar units in combination with a hinge atom and a stereogenic center, introduces a planar chirality that defines the molecular topology of these [15]-macrodilactones.

A flexible and unified strategy for syntheses of cladospolides A, B, C, and iso-cladospolide B.

A simple, efficient and flexible strategy for the syntheses of cladospolides A-C and iso-cladospolide B is reported here. This strategy involves Julia-Kocienski olefination and Yamaguchi macrolactonization as key steps, starting from either d-ribose or suitable tartaric acid esters. Although our initial efforts towards cladospolide A involving a ring closing metathetic approach were not successful, changing the mode of ring closure and the use of Julia-Kocienski olefination for the construction of the key intermediate solved this issue and paved the way for the completion of total syntheses of this class of natural products.